PAPERS PRESENTED TO

IMPACTS AND THE ORIGIN, EVOLUTION,

AND EXTINCTION OF LIFE

A RUBEY COLLOQUIUM

February 9 & 10, 2002

Department of Earth and Space SciencesUniversity of California, Los Angeles

Held in Room 3400 Boelter Hall, UCLA

Sponsored byUCLA Earth and Space Sciences Department

UCLA Institute of Geophysics and Planetary PhysicsNASA Astrobiology Institute

Cover art © Don Davis

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CONTENTSDoes diversification and extinction in North American mammals correlatewith the Cenozoic impact record?

John Alroy……………………………………………………………………………….. 1

Compiling the evidence for impact at seven mass extinctionsWalter Alvarez………………………………………………………………………….. 3

Asteroid impact tsunamisErik Asphaug, Steve Ward, and Don Korycansky………………………………………. 8

Fullerenes and interplanetary dust (IDPs) at the Permian-Triassicboundary

Luann Becker and Robert J. Poreda…………………………………………………….. 10

Laboratory Studies of Shock-Induced Amino Acid Polymerization andImplications for Impact-Generated Production of Biomolecules

Jennifer G. Blank……………………………………………………………………….. 12

High-precision geochronology and the geological record of catastrophicevents

S.A. Bowring…………………………………………………………………………….. 14

The impact crater as a habitatC.S. Cockell…………………………………………………………………………….. 16

Mass extinctions in the Phanerozoic: a single cause and if yes, which?Vincent Courtillot……………………………………………………………………….. 18

Reliability of the geological record of meteorite impacts: Evidence fromSpain, and implications for astrobiology

Enrique Díaz-Martínez………………………………………………………………….. 19

Analysis of the support for and against an extra-terrestrial impact at thePermo-Triassic boundary

Douglas H. Erwin……………………………………………………………………….. 20

The Extraterrestrial 3He Record: How Far Back Can We Go?K.A. Farley, S. Mukhopadhyay, and A. Montanari…………………………………….. 22

Plankton and Isotope changes at the late Neoproterozoic Acraman impactejecta layer

Kathleen Grey, Malcolm R. Walter, and Clive R. Calver……………………………….. 23

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Mars, Panspermia, and the Origin of Life: Where did it all begin?Joseph L. Kirschvink…………………………………………………………………….. 25

The ESF scientific program: Response of the Earth System to ImpactProcesses (IMPACT)

Christian Koeberl…………………………………………….………………………….. 26

Environmental consequences of impact cratering events as a function ofambient conditions on Earth

David A. Kring………………………………………………………………………….. 28

Distinguishing comet and asteroid materials in impact depositsFrank T. Kyte………………………………………………………………………….. 31

Environmental and sedimentological effects of Archean impacts recordedin the 3.5-3.2 Ga Barberton Greenstone Belt, South Africa

Donald R. Lowe, and Gary R. Byerly………………………………………………….. 34

Origin of water on Earth and MarsJ.I. Lunine……………………………………………………………………………….. 37

Interstellar panspermiaH. J. Melosh…………………………………………………………………………….. 39

The asteroid and comet impact hazardDavid Morrison…………………………………………………………..…………….. 41

3He flux from the late Cretaceous to the early Cenozoic: Constraining thenature of extraterrestrial accretionary events

S. Mukhopadhyay, K.A. Farley, A. Montanari………………………………………….. 43

Climatic effects produced by stratospheric loading of S-bearing gasesreleased in the Chicxulub impact event

E. Pierazzo……………………………………………………………….…………….. 45

Planet formation and impactsThomas R. Quinn……………………………………………………………………….. 47

Bombardment of the Hadean Earth: Wholesome or deleterious?Graham Ryder………………………………………………………………………….. 49

Spherule event horizons: The other (and better?) record of impacts in earlyEarth history

Bruce M. Simonson and Scott W. Hassler…………………………………………….. 53

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Timing of (mass)extinctions at the K/T boundaryJan Smit………………………………………………………………………………….. 57

Molecular signatures of microbial lifeRoger E. Summons………………………………………………………………..…….. 59

Comparing the P/T, T/J and K/T events: New insights from new fieldwork

Peter Ward…………………………………………………………………….……….. 60

Large aerial bursts; an important class of terrestrial accretionary eventsJohn T. Wasson………………………………………………………………………….. 61

The comet and asteroid impactor flux on the EarthPaul R. Weissman……………………………………………………………………….. 64

Before uniformitarionism: Impacts in the HadeanKevin Zahnle…………………………………………………………………………….. 66

1

DOES DIVERSIFICATION AND EXTINCTION IN NORTH AMERICAN MAMMALSCORRELATE WITH THE CENOZOIC IMPACT RECORD?

John Alroy. NCEAS/The Paleobiology Database, University of California, Santa Barbara, CA930101

The few major mass extinction events in the fossil record have received intense study.However, background rates of extinction have proven difficult to quantify, leaving theirconnection to large-scale physical disturbances unknown. Key problems include poor temporalcorrelations and calibrations; varying time interval durations (Foote 1994); confoundinggeographic, paleoenvironmental, and taphonomic signals (e.g., Allison and Briggs 1993); themisleading use of higher taxa as proxies for species (Sepkoski and Kendrick 1993); uncorrectedsynonymies and misidentifications; the use of biased extinction rate metrics (Foote 2000); andmost importantly, variation through time in the overall amount of fossil data (Raup 1972, 1976).

A compilation of published literature concerning the Cenozoic mammals of North America(Alroy 1996, 2000) offers good control for all of these bias factors, with an entirely quantitativetime scale divided uniformly into 1.0 m.y. bins. The data set is a component of the PaleobiologyDatabase (Alroy et al. 2001). Previous analyses (Alroy et al. 2000) showed that gross changes inglobal climate recorded by oxygen isotope analyses of benthic foraminiferans have no impact onthe extinction rates, taxonomic diversity, taxonomic composition, or body mass distributions.Instead, major biotic patterns, such as a dynamic equilibrium in taxonomic diversity (Stucky1990; Alroy 1996) and a long-term trend of increase in body mass (Alroy 1998), are governed byecological interactions such as competition.

Here I present a very preliminary test of the hypothesis that major bolide impacts drivebackground extinction rates (Raup 1991). Fortunately, a comprehensive global compilation ofimpact crater dates and diameters has been published (Jetsu 1997). Unfortunately, the number ofknown post-K-T boundary Cenozoic craters is not very large, many of them have poorlyconstrained ages, and the distribution is clumped in the late Neogene, so many older cratersremain to be discovered or no longer exist.

Assuming that large craters are more likely to be sampled, I selected only the largest knowncrater within each 1.0 m.y.-long temporal bin. For binning purposes, averages of crater ageswere used when ranges were given. For the 18 bins that have a crater estimate, there is a near-zero correlation between maximum crater diameters and either extinction rates (rank-ordercorrelation rS = +0.097; n.s.) or origination rates (rS = -0.047; n.s.). Bins with craters average thesame extinction rates (Mann-Whitney U-test: t = 1.142; n.s.) and origination rates (Mann-Whitney U-test: t = 0.254; n.s.) as other post-K-T bins without craters. All of the five knownmajor mammalian biotic transitions during the Cenozoic (Alroy et al. 2001) are within 2 m.y. ofa known crater. However, in every case the crater has a diameter < 20 km, and the two largestcraters -- one 45 km and the other 100 km -- occur at times of unremarkable biotic change.

Although additional connections between the impact record need to be explored, perhapsusing other kinds of proxies, these results do challenge the "kill curve" hypothesis of Raup(1991). At least for the moment, intrinsically-governed models of biotic dynamics remain our

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best explanatory framework for understanding large-scale evolutionary phenomena.

ReferencesAllison, P. A., and D. E. G. Briggs. 1993. Paleolatitudinal sampling bias, Phanerozoic species

diversity, and the end-Permian extinction. Geology 21:65-68.Alroy, J. 1996. Constant extinction, constrained diversification, and uncoordinated stasis in

North American mammals. Palaeogeography, Palaeoclimatology, Palaeoecology 127:285-312.

--------. 1998. Cope's rule and the dynamics of body mass evolution in North Americanmammals. Science 280:731-734.

--------. 2000. New methods for quantifying macroevolutionary patterns and processes.Paleobiology 26:707-733.

Alroy, J., P. L. Koch, and J. C. Zachos. 2000. Global climate change and North Americanmammalian evolution. Paleobiology 26, supplement:259-288.

Alroy, J., C. R. Marshall, and 23 others. 2001. Effects of sampling standardization on estimatesof Phanerozoic marine diversification. Proceedings of the National Academy of Sciences,USA 98:6261-6266.

Foote, M. 1994. Temporal variation in extinction risk and temporal scaling of extinctionmetrics. Paleobiology 20:424-444.

--------. 2000. Origination and extinction components of taxonomic diversity: general problems.Paleobiology 26, supplement:74-102.

Jetsu, L. 1997. The "human" statistics of terrestrial impact cratering rate. Astronomy andAstrophysics 321:L33-L36.

Raup, D. M. 1972. Taxonomic diversity during the Phanerozoic. Science 177:1065-1071.--------. 1976. Species diversity in the Phanerozoic: an interpretation. Paleobiology 2:289-297.--------. 1991. A kill curve for Phanerozoic marine species. Paleobiology 17:37-48.Sepkoski, J. J., Jr., and D. C. Kendrick. 1993. Numerical experiments with model monophyletic

and paraphyletic taxa. Paleobiology 19:168-184.Stucky, R. K. 1990. Evolution of land mammal diversity in North America during the

Cenozoic. Pp. 375-432 in Current mammalogy, Vol. 2 (H. H. Genoways, ed.). PlenumPress, Mt. Kisco, New York, 596 pp.

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COMPILING THE EVIDENCE FOR IMPACT AT SEVEN MASS EXTINCTIONS

Walter Alvarez, Department of Earth and Planetary Science, University of California,Berkeley, CA 94720-4767 (platetec@socrates.berkeley.edu)

In 1981, at the first Snowbird Conference, on “Geological implications of impacts of largeasteroids and comets on the Earth,” those of us who were impressed by the evidence for impactat the Cretaceous-Tertiary (KT) boundary hoped, and probably even expected, that within a fewyears there would be strong evidence for impact at each of the five major mass extinctionsidentified by Raup and Sepkoski (1982). That has manifestly not been the case. Evidence ofmany kinds for a KT impact accumulated through the 1980s and culminated in the early 1990swith the recognition of the Chicxulub Crater, in the Yucatán Peninsula of Mexico. Chicxulub isthe largest impact structure known on Earth and dates precisely to the KT boundary (Hildebrandet al., 1991; Smit et al., 1992). Additional evidence for a KT impact has emerged since then.

Finding evidence for impact at other mass extinction horizons has been much more difficult.The only other mass extinction level at which abundant evidence for impact has been found isthe Eocene-Oligocene boundary. This is not even one of the five major Raup-Sepkoski events,and both the extinction and the impact evidence are spread out stratigraphically.

Through the 1980s, the major competing causal hypothesis for the KT extinction wasvolcanism, with some volcanists attributing the KT extinction to explosive volcanism and othersto flood basalt eruption of the Deccan Traps in India. The recognition of the Chicxulub structureas the largest impact crater known on Earth (Hildebrand et al., 1991) and the discovery of itsejecta precisely at the biostratigraphic horizon of the extinction (Smit et al., 1992; Alvarez et al.,1992) seemed to settle the case in favor of impact. Recently, however, the impact-volcanismdebate has reopened in a new formulation (Courtillot, 1999; Wignall, 2001). In this view,foreshadowed by an early correlation of flood basalts and mass extinctions (Rampino andStothers, 1988), flood basalts provide a general causal explanation for mass extinctions. The KTis seen as an exceptional case where a large impact happened to occur during a time of flood-basalt eruption, producing a sudden spike of extinctions within a fauna already stressed andundergoing more gradual extinction due to the effects of Deccan volcanism.

As Wignall (2001, p. 23) states, “By far the most compelling evidence for a link betweenvolcanism and extinctions comes from the comparison of the ages of flood basalt provinces andmass extinction events….” Three of Raup and Sepkoski’s five major events correlate closelywith major flood basalts (Siberian Traps @ ~250 Ma: PT; Central Atlantic Magmatic Province,or CAMP, @ ~200 Ma: TJ; Deccan @ ~65 Ma: KT). In addition, the newly recognizedEmeishan flood basalts in southwest China appear to correlate with the Middle Permian-LatePermian mass extinction, which also has recently been recognized as a major event separate fromthe PT extinction a few Myr later (Jin et al., 1994; Stanley and Yang, 1994); this event occurredat or about the Capitanian-Wuchiapingian (CW) boundary. Finally, there is a preliminary reportthat an earlier flood basalt in Siberia correlates with the Late Devonian, or Frasnian-Fammenian(FF) mass extinction (Vincent Courtillot et al., in press and personal communication).

Although I have long been a proponent of impact at the KT boundary, I hold no brief for allextinctions being caused by impact. If the evidence for a flood basalt-extinction link is

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compelling, we should accept that conclusion. However, before accepting it, we should carefullyexamine the evidence.

There are inherent asymmetries in comparing the evidence for impact and volcanism ascompeting explanations for mass extinctions. (1) Flood basalts typically cover areas of order105-106 km2, compared to 103-104 km2 for large impact craters; thus flood-basalt provinces aremore likely to be preserved and found. (2) Several varieties of ejecta can be spread worldwideand end up in the stratigraphic record, where they provide proxies for impact. Their absence at amass-extinction horizon thus weakens a claimed impact-extinction link. Distal proxies for floodbasalts have not been found in the stratigraphic record, so proposed volcanism-extinction linksare not falsifiable in this way. (3) Flood basalts erupt over intervals of one to a few millionyears, and if an extinction occurs in that interval, the flood basalt will be considered a candidateexplanation for the extinction. Impacts, being instantaneous and recorded as very thinstratigraphic horizons of ejecta, are easier to falsify, on a chronological basis, as an explanationfor any given extinction.

As noted above, volcanism as an explanation for extinction is supported primarily by agecorrelation, based on radiometric dating of the volcanic rocks or on biostratigraphic constraintsfrom units above and below a flood basalt. Impact as an explanation for extinction is based onvarious proxies in the stratigraphic record, and in three cases on craters dated radiometrically orbiostratigraphically (Chicxulub, Popigai, Chesapeake).

It would be useful to the community of researchers to have a compilation of evidence forimpact and for volcanism at prominent extinction levels. This is probably something that shouldbe prepared by a group of workers experienced in the field. The following table of evidence forimpact at times of mass extinction is intended as a start in this direction. It is based on studies Iknow about, but there are almost surely other lines of evidence that should be added. I havechosen seven mass extinctions — the five major extinctions from Raup and Sepkoski (1982),plus the recently recognized Capitanian-Wuchiapingian boundary extinction, and the Eocene-Oligocene biotic crisis, which is less important but has yielded several lines of evidence forimpact.

An important but difficult task in developing a compilation of this sort would be to makejudgements on which lines of reported evidence have subsequently been confirmed or falsified.Each line of evidence involves (1) observational and/or measurement data, and (2) aninterpretation of those data. Both data and interpretation as subject to confirmation or rejectionin studies postdating the original report. Although there may by now be widely acceptedconclusions on some lines of evidence, others remain controversial or untested. The UCLAWorkshop should be an appropriate forum for discussing whether there is a feasible way ofevaluating the current level of acceptance of different lines of evidence, how such a compilationmight be prepared, and how a parallel compilation of data on flood-basalt provinces might beconstructed.

I would welcome e-mail comments, and additions or corrections to the preliminary table.

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Table 1. A compilation observations cited as evidence in support of an impact at massextinctions.

SOURCE PROXY DATE FIRST REFERENCE(s)Eocene-Oligocene

Target Spherules 1973 (Glass et al., 1973)Bolide Iridium 1993 (Ganapathy, 1982; Alvarez et al., 1982)Target Coesite 1993 (Glass and Wu, 1993)Target Shocked quartz 1993 (Glass and Wu, 1993)Target Chesapeake crater 1996 (Poag, 1996)Target Popigai crater 1997 (Bottomley et al., 1997)Bolide Ni-rich spinels 1998 (Pierrard et al., 1998)Bolide 3He anomaly 1998 (Farley et al., 1998)Target Shocked zircon 2001 (Glass and Liu, 2001)

Cretaceous-TertiaryBolide Iridium 1980 (Alvarez et al., 1980)Bolide PGE ratios 1980 (Ganapathy, 1980; Asaro et al., 1980;

Kyte et al., 1980)Target Spherules - microkrystites 1981 (Smit and Klaver, 1981)Bolide Os isotopic ratios 1983 (Luck and Turekian, 1983)Target Shocked quartz 1984 (Bohor et al., 1984)Bolide Rhodium 1988 (Bekov et al., 1988)Target Stishovite 1989 (Bohor et al., 1984; McHone et al., 1989)Bolide? Extraterrestrial amino acids 1989 (Zhao and Bada, 1989)Target Spherules - microtektites 1990 (Izett et al., 1990)Target Shocked zircon 1990 (Bohor et al., 1990)Target Chicxulub Crater 1991 (Hildebrand et al., 1991)? Diamonds 1991 (Carlisle and Braman, 1991)Bolide Ni-rich spinels 1991 (Robin et al., 1991)Biota? Fullerenes 1994 (Heymann et al., 1994)Bolide Chondritic Cr isotopes 1998 (Shukolyukov and Lugmair, 1998)Bolide Fullerenes with He 2000 (Becker et al., 2000)Bolide Nanophase Fe-rich material 2001 (Wdowiak et al., 2001)

Triassic-JurassicTarget Shocked quartz 1992 (Bice et al., 1992)

Permian-TriassicBiota Abrupt extinction 1998 (Bowring et al., 1998)Bolide Fullerenes with 3He 2001 (Becker et al., 2001)Bolide Nanophase Fe-rich material 2001 (Verma et al., 2001)

Capitanian-Wujiapingian— NO EVIDENCE — —

Frasnian-FamennianBiota Abrupt extinction 1970 (McLaren, 1970)Bolide Iridium 1983 (Nicoll and Playford, 1993)Target Glassy microtektites 1992 (Wang, 1992; Claeys et al., 1992)

Ordovician-Silurian— NO EVIDENCE — —

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REFERENCESAlvarez, L.W., Alvarez, W., Asaro, F., and Michel, H.V., 1980, Extraterrestrial cause for the

Cretaceous-Tertiary extinction: Science, v. 208, p. 1095-1108.Alvarez, W., Asaro, F., Michel, H.V., and Alvarez, L.W., 1982, Iridium anomaly approximately

synchronous with terminal Eocene extinctions.: Science, v. 216, p. 886-888.Alvarez, W., Smit, J., Lowrie, W., Asaro, F., Margolis, S.V., Claeys, P., Kastner, M., and

Hildebrand, A.R., 1992, Proximal impact deposits at the Cretaceous-Tertiary boundary in theGulf of Mexico: A restudy of DSDP Leg 77 Sites 536 and 540: Geology, v. 20, p. 697-700.

Asaro, F., Michel, H.V., Alvarez, L.W., and Alvarez, W., 1980, Results of a dating attempt -Chemical and physical measurements relevant to the cause of the Cretaceous Tertiaryextinctions: Lawrence Berkeley Laboratory Report, v. LBL-11613.

Becker, L., Poreda, R., and Bunch, T., 2000, Fullerenes: An extraterrestrial carbon carrier phasefor noble gases: Proceedings of the National Academy of Sciences, v. 97, p. 2979–2983.

Becker, L., Poreda, R.J., Hunt, A.G., Bunch, T.E., and Rampino, M., 2001, Impact event at thePermian-Triassic boundary; evidence from extraterrestrial noble gases in fullerenes: Science,v. 291, p. 1530-1533.

Bekov, G.I., Letokhov, V.S., Radaev, V.N., Badyukov, D.D., and Nazarov, M.A., 1988,Rhodium distribution at the Cretaceous/Tertiary boundary analysed by ultrasensitive laserphotoionization: Nature, v. 332, p. 146-148.

Bice, D.M., Newton, C.R., McCauley, S.E., Reiners, P.W., and McRoberts, C.A., 1992, Shockedquartz at the Triassic-Jurassic boundary in Italy: Science, v. 255, p. 443-446.

Bohor, B.F., Foord, E.E., Modreski, P.J., and Triplehorn, D.M., 1984, Mineralogic evidence foran impact event at the Cretaceous-Tertiary boundary: Science, v. 224, p. 867-869.

Bohor, B.F., Betterton, W.J., and Foord, E.E., 1990, Shocked zircon and chromite in K/Tboundary claystones: Meteoritics, v. 25, p. 350.

Bottomley, R., Grieve, R., York, D., and Masaitis, V.L., 1997, The age of the Popigai impactevent and its relation to events at the Eocene/Oligocene boundary: Nature (London), v. 388,p. 365-368.

Bowring, S.A., Erwin, D.H., Jin, Y.G., Martin, M.W., Davidek, K., and Wang, W., 1998, U/Pbzircon geochronology and tempo of the end-Permian mass extinction: Science, v. 280, p.1039-1045.

Carlisle, D.B., and Braman, D.R., 1991, Nanometre-size diamonds in the Cretaceous Tertiaryboundary clay of Alberta: Nature, v. 352, p. 708-709.

Claeys, P., Casier, J.-G., and Margolis, S.V., 1992, Microtektites and mass extinctions: evidencefor a Late Devonian asteroid impact: Science, v. 257, p. 1102-1104.

Courtillot, V., 1999, Evolutionary catastrophes; the science of mass extinction: Cambridge,Cambridge University Press, 173 p.

Farley, K.A., Montanari, A., Shoemaker, E.M., and Shoemaker, C.S., 1998, Geochemicalevidence for a comet shower in the late Eocene: Science, v. 280, p. 1250-1253.

Ganapathy, R., 1980, A major meteorite impact on the Earth 65 million years ago: evidencefrom the Cretaceous-Tertiary boundary clay: Science, v. 209, p. 921-923.

Ganapathy, R., 1982, Evidence for a major meteorite impact on the earth 34 million years ago:Implications for Eocene extinctions.: Science, v. 216, p. 885-886.

Glass, B.P., Baker, R.N., Störzer, D., and Wagner, G.A., 1973, North American microtektitesfrom the Caribbean Sea and Gulf of Mexico: Earth and Planetary Science Letters, v. 19, p.184-192.

Glass, B.P., and Wu, J., 1993, Coseite and shocked quartz discovered in the Australatian andNorth American microtektite layers: Geology, v. 21, p. 435-438.

Glass, B.P., and Liu, S., 2001, Discovery of high-pressure ZrSiO4 polymorph in naturallyoccurring shock-metamorphosed zircons: Geology, v. 29, p. 371-373.

Heymann, D., Chibante, L.P.F., Brooks, R.R., Wolbach, W.S., and Smalley, R.E., 1994,Fullerenes in the Cretaceous-Tertiary boundary layer: Science, v. 265, p. 645-647.

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Hildebrand, A.R., Penfield, G.T., Kring, D.A., Pilkington, M., Camargo Z., A., Jacobsen, S.B.,and Boynton, W.V., 1991, Chicxulub crater: a possible Cretaceous/Tertiary boundary impactcrater on the Yucatán Peninsula, Mexico: Geology, v. 19, p. 867-871.

Izett, G.A., Maurrasse, F.J.-M.R., Lichte, F.E., Meeker, G.P., and Bates, R., 1990, Tektites inCretaceous-Tertiary boundary rocks on Haiti: U.S. Geological Survey Open-File Report, v.90-635, p. 1-31.

Jin, Y., Zhang, J., and Shang, Q., 1994, Two phases of the end-Permian mass extinction:Canadian Society of Petroleum Geologists Memoir, v. 17, p. 813-822.

Kyte, F.T., Zhou, Z., and Wasson, J.T., 1980, Siderophile-enriched sediments from theCretaceous-Tertiary boundary: Nature, v. 288, p. 651-656.

Luck, J.M., and Turekian, K.K., 1983, Osmium-187/Osmium-186 in manganese nodules and theCretaceous-Tertiary boundary: Science, v. 222, p. 613-615.

McHone, J.F., Nieman, R.A., Lewis, C.F., and Yates, A.M., 1989, Stishovite at the Cretaceous-Tertiary boundary, Raton, New Mexico: Science, v. 243, p. 1182-1184.

McLaren, D.J., 1970, Time, life and boundaries: Journal of Paleontology, v. 44, p. 801-815.Nicoll, R.S., and Playford, P.E., 1993, Upper Devonian iridium anomalies, conodont zonation

and the Frasnian-Famennian boundary in the Canning Basin, Western Australia:Palaeogeography, Palaeoclimatology, Palaeoecology, v. 104, p. 105-113.

Pierrard, O., Robin, E., Rocchia, R., and Montanari, A., 1998, Extraterrestrial Ni-rich spinel inupper Eocene sediments from Massignano, Italy: Geology, v. 26, p. 307-310.

Poag, C.W., 1996, Structural outer rim of Chesapeake Bay impact crater; seismic and bore holeevidence: Meteoritics, v. 31, p. 218-226.

Rampino, M.R., and Stothers, R.B., 1988, Flood basalt volcanism during the past 250 millionyears: Science, v. 241, p. 663-668.

Raup, D.M., and Sepkoski, J.J., Jr., 1982, Mass extinctions in the marine fossil record: Science,v. 215, p. 1501-1503.

Robin, E., Boclet, D., Bonte, P., Froget, L., Jehanno, C., and Rocchia, R., 1991, The stratigraphicdistribution of Ni-rich spinels In Cretaceous-Tertiary boundary rocks at El-Kef (Tunisia),Caravaca (Spain) and Hole-761C (Leg-122): Earth And Planetary Science Letters, v. 107, p.715-721.

Shukolyukov, A., and Lugmair, G.W., 1998, Isotopic evidence for the Cretaceous-Tertiaryimpactor and its type: Science, v. 282, p. 927-929.

Smit, J., and Klaver, G., 1981, Sanidine spherules at the Cretaceous-Tertiary boundary indicate alarge impact event: Nature, v. 292, p. 47-49.

Smit, J., Montanari, A., Swinburne, N.H.M., Alvarez, W., Hildebrand, A.R., Margolis, S.V.,Claeys, P., Lowrie, W., and Asaro, F., 1992, Tektite-bearing, deep-water clastic unit at theCretaceous-Tertiary boundary in northeastern Mexico: Geology, v. 20, p. 99-103.

Stanley, S.M., and Yang, X., 1994, A double mass extinction at the end of the Paleozoic Era:Science, v. 266, p. 1340-1344.

Verma, H.C., Upadhyay, C., Tripathi, R.P., Tripathi, A., Shukla, A.D., and N.Bhandari, 2001,Nano-sized iron phases at the K/T and P/T boundaries revealed by Mössbauer spectroscopy[extended abstract]: Lunar and Planetary Science, v. 32.

Wang, K., 1992, Glassy microspherules (microtektites) from an Upper Devonian limestone:Science, v. 256, p. 1547-1550.

Wdowiak, T.J., Armendarez, L.P., Agresti, D.G., Wade, M.L., Wdowiak, S.Y., Claeys, P., andIzett, G., 2001, Presence of an iron-rich nanophase material in the upper layer of theCretaceous-Tertiary boundary clay: Meteoritics and Planetary Science, v. 36, p. 123-133.

Wignall, P.B., 2001, Large igneous provinces and mass extinctions: Earth-Science Reviews, v.53, p. 1-33.

Zhao, M., and Bada, J.L., 1989, Extraterrestrial amino acids in Cretaceous/Tertiary boundarysediments at Stevns Klint, Denmark: Nature, v. 339, p. 463-465.

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ASTEROID IMPACT TSUNAMIS

Erik Asphaug, Steve Ward , and Don Korycansky. Earth Sciences Department, University ofCalifornia, Santa Cruz, CA U.S.A. 95064 (asphaug@es.ucsc.edu)

Asteroid impacts are hazardous in a variety of ways, from local shock effects (being "hit onthe head") to long-term perturbations of the global climate. At present we focus upon the subsetof impacts whose primary mode of devastation is through impact-generated tsunamis. Two-thirds of all meteorites strike the ocean, but for those larger than a few kilometers (such as theK/T bolide) the generation of waves may be of little consequence compared with the noxious andparticulate loading of the atmosphere and the rain of fireball debris. But asteroids and cometssmaller than ~1 km diameter might not be capable of causing global climate perturbation; forthese, tsunamis may present the most efficient means of propagating impact energy to distantshores, and thus may represent the brunt of their hazard. In the modern era, when most of theworld‚s population and economic infrastructure exists within a short distance from the coast, thethreat of a basin-wide tsunami spawned from a pelagic impact cavity must be taken seriously.

We have investigated the generation, propagation, and probabilistic hazard of tsunamisspawned by oceanic impacts, and are now in the process of refining those estimates. Our methodis to link the depth and diameter of parabolic impact craters to asteroid density, radius, andimpact velocity by means of elementary energy arguments and crater scaling rules. Then, lineartsunami theory illustrates how these transient craters evolve into vertical sea surface waveformsat distant positions and times. By measuring maximum wave amplitude at many distances froma variety of impactor sizes, we derive simplified attenuation relations that account both forgeometrical spreading and frequency dispersion of tsunami on uniform depth oceans.

As expected, tsunami wavelengths contributing to the peak amplitude coincide closely withthe diameter of the transient impact crater. For impactors smaller than a few hundred metersdiameter, crater widths are less than or comparable to mid-ocean depths. As a consequence,dispersion increases the 1/sqrt(r) long-wave decay rate to nearly 1/r for tsunami from thesesources. We apply linear shoaling theory at the wavelength associated with peak tsunamiamplitude corrects for amplifications as the waves near land. The application of linear shoalingtheory is probably adequate, insofar as nonlinear effects are highly sensitive to local coastlinebathymetry and near-shore topography: we can only model the linear component of shoaling forgeneric tsunamis in any case. But the application of linear theory in modeling tsunamipropagation is something we are currently exploring with computational hydrodynamicaltechniques.

By coupling this tsunami amplitude/distance information with the statistics of asteroid falls,we assess the probabilistic hazard of impact tsunamis by integrating contributions over alladmissible impactor sizes and impact locations. In particular, the tsunami hazard, expressed asthe Poissonian probability of being inundated by waves from 2 to 50 meter height in a 1000 yearinterval, is computed at both generic (generalized geography) and specific (real geography) sites.For a conservative estimate of the impactor flux, a generic site with 180 degrees of oceanexposure and a 6,000 km reach admits a 1:23 chance of an impact tsunami of 2 meter height or

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greater in 1000 years. The likelihood drops to 1:58 for a 5 meter wave, and to 1:476 for a 25meter wave. Specific sites of Tokyo and New York have 1:38 and 1:76 chances of suffering animpact tsunami greater than 5 m in this millennium.

As scientific and political agencies attempt to design plans for safeguarding the humanfuture, one of the cornerstone issues is to determine what diameter impactor is most dangerous.From this preliminary analysis, which does not consider all aspects of the impact hazard, andwhich presumes linear tsunami theory for its analysis of wave propagation, we conclude that themost hazardous asteroid is about 200 m diameter, of which there are tens of thousands onpotentially Earth-crossing orbits.

REFERENCES:

Gersonde, R., Kyte, F.T, Bleil, U., Diekmann, B., Flores, J.A., Gohl, K., Grahl, G., Hagen, R.,Kuhn, G., Sierro, F.J., Völker, D., Abelmann, A., and Bostwick, J.A. 1997. Geological recordand reconstruction of the late Pliocene impact of the Eltanin asteroid in the Southern Ocean.Nature 390, 357-363.

Nemtchinov, I.V., V.V. Svetsov, I.B. Kosarev, A.P. Golub, O.P. Popova, V.V. Shuvalov, R.E.Spalding, C. Jacobs and E. Tagliaferri 1997. Assessment of kinetic energy of mete-oroidsdetected by satellite-based light sensors. Icarus 130, 259-274.

Toon, O.B., K. Zahnle, R.P. Turco and C. Covey 1994. Environmental perturbations caused byasteroid impacts. In Hazards due to Comets and Asteroids, ed. T. Gehrels (Univ. ArizonaPress, Tucson), 791-826.

Ward, S.N. and E. Asphaug 2000. Asteroid impact tsunami: A probabilistic hazard assessment.Icarus 145, 64-78.

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FULLERENES AND INTERPLANETARY DUST (IDPs) AT THE PERMIAN-TRIASSICBOUNDARY

Luann Becker1 and Robert J. Poreda2. 1Department of Geological Sciences, Institute ofCrustal Studies, University of California, Santa Barbara, CA 93106, U.S.A. 2Department of Earthand Environmental Sciences, University of Rochester, Rochester, NY 14627, U.S.A.

In our recent Science paper (1) we presented new evidence that an impact occurred ~250million years ago at the Permian-Triassic boundary (PTB) triggering the most severe massextinction in the history of life on Earth. We used a new extraterrestrial tracer, fullerene; a thirdform of carbon besides diamond and graphite. By exploiting the unique properties of thismolecule to trap noble gases inside of its caged structure (helium, neon, argon), we candetermine the origin of the fullerenes (i.e. extraterrestrial, ET, or terrestrial). So far, we havefound fullerenes with ET helium associated with extinction events in five locations at the 65 myrold Cretaceous-Tertiary boundary (KTB) and in two locations in the 250 myr old PTB (1,2).While it was previously suggested that the fullerenes isolated from some KTB sediments may beassociated with global wildfires triggered by the impact event (i.e. terrestrial) it has now beenaccepted that the KTB fullerenes are extraterrestrial, delivered exogenously to the Earth duringthe impact event (3,4).

Recently Mukhopadhyay et al., (5) have suggested that asteroidal or cometary impacts can bedetermined by measuring the flux of helium-3 in bulk sediments (a proxy for the accretion ofinterplanetary dust or IDPs) over geologic time. Measurement of a near constant flux of helium-3 (3He) in sediments associated with a discrete boundary event, like the KTB, would beconsistent with an asteroidal impact (5) while an enhanced signature for IDPs coupled withmultiple impacts, like those during the Eocene (6), is consistent with a cometary event. Incontrast, a separate study by Farley and Mukhopadhyay, of 3He in volcanic ash layers across theMeishan and Shangshi PTB, indicated no signal for 3He leading them to suggest that a largeimpact did not accompany the extinction at the PTB (7). However, as pointed out in (8) thedifferences in bulk 3He concentrations reported in (7) appear to be directly attributed to sampleselection and preparation. Perhaps more importantly, the preservation of the ET signature insediments is directly related to sedimentation rates and the lithology (e.g. clay vs. volcanic ash).

For example, Farley and Patterson (9) have proposed that the flux of 3He measured in somemarine sediment cores co-vary with the 100-kyr component (eccentricity) of the climate signal(i.e. Milankovitch). Subsequent investigations of marine sediments by Marcantonio et al., (10)compared the 3He flux with thorium-230 (230Th) accumulation (an independent measurement ofsedimentation rate) and found that the mean flux of IDPs over the past 200-kyrs was essentiallyconstant (± 5%). The local burial rates of 3He and 230Th on the other hand, varied by a factor of5 over the past 450 and 200 kyr periods. These variations are consistent with sediment focusing,or lateral advection, and to the deep currents responsible for particle transport. It was also shownthat some deep-sea sediment have higher 3He and 4He concentrations and low 3He/4He ratios,consistent with an older crustal source containing larger amounts of nucleogenic and radiogenichelium (10). Thus, variations in 3He/4He ratios can be attributed to the mixing of similar IDPsbut different terriginous sources.

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One of the difficulties in assessing the true nature of 3He (e.g. terrestrial vs. extraterrestrial)in bulk sediments is that, unlike fullerene, the ‘carrier’ of the ET helium in IDPs has not beenproperly identified. Thus, it is crucial to look for the IDP-carrier in sediments to establishwhether or not 3He is truly a proxy of asteroidal or cometary impact or is simply a result ofsediment focusing (i.e. terrestrial). The identification of the IDP-carrier will provide importantconstraints on the IDP flux and the types of material contributing to the overall helium signaturein deep-sea sediments.

In this work we will present data on the IDP-carrier and the associated trapped noble gascompositions in some PTB sediments. We will also show that the abundances of IDPs arestrongly dependent upon sedimentation rates and lithology. Moreover, the IDP noble gassignature can be uniquely de-coupled from fullerene demonstrating that two separate tracers arepresent (direct flux of IDPs for 3He vs. giant impact for fullerene). The implications of our newresults support previous findings (1,2) that a large impact did accompany the mass extinction atthe PTB. In addition, the identification of a distinct IDP component in sediments redefines therole of cosmic dust and impact events in the geologic record.

References: (1) L. Becker et al., Science 291, 1530 (2001); (2) L. Becker, R. J. Poreda and T. E.Bunch Proc. of Nat. Acad. Sci 97, 2979 (2000); (3) D. Heymann et al., Science 256, 545 (1994);(4) P. Harris, R. D. Vis and D. Heymann Earth & Planetary Science Letters 183, 355 (2000); (5)Mukhopadhyay et al., Science 291, 1952 (2001); (6) Farley et al., Science 280, 1250 (1998); (7)S. Mukhopadhyay and K. A. Farley Science 293, U1 (2001); (8) L. Becker and R. J. PoredaScience 293, U3 (2001); (9) K. A. Farley and D. B. Patterson Nature 378, 600 (1995); (10)Macantonio et al., GCA 62, 1535 (1998).

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LABORATORY STUDIES OF SHOCK-INDUCED AMINO ACID POLYMERIZATIONAND IMPLICATIONS FOR IMPACT-GENERATED PRODUCTION OFBIOMOLECULES

Jennifer G. Blank. Shock Physics Group, Lawrence Berkeley National Laboratory, LivermoreCA 94551 (blank4@llnl.gov)

The discovery of relatively high abundances of organic compounds and amino acids inmeteorites, interplanetary dust particles (IDPs), and molecular clouds suggests that there arerelatively efficient mechanisms for producing fairly complicated organic compounds in space[e.g., 1,2]. In many cases, the concentration of organic compounds in extraterrestrial materialscould exceed that which may have formed on the early Earth by several orders of magnitude [3].This observation has led a number of investigators to propose that, while life itself may havearisen on the Earth, the building blocks of life – poly-aromatic hydrocarbons (PAHs) and aminoacids – may have been delivered to Earth by cometary impact.

The survivability of organic compounds during a cometary crash remains the largestuncertainty facing this theory. In order to estimate the survival rate of organic compoundsduring impact, past investigators [e.g., 4,5] have modeled the shock pressures and temperaturesin various parts of a water-ice comet and predicted, qualitatively, the fraction of organic materialthat might survive given this host environment and an estimate of the flux of impactors to theplanet. More recent work [6] has incorporated simplified rate expressions for the pyrolysis ofamino acids subjected to the impact temperatures in a cometary collision and suggests that a fewweight percent of the original organic payload could survive an impact at velocities up to 15km/s. These initial estimates of payload survival are probably inaccurate, because thebreakdown rates of the amino acids were derived from experiments conducted at 1 bar. Treatingthe destruction of organic compounds as a process solely of pyrolysis ignores the effect ofpressure on both the reaction mechanisms and kinetic pathways associated with breakdown.While little is known of the influence of extreme high pressure on organic reactions, even lessdirect information has been gleaned about organic reactions in the dynamic, high-pressureregime relevant to impact processes.

My group conducts experiments in the laboratory to investigate shock-induced chemicalchanges in organic liquids. The scope of this work is deceptively simple: we shock our liquids toextreme temperatures and pressures, recover them, and characterize their resulting chemicalevolution as a function of pressure, temperature, and duration of the high-pressure impact pulse.Here, we apply our methods to test (1) the viability of comets to transport organic compounds ofextraterrestrial origin to the Earth and (2) the potential for formation of biologically importantcompounds using the energy released during large-scale impacts. We use aqueous solutionsdoped with organic compounds as a comet proxy. Since initial demonstration of the feasibilityof these laboratory studies [7], we have expanded both our range of impact pressures and theclasses of compounds under investigation. In this study, we focus on 6 amino acids (Gly, Pro,Phe, Amb, Lys, Nor), their survival and polymerization resulting from shock loading underpressures relevant to a low-angle impact of a comet with a rocky planet.

We perform our laboratory experiments using a 6.5-m, two-stage light-gas gun with a 35-mmbarrel i.d. The gun consists of 3 major parts: a breech containing gunpowder, a pump tube filledwith a light gas (N2), and a barrel for guiding a high-velocity projectile to the target. The barrelconnects to a steel experiment tank, in which sits our sample. To simulate the impact, we fire thegun and propel a metal impactor into collision with the stationary target: our liquid sampleencased in a stainless steel capsule. Capsule design (arguably the most challenging aspect of ourexperiment) was guided loosely by that of previous recovery vessels [7-9], optimized to ourparticular gun geometry, and allows for approximately 0.1 cm3 of aqueous solution (containing1-10 mg of dissolved amino acids) to be contained inside. Impactor plates, fabricated from 1.5-

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5.0 mm-thick planar stainless steel, copper, and tantalum sheets, hit the targets at velocities of0.5-2.0 km/s, generating pressures within the samples of 11-33 GPa. Initial, 2D hydro-codemodeling of our experiments suggest we achieved uniform loading across most of the liquid fortimes on the order of a millisecond. Peak pressures and temperatures lasted for ≈ 2-8microseconds, and the duration of these peak conditions was directly proportional to thethickness of the impactor plates. These laboratory conditions are comparable to those computedfor low-angle, comet-earth collisions, with the exception that the duration of the shockcompression was significantly shorter.

Following an experiment, the capsule is perforated using a home-made vampire valve, andmaterial is extracted by syringe. Post-experiment analysis by liquid chromatography/massspectrometry revealed large fractions of the initial amino acids survived exposure to the shockevents. More importantly, the dominant reaction products were all possible dipeptide and cyclic-dipeptide pairings of the initial amino acids. Higher-order, linear peptides were also produced,though to a dimished degree. We observed distinct differences in response among the aminoacids to pressure and temperature and shock pulse duration. Our results will be discussed in thecontext of the role of exogenous delivery of biomolecules to the early Earth.

References:[1] E. Anders (1989) Nature, 342:255-257.[2] W.M. Irvine (1998) Origins Life Evol. Biosphere, 28: 365-383.[3] C.F. Chyba & C. Sagan (1992) Nature, 355:125-132.[4] P.J. Thomas P. J. & L. Brookshaw (1996) in: Comets and the Origin and Evolution of

Life, P. J. Thomas et al. (eds.), 131-145.[5] J.G. Blank & G. H. Miller (1998) in: 21st Intl. Symp. Shock Waves, A.F.P. Houwing et

al. (eds.),1467-1472.[6] E. Pierazzo & C.F. Chyba (1999) Meteoritics Planet. Sci., 34, 909-918.[7] J.G. Blank et al. (2001) Origins Life Evol. Biosphere, 31: 1-38.[8] G.T. Gray III (2000) ASM Handbook 8: 1-9.[9] L.L. Davis et al.(1995) Rev. Sci. Instr., 66:3321-3326.

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HIGH-PRECISION GEOCHRONOLOGY AND THE GEOLOGICAL RECORD OFCATASTROPHIC EVENTS

S.A. Bowring, Massachusetts Institute of Technology, Dept. Earth, Atmospheric and PlanetarySciences, Bldg. 54-1126, Cambridge, MA 02139, USA.

High-precision geochronology is essential for understanding mass extinctions, bioticrecoveries, and other major events in Earth history such as Neoproterozoic global glaciations andthe Cambrian radiation. Of particular importance for evaluating extinction mechanisms andrecoveries is a precise temporal framework for the distribution of fossils and chemostratigraphicsignals in the stratigraphic record before, during, and following an extinction event. A majorunanswered question is whether the six major Phanerozoic extinctions share a commonmechanism(s). However, of the six major extinctions, the tempo of only two (K/T and P/Tr) arereasonably well constrained.

Critical tests of extinctions caused by global catastrophes such as bolide impacts or volcaniceruptions involve establishing the contemporanity of paleontologically or chemostratigraphicallydefined boundaries in both marine and terrestrial settings at maximum possible resolution (0.1 %or better). Tests of models for volcanically influenced extinctions (e.g. SiberianTraps andPermo/Triassic) must rely on precise chronometric constraints on both extinction horizons andsuspected volcanic centers. I will review the temporal constraints on the major extinction events.At present, much confusion exists regarding the age, duration, and effects of the Siberian trapsand the Emishan basalts and age estimates vary by 6-8 Ma.

Geochronological tools which are the most useful for resolving the tempo of extinctionevents are U-Pb (zircon) and 40Ar/39Ar (feldspar,biotite, hornblende). Ar-Ar geochronology isa relative dating technique that relies on referencing the age of an unknown sample to a standard.In contrast, U-Pb zircon geochronology exploits two independent decay schemes (238U-206Pband 235U-207Pb) which allows for the evaluation of closed-system behavior. However due touncertainties in decay constants for K and the use of different flux monitors (e.g. Renne et. al;,1998;1999; Schmitz and Bowring, 2001) comparisons of dates using the two systems at the1000-500K year level is impossible. Both U-Pb zircon geochronology and Ar-Ar geochronologycan be complicated by open-system behavior such as complex daughter-product loss andinheritance (e.g. Bowring et al 1998; Mundil et al., 2000). In particular, in many pyroclasticeruptions significant quantities of slightly older volcanic material can be incorporated into theeruption column giving rise to a complex population of phenocrysts. In addition, zircons insilicic magma chambers may have non-negligible residence times prior to eruption. In principle,Ar-Ar geochronology could be used to more precisely determine the time differential betweenclosely spaced ash-beds than U-Pb because of the relative ease in taking a weighted mean ofhundreds of analyses per ash-bed. However, this approach has not been widely applied. In anycase the geochronological approach that must be followed to constrain extinction events usingeither technique is time and labor intensive: analysis of many ash-beds in stratigraphic orderfrom multiple sections, multiple samples from the same horizon, and many minerals from eachash-bed. With this approach the tempo of mass extinctions, recoveries, and geological eventsthroughout the Phanerozoic can be determined with high-precision and help constrain themechanisms of extinction.

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References: [1] Renne, P.R., et al. (1998) Science 282, 1840-1841. [2] Renne, P.R. et al. (1998)Chem. Geol. 145, 117-152. [3] Schmitz, M.D. and Bowring, S.A. (2001) Geochim CosmochimActa 65 (15): 2571-2587. [4] Bowring , S.A. et al. (1998) Science 284, 1039-1045. [5] Mundil,R. etal. (2001) Earth Planet Sci Lett 187 (1-2): 131-145 .

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THE IMPACT CRATER AS A HABITAT

C.S. Cockell, British Antarctic Survey, High Cross, Madingley Road, Cambridge. CB3 0ET.UK. (csco@bas.ac.uk)

Impact craters contain ecosystems that are often very different from the ecosystems thatsurround them. On Earth over 150 impact craters have been identified in a wide diversity ofbiomes. All natural events that can cause localized disruption of ecosystems have quite distinctpatterns of recovery. Impact events are unique in that they are the only extraterrestrialmechanism capable of disrupting an ecosystem locally in space and time. Thus, elucidating thechronological sequence of change at the sites of impacts is of ecological interest. Three phases ofrecovery are suggested following impact events (Cockell and Lee, in press). The Phase ofThermal Biology, a phase associated with the localized, ephemeral thermal anomaly generatedby an impact event. The Phase of Impact Succession and Climax, a phase marked by multipleprimary and secondary succession events both in the aquatic realm (impact crater-lakes) andterrestrial realm (colonization of paleolacustrine deposits and impact-generated substrata) thatare followed by periods of climax ecology. In the case of large-scale impact events (>104 Mt),this latter phase may also be influenced by successional changes in the global environment.Finally, during the Phase of Ecological Assimilation, the disappearance of the surface geologicalexpression of an impact structure results in a concomitant loss of ecological distinctiveness. Inextreme cases, the impact structure is buried. Impact succession displays similarities anddifferences to succession following other agents of ecological disturbance, particularlyvolcanism.

I will describe work undertaken at the Haughton impact structure in the Canadian HighArctic and at the Tswaing impact crater in South Africa. The research focuses on how thegeological units associated with the impact structures (such as breccia, impact-shocked blocksetc.) influence the biological colonisation of the sites. In the case of Haughton, we find that themechanisms of recolonization of the impact breccia melt-sheet bear resemblances to patterns ofrecolonization of volcanic substrates that have been observed directly after recent volcaniceruptions (Cockell et al., 2001a). In the case of impact shocked target rocks, we have observedthat shock metamorphosis reduces the density of the rock and increases porosity, therebyproviding a habitat for microbial invasion by cyanobacteria, specifically Chroococcidiopsis sp.

The research at Tswaing involved a study of the crater to try to quantitatively estimate thesubsurface biomolecular signature left in the crater by microbial communities (Cockell et al.,2001b). We demonstrated the potential for an enormous signature (>50,000 metric tons) ofrecalcitrant UV-screening biomolecules such as carotenoids and scytonemin left by thecyanobacterial communities. This work was intended to demonstrate that if one examinedmartian impact craters that has aqueous environments, even simple craters with diameters lessthan 1 km might be expected to have biological signatures if there was any life at all on Mars.The absence of any signature might be a good indication that such habitats were never colonized.The work dovetailed with an on-going study with Nadine Barlow to quantify impact excavationdepths on Mars for various crater sizes and to quantify the depths that are necessary to prospectfor subsurface signatures of biological activity based on terrestrial craters with a corresponding

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estimation of the required observed crater diameters needed to prospect to these depths (Cockelland Barlow, 2001).

Understanding the patterns of post-impact colonization and succession inside a crater areimportant for understanding how impact events can influence local catastrophic change inecosystems and how impact craters can act as unique cradles for the development of novelecosystems during the process of recovery. These observations have astrobiological implications.

References

Cockell, C.S., Lee, P., Schuerguer, A., Hidalgo, L., Jones, J., Stokes, D. 2001a. Microbiologyand vegetation of micro-oases and polar desert, Haughton Impact Crater, Devon Island,Canadian High Arctic. Arctic, Alpine and Antarctic Research, 33, 306-318.

Cockell, C.S., Brandt, D., Hand, K., Lee, P. 2001b Microbial mats in the Tswaing impact crater,South Africa – Results of a South African exobiology expedition and implications for thesearch for biological molecules on Mars. LPSC Abstracts.

Cockell, C.S. and Barlow, N. 2001. Impact excavation and the search for subsurface life onMars. Icarus (in press).

Cockell, C.S. and Lee, P. 2001. The biology of impact craters - a review. Biological Reviews (inpress).

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MASS EXTINCTIONS IN THE PHANEROZOIC: A SINGLE CAUSE AND IF YES,WHICH?

Vincent Courtillot Moore Scholar (on leave from University Paris 7 and IPG) Division ofGeological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125,USA

Impacts, flood basalts, but also sea-level changes, anoxic events, and even mechanismsrelated only to biological population dynamics are considered as possibles agents of massextinction. I will review recent progress in dating continental flood basalts and show that in anincreasing number of cases (say between 5 and 10, thus not all) correlation with a massextinction, or/and a major anoxic event (or thermal event), sometimes without mass extinction, iscompatible with the most accurate data:

Ethiopian - lower/upper Oligocene, Greenland - late Paleocene thermal maximum, Deccan -K/T, Carribean and Madagascar - Cenomanian/Turonian, Ontong/Java - Barremian/Aptian,Parana - end of Jurassic (???), Karoo - early Toarcian, Central Atlantic Magmatic Province - endof Triassic, Siberian traps - Permo/Triassic, Emeishan - end-Guadalupian.

The traps responsible for the 360 million year Frasnian-Famennian extinction may have beenfound, completing the record now back to the Devonian.

In contrast, the KT impact remains the only well documented case and many impacts do notcorrelate with a mass extinction.

That both impacts and volcanism occurred and correlate with some mass extinctions nowseems established, though many aspects remain to be tested (see W. Alvarez abstract whichraises important points on falsifiability). One can now ask what is the share of impact versusDeccan in the KT extinction, and how much the impact would have achieved alone without theflood basalt. Sea level variations will also be considered and can rather readily be associated withflood basalts, suggesting (at least to me) that internal pulsations of Earth geodynamics and platetectonics exert the principal but certainly not the only) control on the few, brief episodes when"survival of the fittest" was replaced by "survival of the luckiest".

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RELIABILITY OF THE GEOLOGICAL RECORD OF METEORITE IMPACTS:EVIDENCE FROM SPAIN, AND IMPLICATIONS FOR ASTROBIOLOGY

Enrique DÍAZ-MARTÍNEZ. Centro de Astrobiología (CSIC-INTA), Crtra. Ajalvir km 4,28850 Torrejón de Ardoz, Madrid, SPAIN <diazme@inta.es>

The rate at which the surface of Earth is being cratered can be measured by analysing the sizeand age of craters found on certain stable areas. Assumptions on the erosion rate, type ofimpactor, target rock properties, etc., can be accounted for, but the crucial consideration iswhether each structure is or is not impact-related. Research on meteorite impacts and impactcraters currently underway at the new Spanish Astrobiology Center includes the review of theregional geological record. A thorough review of the existing evidence has demonstrated howvulnerable is the cathalog obtained from the record, as new techniques are used in hypothesistesting and the search for new potential sites. The few Spanish case examples represent thevariety of possibilities in the assessment of impact craters and shock-metamorphosed rocks: onethat now is but previously was not considered an impactite (El Gasco pumice: established in1953 as volcanic, and now considered as impact-related), one that now is not but previously wasconsidered an impact crater (Azuara structure: established in 1985 as impact-related, and nowwithdrawn from databases), and one that was and still is considered a possibility (Hervíasstructure: established in 1990 and still unconfirmed as impact-related).

The reliability of the interpretations depends on the unequivocal character of the criteria usedin the determination of shock metamorphism. This includes the use of electron microbeamtechniques (SEM, EDXRA, EPMA), detailed geochemistry of PGEs and Os isotopes (ICP-MS,neutron activation), TEM identification of PDFs, etc. Because most of these techniques are notreadily available to researchers in many countries, it is only through scientific cooperation thatthe impact hypothesis can be tested, including joint cooperative research with access to thespecific laboratory techniques. Some impact events have been proposed and accepted based onlyon equivocal evidence which may have alternative interpretations. Applications of impactresearch in astrobiology include estimates of cratering rates, effects on atmosphere (volume,composition, evolution), survivability of microorganisms (meteorite and target), mass extinctionsand climate change, and more. It is therefore important to confirm with unequivocal evidenceeach impact event that is proposed, before it is included in the databases to be used in estimatesand calculations.

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ANALYSIS OF THE SUPPORT FOR AND AGAINST AN EXTRA-TERRESTRIALIMPACT AT THE PERMO-TRIASSIC BOUNDARY

Douglas H. Erwin. Department of Paleobiology, MRC-121, Smithsonian Institution,Washington, DC 20560 (erwin.doug@nmnh.si.edu)

Two severe mass extinctions brought the Paleozoic to a close: one at the end of theGuadalupian, or Middle Permian (ca. 260 Ma) and a second at the close of the ChanghsingianStage (251.6 Ma). These were the most severe extinctions of the Phanerozoic and triggered anextensive reorganization of marine ecosystems and less pervasive changes in terrestrialecosystems. The marine extinctions were selective, with epifaunal, suspension feeders moreheavily effected than other clades, although significant variations occurred even within theseclades. Relatively little is known about the first extinction pulse: the marine extinction wassevere although some estimates appear to have been exaggerated by sampling effects, and theextinction appears to correspond with a major marine regression. Whether a mass extinctionoccurred at the same time among terrestrial organisms remains unclear, in part due to difficultiesin correlating between marine and terrestrial sections. At this point relatively few possible causescan be excluded from consideration. In south China the Changhsingian marine extinction isnearly catastrophic, occurring in less than 500 ky. (Bowring et al., 1998; Jin et al., 2000). U/Pbsingle crystal zircon dating of at least two sections other than Meishan with tight biostratigraphiccorrelation confirm the U/Pb dates for the Meishan section. On land, vertebrates, plants andinsects all experienced major extinctions. The Changhsingian event coincides with a drop of13C from about +2 to -2 per mil in both marine and terrestrial sections (although with someevidence of a latitudinal gradient in the isotopic shift: Krull, et al., 2000); shifts in sulfur andstrontium isotopes; with the eruption of the massive Siberian continental flood basalts; and withevidence of deep and shallow-water marine anoxia. Additionally, there is growing evidencefrom Russia, Australia and possibly South Africa for rapid global warming at the boundary andinto the earliest Triassic. The increase in fungal spores, interpreted as evidence of disruption ofterrestrial ecosystems, and onset of deep-water anoxia both begin before any evidence ofdisruption of shallow marine ecosystems.

The causes of the great end-Permian mass extinction must be consistent with this evidence.Although the cause of the extinction events remain unclear, significant advances have been madein the past decade, and a series of firm constraints on speculation have been established (Erwin,Bowring and Jin, 2002). Leading contenders for the cause are the climatic effects, including acidrain and global warming from the eruption of the Siberian flood basalts; marine anoxia or carbondioxide poisoning; or an extra-terrestrial impact. Although no conclusive evidence for extra-terrestrial impact has been produced, much of the available data is consistent with such amechanism. The principle pieces of evidence that are not consistent with an impact mechanismare: the early onset of deep water anoxia, the early onset of the fungal spike (at least 500 ky.before the marine extinction in S. China) and possibly the duration of the vertebrate extinctionsin the Karoo of South Africa. There has been extensive speculation that the Siberian floodbasalts may have been impact-induced. Beyond the general problems with such a hypothesis(see Melosh abstract) the Siberian flood basalts include four identified centers spread over atleast 1000 km. Any impactor sufficient to trigger massive flood basalts would have necessarilyproduced impact evidence far more extensive than that seen at the KT boundary. The most

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intriguing possibility is that the greatest mass extinction of the Phanerozoic left signals verysimilar to the end-Cretaceous mass extinction but was produced by entirely earth-boundprocesses. If true, this would tell us much more about the nature of ecosystems and how they failthan would identification of another impact event.

References:Bowring, S. A., D. H. Erwin, Y.G., Jin, M. W. Martin, K. Davidek, and W. Wang. 1998.

Geochronological constraints on the end-Permian mass extinction. Science 280:1039-1045.Erwin, D. H., Bowring, S. A., and Jin, Y. G. 2002. The End-Permian Mass Extinctions. In:

Catastrophic Events and Mass Extinctions: Impacts and Beyond. C. Koeberl and K. G.MacLeod, eds. Geological Society of America Special Paper

Jin, Y. G., Y. Wang, W. Wang, Q. H. Shang, C. Q. Cao, and D. H. Erwin. 2000. Pattern ofMarine mass extinction near the Permian-Triassic boundary in South China. Science289:432-436

Krull, E.S., Retallack, G.J., Campbell, H.J., and Lyon, G.L., 2000, 13Corg chemostratigraphy ofthe Permian-Triassic boundary in the Maitai Group, New Zealand: evidence for high-latitudinal methane release: New Zealand Journal of Geology and Geophysics, 43:21-32.

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THE EXTRATERRESTRIAL 3HE RECORD: HOW FAR BACK CAN WE GO?

K.A. Farley1, S. Mukhopadhyay1, and A. Montanari2. 1Division of Geological and PlanetarySciences, California Institute of Technology, Pasadena, CA 91125 U.S.A. 2OssoveratorioGeologico do Coldigioco, I-62020 Frontale di Apiro, ITALY

Cosmochemical and planetary differentiation processes have caused the Earth to beextremely depleted in the non-radiogenic isotope 3He compared to extraterrestrial bodies, and asa consequence the presence of even tiny amounts of extraterrestrial matter can be detected inseafloor sediments. Measurements made almost 40 years ago confirm the presence ofextraterrestrial 3He, and more recent work indicates that the majority of this helium is derivedfrom the accumulation of interplanetary dust particles (IDPs) in the sediments. Only the smallestof IDPs (perhaps a few to a few tens of microns in diameter) can escape intense atmosphericheating and so retain He; in contrast larger IDPs and bolides are so intensely heated that theylose their He, leaving no obvious 3He record in the sediments. Sediments thus provide a potentialarchive of IDP accretion over geologic time.

IDPs are thought to derive from asteroid collisions and from active comets, in presentlyunknown proportions. Because the solar system residence time of IDPs (< 105 yrs) is shortcompared to the recurrence period of significant dust producing events (major asteroid collisions,comet showers), the sedimentary 3He record provides a means to identify when and how oftensuch events occur. For example, a 3 Myr episode of enhanced 3He accretion correlated with twolarge impact events in the Late Eocene (Popigai, Chesapeake Bay) may be the signature of ashower of long period comets, several of which struck Earth (Farley et al., 1998).

Because 3He is sensitive to the delivery of IDPs rather than large bolides, it is an excellentcomplement to other impact indicators such as platinum group metals, shocked quartz, etc. Giventhe paucity of unambiguous evidence for impacts in the geologic record, IDP-hosted 3He mayprovide a useful new method for identifying and evaluating periods of probable extraterrestrialimpacts, impact recurrence intervals and possible mechanisms, and their role in terrestrialprocesses. For example, many of the major extinction boundaries of the Mesozoic have at onetime or another been attributed to impact events, yet no compelling evidence exists to confirmthese proposals. These boundaries are a current focus of our research. To date, our limitedanalyses reveal little or no extraterrestrial 3He in association with the Permian-Triassic (250 Ma),Triassic-Jurassic (208 Ma) and Jurassic-Cretaceous (144 Ma) boundaries. At face value theseobservations argue against a comet-shower like event at any of these boundaries.

It is clear from several different studies that IDP-hosted 3He survives diffusional anddiagenetic loss for at least 75 Myr in sediments. But how far back can the record be taken? Canour null results be attributed to 3He loss rather than absence of substantial extraterrestrial events?Analyses of the 450 Myr old Kinnekule limestone reveal the presence of extraterrestrial 3He andso suggest long-term He preservation, but this limestone is atypical in that it hosts anextraordinary abundance of fossil meteorites. Laboratory step-heating experiments designed toestablish He diffusivity under natural conditions suggest only partial loss of He through theMesozoic, but the uncertainties in such measurements are large. Systematic experiments are inprogress to establish if and when extreme He loss becomes apparent.

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PLANKTON AND ISOTOPE CHANGES AT THE LATE NEOPROTEROZOICACRAMAN IMPACT EJECTA LAYER.

1Kathleen Grey, 2Malcolm R. Walter, and 3Clive R. Calver. 1Geological Survey of WesternAustralia, 100 Plain Street, East Perth, Western Australia, 6004, 2Australian Centre forAstrobiology, Macquarie University, NSW, Australia 2109, 3Mineral Resources Tasmania, POBox 56, Rosny Park, Tasmania, Australia 7018.

The c.580 Ma Acraman structure is one of the ten largest and best-documented impactstructures in the geological record and meets all recognition criteria for large bolide impacts. Theimpact site (Lake Acraman, Gawler Craton, South Australia) displays typical crateringmorphology, geophysical characteristics, shock metamorphic features (devitrified melt particles,shatter cones, and shocked quartz), and is associated with microspherules, an iridium anomaly,evidence of a tsunami, and a debris layer. A collapse crater ~90 km in diameter indicates impactby a 4.8 km-diameter bolide. The associated ejecta layer is known from 12 outcrops >200 km Eof the impact site, in drillhole WWD1 near Lake Torrens, and from drillholes >500 km W of theimpact structure (Lake Maurice West 1, Observatory Hill 1, and as redeposited ejecta in Munta1). The ejecta blanket is consistent in extent with patterns for 80-90 km diameter lunar craterssuch as Tycho and Copernicus.

The ejecta horizon is a significant synchronous marker layer that provides an appropriatedatum for plotting relative distributions of planktonic acritarch species observed duringbiostratigraphic studies of the terminal Proterozoic of Australia. As data compilation progressed,a remarkable coincidence between the first appearance of the acanthomorphs (spiny acritarchs)after the Marinoan glaciation, a negative δ15Corg excursion, and the Acraman impact ejecta layerbecame apparent. The observed biotic changes are radical. An older leiosphere-dominatedpalynoflora is abruptly succeeded by a complex acanthomorph-dominated palynoflora. There is arapid increase in abundance, size, morphological complexity, and taxonomic diversity. Thesechanges indicate a major evolutionary radiation in the green algae. The actual or presumedposition of the transition matches a negative δ 13Corg excursion that coincides with the Acramanimpact layer, and indicates a rapid fall in organic productivity followed by a steady rise.Preliminary results suggest the Ediacarian acritarch diversification may be a recovery eventfollowing a bolide impact, which may also have triggered other significant biotic changes.

Whether the events are related or fortuitous requires further investigation. Evidence for arelationship between the impact event and the observed changes in acritarch assemblages is stillcircumstantial, but the demonstrated large bolide impact supplies a plausible explanation fordramatic biotic changes that are otherwise difficult to explain.

ReferencesWalter, M. R., Veevers, J. J., Calver, C. R., Gorjan, P. & Hill, A. C. Dating the 840–544 Ma

Neoproterozoic interval by isotopes of strontium, carbon, and sulfur in seawater, and someinterpretative models. Precambrian Res. 100, 371–433 (2000).

Grey, K. Ediacarian palynology of Australia. Memoirs of the Australian Association ofPalaeontologists (in press).

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Calver, C. R. Isotope stratigraphy of the Ediacarian (Neoproterozoic III) of the Adelaide RiftComplex, South Australia, and the overprint of water column stratification. PrecambrianRes. 100, 121–150 (2000).

Calver, C. R. & Lindsay, J. F. Ediacarian sequence and isotope stratigraphy of the Officer Basin,South Australia. Austral. J. Earth Sci. 45, 513–532 (1998).

Gostin, V. A., Haines, P. W., Jenkins, R. J. F., Compston, W. & Williams, I. S. Impact ejectahorizon within late Precambrian shales, Adelaide Geosyncline, South Australia. Science 233,198–200 (1986).

Gostin, V. A., Keays, R. R. & Wallace, M. W. Iridium anomaly from the Acraman impact ejectahorizon: impacts can produce sedimentary iridium peaks. Nature 340, 542–544 (1989).

Wallace, M. W., Gostin, V. A. & Keays, R. R. Discovery of the Acraman impact ejecta blanketin the Officer Basin and its stratigraphic significance. Austral. J. Earth Sci. 36, 585–587(1989).

Wallace, M. W., Gostin, V. A. & Keays, R. R. Acraman impact ejecta and host shales – evidencefor low-temperature mobilization of iridium and other platinoids. Geology 18, 132–135(1990a).

Wallace, M. W., Gostin, V. A. & Keays, R. R. Spherules and shard-like clasts from the LateProterozoic Acraman impact ejecta horizon, South Australia. Meteoritics 25, 161–165(1990b).

Wallace, M. W., Gostin, V. A. & Keays, R. R. Sedimentology of the Neoproterozoic Acramanimpact-ejecta horizon, South Australia. AGSO J. Austral. Geol. Geophys. 16, 443–451(1996).

Williams, G. E. The Acraman impact structure: source of ejecta in late Precambrian shales,South Australia. Science 233, 200–203 (1986).

Williams, G. E. Acraman, South Australia: Australia’s largest meteorite impact structure. Proc.R. Soc. Vic. 106, 105–127 (1994).

Williams, G. E., Schmidt, P. W. & Boyd, D. M. Magnetic signature and morphology of theAcraman impact structure, South Australia. AGSO J. Austral. Geol. Geophys. 16, 431–442(1996).

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MARS, PANSPERMIA, AND THE ORIGIN OF LIFE: WHERE DID IT ALL BEGIN?

Joseph L. Kirschvink, Division of Geological and Planetary Sciences, California Institute ofTechnology, Pasadena CA 91125, USA. (Kirschvink@caltech.edu)

Recent paleomagnetic studies on the Martian meteorite ALH84001 have shown that this rocktraveled from Mars to Earth with an internal temperature entirely below 40ºC. Dynamicalstudies indicate that the transfer of rocks from Mars to Earth (and to a limited extent, vice versa)can proceed on a biologically-short time scale, making it likely that organic hitchhikers havetraveled between these planets many times during the history of the Solar system. These resultsdemand a re-evaluation of the long-held assumption that terrestrial life first evolved on Earth.

I will review first the current controversies about what we do and do not know about thedirect fossil record of life in the Solar System, including early Archean microfossils andstromatolites from Australia and putative magnetofossils in the ALH84001 meteorite. Finally, Iwill try to compare the probable environments of the early Earth with that of early Mars in orderto evaluate which of these two bodies, during the first half-billion years of the solar system,might have produced an environment most suitable for the origin of life and the evolution ofbiochemical electron transport chains based on redox chemistry.

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THE ESF SCIENTIFIC PROGRAM: RESPONSE OF THE EARTH SYSTEM TOIMPACT PROCESSES (IMPACT)

Christian Koeberl, Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090Vienna, Austria

The program was launched in 1998 for a period of five years. Funding organizations fromthe following countries are participating in the program: Austria, Belgium, Finland, France,Germany, Hungary, Norway, Portugal, Sweden, Switzerland, Czech Republic and the UnitedKingdom. In 2000, Spain and Russia joined the IMPACT program. With the additional funding,the annual budget amounts to 586 kFF. In 2001, Estonia joined as well.

The ESF Impact program aims at understanding meteorite impact processes and their effectson the Earth System, including environmental, biological, climatic and geological changes, andconsequences for the biodiversity of ecosystems and global evolution of the planet. Three mainareas, each coupled with and dependent on the others and following a time sequence, can beidentified: i) Understanding the physical and chemical constraints of the impact process, ii)documenting the mechanism of transfer of the enormous energy released during an impact eventto the atmo-, hydro-, bio-, and geosphere, - this is the least understood part in the chain that maylink impact events with environmental changes - and iii) studying the short- and long-termeffects on the environment. A main goal of the program is the development and expansion ofclose cooperation between European (and other) scientists working in the field of impact studies.

The main activities of the program are: organization of workshops, support of internationalscientific exchanges through grants to young scientists (the "Mobility Grants" scheme),development of materials for teaching impact studies at undergraduate and graduate levels, and ashort course program aimed at introducing student and recent graduates to impact processes andtheir role in planetary evolution. One workshop on “Impacts and the Early Earth” was held inDecember 1998 in Cambridge, UK. Two workshops were held in each, 1999 and 2000. Twoworkshops were held in 2001 at the extreme southern and northern parts of the continent. FromMay 19-25, 2001, a workshop on “Impact Markers in the Stratigraphic Record” was held inGrenada, Spain, combined with a field trip to the famous nearby Cretaceous-Tertiary Boundarylocations of Caravaca and Agost. Another workshop took place from Aug. 29 – Sept. 3, 2001 inSvalbard (Spitsbergen), Norway, on “Submarine craters and ejecta crater correlations”. A specialsession on “Icy Impacts and Icy Targets” was included to provide a link with the planetaryscience community. Workshops are typically attended by about 50 to 80 researchers from allover the world, but mainly from Europe.

For the year 2002, two further workshops are planned. Planning for one of them, on “ImpactTectonism”, has progressed very well. The meeting will be held at the large Siljan impactstructure, in Mora, Sweden, 31 May – 3 June 2002, and will include field excursions. A secondworkshop, concentrating on the link between the geosciences and astronomy communities, willbe held in October 2002 in the Czech Republic. The IMPACT program will be extended untilmid-2003 (as program activities only started in the fall of 1998). Thus, a final (10th) workshop on“The Biology of Impact Craters” is planned for April 2003 in Cambridge, UK.

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Since 1999 we have been conducting annual short courses on "Impact Stratigraphy" at theOsservatorio Geologico di Coldigioco (OGC) near Ancona in Italy. The third such course washeld in May 2001. The course was attended by 16 students from all over Europe. These courseshave been an enormous success. Another short course was held in November of 2000 at the Riescrater (Germany) on shock metamorphism. Two other short courses on "The Geophysics ofImpact Structures", and “Mathematical Modelling of Impact Events” are still planned, but thedates have not yet been set. These courses will most likely take place in Scandinavia and Russia,respectively. The program maintains a web page and an electronic discussion list (seehttp://psri.open.ac.uk/esf/). In addition, several young European scientists have benefited fromthe “Mobility Grant” scheme of the IMPACT program, which allows them to obtain support forshort-time visits at other institutions for joint research.

One of the characteristics of the program is that all activities should be documented bypublications, including the scientific proceedings of each of the workshops, as well as shortcourse contents. The first two volumes have been published in 2000 in the series “Lecture Notesin Earth Sciences”, Springer-Verlag, Heidelberg-Berlin (Gilmour and Koeberl, Impact and theEarly Earth, LNES v. 91; Montanari and Koeberl, Impact Stratigraphy LNES v. 93). In 2001,Springer Verlag agreed to launch a new book series entitled “Impact Studies”, which will carrypublications of the IMPACT program, but is open for other impact-related books and editedvolumes as well. The chairman of the IMPACT program was asked to serve as the editor of thenew series. The first volume of this series will be the proceedings of the ESF IMPACT meetingin Quillan, France (E. Buffetaut and C. Koeberl, eds., Geological and Biological Effects ofImpact Events; Springer Verlag, Heidelberg-Berlin, ISBN 3-540-42286-2; to be related at theend of 2001). The next volumes will be the proceedings of the 2000 IMPACT meeting inFinland, on Impacts in Precambrian Shields, edited by J. Plado and L. Pesonen; the camera-readymanuscript will be sent to Springer at the end of 2001 for publication in 2002. The proceedingsof the international conference on “Catastrophic Events and Mass Extinctions: Impacts andBeyond”, which was held in Vienna, Austria, in July 2000 and which was co-sponsored by theIMPACT program, will appear as Geological Society of America Special Paper (GSA-SP) no.356 in March 2002. This volume, edited by C. Koeberl and K.G. MacLeod, carries 56 individualpapers, making it the largest of any of the “Snowbird” series books, which were all published byGSA.

Contacts to the U.S. impact community have shown that there still is no similar program oractivity in the U.S. that would make collaboration between the U.S. and Europe in this fieldeasier. Details on the IMPACT program can be found at the IMPACT program website(http://pssri.open.ac.uk/esf/)

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ENVIRONMENTAL CONSEQUENCES OF IMPACT CRATERING EVENTS AS AFUNCTION OF AMBIENT CONDITIONS ON EARTH

David A. Kring. Lunar and Planetary Laboratory, Department of Planetary Sciences, Universityof Arizona, Tucson, AZ, U.S.A.

The most important biologic effects of an impact event, including extinction, are produced byimpact-generated environmental changes. Depending on the energy involved in the impactevent, the environmental changes can be local, regional, and/or global. The best studied caseinvolves the Chicxulub impact event, 65 million years ago, which has been linked to theCretaceous/Tertiary (K/T) mass extinction event that claimed ~47% of the genera and 76 ± 5 %of the species living at that time (Jablonski, 1991). This effectively ended the Mesozoic Era andthe subsequent radiation ushered in our own era.

For the Chicxulub impact event to have caused any extinctions, the environmental changeshad to extend throughout the habitat range of an organism and overwhelm its ability to adapt. Inthe case of regional environmental effects, this means the changes had to exceed the migratorycapacity of a species or endure longer than its dormant capacity. In the case of globalenvironmental effects, the changes had to be rapid relative to the time scale of evolutionaryadaptation or last longer than the dormant capacity of a species.

Individual species have different migratory capacities, dormant capacities, and evolutionarytime scales, so it is not surprising that the level of extinction was not homogeneous amongdifferent groups of organisms. However, the heterogeneity in the fossil record also reflects otherparameters. One of the most important is a species’ relationship to its ecosystem, because theloss of life that occurred 65 million years ago did not involve a few isolated organisms, but rathercaused the collapse of entire ecosystems throughout the world.

The link between impact cratering processes and the biologic evolution of the Earth isrelatively new and we are still trying to understand the implications of this revelation. The linkbetween impact cratering and biologic evolution has led to detailed studies of the Chicxulubimpact event and its environmental effects (e.g., see Kring, 1993, 2000, and Pierazzo et al., thisvolume, for reviews). It has also generated studies of other known impact events, albeit in lessdetail, at over a dozen locations on Earth (e.g., Heissig, 1986; Aubry et al., 1990; Kring et al.,1996; Kring, 1997) in an effort to determine the magnitude of environmental changes needed tocause biologic consequences.

In general, it is recognized that the largest or most energetic impact events are likely toproduce the most severe environmental changes. This has led to a series of studies examiningthe environmental effects of hypothetical impact events of various sizes (e.g., see Toon et al.,1997, for a review). However, this approach is incomplete, because the environmentalconsequences of an impact event depend on far more parameters than just the energy of thecollision.

For example, one of the environmental consequences of the Chicxulub impact event is a largeperturbation in the carbon cycle, produced in part by the vaporization of carbonate in target

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lithologies, vaporization of carbon-bearing materials in the projectile, and post-impact wildfires,all of which injected >104 GT CO2, 103 GT CO, 102 CH4 into the atmosphere (e.g., Pierazzo etal., 1998; Kring and Durda, 2001). The amounts of these climatically active gases generated bythe Chicxulub impact event are quite large compared to the perturbations being caused today byfossil fuel burning (5.4 ± 0.5 GT/yr; Dixon et al., 1994) and likely had a role in the subsequentmass extinction event. The environmental consequences, however, of a similarly sized impactevent in the Archean, when ambient atmospheric pCO2 was much greater and stands ofvegetation that could be consumed by fire did not exist, would be much different. That is, theenvironmental outcome of the impact event and subsequent biologic effects are a function ofEarth’s ambient conditions, not just the energy of the impact event.

A broader goal, therefore, is to determine the range of environmental perturbations for severalother relevant parameters, including impactor composition, target composition, target location(e.g., ocean vs. continental shelf vs. a mid-continent terrain), troposphere composition,stratosphere composition, the geographic positions of continents, and the areal fraction of theEarth covered by continents vs. areal fractions of the Earth covered by ice and liquid water.Each of these issues will be explored in this chapter in an effort to understand the relativeimportance of different impact-generated environmental perturbations as a function of ambientconditions on Earth. Special attention will be given to those intervals in time when other massextinctions have been identified in the fossil record (Late Ordovician, Late Devonian, Permian-Triassic boundary, and Triassic-Jurassic boundary) and when particularly large impact events areknown to have occurred (e.g., Vredefort, Sudbury, and Manicouagan).

ReferencesAubry, M.-P., F.M. Gradstein, and L.F. Jansa, 1990, The late early Eocene Montagnais bolide:

No impact on biotic diversity, Micropaleontology 36, 164-172.Dixon, R.K., S. Brown, R.A. Houghton, A.M. Solomon, M.C. Trexler, and J. Wisniewski, 1994,

Carbon pools and flux of global ecosystems, Science 263, 185-190.Heissig, K., 1986, No effect of the Ries impact event on the local mammal fauna, Modern

Geology 10, 171-179.Jablonski, D., 1991, Extinctions: A paleontological perspective, Science 253, 754-757.Kring, D.A., 1993, The Chicxulub impact event and possible causes of K/T boundary

extinctions, in Proc. First Annual Symp. of Fossils of Arizona, D. Boaz and M. Dornan(eds.), Mesa Southwest Museum and Southwest Paleontological Society, Mesa (Arizona), pp.63-79.

Kring, D.A., 1997, Air blast produced by the Meteor Crater impact event and a reconstruction ofthe affected environment, Meteoritics Planet. Sci. 32, 517-530.

Kring, D.A., 2000, Impact events and their effect on the origin, evolution, and distribution oflife, GSA Today 10, no. 8, 1-7.

Kring, D.A. and D.D. Durda, 2001, The distribution of wildfires ignited by high-energy ejectafrom the Chicxulub impact event, Lunar Planet. Sci. XXXII, Abstract #1447, Lunar andPlanetary Institute, Houston (CD-ROM).

Kring, D.A., H.J. Melosh, and D.M. Hunten, 1996, Impact-induced perturbations of atmosphericsulfur, Earth Planet. Sci. Lett. 140, 201-212.

Pierazzo, E., this volume.

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Pierazzo, E., D.A. Kring, H.J. Melosh, 1998, Hydrocode simulation of the Chicxulub impactevent and the production of climatically active gases, J. Geophys. Res. 103, 28607-28625.

Toon, O.B., K. Zahnle, D. Morrison, R.P. Turco, and C. Covey, 1997, Environmentalperturbations caused by the impacts of asteroids and comets, Rev. Geophys. 35, 41-78.

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DISTINGUISHING COMET AND ASTEROID MATERIALS IN IMPACT DEPOSITS

Frank T. Kyte. Center for Astrobiology, Institute of Geophysics and Planetary Physics, UCLA,Los Angeles, CA 90095-1567 (kyte@igpp.ucla.edu).

An important part of the history of impacts on Earth, and their influence on the terrestrialenvironment and biotic evolution, is the provenance of the impacting bolides. This will reflectthe history of the large-body object flux in the inner solar system. The physical and chemicalproperties of projectiles, as well as their orbital evolution, has influenced the dynamics and therelative timing of impact events. Possible impact scenarios include random impacts by individualasteroids or comets, or clusters of impacts due to major collisions in the asteroid or Kuiper belts,or large perturbations of the Oort cloud of comets. Over the last several years, a combination oftrace element, isotopic, and petrologic data have yielded significant insights into this impacthistory.

The trace element chemistry of sediments, in particular the concentration of siderophiles (e.g.,Ir), is a useful tool to detect impacts (e.g., [1,2]) and provides supporting evidence for suspectedimpact deposits (e.g., [3,4]) However, siderophiles are not especially useful in distinguishingbetween types of projectiles (e.g., [5]). Interelement abundances of PGEs can distinguish achondritic signature, but since most asteroids, and probably all comets are chondritic, these datado little to distinguish between chondritic source materials. Perhaps the most significantchemical argument used to constrain provenance, is that the total amount of Ir in the globalCretaceous-Tertiary (KT) boundary ejecta layer is considerably less than that expected by a low-velocity, 10 km asteroid impact [6] and is most consistent with the impact of a high-velocity,low-Ir comet. Alternatively, much of the Ir may have been buried in the Chicxulub crater and/orejected to escape velocity.

Isotopic systems of He and Cr provide insights into the impact record. 3He concentrations insediments can measure of the flux of interplanetary dust [7]. Increased flux of 3He in late Eocenesediments is evidence of a comet shower [8], but no large increase in 3He at the KT boundaryargues against a comet shower at 65 Ma [9]. Reports of 3He in fullerenes from KT boundary [10]and Permian-Triassic (PT) boundary sediments [11] have been cited as evidence of carbonaceouschondrite materials in both deposits. However, this is inconsistent with Ir (e.g., [12]) and 3Heconcentrations [13] measured in similar samples from the PT boundary. Independentconfirmation of the positive results will be necessary to establish a PT impact event. The Cr-isotopic system can distinguish between different solar system objects, (e.g., terrestrial, martian,asteroidal [14-16]). This system provided the first unequivocal isotopic evidence of anextraterrestrial component in KT boundary sediments [17] and in early Archean spherule bedsfrom the Barberton Greenstone Belt [18-19]. In each of these cases, Cr-isotope abundances aresimilar to those measured in carbonaceous chondrites. Whether these data allow for a cometarysource or not is unknown.

Another important source of information for projectile provenance is actual meteoriticmaterials that survive impacts or other accretionary events. Only a few such studies exist.Meteorites in ejecta from the late Pliocene impact of the km-sized Eltanin asteroid [20-21]provided evidence that unmelted meteorites can survive hypervelocity impact. These meteorites

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are from a differentiated, basaltic object so the Eltanin asteroid was probably from the innerregions of the asteroid belt. A 2.5 mm fossil meteorite from a North Pacific KT boundary site(LL44-GPC3; [22]) is likely from a carbonaceous chondrite that experienced aqueous alterationon its parent body. This is consistent with the Cr-isotopic composition of KT sediments [17] andmay indicate an asteroid impact at the KT. Smaller (~200 µm) particles from a nearby KT site(DSDP 577) are also possible KT meteorite fragments [23] and recent work at site 577 by thisauthor is consistent with those results. Many more pieces of the KT bolide can probably berecovered from North Pacfic sediments with shallow burial depths (<100 m). An interestingphenomenon not linked to an impact is the discovery of 40 fossil meteorites in a single quarry ofLower Ordovician (480 Ma) marine limestones. Schmitz and Tassinari [24] estimate themeteorite flux that produced this deposit was one or two orders of magnitude greater than today.They speculate that this was from disruption of the L chondrite parent body in the asteroid beltand propose an enhanced flux of asteroids to Earth for about 30 m.y. at that time.

These data, as a whole, provide an evolving understanding of a small part of the record ofimpacts that can be discerned from sediment deposits.1. Late Pliocene – a km-sized projectile from the asteroid belt – probably a solitary event.2. Late Eocene – a shower of dust and projectiles, including two large craters (Popigai and

Chesepeake Bay) – probably a ~3 Ma disturbance of the Oort cloud.3. KT Boundary – probably a solitary projectile, with affinities to carbonaceous chondrite

meteorites – cometary vs. asteroidal origin has not been proven (this author favors the latter).4. PT Boundary – whether mass extinction was related to an impact remains an elusive and still

disputed, but intriguing possibility.5. Early Ordovician – major collision in the asteroid belt and a rain of projectiles on the Earth?6. Early Archean – at least three major impact events over a 30 m.y. interval – could be tail of

the Late Heavy Bombardment (?), or evidence of a catastrophic event in the asteroid belt.

A better undertanding of the Cr-isotopic composition of cometary material would helpresolve some of the present uncertainties. Are comets similar to carbonaceous chondrites ordistinct from known meteorite groups? It may be possible to resolve this question by directsampling of cometary materials from the late Eocene. High concentrations of unusual cosmicspherules occur in North Pacific core LL44-GPC3 [25] at the same interval as the 3He anomaly.Work is currently underway to attempt to isolate meteorite fragments, or at least a sufficientquantity of the spherules from these sediments to allow characterization of this cometarycomponent.

References.1. Alvarez L.W., Alvarez W., Asaro F., and Michel H.V. (1980) Extraterrestrial cause for the

Cretaceous–Tertiary extinction. Science 208, 1095-1108.2. Kyte F.T., Zhou Z., and Wasson J.T. (1981) High noble metal concentrations in a late Pliocene

sediment. Nature 292, 417-420.3. Ganapathy R., (1982) Evidence for a major meteorite impact on the Earth 34 million years

ago: implications for Eocene extinctions. Science 216, 885-886.4. Lowe. D.R., Byerly G.R., Asaro F. and Kyte F.T. (1989) Geological and geochemical

evidence for a record of 3,400 Ma-old terrestrial meteorite impacts. Science 245, 959-962.

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5. Kyte F.T. and Wasson J.T. (1982) Geochemical constraints on the nature of large accretionaryevents. Geol. Soc. Amer. Spec. Paper 190, 235-242.

6. Hildebrand et al. (1994) The Chicxulub crater and its relation to the KT boundary ejecta andimpact-wave deposits. Snowbird III, LPI Contribution 825, 49-50 (abs).

7. Farley K.A., (1995) Cenozoic variations in the flux of interplanetary dust recorded by He-3 ina deep-sea sediment. Nature 376,153-156.

8. Farley K.A., Montanari A., Shoemaker E.M., and Shoemaker C.S. (1998) Geochemicalevidence for a comet shower in the Late Eocene. Science 280,1250-1253.

9. Mukhopadhyay S., Farley K.A., and Montanari A. (2001) A 35 Myr record of helium inpelagic limestones from Italy: implications for interplanetary dust accretion from the earlyMaastrichtian to the middle Eocene. Geochim. Cos. A, 65, 653-669.

10. Becker L., Poreda R.J., and Bunch T.E. (2000) Fullerenes: An extraterrestrial carbon carrierphase for noble gases. Proc. Nat. Acad. Sci. 97, 2979-2983.

11. Becker L., Poreda R.J., Hunt A.G., Bunch T.E. and Rampino M. (2001) Impact event at thePermian-Triassic boundary: Evidence from extraterrestrial noble gases in fullerenes Science291, 1530-1553.

12. Zhou L. and Kyte F.T. (1988) The Permian-Triassic boundary event: A geochemical studyof three Chinese sections. Earth Plan. Sci. Lett. 90, 411-421

13. Farley K.A. and Mukhopadhyay S., (2001) An extraterrestrial impact at the Permian-Triassicboundary? Science 293, U1-U3.

14. Papanastassiou D.A. (1986) Chromium isotopic anomalies in the Allende meteorite.Astrophys. J. 308, L27-L30.

15. Lugmair G.W., and Shukolyukov A. (1998) Early solar system timescales according to 53Mn-53Cr systematics. Geochim. Cos. A, 62, 2863-2886.

16. Shukolyukov A., and Lugmair G.W. (1999) The 53Mn-53Cr isotope systematics of theenstatite chondrites. Lunar Planet. Sci. 30, abstract #1093 (CD-ROM).

17. Shukolyukov A., and Lugmair G.W. (1998) Isotopic evidence for the Cretaceous-Tertiaryimpactor and its type. Science 282, 927-929.

18. Shukolyukov A., Kyte F.T., Lugmair G.W., Lowe D.R., and Byerly G.R. (2000a) The oldestimpact deposits on Earth — First confirmation of an extraterrestrial component. in Impactsand the Early Earth. Springer-Verlag, Heidelberg, Lec. Notes in Earth Sciences, 92, 99-116.

19. Shukolyukov A., Kyte F.T., Lugmair G.W., Lowe D.R., and Byerly G.R. (2000b) EarlyArchean spherule beds - confirmation of impact origin . Met. Plan. Sci. 35, A146-A147 (abs).

20. Kyte F.T. and Brownlee D.E. (1985) Unmelted meteoritic debris in the Late Pliocene Iranomaly: evidence for the impact of a nonchondritic asteroid. Geochim. Cos. A, 49, 1095-1108.

21. Gersonde R., et al. (1997) Geological record and reconstruction of the late Pliocene impactof the Eltanin asteroid in the Southern Ocean, Nature, 390, 357-363.

22. Kyte F.T. (1998) A meteorite from the Cretaceous-Tertiary boundary. Nature 396, 237-239.23. Robin E., Froget L., Jéhanno C., and Rocchia R. (1993) Evidence for a K/T impact event in

the Pacific Ocean. Nature 363, 615-617.24. Schmitz B. and Tassinari M. (2001) A rain of ordinary chondrites in the early Ordovician.

Met. Plan Sci. 36, A183 (abs.)25. Kyte F.T. and Bostwick J.A. (1995) Magnesioferrite spinel in Cretaceous-Tertiary boundary

sediments of the Pacific basin: Hot, early condensates of the Chicxulub impact? Earth Plan.Sci. Lett. 132, 113-127.

34

ENVIRONMENTAL AND SEDIMENTOLOGICAL EFFECTS OF ARCHEANIMPACTS RECORDED IN THE 3.5-3.2 GA BARBERTON GREENSTONE BELT,SOUTH AFRICA

Donald R. Lowe1, and Gary R. Byerly2. 1Department of Geological and EnvironmentalSciences, Stanford University, Stanford, CA 94305-2115 USA. 2Department of Geology andGeophysics, Louisiana State University, Baton Rouge, LA 70803 USA

Rocks in the Barberton Greenstone Belt (BGB), South Africa, include at least 4 layerscontaining spherules produced by large meteorite impacts on the Archean Earth. Termed S1through S4, these layers were initially described and interpreted by Lowe and Byerly (1986) andsubsequently in greater detail by Lowe et al. (1989), Kyte et al. (1992), and Byerly and Lowe(1994). A key step in the confirmation of these units as the products of impacts was thediscovery of extraterrestrial chrome in S4 (Shukolyukov et al., 2000a) and subsequently in S2(Shukolyukov et al., 2000b).

The 10-13 km of deformed sedimentary and volcanic rocks in the BGB comprise threelithostratigraphic groups. The Onverwacht Group, up to 9 km thick and ranging from 3,555 to3,260 Ma, includes, from base upward, the Komati, Hooggenoeg, Kromberg, and MendonFormations. All consist largely of mafic and ultramafic volcanic rocks. The upper three includeinterbedded layers of silicified sediments and pyroclastic materials (cherts). The overlying FigTree Group (3,260-3,225 Ma) includes 100-600 m of felsic pyroclastic and volcaniclastic rocks,sandstone and conglomerate, chemical sediments, and chert. The uppermost unit, the MoodiesGroup (<3,225 Ma, >3,109 Ma), includes up to 3000 m of quartzose sandstone andconglomerate.

S1 occurs in a 1-3 m-thick layer of chert, H4c of Lowe and Byerly (1999), in the upper halfof the Hooggenoeg Formation. Its age is <3,475 and >3,445 Ma. H4c can be traced for over 15km along strike within a sequence of mafic volcanic rocks. The lower 1-2 m consists of blackcarbonaceous chert, banded chert, and silicified pyroclastic fall deposits. All beds are thin, fine-grained, and lack evidence of current or wave activity. Deposition took place in quiet water wellbelow local wave base. In the upper part of the unit, S1 is a 10-50 cm-thick unit of spherules (5-30%) mixed with sand-sized clastic and volcaniclastic debris, including some black chert clastsrepresenting ripped up carbonaceous and muddy sediments. The beds are commonly cross-laminated to cross-bedded. S1 is overlain by fine-grained carbonaceous and volcaniclasticsediments marking a return to quiet-water sedimentation.

S2 is present in a number of structural belts in the southern part of the BGB at the base of theFig Tree Group, dated at 3,256 Ma based in a felsic tuff a few meters above S2. It overlies blackand banded cherts at the top of the Onverwacht Group. S2 everywhere consists of ripped-upchert clasts, chunks of ultramafic volcanic rock, and sand-sized detrital chert grains mixed with<5% sand-sized spherules. In most sections, S2 is 20-100 cm thick, but it ranges up to 3-4 m.Where thickest, it includes blocks of underlying chert up to 50-75 cm across. Although theunderlying cherts and overlying Fig Tree sediments were deposited under quiet, low-energyconditions, S2 represents a period of anomalous high-energy, erosive current and/or waveactivity.

35

S3 occurs in the lower 100 m of the Fig Tree Group in southern areas but marks the Fig TreeOnverwacht contact in the north, where an immediately underlying felsic tuff has been dated as3,243 Ma. It is the most widely distributed spherule layer in the BGB and occurs in sectionsdeposited across a wide range of depositional environments. In the southern BGB, it isinterbedded with fan-delta to shallow shelf clastic and volcaniclastic units. Along the northernedge of the BGB, S3 occurs in quiet, deep-water sections as a 20-30 cm-thick layer of nearlypure spherules characterized by abundant chrome spinels and high-Ir contents. In all sections, itshows evidence of working by current or wave activity, even in deep-water sections whereadjacent units reflect deposition under very low-energy, wave- and current-free conditions.

S4 is known from a single outcrop, where it occurs 8 m above S3 and consists of 10-15 cm ofnearly pure chlorite spherules. It is interbedded with and eroded laterally by coarse, probablynon-marine fan-delta clastic units.

Lowe and Byerly and Lowe et al (1989) interpreted the spherules in S1-S4 as thecondensation products of rock vapor clouds ejected above the earth's atmosphere following largeimpacts. Modeling based on spherule size, bed thickness, and Ir fluence suggests very largeimpactors, 20-50 km in diameter. Modeling of impact bed compositions suggest little or nocontinental material in any of the impact targets. The falling condensate spherules probablyformed layers that extended over most of the Earth's surface. S1-S4 are composed only ofspherules or spherules mixed with debris that appears to be derived by local erosion. They lackcoarse ballistic ejecta and were apparently deposited far away from the impact sites.

These beds carry a number of significant inferences about the Archean Earth: (1) S1-S3 allshow sections deposited under quiet, often deep-water conditions but within which the spherulebeds represent major but short-lived current and wave events that widely resulted in erosion ofunderlying sedimentary and volcanic units. These events appear to reflect the passage of largetsunamis that swept the world's oceans following the impacts. (2) The widespread evidence oftsunami activity accompanying the impacts suggests that these layers and the associated parts ofthe greenstone-belt sequence were deposited in the oceans and not in local lakes, as has beensuggested by some investigators. (3) The presence of tsunami deposits in all three marine impactlayers (S1-S3) suggests that there were no large continent-sized land masses to mitigate theglobal effects of the tsunamis. (4) The coincidence of two major impact layers, S2 and S3, withthe diachronous transition from Onverwacht mafic and ultramafic volcanism to Fig Tree felsicvolcanism, uplift, and orogenesis suggest that impacts may have triggered reorganization of thesurficial tectonic system. (5) The presence of at least three and perhaps 4 major impact layers>10-15 cm thick in the Fig Tree Group deposited over an interval of <35 ma and probably <15ma suggests the possibility that either the flux of large (>10 km diameter) impactors was veryhigh as late as 3,250 Ma or the impact rate varied significantly over the interval represented byrocks in the BGB.

ReferencesByerly, G.R., and Lowe, D.R., 1994, Spinel from Archean impact spherules: Geochim.

Cosmochim. Acta 58, 3469-3486.

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Lowe, D.R., and Byerly, G.R., 1986, Early Archean silicate spherules of probable impact origin,South Africa and Western Australia: Geology 14, 83-86.

Lowe, D.R., and Byerly, G.R., 1999, Stratigraphy of the west-central part of the BarbertonGreenstone Belt, South Africa: in Lowe, D.R., and Byerly, G.R., eds., Geologic evolution ofthe Barberton Greenstone Belt, South Africa: Geol. Soc. America Special Paper 329, 1-36.

Lowe, D.R., Byerly, G.R., Asaro, F., and Kyte, F.T., 1989, Geological and geochemical recordof 3400-Million-Year-Old Terrestrial Meteorite Impacts: Science 245, 959-962.

Kyte, F.T., Zhou, L., and Lowe, D.R., 1992, Noble metal abundances in an Early Archeanimpact deposit: Geochim. Cosmochim. Acta 56, 1365-1372.

Shukolyukov, A., Kyte, F.T., Lugmair, G.W., Lowe, D.R., and Byerly, G.R., 2000a, The oldestimpact deposits on Earth – First confirmation of an extraterrestrial component: in Gilmour, I.,and Koeberl, C., eds., Impacts and the Early Earth: Heidelberg, Germany, Springer-VerlagLecture Notes in Earth Sciences p. 99-116.

Shukolyukov, A., Kyte, F.T., Lugmair, G.W., Lowe, D.R., and Byerly, G.R., 2000b, EarlyArchean spherule bed S3 – Confirmation of impact origin: Meteoritical Society, 63rd AnnualMeeting, Abstracts.

37

ORIGIN OF WATER ON EARTH AND MARS

J.I. Lunine. LPL, U. Arizona, Tucson AZ 85721 USA

Morbidelli et al. (Meteoritics and Planetary Science, v. 35, 1309, 2000) quantified thedelivery of water to the forming Earth from a number of solar system sources, and concludedthat planet-sized bodies in the primordial asteroid belt constitute the largest source-- oneconsistent with the deuterium-to-hydrogen ratio (D/H) in chondrites and Earth's ocean. In thepresent paper I explore the implications of our work for the delivery of water to Mars during thatplanet's formation, assess whether the results are consistent with constraints on the early D/H ofMartian water, and then reexamine the whole picture in light of additional isotopic data.

While the D/H of the present Martian atmosphere almost certainly reflects strongfractionation during loss of the atmosphere over geologic time, the SNC meteorite record of theancient Martian crust provides important constraints on the original water inventory. Leshin(Geophysical Research Letters, v. 27, 2017, 2000) obtained a D/H ratio twice that of StandardMean Ocean Water (SMOW) in the meteorite QUE94201, and interprets this as the signature ofan early, rapid episode of atmospheric doubling of D/H from the SMOW value. However, analternative view would be that the 2 x SMOW value of D/H is a primordial value and that Marsreceived a different mix of high vs. low D/H water during its formation, relative to the Earth. Ifso, where does the high D/H water come from, and with how much water might the crust of Marshave been initially endowed? It is possible to answer these questions in the context of theMorbidelli et al. dynamical simulation of planet growth.

Our results are as follows, under the plausible assumption that Mars is not struck by a largeplanetary embryo from the asteroid belt, in contrast to the Earth (otherwise Mars grows too big).

§ Mars acquires its water from a roughly comparable mix of asteroidal sized bodies, in theasteroid belt and in the outer solar system (i.e., "comets").

§ The total water Mars accretes is 1/20 of an Earth ocean, which is enough to explain thegeological features on Mars indicative of water erosion, but toward the low end of the rangeof estimates of early water abundance on Mars.

§ The D/H ratio of martian water, prior to atmospheric escape, is 1.6 times SMOW, which isconsistent with the picture obtained from Leshin's analysis of the SNC meteorite QUE94201.

If the above model for the origin of water is correct, then I propose the following sequence ofevents regarding the origin and evolution of water on Earth and Mars:

§ Massive primordial asteroid belt is stirred up by Jupiter formation§ Large asteroids form as part of inner planet accretion§ Earth accretes water-rich embryos late, from > 2.5 AU§ Final Earth water = 3 Earth oceans with D/H =150 ppm=SMOW§ Mars does not accrete an embryo (but could have)§ Mars receives water from mixture of asteroids and comets§ Final Mars water = 0.05 Earth oceans; D/H=1.6 SMOW§ Earth crustal water system mostly retained over its history

38

§ Mars water mostly lost; current atmospheric D/H= 5 SMOW§ Earth water will undergo escape in 1-2 gyr, (J. Kasting, Icarus v. 74, 472, 1988); D/H will

rise

The Morbidelli et al model can be tested by examining isotopes other than deuterium-hydrogen. Indeed, the nearly identical isotopic pattern in stable oxygen between Earth and Moonseverely constraints the amount of asteroidal (chondritic) material acquired by Earth to abouthalf our desired value. Nonetheless, we still can generate several oceans worth of water on Earthand satisfy the oxygen isotopic data. Alternative models for the supply of water to Earth, namelythat local planetesimals were hydrated but had a different oxygen isotope ratio from thechondrites, would not negate our hypothesis for Mars. However, it is hard to understand, in anebula cool enough to allow hydrated minerals at 1 AU, why Mars was not endowed with anenormous inventory of water that today ought still to be in evidence.

Additional, more general references beyond those cited above:

§ Lunine, J.I. 1999. Earth: Evolution of a Habitable World. Cambridge U. Press.§ Ward, P. and Brownlee, D. 2000. Rare Earth, Copernicus Books, New York.§ Lunine, J.I. 2001. The occurrence of Jovian planets and the habitability of planetary systems.

Proc. National Acad. Sciences 98, 809-814.

39

INTERSTELLAR PANSPERMIA

H. J. Melosh, Lunar and Planetary Lab., University of Arizona, Tucson AZ 85721(jmelosh@lpl.arizona.edu).

Introduction: It is now generally accepted that meteorite-size fragments of rock can beejected from planetary bodies. Numerical studies of the orbital evolution of such planetaryejecta are consistent with the observed cosmic ray exposure times and infall rates of thesemeteorites. All of these numerical studies agree that a substantial fraction (up to 1/3) of theejecta from any planet in our solar system is eventually thrown out of the solar system duringencounters with the giant planets Jupiter and Saturn. Here, I examine the probability that suchinterstellar meteorites might be captured into a distant solar system and fall onto a terrestrialplanet in that system. The implications of these results for the possibility of interstellarpanspermia are examined.

Orbital Evolution of Planetary Ejecta: Currently, we believe that planetary meteoritesejected from their planets of origin by large impacts through the process of spallation. Onceejected from a parent planet, such meteorites make many close passes to other planets or interactwith orbital resonances created by the giant planets. Because of these encounters the orbits ofthe ejecta alter with time and the ejected rocks may eventually end up falling onto another planetor leaving the solar system. A number of estimates suggest that roughly 15 potentiallyrecoverable planetary meteorites per year fall on the Earth.

Orbital evolution simulations suggest that about as many Martian meteorites are ejected byJupiter as fall on Earth each year. Currently, about 15 rocks greater than 10 cm diameter thatoriginate on the surface of a terrestrial planet leave the solar system each year. These meteoritesexit the solar system with velocites in the vicinity of 5±3 km/sec

It is much more likely that an approaching interstellar meteorite will be captured into a boundorbit by a hypothetical giant planet in the target system than that it will directly impact aterrestrial planet. Just as Jupiter is the main agent of ejection from our solar system, it may alsoserve as the main entry point for interstellar meteorites. Once captured into the stellar system themeteorites have a much higher probability of eventually striking a terrestrial planet belonging tothat system

The capture probability of interstellar meteorites is a decreasing function of the approachvelocity. I find that the probability drops rapidly at approach velocities higher than about 1km/sec, but even at this velocity the cross section approaches 1 AU2—far higher than that fordirect impact on a terrestrial planet. The overall integral of cross section under the curve is about40. Using the velocity dispersion of ±20 km/sec for stars near the Sun, the convolution of thevelocity distribution and the velocity-dependent cross section yields an over-all capture crosssection of about 1 AU2. This cross section indicates that about 1 meteorite ejected from a planetbelonging to our solar system is captured by another stellar system every 100 Myr.

Impact on a Terrestrial Planet: It is not enough that a potentially life-transportingmeteorite should be captured into another solar system. It must also find its way onto the surface

40

of a planet within a habitable zone. The capture simulation program indicates that the orbits ofcaptured interstellar meteorites are very comet-like, with semimajor axes of typically severalhundred AU. For a wide range of assumptions about the location and size of a target terrestrialplanet the probability that it impacts the planet is only about 10-4. This translates to only a veryslim chance that life can be transported from one stellar system to another. It seems that theorigin of life on Earth will have to be sought within the confines of the solar system itself, notabroad in the galaxy.

41

THE ASTEROID AND COMET IMPACT HAZARD

David Morrison. NASA Astrobiology Institute, NASA Ames Research Center

The discipline of astrobiology includes the dynamics of biological evolution. One of themajor ways that the cosmos influences life is through the catastrophic environmental disruptionscaused when comets and asteroids impact a planet. We now recognize that such impacts haveplayed a major role in determining the evolution of life on Earth, and presumably the same sortof influences work on other potentially habitable planets.

The small fraction of the asteroids with Earth-crossing or Earth-approaching orbits constitutethe major impacting population on Earth. The time-averaged impact flux as a function ofprojectile energy can be derived from lunar cratering statistics, although we have littleinformation on the possible variability of this flux over time. Effects of impacts of variousenergies can be modeled, using data from historic impacts (such as the KT impactor 65 millionyears ago) and the observed 1994 bombardment of Jupiter by fragments of comet Shoemaker-Levy 9. It is of particular interest to find from such models that the terrestrial environment ishighly vulnerable to perturbation from impacts, so that even such a small event as the KT impact(by a projectile roughly 15 km in diameter) can lead to a mass extinction. Similar considerationsallow us to model the effects of still smaller (and much more likely) impacts, down to the size ofthe asteroid that exploded in Tunguska in 1908 (energy about 15 megatons).

Combining the impact flux with estimates of environmental and ecological effects revealsthat the greatest contemporary hazard is associated with impactors near one million megatonsenergy (about 1-2 km diameter for an asteroid). The current impact hazard is significant relativeto other natural hazards, and arguments can be developed to illuminate a variety of public policyissues. These include the relative risk of different impact scenarios and the associated costs andprobability of success of countermeasures. Impacts represent the extreme case of a hazard of lowprobablity but great consequences.

The first priority in any plan for defense against impactors is to survey the population ofEarth-crossing asteroids (NEAs) and project their orbits forward in time. This is the purpose ofthe Spaceguard Survey, which has already found more than half of the NEAs larger than 1 km indiameter. If there is a NEA on a collision course with Earth, it can be discovered and the impactpredicted with decades or more of warning. It is then possible to consider how to deflect ordisrupt the NEA. Unlike other natural hazards, the impact risk can be largely eliminated, givensufficient advanced knowledge to take action against the threatening projectile.

ReferencesNASA Impact Hazard Website: http://impact.arc.nasa.govNASA NEO Program Office website: http://neo.jpl.nasa.govAhrens, T.J. and A.W. Harris: Deflection and fragmentation of near-Earth asteroids. Nature

360:429-433 (1992)Chyba, C.F., P.J. Thomas, and K.J. Zahnle: The 1908 Tunguska explosion: atmospheric

disruption of a stony asteroid. Nature 361:40-44 (1993)

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Chapman, C.R. and D. Morrison: Impacts on the Earth by asteroids and comets: assessing thehazard. Nature 367:33-39 (1994)

Gehrels, T. (editor): Hazards Due to Comets and Asteroids, University of Arizona Press, pp 1300(1994)

Morrison, D., C.R. Chapman, and P. Slovoc: The impact hazard. In Hazards Due to Comets andAsteroids (T. Gehrels, editor), University of Arizona Press, 59-92 (1994)

Lewis, J.: Rain of Iron and Ice; The Very Real Threat of Comet and Asteroid Bombardment.Addison-Wesley, pp 236 (1996)

Toon, O.B., K. Zahnle, D. Morrison, R.P. Turco, and C. Covey: Environmental pertubationscaused by the impacts of asteroids and comets. Reviews of Geophysics 35:41-78 (1997)

Asphaug, E., S.J. Ostro, R.S. Hudson, D.J. Scheeres, and W. Benz: Disruption of kilometre-sizedasteroids by energetic collisions. Nature 393:437-440 (1998)

Harris, A.W.: Evaluation of ground-based optical surveys for near-Earth asteroids. PlanetarySpace Science 46:283-290 (1998)

Binzel, R. P. The Torino Impact Hazard Scale. Planetary & Space Science 48: 297-303 (2000)Bottke, W. F., R. Jedicke, A. Morbidelli, J.-M. Petit and B. Gladman. Understanding the

Distribution of Near-Earth Asteroids. Science 288: 2190-2194 (2000).Milani, A., S.R. Chesley, G.B. Valsecchi: Asteroid close encounters with Earth: Risk

assessment. Planetary & Space Science 48: 945-954 (2000)Ward S.N. and E. Asphaug: Asteroid impact tsunami: A probabilistic hazard assessment. Icarus

145: 64-xx (2000)

43

3HE FLUX FROM THE LATE CRETACEOUS TO THE EARLY CENOZOIC:CONSTRAINING THE NATURE OF EXTRATERRESTRIAL ACCRETIONARYEVENTS

S. Mukhopadhyay1, K.A. Farley2, A. Montanari3. 1 Department of Terrestrial Magnetism,Carnegie Institution of Washington, Washington, DC 20015, U.S.A. 2Department of Geologicaland Planetary Sciences, Caltech, Pasadena, CA 91125, U.S.A. 3Osservatorio Geologico diColdigioco, 62020 Frontale di Apiro, Italy

Interplanetary dust particles (IDPs) are tremendously enriched in 3He, compared to terrestrialmatter, because of the presence of implanted solar wind helium (e.g., Merrihue, 1964; Nier andSchlutter, 1990). Accumulation of IDPs imparts high helium concentrations and high 3He/4Heratios to many deep-sea sediments, and the 3He/4He ratios can be used to fingerprint the fractionof extraterrestrial 3He in the sediments. The extraterrestrial 3He in sediments is most sensitive tothe accretion of IDPs ≤ 35 µm because larger IDPs undergo frictional heating upon atmosphericentry and lose their helium (Farley et al., 1997). Like large IDPs, large bodies (> few meters indiameter) are intensely heated and vaporized upon impact and, hence, do not contribute 3He tothe sedimentary record.

IDPs are derived from collisions in the asteroid and Kuiper belts and from active comets. Thegeologic record of the IDP accretion rate is a powerful tool to investigate major events occurringin the solar system, such as the dynamics and recurrence interval of collisions in the asteroid beltand comet showers (e.g., Farley et al., 1998). For example, Farley et al. (1998) demonstrated thatan enhanced 3He accretion rate in the Late Eocene was temporally correlated with multipleterrestrial impact features, specifically the Chesapeake Bay and Popigai impact craters, and Iranomalies in the stratigraphic record. The pattern of 3He flux in the Late Eocene and closetemporal association with multiple impacts is most consistent with an enhanced solar systemdustiness associated with a comet shower (Farley et al., 1998).

To understand the delivery history of IDPs over geologic time and its relationship toterrestrial impacts, particularly at the K/T boundary, He abundance and isotopic compositionwere measured in a suite of pelagic limestones exposed in the Italian Apennines that weredeposited between ~74 to ~39 Ma (Mukhopadhyay et al., 2001a). The measurements indicatethat the extraterrestrial 3He accretion rate in the Maastrichtian was fairly constant. Minorfluctuations in the IDP accretion rate are present, which probably reflect random events in theasteroid or Kuiper belts.

High-resolution 3He data across the K/T boundary (63.9 to 65.4 Ma) indicate a low andinvariant IDP accretion rate (Mukhopadhyay et al., 2001b). Thus, the K/T impactor was notassociated with enhanced solar system dustiness and, hence, not a member of a comet shower.Instead, the K/T impactor is more likely to have been a single earth-crossing asteroid or comet.3He as a constant-flux proxy of sedimentation rate implies deposition of the K/T boundary clayin 10 ± 2 kyr and the impact ejecta layer in less than 60 years. These results indicate that themass-extinction at the K/T boundary was catastrophic, suggesting that the impact was the maindriving force behind the K/T biotic calamity.

44

We observe a 2-4 fold increase in the 3He accretion rate close to the Paleocene/Eoceneboundary followed by a factor of three decay over a ~4-5 Myr period. This increase does notexhibit the temporal pattern expected from a comet shower arising from a gravitationalperturbation of the Oort cloud (Hut et al., 1987; Farley et al., 1998). Furthermore, there are noknown PGE anomalies despite measurements covering the time interval (e.g., Kyte and Wasson,1986). Instead, our data are more consistent with an increase in solar system dustiness resultingfrom a major collision in the asteroid belt or, alternatively, the Kuiper belt.

Based on the 3He accretion rate between 74 to 39 Ma, we find no evidence for recurrentcomet showers with periods less than 38 Myrs. In general, in combination with previous 3Hemeasurements (Farley, 1995), the recent data do not support either models that predict periodicimpacts, or the hypothesis that comet showers drive terrestrial mass extinction events.

ReferencesFarley K. A. and Patterson D. B. (1995) A 100-kyr periodicity in the flux of extraterrestrial 3He

to the sea floor. Nature 378, 600-603.Farley K. A., Love S. G., and Patterson D. B. (1997) Atmospheric entry heating and helium

retentivity of interplanetary dust particles. Geochim. Cosmochim. Acta 61, 2309-2316.Farley K. A., Montanari A., Shoemaker E. M., and Shoemaker C. S. (1998) Geochemical

evidence for a comet shower in the Late Eocene. Science 280, 1250-1252.tide. Icarus 116, 255-268.Hut P., Alvarez W., Elder W. P., Hansen T., Kauffman E. G., Keller G., Shoemaker E. M., and

Weissman P. R. (1987) Comet showers as a cause of mass extinctions. Nature 329, 118-126.Kyte F. T. and Wasson J. T. (1986) Accretion rate of extraterrestrial matter: Iridium deposited 33

to 67 million years ago. Science 232, 1225-1229.Merrihue C. (1964) Rare gas evidence for cosmic dust in modern Pacific red clay. Ann. N.Y.

Acad. Sci. 119, 403-411.Mukhopadhyay S., Farley K. A., and Montanari A. (2001a) A 35 Myr record of helium in

pelagic limestones from Italy: Implications fro interplanetary dust accretion from the earlyMaastrichtian to the middle Eocene. Geochem. Cosmochim. Acta 65, 653-669.

Mukhopadhyay S., Farley K. A., and Montanari A. (2001b) A short duration of the Cretaceous-Tertiary boundary event: Evidence from extraterrestrial Helium-3. Science 291, 1952-1955.

Nier A. O. and Schlutter D. J. (1990) Helium and neon isotopes in stratospheric particles.Meteoritics 25, 263-267.

45

CLIMATIC EFFECTS PRODUCED BY STRATOSPHERIC LOADING OF S-BEARINGGASES RELEASED IN THE CHICXULUB IMPACT EVENT

E. Pierazzo. Lunar and Planetary Laboratory, Univ. of Arizona (betty@lpl.arizona.edu)

The Chicxulub structure, on the Yucatán Peninsula, Mexico, was produced 65 Myr ago, at theend of the Cretaceous Period [1]. The impact occurred on a partially submerged platformconsisting of a thick (3 km) sequence of carbonates and evaporites overlying a continental crust,and produced a multiring structure, whose transient cavity diameter is estimated to be around100 km [2]. Estimates of the amount of evaporitic deposits in the sediments range from about50% [3] to 30% [4], although lower limits of 10% have also been suggested (Claeys, personalcommunication). A shallow sea (few tens of meters) was covering the region, suggesting thatthe sedimentary layer was probably saturated with water. In addition to the direct short-termeffects of the impact (e.g., see [5]), the presence of evaporites in a water-saturated sedimentarylayer may have caused, through the release in the atmosphere of large loads of S-bearing gasesand water vapor, a longer-lasting strong and abrupt climate shift, possible key to the K/Tboundary mass extinction. Hydrocode simulations [6,7] indicate that the amount of S injected inthe stratosphere ranged between about 75 and 270 Gt, depending also on projectile type andimpact speed, with a lower limit of 25 Gt under the assumption that evaporites constituted only∼10% of the sedimentary layer. Important climate changes were initially attributed to the CO2

released from the carbonates as well. However, impact modeling studies [6,7] indicate that theCO2 released in the impact is not enough to produce a strong effect on the climate.

The evolution of the sulfur-bearing gases injected in the stratosphere by the impact event isinvestigated using the Sulfate Aerosol Model (SAM) developed for this work [8]. The SO2, H2O,and sulfate aerosols are distributed uniformly over the globe (partially justified by the fastworldwide expansion, well beyond the stratosphere, of the impact plume). The microphysicalmodel for the formation and evolution of the stratospheric aerosols follows that described in [9].The aerosols are continuously formed by combining impact-produced SO2 and SO3 with H2O,and their evolution is described by processes like coagulation, growth, gravitational settling anddiffusion for various particle sizes.

Climate Forcing: Climate forcing is defined as the change in the Earth’s total radiativebalance at the tropopause and is measured in units of heat flux (W/m2). The climate forcing dueto stratospheric S-injection is assessed by coupling the SAM to the Column Radiation Model(CRM), a standalone version of the radiation model used in CCM3 [10], the general circulationmodel of the National Center for Atmospheric Research (NCAR). The CRM determine theradiation balance and heating rates at various atmospheric levels for the perturbed atmosphere,which then can be compared to those for the unperturbed atmosphere to determine the climateforcing due to the stratospheric aerosols. The model has been tested and tuned using the well-recorded and studied Pinatubo eruption of June 1991. As a result of the eruption, in 1992 thetemperature of the lower troposphere showed a global decline of ∼0.5°C, with much of thedecrease occurring in the Northern Hemisphere (0.7°C).

The impact-related injection of S-bearing gases from the sedimentary layer (S>25 Gt), isresponsible for a negative forcing about two orders of magnitude stronger than the Pinatubo

46

volcanic eruption, persisting globally for about two years after the impact [8]. Even without thecontribution from the sedimentary layer, the forcing due to the S from the projectile alone wouldbe over 30 times larger than for the Pinatubo case. For comparison, the climate forcingassociated with the largest CO2 injection estimated by [6] (for a 100 km transient crater) is onlyaround 2 W m-2 (i.e., warming with the adopted convention). A forcing larger than the maximumforcing for the Pinatubo eruption, i.e., around -5 W m-2, persists for 4 to 5 years. This timescalemay be enough to cool down the ocean’s surface layer, but it does not affect the deep ocean. It isthe deep ocean that, thanks to its high heat capacity, plays a crucial role as moderator of theEarth’s climate. This implies that, on a geologically short timescale (i.e., hundreds of years) theclimate was able to fully recover to pre-impact conditions.

Climate Sensitivity: Climate sensitivity is the mean change in global temperature that occursin response to a specific forcing. The CRM is particularly useful for studying the Earth’s energybudget and the radiative forcing of greenhouse gases and aerosols. Unfortunately, the responsesof the climate to the forcing, like surface temperature, are not a simple function of the forcing,and can only be satisfactory modeled by a complete 3-D GCM. An initial assessment of climatesensitivity to the sulfate loading has been done by coupling NCAR’s single column model(SCCM) to the SAM. SCCM is equivalent to a grid column of the more complete global climatemodel CCM3 [10] where the performance of the parameterized physics for the column isevaluated in isolation from the rest of the large-scale model. While lacking the more completefeedback mechanisms available to an atmospheric column imbedded in the global model, itprovides an inexpensive first look at the response of the system to the forcing introduced by aparticular parameterization.

The presence of the S-bearing gases and sulfate aerosols (strong LW absorbers) initially in theupper atmospheric layer of the model produces a significant change in the atmospheric radiationfluxes. This results in a strong heating of the stratosphere accompanied by a strong cooling atthe Earth’s surface [11]. Compared to a Pinatubo-type eruption, the model estimates that in theuppermost layer the temperature increases by (at least) several tens of degrees, more than anorder of magnitude that associated with Pinatubo. At the surface the impact-produced cooling isaround several degrees, again at least an order of magnitude that associated with Pinatubo.

References: [1] Hildebrand A.R. et. al. (1991) Geol., 19, 867. [2] Morgan J. et al. (1997)Nature 390, 472. [3] López-Ramos E. (1975) in The Ocean Basins and Margins Vol. 3 (Nairn-Stehli eds.), 257. [4] Ward P.D. et al. (1995) Geology 23, 873. [5] Toon O.B. et al. (1997) Rev.Geophys. 35, 41. [6] Pierazzo E. et al. (1998) JGR, 103, 28607. [7] Ivanov B.A. et al. (1996)Geol. Soc. Am. Spec. Paper 307, 125. [8] Pierazzo E. (2001) LPSC XXXII, Abst. #1196. [9]Turco R.P. et al. (1979) J. Atmo. Sci., 36, 699. [10] Kiehl J.T. et al. (1996) NCAR/TN-420+STR,152 pp. [11] Pierazzo E. & Hahmann A.N. (2001) AGU Fall Meeting, Abst. #6301.

47

PLANET FORMATION AND IMPACTS

Thomas R. Quinn. Astronomy Dept., University of Washington, Seattle WA 98195-1580,U.S.A.

The formation and subsequent orbital evolution of the Giant planets plays a significant role indetermining the impact rate of asteroids and comets on Earth. First, the planet formation processcontrols the size and distribution of the reservoirs of small bodies that are the ultimate source ofEarth impactors. Second, the gravitational influence of the large planets determine the deliveryof potential impactors from these reservoirs into Earth crossing orbits. For example, it has beenproposed (Wetherill, 1994) that the absence of Jupiter size planets in a planetary system wouldresult in a cometary flux from the Oort cloud into the terrestrial region 1000 times greater than inour Solar System with clear consequences to the impact rate. On the other hand the formation ofthe Oort cloud is determined by the mass and orbits of the outer planets (Duncan, Quinn andTremaine, 1987; also see a review by Duncan and Quinn, 1993). Likewise the delivery ofasteroids from the main belt into Near Earth objects involves orbital resonances with Jupiter.However, the presence of the asteroid belt itself, as opposed to a fifth terrestrial planet, may bedue to the presence of Jupiter during the planetesimal accretion phase of planet formation.

The difficulty in sorting out these issues is exacerbated by our lack of knowledge of theplanet formation process. The standard paradim for the formation of our own planetary systeminvolves the condensation of volatiles into grains, the aggregation of those grains intoplanetesimals, and the collisions of those planetesimals to build up proto-planets and the planetswe see today (see a review by Lissauer, 1993). This model has been very successful inexplaining the the properties of our own planetary system including: the composition of theterrestrial planets in contrast to the gas giants, the spins of the terrestrial planets, the formation ofthe moon, and, of course, the presence of the small bodies including the asteroid belt, the Kuiperbelt, and the Oort comet cloud. This model was badly shaken by the discovery of ``hot Jupiters''around other stars (see Marcy, Cochran, and Mayor, 2001 for a review of planets discoveredaround other stars): low density bodies were found where the models predicted planets with thedensity of Mercury. Therefore, we have been forced to consider modifications to the standardplanet formation paradim, and completely different senarios for planet formation. Orbitalmigration of gas giants either through interation with a gas disk (Lin, Bodenheimer, andRichardson, 1996) or by scattering of planetesimals (Murray et al., 1998) provides anexplanation for these planets close to the parent star, and this would have significantconsequences for small body reservoirs. A complete different theory for planet formation is thegravitational instability model (Boss, 1997), but the origin of comets and asteroids in such amodel is unclear.

References:

Boss, A.~P. 1997. ``Giant planet formation by gravitational instability.'' Science 276, 1836-1839.Duncan, M.~J.~and T.~Quinn 1993. ``The long-term dynamical evolution of the solar system.''

Ann. Rev. Astron. Astrophys., 31, 265-295.Duncan, M., T.~Quinn, and S.~Tremaine 1987. ``The formation and extent of the solar system

comet cloud.'' Astron. J., 94, 1330-1338.

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Lin, D.~N.~C., P.~Bodenheimer, and D.~C.~Richardson 1996. ``Orbital migration of theplanetary companion of 51 Pegasi to its present location.'' Nature, 380, 606-607.

Lissauer, J.~J. 1993. ``Planet formation.'' Ann. Rev. Astron. Astrophys., 31, 129-174.Marcy, G.~W., W.~D.~Cochran, and M.~Mayor 2000. ``Extrasolar Planets around Main-

Sequence Stars.'', Protostars and Planets IV, 1285.Murray, N., B.~Hansen, M.~Holman, and S.~Tremaine 1998. ``Migrating Planets." Science,

279, 69.Wetherill, G.~W., 1994, "Possible consequences of absence of Jupiters in planetary systems'',

Ap&SS, 212, 23.

49

BOMBARDMENT OF THE HADEAN EARTH: WHOLESOME OR DELETERIOUS?

Graham Ryder, Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston TX 77058,U.S.A.

Carbon isotopic and geochronological evidence led Mojzsis et al. (1996) and Mojzsis andHarrison (2000) to infer that life existed on Earth in bacterial form in the oceans at 3.85 Ga. If so,then Earth’s life originated at that time or at some time during the Hadean Era. The lunar impactrecord for the post-Akilia interval ~3.8 Ga to ~3.0 Ga shows that even this late in solar systemevolution, the impact rate in the Earth-Moon system averaged ~ 15 times the present rate,declining from more than 100 times to within a factor of about 2 of the current rate. A basin-forming time of even higher impact intensity with larger impactors preceded that interval. Thus,it is generally and reasonably assumed that the origin of life on Earth took place under a muchhigher impact flux than life has had to endure during its subsequent evolution (e.g., Chyba,1993). The general effect of this bombardment is assumed to be the frustration of the origin oflife (e.g., Maher and Stevenson, 1988; Sleep et al., 1989; Oberbeck and Fogleman, 1989), i.e.,that the environment was deleterious.

However, both of these assumptions can be questioned because of the lack of independentevidence for the timing and intensity of impacting at any given time in the Hadean, anddepending on what it takes for an impact to have eradicating effects. The Earth’s own geologicalrecord for events in the Hadean Era is so sparse that nothing can be inferred directly from itabout impact events, impact rates, or impactor populations. All claims about them are based ontheoretical models and evidence from extraterrestrial bodies, including meteorites and the Moon.Among terrestrial rocks ~3.85 Ga and slightly younger (the Akilia and Isua sequences) there is asyet no unequivocal evidence of a heavy bombardment (Ryder et al., 2000; Koeberl et al., 2000;Anbar et al., 2001). Whether this is because the chronology is incorrect, or there was no heavybombardment, or that the processes of sedimentation and erosion were such as to precludepreservation of measurable impact signatures remains to be determined.

The record of the Moon is our best potential template for the impact history of the HadeanEarth: It is the only body for which we have both a stratigraphic record of events and somecalibration of absolute chronology using samples. The Moon also provides indications of thestate of its crustal preservation, and chemical and petrographic information about impact eventsand impactors, all of which provide constraints on the impact history. Furthermore, the Moon hasbeen in Earth orbit since its origin, so the two bodies have been subject to a common populationof heliocentric impactors. Scaling of the Earth’s record to allow for its attraction of a greaternumber of impactors and their greater impact energy is reasonably understood (e.g., Zahnle andSleep, 1997). It is critical to establish what the lunar record actually is based on observations ofthe Moon and lunar samples, rather than on theoretical models alone.

Photogeological techniques have produced a relative stratigraphic column, based on majorbasin formation. The major divisions are defined by the formation of the Nectaris basin, theImbrium basin, and the Orientale basin, leading to the Pre-Nectarian System, the NectarianSystem, and the Lower Imbrian Series (Wilhelms, 1987; Stöffler and Ryder, 2001). The Pre-Nectarian System contains about 30 recognized impact basins, including the oldest and largest

50

(South Pole – Aitken). The Nectarian contains 12 basins, including Nectaris. The Lower Imbriancontains only the Imbrium, Schroedinger, and Orientale basins. The younger Upper Imbrianconsists of basin-free mare lavas and smaller craters.

This stratigraphic column has been partly calibrated against absolute ages, requiringgeological interpretation of the context of particular rocks. Nectaris and Imbrium are inferred tobe 3.90 +/- 0.1 Ga and 3.85 +/- 0.1 Ga, respectively (see Hartmann et al., 2000 for recentdiscussion). These ages are consistent with absolute ages for Serenitatis and Crisium of close to3.89 Ga, and show that 13 basins (and possibly 15) were formed in a brief period of less than 60million years, with the ~ 30 Pre-Nectarian basins at some earlier time. What this means dependson whether the basin record is saturated or in production. If the entire post-crustal formationrecord is preserved, then there was certainly a cataclysmic bombardment at ~ 3.90 Ga that wasunprecedented in the previous 500 million years.

Most lunar impact melt rocks sampled have little geological context. However, the best wayto date impacts is with melt produced by impact melting; most other crater ejecta and depositsare produced at lower temperatures that rarely reset (just disturb) radiogenic systems, even K-Arsystems (e.g., Deutsch and Schärer, 1994). A wide compositional variety of impact melts hasbeen obtained, and nearly all give radiogenic ages that are less than 3.95 Ga. This observationwas made early in lunar sample studies and gave rise to the cataclysmic bombardment concept(Tera et al., 1974). If a continually declining bombardment following crustal formationoccurred, the melts produced by it are not in the sample collection. One possibility is that theyhave been remelted/reset by the youngest major events (Hartmann, 1975). This would haveconverted much of the upper crust to impact melt; samples show that this is not the case. Manyold rocks exist, but all but a few are not impact melts. The availability of old anorthosites,norites, troctolites, and volcanic rocks suggests strongly that numerous old impact melts wouldhave been preserved and collected, if they had ever existed (Ryder, 1990). Another concept, thatthe collection is biased to include only the melts from the latest basins (e.g., Imbrium; Wetherill,1981), is incompatible with geological arguments for collection, lack of equilibration in melts,and the wide variety of melt compositions that have been sampled and dated as actually distinct –though similar – in age. Recent data for meteorite samples also suggest that old impact melts arerare, and that the ~ 3.9 Ga ages are global (Cohen et al., 2000). Other possibilities for biasedcollection have not been developed in the literature.

Two further points suggest that there was a late intense bombardment rather than acontinuous decline from accretion: The low levels of siderophile elements in the lunar crust assampled, and the preservation of crustal structure and lithology as seen from samples and orbit(see discussion in Hartmann et al., 2000).

The flux to the Moon can be expressed in terms of mass accretion rather than basin or craterformation. The sizes of the basin-forming projectiles can be reasonably inferred, and for theNectarian-Lower Imbrian times provide a minimum mass flux to the Moon of 2 x 1021 g (Ryder,submitted 2001). If the interval was as much as 100 million years, then the flux was 2 x 1013 g/yover this period, higher by an order of magnitude than any potential curve that declinescontinuously from accretion to the rate inferred for the older mare plains. This rate cannot beextrapolated increasingly back into Pre-Nectarian times because the Moon would have added

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masses far in excess of itself in post-crust formation times, showing that this Nectarian-LowerImbrian episode was a distinct and cataclysmic set of events. In that Nectaris is an arbitraryboundary, not representing any geological change, then it is most likely that the previous ~ 30basins were part of this same cataclysm and not much older (not older than 4.0 Ga, possibly noteven 3.95 Ga).

These lines of evidence suggest that the ancient lunar bombardment, in the period 4.4 – 3.9Ga, has been exaggerated in most studies. It was instead comparatively benign, and for the mostpart could have been within an order of magnitude of the present flux, following an intenseaccretionary episode that was complete by ~ 4.45 Ga at the latest. When scaled to the Earth, eventhe late cataclysm does not produce ocean-evaporating, globally-sterilizing events (based onmodels in Zahnle and Sleep, 1997). The rooted concept that such sterilizing events took place isbased on the extrapolation on a non-existent lunar record to the Earth’s Hadean. The Earth from~ 4.4. Ga to ~ 3.8 Ga was comparatively peaceful, and the impacting itself might have beenthermally and hydrothermally beneficial and wholesome, rather than deleterious. The origin oflife could have taken place at any time between 4.4 Ga and 3.85 Ga, given the current impactconstraints, and there is no justification for the claim that life originated (or re-originated) as lateas 3.85 Ga in response to the end of hostile impact conditions.

References:Anbar, A..D., K.J. Zahnle, G.L. Arnold, and S. J. Mojzsis, Extraterrestrial iridium, sediment

accumulation and the habitability of the early Earth's surface, J. Geophys. Res., 106, 3219-3236, 2001.

Chyba, C.F., The violent environment of the origin of life: Progress and uncertainties, Geochim.Cosmochim., 57, 3351-3358, 1993.

Cohen, B.A., T.D. Swindle, and D.A. Kring, Support for the lunar cataclysm hypothesis fromlunar meteorite impact melt ages, Science, 290, 1754-1756, 2000.

Deutsch, A. and U. Schärer, Dating of terrestrial impact events, Meteoritics, 29, 301-322, 1994.Hartmann, W.K., "Lunar cataclysm": A misconception?, Icarus, 24, 181-187, 1975.Hartmann, W.K., G. Ryder, L. Dones, and D. Grinspoon, The time-dependent intense

bombardment of the primordial Earth/Moon system, in Origin of Earth and Moon, edited byR.M. Canup and K. Righter, pp. 493-512, University of Arizona Press, Tucson, 2000.

Koeberl, C., W.U. Reimold, I. McDonald, and M. Rosing, Search for petrographic andgeochemical evidence for the late heavy bombardment on Earth in Early Archean rocks fromIsua, Greenland, in Impacts and the Early Earth (I. Gilmour and C. Koeberl, Eds.), Springer,73-97, 2000.

Maher, K.A. and D.J. Stevenson, Impact frustration of the origin of life, Nature, 331, 612-614,1988.

Mojzsis, S.J., and T.M. Harrison, Vestiges of a beginning: Clues to the emergent biosphererecorded in the oldest known rocks, GSA Today, 10, no.4, 1-6, 2000.

Mojzsis, S.J., G. Arrhenius, K.D. McKeegan, T.M. Harrison, A.P. Nutman, and C.R.L. Friend,Evidence for life on Earth before 3800 million years ago, Nature, 384, 55-59, 1996.

Oberbeck, V.R. and G. Fogleman, Estimates of the maximium time required to orginate life,Origins of Life, 19, 549-560, 1989.

Ryder, G., Lunar samples, lunar accretion, and the early bombardment of the Moon, Eos, 71,313, 322-323, 1990.

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Ryder, G., C. Koeberl, and S. J. Mojzsis, Heavy bombardment of the Earth at ~ 3.85 Ga: Thesearch for petrographic and geochemical evidence, in Origin of Earth and Moon, edited byR.M. Canup and K. Righter, pp. 475-492, University of Arizona Press, Tucson, 2000.

Sleep, N.H., K.J. Zahnle, J.F. Kasting, and H.J. Morowitz, Annihilation of ecosystems by largeasteroid impacts on the early Earth, Nature, 342, 139-142, 1989.

Stöffler D., and G. Ryder, Stratigraphy and isotope ages of lunar geologic units: Chronologicalstandard for the inner solar system, Space. Sci. Rev, 96, 7-53, 2001.

Tera, F., D.A. Papanastassiou, and G.J. Wasserburg, Isotopic evidence for a terminal lunarcataclysm, Earth Planet Sci. Lett., 22, 1-21, 1974.

Wetherill, G.W., Nature and origin of basin-forming projectiles, in Multi-Ring Basins, Proc.Lunar Planet. Sci. Conf. 12A (P.H. Schultz and R.B. Merrill, Eds.), 1-81, 1981.

Wilhelms, D.E., The Geologic History of the Moon, U.S.G.S. Prof. Paper, 1348, 302 pp., 1987.Zahnle, K.J. and N.H. Sleep, Impacts and the early evolution of life, in Comets and the Origin

and Evolution of Life (P.J. Thomas, C.F.Chyba, C.P. McKay, Eds.), Springer-Verlag, NewYork, 175-208, 1997.

53

SPHERULE EVENT HORIZONS: THE OTHER (AND BETTER?) RECORD OFIMPACTS IN EARLY EARTH HISTORY

Bruce M Simonson1 and Scott W Hassler2. 1Geology Department, Oberlin College, Oberlin OH44074, U.S.A. (bruce.simonson@oberlin.edu) 2John F. Kennedy University, Orinda, CA 94563,U.S.A. (shassler@jfku.edu)

Discrete layers rich in millimeter-scale spherules of former silicate melt that persist laterallyfor tens to hundreds of kilometers have been detected in various early Precambrian stratigraphicunits. In my talk, I will describe spherule layers in 6 late Archean to Paleoproterozoic formations(Table 1), interpret them as distal impact ejecta, and suggest that they should be used to constrainthe incidence and environmental effects of large impacts on the early Earth.

Table 1. Selected data on early Archean to early Paleoproterozoic impact spherule layers(source: Simonson et al. 2001).

HostFormation

Group Location Est. Thicknessof Spherules

EstimatedAge

Grænsesø Vallen South Greenland 180 mm 1.9-2.0 GaDales Gorge Hamersley Western Australia 50 mm ca. 2.49 GaWittenoom Hamersley Western Australia 10 mm 2,541+18/-15 MaCarawine Hamersley Western Australia ≤250 mm ? 2,548 +26/-29 MaJeerinah Fortescue Western Australia 100 mm? 2.63 GaMonteville Ghaap South Africa 50 mm ? 2.64 Ga

In each of these formations, the spherules are restricted to a single layer or thin stratigraphiczone amidst shales and fine-grained carbonates. The latter were deposited below wave base indeep shelf environments and commonly contain a number of intercalated “event” beds, mainlyturbidites and normally graded tuffs. The spherule-rich layers are also event deposits, but theyshow a distinctive suite of sedimentary structures. Based on these structures, we infer thefollowing sequence of events during the deposition of the spherule layers: 1) high-energy scour,resulting in local transport of meter-scale rip-up clasts, 2) deposition of spherules and other sand-size detritus under the influence of waves, 3) reworking by syn- to post-wave offshore-directedcurrents, and 4) later reworking by sediment gravity flows (Hassler and Simonson 2001). Weattribute most of this sequence to tsunami waves triggered by an impact, although the sedimentgravity flows may represent later events.

Petrographically, the spherules differ from volcaniclasts in the associated tuffs. The spherulesare highly rounded and comparable in shape to known impact ejecta such as microtektites,whereas the volcaniclasts have angular shapes and other characteristics typical of hydrovolcanictephra. The spherules have been largely replaced by authigenic K-feldspar during diagenesis, butmost show relict crystal fabrics internally. Fibrous crystal sprays radiate inwards from the edgesof most spherules, but a minority have textures similar to those of partially crystallized basalts.Most or all of the spherule layers are enriched in siderophile elements, particularly iridium,relative to the non-volcanic and tuffaceous strata associated with them (Simonson et al. 1998,2000). The most careful analysis done to date suggests the interelement ratios of the PGEs in the

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late Monteville layer (Table 1) are relatively close to chondritic (Simonson et al. 2000). Giventhese characteristics, we interpret the spherules as droplets of ballistic melt and/or vaporcondensate generated by large impacts. The K/T boundary layer is the only known Phanerozoicoccurrence with impact spherules as large and potentially as thick as these late Archean toPaleoproterozoic occurrences. This suggests the Precambrian spherule layers are products ofimpacts by projectiles on the order of 10 km in diameter.

Researchers have traditionally used the size and abundance of craters to assess the history ofimpacts on Earth. We believe Precambrian spherule layers offer a second line of evidence thatcan be used to constrain this history, and one that has greater potential for assessing theenvironmental effects (or lack thereof) of large impacts on the early Earth. Large craters canprovide unmistakable evidence of impact, but ejecta layers from large impacts are much easier tofind in the sense that they blanket much larger areas. Given the absence of metazoa to burrowsediment, the thinnest of ejecta layers can be preserved with no disruption in an appropriatePrecambrian succession. Moreover, ejecta layers represent our best source of data on largeimpacts in ocean basins because most craters formed by impacts in oceanic lithosphere havebeen destroyed by subduction. To appreciate what spherule layers can add to our understandingof impacts in early Earth history, consider this: the oldest terrestrial impact craters we haverecognized formed about 2.0 Ga, yet all but one of the Precambrian spherule layers listed inTable 1 are significantly older.

Using spherule layers to shed light on large impacts in early Earth history will have bothadvantages and disadvantages. One advantage is that a researcher who is familiar with impactspherules can find evidence for a major impact thousands of kilometers from the site where it hitwith just a hand lens. Another big advantage of spherule layers over craters is that they arealways found in stratigraphic context. This will facilitate the determination of whether or notsignificant environmental change took place at the time of impact, e.g. by analyzing biomarkersin and isotopic compositions of strata above and below a given spherule layer. The K/T boundaryimpact certainly resulted in major changes in Earth's surface environments and the life thatinhabited them. Large impacts could have caused equally significant changes in, for example, thechemistry of the atmosphere or the global circulation of the oceans more than once during thePrecambrian. This in turn could have produced dramatic (and perhaps interrelated) changes inthe distributions of organisms and sediments. The most impressive thing we have noted is thatthe strata directly above and below the spherule layers appear to be quite similar in most cases.This suggests these impacts caused no immediate significant environmental change in deep shelfenvironments, but this possibility needs to be tested more rigorously and in a broader range ofpaleoenvironments.

One of the disadvantages of using spherule layers as a record of large impacts is that thinlayers are vulnerable to removal from the stratigraphic record via erosion, tectonic deformation,and/or diagenetic alteration. Most of the Precambrian spherule layers found to date have beenfound in two restricted geographic areas (Table 1). These areas include some of the best-preserved Archean and Paleoproterozoic successions on Earth, which suggests a preservationalbias. The textures that make spherules so distinctive are probably rapidly obscured bymetamorphism, and undeformed strata are progressively harder to find in older successions.Finally, the conditions by which an impact creates primarily spherules and not other types of

55

ejecta are not well understood. Only a few Phanerozoic impact layers consist predominantly ofspherules (Grieve 1998), and one of the most widespread (the Australasian strewn field) has yetto be linked to a crater. Therefore it is difficult to determine the exact relationship between thesize of an impact crater or projectile and the size, thickness, and abundance of the spherules itproduces.

Even though spherule layers are potentially easy to erode and difficult to interpret, knownoccurrences in the Hamersley succession indicate at least three large impacts happened within ca.140 million years around the Archean/Proterozoic boundary. This implies a "recurrence interval"of approximately 70 million years, which is similar in magnitude to the recurrence interval forlarge impactors in the Phanerozoic (Chapman and Morrison 1994). Similar spherules occur inlayers at 4 different stratigraphic levels in the early Archean Barberton Greenstone Belt of SouthAfrica (Lowe et al. 1989). Byerly et al. (1996) established that these four spherule layers weredeposited within a time span of some 300 million years, which yields a roughly comparable"recurrence interval" of 100 million years. However, these "recurrence intervals" could getdramatically shorter with the discovery of additional spherule layers. Moreover, the relictcrystallization textures commonly observed in early Precambrian spherules have not beenreported from any Phanerozoic impact spherules, so factors other than the flux of impactors needto be taken into consideration. One possibility is that more impactors hit basaltic target rocksearly in Earth history due to the volume of continental crust being smaller and/or the oceansbeing shallower on average (Simonson and Harnik 2000).

In summary, a small but growing number of layers rich in spherules and best interpreted asdistal ejecta from large impacts are being found in early Precambrian successions. Thesespherule layers can teach us things about impacts in early Earth history we are unlikely to learnfrom studies of craters and proximal ejecta alone, particularly concerning the environmentaleffects of large impacts. The study of impact spherule layers is in its infancy. We predict morespherule layers will be found, and as they are, we will gradually increase our understanding ofnot only how they formed, but also the impacts that formed them and their effects on the earlyEarth’s surface.

REFERENCES CITED:

Byerly G.R., Kröner A., Lowe D.R., Todt W. and Walsh M.W. 1996 Prolonged magmatism andtime constraints for sediment deposition in the early Archean Barberton greenstone belt:evidence from the Upper Onverwacht and Fig Tree Groups. Precambrian Research 78, 125-138.

Chapman C. R. and Morrison D. 1994. Impacts on the Earth by asteroids and comets: assessingthe hazard. Nature 367, 33-40.

Grieve R. A. F. 1998 Extraterrestrial impacts on earth: the evidence and the consequences. inGrady M. M.; Hutchison R., McCall G. J. H. and Rothery D. A. eds., Meteorites: Flux withTime and Impact Effects. Geological Society London Special Publication 140, 105-131.

Hassler S.W. and Simonson B.M. 2001 The sedimentary record of extraterrestrial impacts indeep shelf environments – Evidence from the early Precambrian.. Journal of Geology 109, 1-19.

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Lowe D.L., Byerly G.R., Asaro F. and Kyte F.J. 1989 Geological and geochemical record of3400-million-year-old terrestrial meteorite impacts. Science 245, 959-962.

Simonson B.M. and Harnik P. 2000 Have distal impact ejecta changed through geologic time?Geology 28, 975-978.

Simonson B.M., Davies, D., Wallace M., Reeves S. and Hassler S.W. 1998 Iridium anomaly butno shocked quartz from Late Archean microkrystite layer: oceanic impact ejecta? Geology 26,195-198.

Simonson B.M., Koeberl C., McDonald I. and Reimold W.U. 2000 Geochemical evidence for animpact origin for a Late Archean spherule layer, Transvaal Supergroup, South Africa. Geology28, 1103-1106.

Simonson B.M., Cardiff M. and Schubel K.A. 2001 New evidence that a spherule layer in thelate Archean Jeerinah Formation of Western Australia was produced by a major impact. Lunarand Planetary Science XXXII, abs. #1141.

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TIMING OF (MASS)EXTINCTIONS AT THE K/T BOUNDARY

Jan Smit, Vrije Universiteit , Amsterdam, Neterlands (smit@geo.vu.nl)

The majority of the extinctions at the Cretaceous/Tertiary boundary can be tied to theChicxulub impact, through correlation with the worldwide ejecta layer. However, the extinctionsmay not be as instantaneous as generally assumed. A problem is the often complex geologicalrecord, yielding a disturbed original sequence of extinction events. Oceanic calcareous plankticbiota (foraminifers, nannofossils) are almost instantaneously annihilated by the first after-effectsof the Chicxulub impact. That can best be observed in outer-shelf, sections at bathyal depth(500-1500m). Here the few mm thick ejecta layer is preserved due to near cessation ofbioturbation caused by widespread anoxia. The overlying few mm to dm thick clay layer isdesignated as 'strangelove ocean' or P0 zone, and represents the 'sterile' oceans, following theimpact event for several thousands of years. The CSDP Chicxulub drilling project (11 dec-2001-febr 2002) may shed some light on this critical period. This period is important for theevolutionary radiation following the Chicxulub event, and recent investigations show severalfirst appearances of new planktic foraminiferal species in this interval. The subsequent radiationof these species is characterized by the rise and fall of successive dominant species, that do notlast long as a species. After just 50.000 years, the associations are stabilized again, and slowevolutionary turnover prevails

Nannofossil extinctions are also tied to the impact event itself, as their abundance dropsimmediately by an order of magnitude. Their tiny size makes them vulnerable for reworking,however, and their true last occurrence is almost impossible to establish. In contrast to theplanktic foraminifers, the nannofossil species that radiate just above the K/T boundary, such asthe tiny N. romeinii, N. parvulum and C. primus already exist 0.5-1 million years before K/T,e.g. in near-shore habitats in the Maastrichtian type area. These species are widely used as indexfossils for the basalmost Danian NP1 zone elsewhere, but will need some revision.

Dinoflagellate1 species are well known for their survival at the K/T boundary, due to theircyst-forming capabilities. The dinocyst abundance in the p0 Zone is, in contrast to calcareousplankton, extremely high. These organisms are therefore wel suited to record the initial climaticchanges following the KT impact. At the very base of the p0 clay in Kef, Tunisia, suddenly anassociation is found of cold, boreal species, followed by an association of tropical species in theoverlying p0 clay. A short cold spell, followed by a prolonged (greenhouse) warm period isinferred from these migration patterns.

Ammonites were thought to be declining to near extinction well before the KT boundary.Peter Ward has subsequently shown them to remain highly diversified all the way up to the K/Tboundary. Recent research revealed baculitid and scaphitid survivors just above the K/Tboundary in the Maastricht type area (Netherlands)2 and the Fox Hills sandstone in SouthDakota3. Rudists and Inoceramid clams are highly characteritic of the Cretaceous period, andtheir disappearance has often been linked to the KT boundary. Yet the Inoceramids disappearabout 3.6 million years before KT after a gradually decline over at least a million years. It is notlikely to add these highly specialized occupants of a niche vacated today to the victims of the

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K/T boundary event. Rudist likewise display a decline prior to the K/T boundary, but there isgood evidence from Italy and Istria (Slovenia) that endemic reefs last up to the K/T boundary.

Research in dinosaur extinction has not yielded radical new insights for a decade or so, nocritical finds have been reported yet that shed new light on their disapparance. Yet the footprintrecord still shows their demise to be related to the impact event. Their preceding slow decline ismore likely an artifact of the geological record, rather than a true decline leading up toextinction.

In conclusion, the picture emerging from KT boundary is that some decline in speciesnumber has taken place in certain habitats before KT. But the crucial turnover of Cesozoic toCainozoic species is strongly related to the K/T boundary impact event, probably through a shorticehouse-greenhouse event.

References1Brinkhuis, H., J. P. Bujak, et al. (1998). "Dinoflagellate-based sea surface temperature

reconstructions across the Cretaceous-Tertiary boundary." Paleogeogr., Paleoclim.,Paleoecol. 141: 67-83.

2Smit, J. and H. Brinkhuis (1996). "The Geulhemmerberg Cretaceous/Tertiary boundary section(Maastrichtian type area, SE Netherlands); summary of results and and a scenario of events".75 (Special Issue Geol. & Mijnbouw): 283-293.

3Terry, D. O., J. A. Chamberlain, et al. (2001). "Marine Cretaceous-Tertiary boundary section insouthwestern South Dakota." Geology 29(11): 1055-1058.

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MOLECULAR SIGNATURES OF MICROBIAL LIFE

Roger E. Summons. Dept Earth Atmospheric and Planetary Sciences, Massachusetts Institute ofTechnology, 77 Massachusetts Ave E34-246, Cambridge MA 02139-4307, U.S.A.

Microscopic organisms are often overlooked as indicators and agents of globalenvironmental change on both short and long timescales. Most microbes leave no visible recordof their former presence and we can only infer their involvement in environmental andgeological processes from chemical clues left in the ocean, the atmosphere and rocks. Molecularmethods are providing new ways to track microbiologically driven processes. DNA cloned fromliving microbes is one component of this and analyses of diagnostic lipids of living, recentlydead and fossilised organisms is another.

Biological marker compounds (biomarkers), which comprise the lipids of extant organismsand also their hydrocarbon (i.e. fossil) counterparts carry diagnostic information in theirchemical structures and in their carbon and hydrogen isotopic compositions. Biomarkers can tellus about the inhabitants of the oceans, lakes, rivers and sediments including sub-surface life, theso-called deep biosphere. They reveal much about the biota of extreme environments, forexample, hydrothermal vents, and are proving an effective means to learn about microbes thatinhabited the early Earth and whose fossilised lipid remains are found trapped in rocks as old as2.7 billion years.

If biological marker compounds are to be used with maximum confidence we need toexamine the degree to which biosynthetic pathways leading to diagnostic lipids closely parallelthe molecular phylogeny that has been established using small subunit r-RNA. The sterols fromeukarya, bacteriohapanepolyols from bacteria and isoprenoid ether lipids from archaea are allmarkers which, with a few minor exceptions, are confined to those domains. A significantamount of new research is directed toward seeking unambiguous biomarkers for taxa withparticular biological or geological significance. For example, studies of hyperthermophilicorganisms that cluster near to the base of the bacterial domain show that they have their ownparticular class of ether lipid.

Biomarkers can be used as chemostratigraphic tools. For example, Early Triassic sedimentsfrom Western Australia show molecular evidence for the radiation of dinoflagellates at that time.In the same sedimentary section, bacteriohopanes which can be used to evaluate the processes ofcyanobacterial photosynthesis and methanotrophy, show dramatic fluctuations in their relativeabundances which might eventually provide valuable information about the biogeochemicalprocessing of carbon following the Permian extinction and subsequent radiation.

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COMPARING THE P/T, T/J AND K/T EVENTS: NEW INSIGHTS FROM NEW FIELDWORK

Peter Ward. Department of Earth and Space Sciences, The University of Washington, SeattleWA 98195-1580, U.S.A.

The 1980 Alvarez Impact Hypothesis changed the paradigm for mass extinction events frommillion to multi-million year, multi-causal events to thousand year (or less) single-cause events.Since the appearance of this seminal hypothesis, investigators have attempted to fit othermembers of the “Big Five” mass extinction events into the short term, single cause category.New research on the P/T and T/J events based on field work in the Karoo of South Africa and theQueen Charlotte Islands of British Columbia have resulted in new information about fossilranges and stable isotope signatures around each of these boundaries.

In the Karoo of South Africa, Ward et al (2000) have demonstrated that the extinction amongmammal-like reptiles was geologically rapid, and coincided with sedimentological evidence forrapid environmental change. However, stable isotope records across this boundary suggest thatmultiple perturbations occurred. It seems unlikely that a single event (such as a single asteroidor comet impact) could have produced this pattern.

In the Queen Charlotte islands, a T/J boundary isotopic anomaly reported by Ward et al.2001 corresponds with a highly significant extinction in marine plankton at both Kunga Islandand Kennecott Point. At Kunga Island more than 40 species of radiolarians disappear at thislevel . Detailed biostratigraphic analysis of Kennecott Point radiolarians shows theirdistributions to be similar to those of Kunga Island; the boundary here, as defined byradiolarians, falls within a 5 meter stratigraphic interval. The highest Triassic ammonoidrecovered at Kennecott Point, Choristoceras rhaeticum is found within this interval as well,based on new collections made in 2001. The end of the isotopic excursion coincides with theappearance of earliest Jurassic radiolarians and, 8 meters above, by the lower (possibly lowest)Hettangian ammonites.

The pattern of sudden extinction and the coincident and short stratal duration of the δ13Corg

excursion at the level of this extinction are compatible with a sudden biological crisis affectingmarine productivity.

These results suggest that a variety of causal hypotheses must still be entertained by thoseattempting to understand mass extinctions.

ReferencesWard, P., D. Montgomery, and R. Smith. 2000. Altered River Morphology in South Africa

Related to the Permian-Triassic Extinction. Science v. 289: 1740-1743.Ward, P. ,Haggart, J., Carter, E., tipper, H., and Evans, T. and others. 2001Sudden productivity

collapse associated with the Triassic/Jurassic boundary mass extinction. Science, Vol. 292,11 May

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LARGE AERIAL BURSTS; AN IMPORTANT CLASS OF TERRESTRIALACCRETIONARY EVENTS

John T. Wasson, University of California, Los Angeles, CA 90095-1567

The remarkable Tunguska event occurred on the morning of 30 Jun 1908. A large (≈40 m)meteoroid was totally disrupted at an altitude of ≈8 km, the resulting explosion (here designatedan aerial burst) having an energy of ≈15 MT (TNT equivalent) [Vasilyev, 1998]. The blastwave leveled trees over an area of about 2000 km2. The first field investigation two decadeslater showed extensive evidence of charring of the forest debris; the maximum thermal pulse atground zero is estimated to be 238 J cm-2, sufficient to heat 0.16 g of dry continental crust to1500 K and melt it.

Tunguska is one of a continuum that extends from events <103 times smaller to events >106times larger. The smallest documented members of the set are the type-III fireballs recorded bythe cameras of fireball networks [Ceplecha et al., 1998] and several events recorded by militarysatellites [Tagliaferri et al., 1995].

Two circumstances are required to generate aerial bursts appreciably larger in magnitudethan Tunguska: (1) the meteoroid must be weak enough to disrupt during atmospheric passage,and (2) the atmospheric entry angle must be relatively oblique. Tunguska's entry angle was 20-45° relative to the horizontal [Bronshten, 1999]. Modeling by Hills and Goda [1999] indicatesthat a friable meteorite (strength of 1•108 dynes cm-2) coming in at an entry angle of 90° and a

geocentric velocity of 18 km s-1 will deposit more than half its energy in the atmosphere if itradius is <100 m. These authors estimate that a 500 m projectile will lose half its energy at ≥10km if its entry angle is 20°.

For Tunguska the blast effects were more dramatic than the thermal effects, but thermaleffects dominate as the size of the event increases. As the size of the burst increases, the centralregion finds itself surrounded by hot atmosphere that is also hot, and loses its ability to cool itselfby radiation in directions other than vertical. Because there is so little mass above the hot gas,expansion results in negligible cooling.

Below this central region we can expect vegetation to be fully incinerated. In moist,vegetated areas or regions covered by water, much of the heat energy is expended on the latentheat of vaporization of H2O and carbonaceous compounds, and little melting of the local soiloccurs. In contrast, if the surface below the burst is desert-like, the radiation is capable ofmelting several mm of sand or rock.

When large (D>100 km) craters such as Chicxulub form, the ejecta is thrown above theatmosphere and scattered all around the globe; its reentry into the atmosphere causes largeamounts of heating, creating conditions much like those resulting from an aerial burst. The chiefenvironmental difference is one of scale.

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Materials which we interpret to be the products of giant aerial bursts are the layered tektitesfound over a region having an area of ≈8•105 km2 in Southeast Asia and the Libyan DesertGlass (LDG) found over a region having an area of ≈7•103 km2 in Western Egypt. These areglassy materials which were formed by the melting of the local continental crust. Compositionsof tektites are about the same as the local continental crust; the LDG is ≈98% SiO2, similar incomposition to the adjacent Great Sand Sea. The layering common to these materials is inferredto have resulted from down-slope flow of a melt sheet; this flow requires that temperaturesremained high (>2400 K) for several minutes.

In the past such materials were ascribed to ejection from craters. However, there is noevidence that large amounts of fully molten (free of unmelted clasts) materials are ever ejectedfrom terrestrial craters, and any such ejecta would be expected to quench when it returns to thesurface rather than to flow tens of cm. In addition, melt production is most efficient at thebottoms of relatively large craters, whereas layered tektites have high 10Be contents requiringthat much or most of the target have been a surficial soil originating <1 m from the surface.

The very low density (1.3 g cm-3) of the asteroid Mathilde [Veverka et al., 1999] suggeststhat it is a flying rubble pile. It is plausible that a sizable fraction of asteroids and comets areprimordial materials that were never compacted, and have essentially no strength. Thus, many ofthe asteroids striking the Earth may be these strengthless objects. In support of this view is thehigh fraction of low-strength fireballs observed by the photographic networks. According toCeplecha et al. [1998], only 32% of photographed meteoroids are as strong as anhydrouschondrites. Another 33% are friable, having strengths similar to CM chondrites, and theremaining 35% are designated “cometary” because they break up very high.

A potential problem is how to account for the very large areas in which the layered tektites andLibyan Desert Glass are found. This seems to require oblique entry into the atmosphere andpossibly (similar to many comets) breakup in deep space prior to atmospheric entry. However,preliminary modeling by James Lyons (pers. comm., 2002) suggests that atmospheric shockwaves transmit energy large (tens of km) distances from the site where most of the energy isdeposited, and that this effect may significantly increase the lateral extent of the heated portionof the atmosphere.

It is therefore plausible that aerial bursts should comprise an important fraction of theaccretionary events occurring on the Earth. When tektite-size events occur, much of the aboveground part of the biosphere will be incinerated, leaving a virgin area that may encouragespeciation.

References:Bronshten V. A. (1999) The nature of the Tunguska Meteorite. Meteorit. Planet. Sci. 34, 723-

728.Ceplecha Z., Borovicka J., Elford W. G., Hawkes R. L., Porubcan V., and Simek M. (1998)

Meteor phenomena and bodies. Space Sci. Rev. 84, 327-471.Hills J. G. and Goda M. P. (1998) Damage from the impacts of small asteroids. Planet. Space

Sci. 46, 219-229.

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Tagliaferri E., Spalding R., Jacobs C., Worden S. P., and Erlich A. (1994) Detection ofmeteoroid impacts by optical sensors in earth orbit. In Hazards Due to Comets and Asteroids,(ed. T. Gehrels), pp. 199-220. University of Arizona Press.

Vasilyev N. V. (1998) The Tunguska meteorite problem today. Planet. Space. Sci. 46, 129-150.Veverka J., Thomas P., Harch A., Clark B., Bell J. F., Carcich B., and Joseph J. (1999) NEAR

encounter with asteroid 253 Mathilde: overview. Icarus 140, 3-16.

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THE COMET AND ASTEROID IMPACTOR FLUX ON THE EARTH

Paul R. Weissman, Jet Propulsion Laboratory, MS 183-601, Pasadena, CA 91109 U.S.A.

Impacts on the Earth by comets and asteroids are common events on geologic time scales.More than 150 terrestrial impact craters and structures have been recognized and studied. Atpresent, it is estimated that a 20 km diameter crater is formed on the Earth (or a comparableimpact occurs in the oceans) about once every 3.5 _ 10 5 years (Grieve 1995). That rate, derivedfrom the cratering record, is consistent with that expected from the observed populations ofcomets and asteroids in Earth-crossing orbits.

The flux of potential impactors on the Earth is made up of: 1) asteroids, derived almostentirely from the main belt of asteroids between the orbits of Mars and Jupiter; 2) short-period orAJupiter-family@ comets derived from the Kuiper belt, a belt of remnant icy planetesimalsbeyond the orbit of Neptune; and 3) intermediate and long-period comets, also known asAHalley-type@ and AOort cloud@ comets, respectively, derived from the Oort cloud, a vastspherical cloud of comets surrounding the solar system at near-interstellar distances (Weissman1996). The Oort cloud comets likely originated as icy planetesimals in between the orbits of thegiant planets, Jupiter through Neptune, and were dynamically ejected out to their current distantorbits during the formation of the solar system. All of the potential Earth impactors are transientpopulations that must be continually resupplied as objects either strike one of the planets (or theSun) or are dynamically ejected from the system due to close planetary encounters. Dynamicallifetimes for near-Earth asteroids are ~1B30 Myr, while for comets they are typically only ~0.5Myr (Levison and Duncan 1997). The difference is caused by the fact that the comets are almostalways on Jupiter-crossing orbits, and Jupiter=s large gravity is able to dispose of objectscrossing its orbit much more quickly.

It is currently estimated that there are ~700 asteroids brighter than absolute magnitude H =18, corresponding to radii larger than ~0.5 km, in Earth-crossing orbits. Approximately half ofthat number have actually been found, thanks to automated surveys that have greatly increasedthe discovery rate in recent years. About 56% of these asteroids are S-type or stony asteroids,while the remainder are C-type or carbonaceous. Also, perhaps 3% of the Earth-crossingasteroids are iron-nickel composition but these do not seem to be present among the ones largerthan 0.5 km radius. The average impact probability for a near-Earth asteroid (NEA) in an Earth-crossing orbit is 2.8 _ 10 _9 year_1 and the mean impact velocity is ~20 km sec_1. Some fraction ofthe NEA=s, perhaps 6B10%, are believed to be extinct cometary nuclei, no longer able toproduce visible comae (Rabinowitz et al. 1995; Bottke et al. 2001).

Comets are made up of roughly equal parts of volatile ices (primarily water ice), silicate dust,and carbonaceous dust, all intimately mixed. Approximately 10 Jupiter-family comets in Earth-crossing orbits have been discovered; the total number is likely not more than 20. The JF cometshave typical impact probabilities of ~1.3 _ 10 _9 year_1, and mean impact velocities of ~23 km sec-

1. Approximately 10 Halley-type comets are also known to be in Earth-crossing orbits, with thetotal number possibly as large as 100. These have much lower impact probabilities of ~0.16 _10_9 year_1 because of their longer orbital periods, but considerably higher impact velocities, ~52

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km sec_1, because of their larger orbital eccentricities and inclinations. About 10 long-periodcomets cross the orbit of the Earth every year, with a mean impact probability of 2.2 _ 10 _9 foreach comet passage and a mean impact velocity of ~56 km sec_1. Although comets likely makeup only a few percent of all terrestrial impactors, their higher impact velocities result in cometsbeing responsible for perhaps 10B20% of all major impacts (Weissman 1990). The sizes ofcometary nuclei are poorly determined but are typically a few kilometers.

The flux of potential impactors will vary stochastically as comets and asteroids are randomlyinjected into Earth-crossing orbits. However, the flux of Oort cloud comets can undergo largefluctuations. The flux of long-period comets varies by about a factor of 4 with a period of ~33Myr as the solar system=s galactic orbit carries the Sun and planets above and below the galacticplane. The solar system has recently passed through the galactic plane so the present cometaryflux is expected to be near a maximum. More substantial variations in the long-period cometflux, up to a factor of ~300, can occur if a star passes directly through the Oort cloud, or if thesolar system encounters a giant molecular cloud in the galaxy. Major cometary showers areexpected to occur every ~300-500 Myr, with more modest showers every 30-50 Myr.Examination of the current orbital distribution of comets suggests that the solar system is notpresently experiencing a cometary shower.

Several mechanisms have been suggested for causing periodic cometary showers from theOort cloud, such as a 10th planet or a distant red dwarf star orbiting the Sun, but all of these havebeen rejected on dynamical grounds and/or for lack of evidence.

ReferencesBottke, W. F., Morbidelli, A., Jedicke, R., Petit, J.-M., Levison, H., Michel, P., and Metcalfe, T.

S. (2001) Debiased orbital and size distributions of near-Earth objects. Icarus, in press.Grieve, R. A. F. (1995) Target Earth: Evidence for large-scale impact events. In Near-Earth

Objects, Annals of the NY Acad. Sci. 822, 319-352.Levison, H. F., and Duncan, M. J. (1997) From the Kuiper belt to Jupiter-family comets: The

spatial distribution of ecliptic comets. Icarus 127, 13-32.Rabinowitz, D., Bowell, E., Shoemaker, E., and Muinonen, K. (1995) The population of Earth-

crossing asteroids. In Hazards Due to Comets and Asteroids, ed. T. Gehrels (Univ. ArizonaPress, Tucson), pp. 285-312.

Weissman, P. R. (1990) The cometary impactor flux at the Earth. In Global Catastrophes inEarth History, eds. V. L. Sharpton and P. D. Ward, Geol. Soc. Amer. SP 247, pp. 171-180.

Weissman, P. R. (1996) The Oort cloud. In Completing the Inventory of the Solar System, eds.T. W. Rettig and J. M. Hahn, ASP Conf. Ser. 107, 265-288.

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BEFORE UNIFORMITARIONISM: IMPACTS IN THE HADEAN

Kevin Zahnle. NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035 U.S.A.

It is more useful to define the Hadean Eon as the time when impacts ruled the Earth than todefine it as the time before the rock record. For decades now it has been obvious that thecoincidence between the timing of the end of the lunar late bombardment and the appearance of arock record on Earth is probably not just a coincidence. I doubt I am pointing out something thatthe reader hasn't long ago given thought to. While the Moon was struck by tens of basin-formingimpactors (~100 km objects making ~1000 km craters), the Earth was struck by hundreds ofsimilar objects, and by tens of objects much larger still. The largest would have been big enoughto evaporate the oceans, and the ejecta massive enough to envelope the Earth in 100 m of rockrain. Smaller impacts were also more frequent. On average, a Chicxulub fell every 100,000years. When one imagines the Hadean one imagines it with craters and volcanos: crater oceansand crater lakes, a scene of mountain rings and island arcs and red lava falling into a steamingsea under an ash-laden sky. I don't know about the volcanos, but the picture of abundant impactcraters makes good sense --the big ones, at least, which feature several kilometers of relief, arenot likely to have eroded away on time scales of less than ten million years, and so there werealways several of these to be seen at any time in various states of decay. The oceans would havebeen filled with typically hundreds of meters of weathered ejecta, most of which was ultimatelysubducted but taking with them whatever they reacted with at the time --CO2 was especiallyvulnerable to this sort of scouring. The climate, under a faint sun and with little CO2 to warm it,may have been in the median extremely cold, barring the intervention of biogenic greenhousegases (such as methane), with on occasion the cold broken by brief (10s to 1000s of years)episodes of extreme heat and steam following the larger impacts. In sum, the age of impactsseems sufficiently unlike the more familiar Archaean that came after that it seems useful to givethis time its own name, a name we already have, and that, if applied to the Hadean that I havedescribed, actually has some geological value.