C H A P T E R 1 9

The Oceans

mcnt derived from the conljnents and hanspodedby slremq wind. and glacial ice. In addition,streams and. to a lesse. exlenl, groundwate. ddglaciers tansport dissolved chemical elements iltothe oceds dd tber€by contribute to the chemicalcomposition ofseawater and to the isotope compo-sition of Sr. Nd, Pb. and other elements that haveradiogenic isotopes- The mdine isotope geochemisrry of ca was discussed in Section 8.2b.

I 9 . 1 S T R O N T I U MI N T H E P H A N E R O Z O I C O C E A N S

The chenical composition of seawater is controlledby the balance of inputs and outpuls of each e1e-men1. The lesulting concentrations of conservaliveelemenis are elated to the salinity of the water,which is denned as the total amount of solid ma!e-rinl in grams conlained in one klogram of seawalerwhen aI the cdbonate has been converted lo theoxide. the bromine and iodine have b@n replacedby chlorine. and all orgadc matter has been com-pletely oxidized. Since salinily is difficult m measure diectly. it is detemined from the clrlorinityof seawater, which is based on the weighl of Agprecipiraled fiom one kilogram of seawater:

Chloriniry (%.) = 0.3285234 Ag (g) (19.1)

436

lre oceds and the basins they occupy playimporrant roles in the geological activity ofthe Eanh, including the deposition of sedi-

The chlorinity is related to the salinity by theequarion (Sverdrup et al., 1942; Ross, 1970)

Salinity (7@): 1.8066 chlorinity (%t (19.2)

The .al n,b ol .earaR|n rhe ope. oced ol31.711dd ls lowered ̂ s n rcsult of dilution by fesbwater dischdged by streams (e-g., the Baldc Sea)or by melrjng of icebergs, and it \s itcrcosed byevaporalive conceniratior (e.9., the Rod Sea) andby tbe formation of ice (e.g.. lhe Weddell Sea

19.la Present Day Seawater

The concenrrauon of sr in \eouarer ar deprh. o''1500 m rs re ls ,ed ro i r \ ,a l in i ry b) an equdr io '

den\eJ by Bem.t e l d l . , la7)r and Br.$ "nLTurekian (i972, 1974):

Sf(ppm) = 0.221+0.0010 salinity (%t

LAdanl ic Oc.an\ ILo 1)

Sr(ppm) : 0.220 + 0.0026 salinity {%d

lPacific Oceani ( lo 4l

Ac.ordrnSl). rhe Sr concenu.rion ol 'landatd\eawaler In rhe Atldntj. O.ed hJ\in8 a.alinjtvol J5-d ii 7.74 ppm. The \alinrty cnd lhe 5rconcentration of surface water in the oceans both

decrcase reSionally in respo.se Lo dilution ofscawatef by mcteorjc water and by mixing wirhdverwater and glacial mcltwater. Conversely, bo r0E salinily and the concentrarioD of Sr indease incase of watd los by excessivc cvaporation andby lhe fomdtion of sea ice. Tbe concentmrionof Sr in selwaref (7.74 pp ) is mofe than onehundred times higher tban lhar of alerage nverv.rer (0.070 !pm). The residence tinc of Sr intl'e oceans, 5.0 x I 0d yea6 (Taylo. and Mcl-eman,1985), causes Sr in the oceans ro be homogcnizcdisotopically by mixingon a limcscale ofabout 1000yeals (Patmer and Edmond. 1989).

The isotope composirion of Sr in seawarerhas been mcasrcd by many investigatos. whoseesults published prio. ro 1960 wcre sunmarizedby Faure (1982). These sudies ircluded anrlysesof ma.ine Sr iiom the No(h Atlantic by Faureer al. (1965), who used a ftree,componcnt mixing modcl lo explain thc obsened numericul valueof rhe 315/365r rario of nodern seawarer. Subsequent lnalyses of narine Sr ftorn the Hudson Blyby Faure et al. (1967) st.cngthened rhe importantconclusion ftar fie pcscnt 3?51165r raLio of scawat€. rs consrant rhroughout all pdrs ofthc ocean.

The numerical valks of rhc 3?5165r rariosof seawater rcponed in the lirerdlurc !rc reconciledby Elerence to interlaborilory sbndards rotrtinelyanalyzed by most invcsti8aton. The numerical values of rhe MSrl6Sf ratios of these standdds ee6 fo l lowsr E&A,0.70800r NBS 987,0.71025. Inaddnion, Ludwig ct!1. (1988) reported 3i5r/365r

ntios of a large Trida.hha shell collccrcd livetrcm the botrom ol the lagoon at Enewerak atollin thc Pacilic Ocean. This sample is idenrified !sEN-l and has been used to standddize measu.c-ments ofthc 3rsr/36sr rarios ofbiogcnic c&bonaresoi lnaine origin (e.9.. Hodclt c1a1.. 1989. 1990iBanera d al., 1991i Carpentcr er al, l99l). Theaveragc 375r/365. rxrios reporred by these invest!

EN-l 0.7091 86 + 0 000001NBS 987 0.71025

A compilation of lverage 3?5r/365r ratios ofserwater and of modcrn mdine biogenic andabiogenic carbonrres in Tlblc 19.1 leaves no room

Stohtiun in the Phanerc.oic Oceans 437

Table r9.r. Averas€ 375r^65r Rarios ofModern Marine Sr Adjusted to 0.710250 forNBS 987 and 0.708000 for E&A

Standud Ref€rcnces'

0.709150.7092200.7091980.709170.709183

o.'709\70.709180.709100.7091830.709184

0.7091820.109172

NBS 987NBS 987E&ANBS 987NBS 9870.70918t0.(J0003

NBS 987NBS 987E&ANBS 987NBS 9870.70918+0.00001

NBS 987NBS 9870.70918+0.00001

I2

3

(NBS only)

5

1(NBS only)

8(NBs only)

"r. Elderlield and Creales. 1981, ceachith Coenachnn. Acta,45:22A1-22\2 2, Hess et.l., 1986. S.ie".., 231 9?9 983. 3. Keto rnd Ja.obsen, 1987, znr,,Pkuvt. S.i. Ltt.. 8:t:27 41. 4. Armom el al.. 1991,aeochnl coenochr,. Attu.55:2883 2894. 5 DePaolo'md rrgram, 1985, kicn.. 221:93a 941. 6. Burkeet d., 1982, 6dal,g_!. l0(10):516-519. ?. Bafttu et al.,r991, in Banon ct al. (Eds.). P,,.. O.eh DriltusPtugttn, S.ientifc Results, l19:731 738. 8 Cdpentoet zt., 1991. eeochh. Cosnochrr 1.k,55:1991-2010.9. Denier er ar. 1998, Crc,,. ceol.. t521325 34a.

ibr doubt tiat Sr in the oceans a1 the presenr lime

s6..(oceans) : 0.70918 + 0.00001(2-)

relalive !o NBs 987 : 0.71025. Thc 3i5r^65r ratiosof marine Sr in Table 19.1 thal wero adjusted ro

438 19. Tha O.eas

0.70800 for E&A were excluded fiom the average

The best available measurements contaiDed inTablc 19.l indicate tbat the 375r/365r ratios ofbiogenic and ino.ganic marnre calcile/dagonite inthe oceans today lre indistinguishable from the37sl"Sr rario ot reawater. The.etbrc. unreplacedskeletal calcium carbonate of mdinc origin ce beuscd to delernine tbe isotope conposition ofSr inthc occans of the geological pas.

l9.lb Pharerozoic Carbonaies

Tinre-depcndcnl vaiations of the 375./365r ratio ofseawate. in Phlnerozoic ti'ne were lirst detededby Peteamn eral. (1970), who measured theisotope composition of Sr in lnreplaced skeletalcalcium carbonltcs of Phanerozoic age.In addiiion,these authors demonstralcd that the Sr in theoceans rcmained isotopically homogeneous in spiteof rhe facr rhat the 315r/165r ratio changcd wirhtimc. Thcse conclNions were confirmed by Vcirerund Compston (1974) based on analyses of alarge nuDrber of marine linestones of knownstrtigraphic !ge. Lalcr, Faure (1982) conpiledmore rhan 100 375r/365r ratios of marinc cdbonareslublisbed by 33 research g.oups prior to l98land ploltcd them in 25 million-year increments.The re$ 1ing cune conlirmed ihat Ihe tSrF6Sr

ol seawaler declincd iregtrldly from aboui 0.7090during the Cambrian Pcriod to about 0.?071 althe Permian Triasric boundary and subsequentlyincrclsed to the presenr value of 0.7092.

All of these etb.ts were suQ.ssed by Burkeer al. (1982). who reconstructcd ihe vdiation oftbe 3?S/'Sr rario of thc occans in PhaDerozoictime iiom S. isolope analysos of 786 samplesof ma.ine llmestones of known stratigralhic ages.Althoneh 937. oflbe samples ftey analyzeddefineda nlnow band in coordlnares of tbe rT5r/365r ratioand gcohgicll agc,77o of the dara points deviatedftom fte mai. fend Burke et al. (1982) aftriburedlbc disc.epanr resuits lo several possible causes:

L The limestones were of lacustrine o.igin.2. The isotope composirion olSrwas contaminated

3. The asigncd stratigraphlc ages were in enor.

4. The rTSrASr ratios vuied on a shon tmescaknot resolved by the samples-

An additional source of enor is the posibility tharradiogenic 37Sr was leached from clay mineral.and fenic oxide particles wilhin the limeston(samples by the acid used to dissolve fteln. Bu.kret al. (1982) attempted to minimize tbls source oieror by analfring only pure limestone smpleiwhose Sr concenradons exceeded 200 ppm ancthat contained lcss than 10% of insoluble residueNevenheless. o(her investigators have used dilut€acetic acid, which is less aggressive than thehydrochlo.ic and nilric acids Dsed by Burke and hncollea$es (e.9. DePaolo md Ingram, 1985i Baile'e t 'a I . .2000).

Many invesligaiors havc conrributed to thestudy of the evolrlion of Lhe isotopic compositionof Sr in the oceans by analyzing lineslones andother Sr-bearing materiah of marine oriSin fronspecllic systems of Phanerozoic age:

Cambrian and Ordovician: Denison et al.,1998. Chen. GeaL., 152:325 340. Ebnefter al.,2O0l, Geochin. Cosrtochin. Actd. 65:2273 2292. Detry er al., 1994. Earth Plonet.Sci. Iztt.. 128:671 681. Monranez etrl.,1996, Geolosr. 2,1(10)1917 920. Kaufmanet ̂ 1., 1996, Geal. Mag., 1331509-533. Coro-yko! et al-. 1995, Sttutigldpht aNlGeologi.aLCamlation. 3(I):l 28. Shields et al.. 2003,Geothin. Cosdochin. Actd. 67:2005 2025.

Silurian and Devodan: Denisonelal., 1997.Chen. Geol., l40rl09-121. Bertram eta1..1992. Eatth Pldnet. Sci. Lett.. 113.239-249.Carpenter etal., 1991, C.o.him. Cos,tuchin.4.r4, 55:1991 2010. Diener et al., 1996. Geo-chini. Cosnachin. Acta, 60:639 652.

Carboniferous: Cummins and Elderfield,1991, Chetu. Geot..118:255 270- Popp er al.,1986, G.ochin. Cosno.hin. Actd, 50:13211328. Bruckschen et al.,1999, Chen. Geal.,161:127 163.

Perrnian and T.isssic: Maftin and Macdolgall,1995. Chen. Geol., 125:73 99. Spdtl^nd P^k.1996, Chen. Gcol., 131:219-234.

Tritssic ind Jurassic: Hallam. 1994, c?ologr, 22:1079 1082. Koepnick er a1., 1990.Chen. Geol. (kotope Gzrr.i. S?cr), 80:321 4t0.

Ju.assic: Jones et al.. )994a. Geochift. Cosha.h i f t Actd, 58:1285-1301. Jones eta l . ,1994b. Cea.hin. Casnochin A.rd. 58:30613074.

Cretac€ous and T€rtiary: Jorcs et al., 1994,Geochin| Cosnothin. A.rd, 58:3061 3074.Meisel er al., 1995, Geolosr, 23(4):313 316.Jenkyns et al., 1995, Prcc. Oceon DtillingPrcstuh\ Scientijc ReMl . 143: 89 97.DePaolo e1al.. 1983, Earth Plaaet. Sci.bn, fl:356 373. Hess et al., 1986, Scpn.e.23t:979 983.

T€rtia.y: Fmeli et rl.,1995, Geolgy. 23:403'u406. Berera er ̂ 1..199t. Prc. OceanDn l l ing P ro g tu n, S. ie tutif . R es ults, I 19 :7 3 1718. Hodell et al., 1949, Eatth Plaret. Sci.krr, 92:165 178. Hodell er al., 1990, Cheb.G?ol. (tsotope Geosci. Sect.), 80:291-301.Hodell et al., 199t, Geolo8]. 19.21 2'7. He\dcNon et al., 1994, Eonh Planet. S.i. Iztt..128:643 651. Hess et al.. 1989. PaLeoceanag,/aph),4:655 679- Capo and DePaolo, 1990.Scien.e,249:51 55. Hodell and Woodruff.1991, Palea.eakography, 9:405-426. Marrin et al.. 1999. Paleoceonograph!, 14:7 4-83.Relnhddl er a1..2000. Chen_ Geol.. 164:331 3:13.

The lugc nunber of 3? 5165. ralios and con€-sponding stratigraphic ages of unaltered carbonateand phosphale samples of marine origin publishedby these and othcr investigatom have been usedlo reconstruct rhe lirne-dependcnt evolution of theisotopic composiion of Sr in rhc oceans during

Burke et al., 1982, Geologl, 10:516 519.Elderfield. 1986, Pdleo. Pateo. Pdteo,57:'7t-90. Veizer. 1989. ,{nr!. Reu Eanh Pldn.t. Sci.,l7:l4l-.'16?. McAdhur, 1991. Tefta Nota,6:331 358. Vei2er et al.. 1991, Poleo. Pdleo.

Strcntiurt in the Phoketuzoic Oceahs 439

Paleo., )32.65- 11- Ye\zet et al.. 1999, Chen.G"or, 161:59-88. Howarth and McAfhur,1991, J- Geol., 105:441-456. Denison et al.,1998. Chetu Geol., 152:325 340. Prokophed Yeize\ 1999. Chen. Geol., 161:225 240-

Smalley et al. (1994) inrioduced a statisiicalmethod for fitnng a cune to rhe available darausing weighling factor based on a ranking of !am-ples in Table 19.2 dedved ftom consideration ofthe type of sdple, the method of dissolution,supporling data lhat indicare the absence of alleFation, biostradgraphic age asigtrment, atrd analyt-ical eEors of measlrement. The authoB assignedhigh reliability to unaltered biogenic cdbonatesand phosphates but considered whole-rock sanples of limestone and chalk to have low rehability.The fitling program LOWESS was used by Smalleyci al. (1994) ro conslrxct a cuwe exrendin8 frcnRecent to ,150 Ma bded on 1300 data points.Before herging 375#65! rarios measured on dif-ferenr mass speclroneters, interlaboratory biasesmust be rcmoved as neitrly as possible by rcferenceto NBS 987 and EN-I. For example, McArthuot al. (2001) corrected 3366 srsl6srralios to NBS987 : 0.710248 and EN I - 0.7091?5, which theyobtained in lhe Radiogenic Isotope Laboratory atRoyal Holloway Univenity ofl-ondo!. The resulling data points in Figure l9-l define the vari-ation of srsl6sr ralios of seawater belween 0and 509 Ma.

The 375r/365r ralio of seawater jn Figure 19.1reached about 0.?091 in Late Cambrid tine(500 Ma) and subsequently fluctuated repearedlyas it declined to 0.7068 in the Late Jurassic(oxfordian) Epoch (158 Ma). More recendy. the3?Sl6sr ratio of seawarcr has been rising wirhonly m'nor fluctuations towdd tbe present valueof 0.70918. Stalting at abour :10 Ma during theBanonian Age of the late mjddle Eocene Epoch, the3?5r/65r mtio of the oceans has increased sreadilyand without significant nuciuations.

The isotopic evolution of S. in the oceans inFigure l9.l is a .ecord of the geological acrivityof the Eartb on a global scale and therefore isof great importdce for the Ea.rth sciences. Thelime-dependent flictuations of the 3?5r/365r ralioof the oceans were caused by changes in theaDounts and isotope compositions of Sr derived

140 19. The Oc?ans

Table 19.2. Reliabitity of various Tlper ofSamples to PRserre rfie 37Sr/osr tatio,,r'

Hi 8h kliabiliry: belemnitc$ nonluminesceDr shcltsof brachiolods, wcll p.escred tcsrs otforaminifers i. decp sea sedinent, red algae,massve anhydrite in marine evapofite, rudlst

Mediun rcL rbiLit!: lnmjnescent br.chiopod shelh.lhick{helled bivalves. conodonts wirh tow

Low 'elidbilia-r conodonh wirh high alrentionrndex. thin{hclled bivalves, tish eeth,echinoids. mmonoids, disseminared dhydrile,loru tests in decply buried sandstoncs.whote rock samptes oi limesronc and clulk

S,!r.?; Snalley er al.. t994.

irom different sorrccs ftar enrercd the oceanincludinB

| . wearhering oi old g.aDitic blscment focks of tb

2. lolcanic activity in rhc oceans i]nd on rh

3. the diagenesis, dolomirizario., and dissoluriool marine carbonatc focks on lhe contincnts anon Lhc conrDcntit shelves.

ceoiogicauy speaking, the Sf isotope cDrle oseaware. records chaDges in tbe rare of seafloospreading and subducrion, the occurence of orogenies, upl i I o f ronl |nen\ ro o$ed by r i t | |nE rn,r f fBe+!are h$ure erupr ions. dd ptobi t c t imar,change lcading cither b continental gtaciarion an(lowering of sealcvel of ro fom.rion of harin,

atsr

0.7090

0.7085

0.7080

4.7075

0.7070

0.7065

variation of the sTsrAsr Balioof Seawater

l

r | ^ l n , I n I ' u " | " I o s l o l e2OO 3OOGeologica lime, Ma

400 500t00

FT.LRE rei Time-dcpendent vdiarion of the 3rsr/$sr rato of scawaler in phrDerczoic rime based on 3366drJ ru 'nrs i ,om 47 pubt ,c . . ron\ id tusred ru " sdbsr 0 - t0248 ror Na, * . r f " " , , , . _- "

K ^Crct rms. I

rurN. i ( . E I . * i . p pcf ln .n.c r - r rbon, fcrou. . O O." . "1"" .J . \ -nxn. U O.Jo\rc ian. € .Cdbdrn fh( Ronan nInerr t . . dent | tJ l tporher i ( ! . eo i \ude. o lglobal lolcaDic adiviry thar caused lhe t?Sl6sr raLi" .i,""*",", ," c""ri"i ,"iip,.".i,). ilil;; ,",,,the diagams of McAdhur et al. (200t).

evaporite deposits lhat decrease the concentatjonof Sr in seawater and make it more susceptible 1ochdge of ils isotope composition.

In addnion, the 37516$ mtios of mdineclrbonates and phosphates can be used to daiesuch materials. especiauy in case th€y were deposiled during the Teniary Period (i-e., posl mid-dlc Eocene). Other Dses of this record include theconelation of slrata of known stratigraphic ages,differentiation between marine and lacustrine cdbonate rocks, and evidence for diagenedc alterationof marine carbonares of known ages. The isotopecomposition of Sr and other elemenh nay alsoecord the 'mpacts of ldge ext ateresrial objectse.9., asteroids and comets) md the resulting peclurbations of the global climale and biospbere:

DePaolo er al., 1983, Earth Planet. Sci. Lett.,64:356 373. Macdougau, 1988, J.!:er.e, 239:485-,186. Martin and Macdougall, 1995,Chen. Geal.,125:73 99. Meisel elal.. 1995.Geolo$. 23(4):313 316. McArthur er al..1994, Eotlh Ploh.L Sci. Lett.. 160:179 1992-

19.1c Mixins Models

The isolope composition of Sr in scawater durinSPhanerozoic time is fte result of mixing of threeisolopic varielies of Sr lhat enter thc ocems primd-ily by djschdge of water by rivcB and hot springsalong midocem ridges and by the interaction of sea!a1er with volcdic rocks erupred wirhin rhe ocearbasins. Tbe mixirg Dodel to be presented here wasonginally proposed by Faure er al. (1965) and waslaler elaboraled by Faure (1977).

According to this model, fte principal sourcesol Sr enlering the oceans ee

l. .ocks of si.lic composition in the cortinentalcnst thar deenriched in radiogenic 3TSfbecause

of their .ge and Rb enrichment:2. volcmic rocks derived from lhe manrle along

midocean ridges. oceanic islands. island arcs,and lava plaleaus on the continents: aDd

3. mdine limestones of Paleozoic. Mesozoic. and

Strcntium also originates from mixed sources,such as ftecambrian gneisses overlain by crosional emnants of marioe limestone of Paleo-zoic age (e.8.. pans of the Superior tecronicprovince of Cdada), volcano sedimentary con-plexes (e.9., the Aplalachjan Mounrains of NorthAmerica), interbedded limestone-shale sequences(e.9.. midwesrern USA and southwesrem Onrdio).and aerosoi particles from the atmosphere. Theexislence of such hybrid sowes meds rhat mix-ing of difierent isolopic vdieties of Sr starts at rhesoure, conrinues during transpori by sirems, andis completed in the oceans. Most of rhe major rive6of lhe world transport Sr jr solulion whose 375r/365r

.atios are the result of mixing of Sr derived frontbe primaJy sources identilied above.

The YSrl6S. ratio of present-day seawarer(0.70918) can be represented by the equaiion

where s = taction of Sr derived from sialicbasemcnl rocks of Precambrim age

r ka.rion of Sr deri\cd from mJnc volla-nic rocks ofMerozoic and Cenozoicages that originated from rhe mude

n = iiactiotr of Sr thai is recycled bydissolution of meine carbonate rocksof Phanerozoic aee on the conlinenlsand on the co.tinental shelves

I + ! + f t = t - 0 ( 1 9 . 6 )

Infomation presenbd in Chapters 17 md l8suppon. rhe rs\umflion lnal rhe !rc.cnr

3-Sr/3/Sr

ratios of S! derived from the ftree plimd)

/ 3 7 s r \ / 3 r s r \ r 8 ' s r rI *s, /.. = I *s"/., + \"s,/, ,

r srsr r*

\ * " / - - (Le 5 )

Str.'.tiun ir the Pha'1erczoi. O..ans 441

(fr]. : u z:' to oos

l s6s. r , ,=o7o4+ooo2

( c ; ) " , = o i o 8 + o o o r

/"s.\\*s.i.-

The 375r/65r raros assisned to Sr fron sialic ddvolcanic sources @ simild to those used by Brass(19?6). The slimate of the 375r/365. rario of marinelimestones of Phanerozoic age (0.708 + 0.001) wasobtained by averaging 52 lalues taken from the Srevolulion curye of McArthur et al. (2001) at 10million-year interrls. The result was 3?5r/365r:

0.70793 + 0.0006 (ld).Based on these a$umptions. rhe three-compo

nent mixing nodel expresses rhe stsl6sr iario ofseawarer at rhe present timc by rhe equation

442 19.Ihe o.eans

: 0.720s + 0.704r + 0.7o8n (19.'7)

F4ualions 19.6 and 19.7 were used to constlactthe three-component tnixing diagram of seawaterin Fisure 19.2 in cooldinates or (37sr/36sr)". (r-coordinate) ud ! (r-coordinale).

The procedure is to set n equal to zerc and toexpress r in terms of , by use of equation 19.6: Ifn = 0 . s + r - 1 . 0 , a n d r : l , .

In this case. equation 19.7 reduces ro

l = l : 0 . ? 2 0 ( r ! l + 0 7 0 4 !

r srsr rl * I : o r n ' l\_ i r . / . "

l ; + l : 0 7 2 0U r / . -

l t a : 0 . 2 . s + u + 0 . 2 = l - 0 , a n d r : 0 . 8 ! ,

l - l = 0 7 2 0 ( 0 8 - u )

+ 0.704r + 0.?08 x 0.2

o.718

0.716

0.714

0.712

0.710

0.708

0,706

4.704o o.2 0,4 0.6 0.8 1,0

HauR! re., Model of the 3?5165r ntio in rheoeans considered as a mixture of Sr conrributedby weathering of young volcanic rocks(0.704 f0.002), old sialic rocks (0.720+0.005),and marine cdboDate rccks (0.708 + 0.001.i. Thecoefficients u, s. and n de denned as lhe fracrionsof Sr contributed (o the oceos by volcanic, sialic,dd mrine carbonale r@ks. lesp4tively.

f f , : 0 ,/ 37s. \l ; : l = 0 . 7 t r 6\ ^"sr / \q

The calculrtiors for n = 0.4. 0.6, 0.8 follow Ihesame pattem. In addition, equation 19.7 is solvedfor different values of r :0, 0.2, 0.4, 0.6. 0.8.The lines representire the selecled values of Dand r de contours within the minng trimgle inFigure 19.2.

The preseni value of the 3?51165! Etio of seawater is represented in Figure 19.2 by a horizonr2lline which is the locus of all combinations of r,!, and n rhat yield m 3?5165r rado of 0.70918.Point A represents the extrene case of m = 0 (no

E.9

5

( * o - ) . = o t o ' u

Strontium in the Phanerozait Ocedht 441

Sr is derived lrom mannc limcstone). ln that case,

0.70918 - 0.720(l - r) + 0.704r

r : 0 . 6 7 r : 0 . 3 3 .

, : 0 . 2 3 a n d r = 0 . 1 7

This case is unrealistic becausc of rhc grcat abundance ofmarine carbonatc focks cxposed to wealhering on the conlinenls during Phlncrozoic time.However. duri.B tbe Arched Eon rhe sisr/36sr

nlio of tbe @eans was controlled primdily by Srderived from young volcanjc rocks and old graniticrocks of tbe conlinental crust bccausc marine cdbonlle @ks were mDch less abundantin early Prccdnbdan time tbar during the Phanerozoic Eor. lnaddilion, old crustalrocks as well as young lolcanicrocks had lowcf 3?Sr'/36Sf rarios in early Precam-brian time than they do at prcsent.

A more likely s.enario 10 explain thc 37Sl6Sr

fttio of the presenl oceans is suggested by thecoordin$cs of point a in Fi8ure 19.2 (- :0.6)

4.2 0.4 06 0.8Sr iiom marine cabonale

P . !

0.6

4.2

- o1 , 0

In this c!sc. wcalhering of the conline.tal crust andofyoung volcanic rocks contribulc 23 and 17% of$e Sr in the oceans, respeclively, whcreas marinclimestonc coDt.ibtrres 607..

Poinl C in FiSure 19.2 implies d even largetconribution of Sr ro lhe oceans (d :0.8) rhrnpoini A. The conesponding vllues of r md r de

Frc'rRE ier Compatible sets of values of theproportions of Sr in $e present oceans conlributcdby 'neine cdbonares of Phane.ozoic age (n).young volcanic rocks (r), a.d sialic basebentrocks of Precanbrian ase (s) derived rromequations 19.6 and 19.7. Nole that r beconesnegative when a : 0.9, which requires Lhalmdlne cdbonate rocks cont.ibute less than 907.of thc Sf entering ihe oceans at the present !ime.

trme at about 158 Ma. The horlbntal li.e labeled0.70685 in FigDre I9.2 is the locus of all pointswhose r, !, .nd n values yield 375r/365r=

0.70685. The most likely mixing proportions arerepresented by poi.t D. which conespords toa :0.6. r = 0.38. and s = 0-02. The model (aspresendy conngured) also makcs clear that the375r/365r rario of seawaler could nol have decreascdto 0.70685 unles fl was less ftan 0.70 becausehigher values ofn cause r to become negative.

These insighti derived tiom Figurc 19.2 suggest that the obseNed flucruations of lhe 375r/365r

fatio of seawater ir Fi8ure l9.l were caused p.i-marily by variations of the proporrions of Sr denvedfiom volcanic and sialic rocks, whereas the con,tribution from marine carbonale rocks rcmainedcompdarlvely constant and ac(ed to stabilize thcesrl6sr ratio in the oceans. Therefore, the par'tem of varidtion of thc 375r/365r ralio of seawaterthroughout Phanerozoic time can be explained byepisodic ircreases jn the inputs of Sr denved fromvolcanic rocks which caused the srsr/36sr ratio ofscawatcr to de.lh.. Each time one of these volcanicoutbursts ended, rhe 315r/365r ratio of suwate.

u : 0 . 0 8 a n d r : 0 . 1 2

The proportions indicated by poinh A and Csuggest th.t moe rhan half of the Sr enreringlbc oceans originates from marine carbonate rocksof Phanerozolc age. The values of I and r inFigure 19.3 decrcasc linearly wilh lncreasing valuesofa unl i l !=0 at n:0.90, which meansrbar less fian 90'ld of 1he Sr in thc prcsenloceans originated from marine carbonale rocks ofPhancrozojc ase. Bruss (1976) concluded rhar Srlrcn marine linestone amounrs to lbout 75% oftle tolal Sr cntering thc oceans.

The Sr isotope mixirg modcl in FiSure 19.2can aho a.counr fo. lhe low 3?5r/365r ratio ofseawale. (0.70685) rh occDred in Lale Jurassic

Mlthq of Sr ln the OceansBz-Sr/a6Sr _ O.ZO91 S

444 19. The Oceans

itr.,"aied again, bLrt never quile reached the valuei! had in Late/Middle Canbrian time.

When viewed ln this perspective. the timedependent vdations of the 37sr/36sr Ftio ofseawaler in Figure 19.1 can be regarded as arecord of the intcnsity of volcanic activily ona global scale. Accordiig 10 thjs jnterpretation,

majof episodcs olvolcanic activity occurred duringrhe Late Ordovician (l). Middle Devonian (II),Middle Cdrboniferous (III). Late Pemian (ry), mdLae Jurassic (V) Epochs. The last decline of the3?sr/36Sr mtio ofseawater, wbich starled during theTriassic Perlod dd bortomed out in Late .lurassictime, coincidcs with the volcanic aclivity thataccompanied lbe opening of the Atlanlic Ocean.After several minor episodes of volcanic activityduring lhc Crctaccous Period and in early Tertiary(Palcogene) time. the 37Sr/sS. ratio of seawater hasbee. rising continDously (owdd its lresent valueof 0.70918.

Ahernatively. the increase of the 375r/365r ratioofseawarer at differcnt times in the geological pas!can be attributed to episodes of orogeny causedby collisions ofcontinents (Jacobsen and Kautinan,r999) :

L Himalayan Tibetan collision during Neogene toRcccnt ('0 Ga).

2. Caledonidn-Appalachian collhion during thePaleozoic Era (-0..1 Ca). and

3. Pan-African collision du.ing lhe Late Proterozoic ELa (^-0.6 Ga).

ln the nnal analysis, the vdialions of theNrSl6Srratio ofseawater in Phanerozoic time werecaused by time dependert changes in lhe fluxesof 37Sr and 365r entering ald leaviry lhe oceans.Numerical models based on Sr fiuxes have been

Brass, 1976. Geochih. Cosno.hin. A.ta,40:721-..730. Brcvart and Afterc.1977. Bull.Sac. Geol. Fronce. 19(6):1253-1257. Gold-stein and Jacobsen, 1987, Chen. Ceot. (Isotope Geosti. Sect.). 66t245-272. Jeobsenand Kaufrnan, 1999, Cle,z Geol.,16l:37 51.Richter and DePaolo. 1987, Edtth Plahet. Sci.Iztt.,83t27-38. Richter and DePaolo, 1988,

Earth Plarct. Sci. INtt.. 9Ot382-394. Padlaha er ̂ 1.,1999, Chetu. Geol., |61:u1 252.cimino er al., 1999, Chen. Geol., 1611253-170. Bemer and Rye, 1992, An. J. S.i.,292t136-148. Richter et a1., 1992, Ear,/,Planet Sci. Lett.,109Jl 23.

19.ld Sr Chronometry (Cenozoic Era)

The systenatic va.iations of the 37sr/36sr ratio ofmarine Sr depicled in Figure l9.l cm be usedto obtain numerical dat?r for mdne cdbonatemd phosphate samples. However, cerlain liDila-

1- The fluctuarions of the marine 375r^65. ratiodoring the Paleozoic ard Mesozoic Ems pemitunique age determinations only when the strati-grapbic age can be used to conslrain the timeinterval fidrin which the rock was deposited.

2. The 375r/365r rario of the sanples must beunaffe.ted by diagenetic alteralion. especially inthe case of sbelh of forminifem, which mycontain crystals of calciDn cdbonare depositedby porcwater pefcolating thrcugh the sediment,

3. The selection of sanples for dating is con-slrained by the reliability criteria lisled inTable l9 . l .

4. The accuracy of the measured 3751165r ratiosmust be confimed by analyses of inlcrlabora-tory isotope standards or of modem seawatercollect€d in the open ocean,

Nevertheless, the mdine-Sr chronometer is the onlyway to diretly dale sedimentary rocks of narineongin. rvhichrealizes a proposal made by wickman(1948). ln addition, the isolope composition or srin carbonate rocks of t owz ag" can be used tocharactdize the environment of deposnion (i.e..mdine vs. lacustrine or estudine).

McArthur er al. (2001) consructed a looklptable tha! facilitaEs tbe convemion of the 37sr/36sr

ratios of samples 1o their coresponding stratigraphic ages ranging from 0 to 509 Ma (The tableis available at j.mcarthu@ucl.ac.qk)

The most favorable conditions for datingmdine cdbonate and phosphate samples exislfor samples of posl-Eo.ene age in the cenozoic

Era (i.e., 40-0 Ma). In this time intedal, lhesrsr/36sr mtio of scawarer increased steadily fromabout 0.70775 to 0.70918 with only one significmichaige ln slope dunng thc etrly Langhian Ageof the Miocene Epocb ar about 16 Ma {Mcturhurer a l . , 2001).

The key rc dalng marine carbonale samplesof Cenozoic agc is to conshrct a stmddd profilebascd on the 3?St36Sr ratios of ureplaced sbelhofplanktonic fo.aniniilra and other marine organ-isns. The stratigraphic agcs ofthe selecEd samplesm lranslared into numerical dares by reference logeological tincscales such a! that of H&tand er at.(1989) and BerggM et al. (1995) dd the pateo-nag.etic limescale of Cdde and K€nr (1995).

To facilirate irterlretrrions based on smalldifferences between P|ecisely detendned 375./365rraiios and to eliminale instrumental bias from thedata, DePaolo and Insram (1985) and Elderficld(1986) denned a A37Sr ptrameter:

: i05 (r9.8)

where the subscripts spl= sample and mc =inodecn cdbonate. Hess et rl. (1986) used a d3rsrpalamder which ftey dcnred by $e equatior

({rsr/N6srrd isrsr/Rdsr),"x 105

eTsr/ssr),*( I9 .9)

Both pammeters require investigatoN to measurethe 3?5r/365r raiio ofmodern cdbonale or se^w,rerGw) on the same mas spectrometer used to ealyzefie samplcs of marine carbonate being dated. whicheliminales intcrlaboratory discrepancies caused byinslrumental efieds. The ,537Sr pdmeler was usedby Hess e1al . (1986, 1989) and Hodel le t a l . (1989.1990) to express the 3?5r/365r rado of individualshelh of planktonic forminifera that were 6rstexamined by scanning electron microscopy rodelect diagenetic alteraLion such as the prescnceof secoDdary calcire crystak, cemenralion, andovergrowlhs. Only shelh thar passed this inspeciionwere Ned ro measure the 3?5r/165r rano.

The .esults in Fieue I 9.4 demonstrare that rhesrsr/36sr mtio of seawarer increased from 40 toabout 15 Ma when lbe rate of growth slowed until

Slrontiun n the Phadera.,'t Oceans

Stronllum in Post-Eoc€ne

M E

445

637sr

0

80

-124

-160

200

4030201 0*"=[(,,])., ffi)."] Geologicalage, MaFGIRE 'e.a lsotope composifion of Sr inposfEocene seawater expressed a! ,37Sr, which isdelined by equation 19.10. The d37sr p&ameter iscalculated from the difference between rhe3?SrASr ralios of mdne carbonate smples andseawaler when both tre measured on rhe samemass spelromeler. The samples are shells ofplanltonic foramioilera taken from severaldificfenr Deep Sea Ddllins Program (DSDP)cores. Dara flom Hess er at. (1986, 1989)and Hodell er ar. (1989, 1990).

aboul 5 Ma. Simild cufles have been constructedby other investigatoB:

Palmer dd Elderfield. 1985 , Nature,31L526528. DePaolo and Ingram, 1985, S.iea.e,227:938-911. DePaolo. 1986. ceolr8). t4:I03 106. Miller etal.,198a. PdteoceanoSnpttt, 3:223 233. Miller etal., 1991. Pateo-eaaogruph!, 6:33-52. Ludwig et al., 1988.Geolog!, 16:113 177. Capo ud DePaolo,1990,Science,249:5I 55.Baneraetal., 1991,Proc. Ocean Drilling PnBrom, Sciehti.f.n?sr l r , l l9 :731-738. Montanan era l . ,1991,

M6 19. The Oceans

New sleue I S trat iB r.,23 :15 7 - 180. Zacbos et al.,1999. Chen GeoL., 161:165 180. Denisoner^1.. 1993, Paleoceanogtuph!. 8:101-126.Oslick er ̂ L, 1994. Pdleo.eakography,9:427#443. Hende6on et al., 1994, Edrth PLdnet. Sci.12u., 1281643 651. Hodell and w@drutr,1994, PaleocearcgtuFhr. 9:405-426. Meadand Hodell, 199 5, P a I e o c e d 4 a I ra p h!. 1 0.32'7346. Farell et al.. 1995. G.o Logy,23:4O3-4O6.MfiiDer al., 1999. PdleaceonoBrdPhf. 14.74-83- McArthur er.1..2001. ./. Ccrl, 109-155 170.

The time inteNal belwen 7.0 Ma (lateMi@ene) and the preseDt was investigatedby Fdell et al. (1995) based on 3?5r/65r rarios of455 samples of lianktonic forminifera talen froma continDous 10Gm core of sediment recovercdat ODP site 758 (latitude 5'23'N. longitude90"21' E) in the Indian Ocean (oDP = OceanDrilling P.ogran). The stratigraphic ages of thesamples were derived flom the magnetostratigraphyof the core, which contained all major nagneticrcversals of lhe time leriod (Seclion 6.4b)- Theages of the polarily rcveBals were mken fromthe 6\ronomcally luned rirescle of Shdcklerorer al. (1995). The time resolution of lhe samplessas 15.000 yeds. the lbrarniniterdl \hell' wekunalrered becau.e ol theiryoung cge. \hallo* dcpll.of burial, md fr€sh appemlce and because thenrsolope rar:o, ol Sr and O m.r.h lho\e ol shel\ oilhe same age recovercd etsewhere.

the 3?Sl6sr ratios were measured on 5-10hed p'cked dnd ulua'oni.all) cleaned pldnl'ronicformin i leru wuh d imereb SreaRr tha.425 umThe 'hell\ wer drssolved in dceric a.id and $ei?SV36Sr ratios were measured on a multicollectorrnrs specLrcmerer olenred in rhe stalrc mode. Themearured rrdo. trere conecred ior bolope fm"rional ron ro 3 'S/33Sr = 0 i lo40 sd $cre adlured ro0. /1025' ior \BS 08. Tne reprodu.rb i | ly ot rh.37Sr/&Sr ratio basd on 136 replicate analyses of\BS 987 sd Ia . 0 6 e\prcr"ed a. 'wo srdd"rddevia l ions In addrdon, FMel l e l d ! lao5) mea\ureo duplide i\orope rdrio. ot 211 samPle\ rhJlyielded a reproducibility of 2l x 10 6, in goodrgreemenr sIh lhe re"ulr. ,or NBs q87. All ol

gsr0.70920

0.70915

0.70910

o.70905

0.70900

0.70895

strenrlum ot latelNooEsne se.water

0 1 2 3 4 5 6 7Geologlcal aqe, lMa

EcuRE ,e5 Fifih-order polynomial fit representi.gthe rime'dependeni varirlion ol the 3?51165. ralioof seawater from 7.0 Ma to thc present. The curveis based on selected planktonic fonminifera lroma I00-m coe in the lndian Ocan (ODP site 758)The 3?Srr6sr mtios are rclalive 10 0.710257 forNBS-987. Replotled from Figure I of Feellet al- (1995).

lheie p'ocedural derrls m n4esary ro appreciate the refinement of tbe isotope evolulion cuNe ofmriDe Sr achieved by Farrell et ar. (1995).

The resulting curye in FiSure 19.5 is a fifth-orderpolynomial titted to the data poinB which accuratelyre8e^s rhe rime dependenr vrr,uon of lhe R SrP"Srralio ol \erq er. alrhough rr may r8\k .hoa_

rem nucruauon.. Tle 3?51165r mtio of sea$alerin Figure 19.5 varies smootl y and nonlinearly withtime with less scatter of dara points fian in previousddra \er, tor rhc dre Neo8ene pubhched b) Hodellelal. (1989, l99l) md other investigatom- Farreller r l . ( lqorJ e: r inr red rhat the t rn.enainry o l rhe 39.of a sample whose measured 3?sr/36sr ratio has aneror of 19 x 10 6 ranges from +0.60to +2.03 Madelending on the slope ofthecune.

19.1e The Cambrian "Explosion"

'lhe lo,.il record conrain. evidence lhrr impoflanrchdrge\occuflcd in the biora duDng rhe I-$I} Csrbr id Lpeh r r .a l -s10 Ma, $hen the Ediadanorganisms of lhe Late Prolerozoic Era" whichlacked skeletons. were replaced by a diversined

Strontiuki ifl th. PruLanhtiar Oceahs

fauna of mollDsks, brachiopods, cchinoderms. andrefibming dchaeocyathid sponges !11 of wbichhale mineral skclctons (Babcock e1 al.. 2001). Thcnpid diversincrtion oi the marinc fauna duringrhe Eely Cambrian coincided with a signilicantincrcasc of rhe tSrI6Sr ralio of seawate. from0.7081 durins the Tommotian Ase (-530 Ma)ro 0.7085 (Eotomian), and ultimarely ro 0.7088(erly Middle Carnbridn), as rcportcd by Deryct al (199:l). The 375r/365r rurio of Cambian seawatercontinucd to risc.nd reached ns highe$ valueof abort 0.7093 during thc traDsilio. from Mid-dle to Late Cambrian al about 5ll Ma (Montanezd al, 1996). These te$tlrs conoborate rneasurements publishcd by olher invesligalorsl

Burke era1. .1982. Ger l . ,s) , L0(10) :5 i6 519.Corokhov et.l.. 1995, Srratist1ph\ and Gealosit Con"lation. 3(l):l 28. Kaufman et al.,1996. Gzol. Mos., 133:509 533. Denisonct!1., 1998, C/'?,,. Gc,l.. 1521325-340. D€nlctal..1991, Ed,1h Planat. Sci. Lett., l28t67l-681 . Ebncth ct aI..2001, G?,rhin. Cos\achim_A. ta. 65( 14):221 3 2292.

The increase of the 3rS/'Sr rarro of scawarerdunng fie Cmbrian Pcriod in Figure 19.6 wascaused b) an increlse in the rate of erosioniouoNing the Pan'Af.ican orogeny. Thc risc ofthe srsr/36sr rario was accompanied by widcllucluations of the isotope conposition of carbonof mlrine carbo.alc rocks (Chapter 27) depositedin Early Crnb.ian timc (Derry et al., 1994). Theflucluations of the isotope co position of clrbonalso coincide with dre dilersincation of fte na.inefauna (Carnb.ian explosion) durin8 the Tommotian.Atdabanian, Borohian, and ToyoDian Ages of theErly Cambnan Epoch.

The connection belween lhe increase of rhcNrSr/N6Sf mtio and rhe lsorope composirion of c{-bon of marine carbonatc rocks Day have resultedfrcm rhe increrse ot lhe biological pfoductivity oftheoccans which was caused by rhe enbanced input0i phosphorus and odref nulrienls lo rhe oceansiouowing the Pun-African ofogeny. The resultingbu.ial of large quantities of biogcnic carbon compounds ls reflecred by lhe obsercd changcs in thenotolc composilion of carbon in manne ca.bonatetuks. A morc spccilic explanalion of fte isotope

- - \

1$r

0.7086

o 7082510 524

Geologica age, Ma

EcuRE 06 variaion of the 3rsl6sr ratio ofseawater durinS lbe Cnmbrian Period rclalive to37 5./365r : 0.71025 for NBS 987. Adapledliom Monlaiiez et al- (1996) and based on theifdata as well as data compiled by them fron Burkeel al. (1982). Derry er al. (1994). od orhen. Thetimescale is by Bowdng et dl. (1993).

composilion of carbon in Cambdan limestones isprcscnred ln tbe context of the Ladionation ofcdbon isotopes by planrs in Chapter 27.

The connection between lhe geochemicalcycles ol carbon. sulfur. and stroniium were mod-eled by Krnp (1989). whcrcas Berner (1991) mod-eled lhe vdation of CO, in the atmosphere dutsing Paleozoic time. Subseqlently, Bcmer and Rye(1992) atrcmpled to calculale the 3?5r/365r .lriosof Phderozoic seawater from the .ates ol weath-ering of silicatc rccks (primdily on the continentodd of burial ofcarbonate rocks in the oceds. Thebesl results arise when rhe 31516$ raio of Srdorivcd by wearhering of silicate rocks is assumcdto vary frcn 0.709 to 0.716 in response to changesin sealevel. Low values (0.709) occur wben sealevelh higb because of increascs in tbe rate of seafloorspreading and rhe resulting input ofmantle derjved

I 9 . 2 S T R O N T I U MI N T H E P R E C A M B R I A NO C E A N S

The isolopic compositlon of Sr in ca(bonate rocksdcposiled in Prccambrlm time is still nol weliknown becausc hccmbrian carbonate ro.ks are

448 19. The Oceans

lcss common than tlose of Phanerozoic age andbecause the chemical, mineralogical, md isotopiccompositions of many Precambrian cdbonate rockshale been altercd (Veizer and Conpston, 1976).In addition, carbonate rocks of Precambrian agerepresent a much longer intenrl of time lhancarbonab rocks of Phanerozoic age but re dimcultto date paleontologically becaus of the scdcityof index fossih and because isotopic methods ofdating de generally not applicable to lhese rocks orare inprecise. Even the distinction belween ma.ineand nonmeine deposilional basins is uncefiaindDring the edliest periods of Earth hislory, wh€nthe salinity of the oceans may have been less than

The alteralion of Precambrian carbonate rocksmay occur initially du.ing diagenesis o. subsequently as a result of f.acturing during structuraldetbrmation. which pemils the deposilion of sec-ondary calcne by bfnes containing Sr whose iso'rope composition ditres from that of the cdbonate Mks. In addilion, dolomitization causes ldgedecreases of Sr dd lesser dereases of Rb concentrations wbich raises the Rb/Sr ratios of thecdbonates. Even though the 87sr/esr ratios of thecarbonale minerals are routinely conected for insitu decay of 3?Rb, the initial 37sl6sr ratios ofdolomites and limestoDes lhat have elevated Rb/Srratios, in mny cases, exceed th€ initjal 3?sl6sr

mtios of samples having iow Rb/Sr ralios.systemalic increases of 3rsl6sr ratios may

also .esL,lt from the release of Sr fiom the silicateand oxide minerals dunng the acid dissolulion ofrhe cdbonaie phases of limestone. The problenca' be minimized by excluding samples containing> 10"/, ofacid-insoluble residue and by using dilutesolutions of weak acids (e.9.,0.5 M acetic acid).lnaddition. some auihoa leleach powdered sampleswith distilled wate. or with dilute solutioos ofmonium acetare betb.e dissolving the carbonatephases in acetic acid (e.s., Gorokhov et al.. 1995.1996, 1998). Hydrochloric acid, even at lowconcentralions. car release 37sr tiom silicate mdoxide minerals and thereby increases the measureds7sl6s. ratios of carbonate phases.

Dolomite ard altered limeslones de ahoedched in Mn and Fe dunng fluid-rock interac-tion and have high Mn/Sr and Fe/S. ralios (Brandmd Veizer, 1980). Therefore, these ratios de useful

crite.ia for idenlifyine cdbonale rocks whose37sl6sr ratios may have been altered after deposition. Therefore, all dolomiie smples and thoselimestones having MrL/Sr >0.6 and Fe/Sr >0.3should be excluded because, in many cases, beir37s/36sr ratios exceed those of unaltered carbonaEGks. However. even samples that satisfy these cri-teria may have been alte.ed (Asmerom et al., l99l:Derry et al., 1994).

19.2a Late Proterozoic Carbonates

A siudy by Asmerom et al. (1991) of cdbonaterocks in the Late Proterozoic Shaler Group onVictoria Island in the Canadim Arcric illustates theprecautions necessary in fte selection of smplesfor malysis. The smples for tbis study weretaken from a meNured section that is underlainunconformably by aD older volcdo sedimenbrycomplex and is ovedain disconfomably by basalticlava flows. The sampl€s originated from preciselyneasred positions within a 1221 m interyal ofa section whose tolal thickness was 3360 n.Informalion reviewed by Asmeron et al. (1991)indicated an age of 880 Ma for the base of thesection and 723 Ma for the youngest rocks arlhe rop. This information was lsed to calculatethe ages of the samples from lheir known heighlabove the base of the section using an equationderived by Deny et al. (1989) for a model of

r : r. +.. r" lr -L)\ . . 40 . /

(19. l0)

where r : age of a sample in Mar0 : age of lhe oldesl rocks at the base of

the seclion (880 Ma)Ao : initial sedimentation raie

(70.24 ']'lt\{a)4 = erosion-raie constant (50 Ma)D : slratigraphic height in meters of a

sanple measured from the base of the

Subslituting these values inlo equadon 19.10 yieldsthe ager of sarnlles based on the measured

height (D) above thc base of ihe seclion studledby Asneron e l a l . ( l99 l ) :

/ I t \I = 8 8 0 + _ s 0 t n l t - l ( l a . L t )

\ ru /u. r4 l

For exabple. thc age of.limesrone sample (Wl-76,oosparire) at D-1992 rn above the base is

/ r o o ) \r : 8 8 0 + 5 0 t n l | = - = l : B t R M a

\ ru Y ^ , .24l

Asmcrom etal. (1991) measured the Rb/Sriatlos of lhc carbonare ninelals of 39 saftplesfrom the Shaler croup and selectcd 17 for fur-ther sudy based on the diterion that the decaycorcction ro the 375r/365r ratio wa! less than0.0001. The 14 limesloncs in rhar ser conrained260 ppn Sr (ill.z 520.3 ppm) and 0.100 ppnRb (0.006-0.897 !pm) on average, whereas thedolomites conuined only 42 lpm Sr and 0.096 ppmRb. The dolomites aho had elevated Mn/Sr ratios(6.2) compded to only 0 67 ibr the llmestones.

Tbe limestone sampler oflhc Late ProterozoicShaler Croup in Figure 19.7 indicare rhat rheesrl6sr ralio of sclwaler varied smoothly from0.70738 to 0.70561 relarivc to 0.710241 for NBS987 provided rhat the secrion contains .o gaps indeposition, thu1ftc dates at ihe rop and bottom wereaccurately construined by the avdilable isotopicage determlnarions. and that fte ages of thesamples were correclly interpolared by the blsin

Sttuntiutn ir the Pre.dtubrian Oc.ans 449

Seawater, Shaler Group

jN

CHUF

37sr

0,708

0.706

0.742

AdditioDal srudies of lare Prcrerozolc carbon-ale rocks have been pcdormed by

Dcfry etal.. 1989. c?o.,th. Cornothiu.A.ta,53:2331 2339. Dery etat.,t992. Geo.hin.Cosnochih. Ad1, 56:1317 1329. KaDfmanet^1..1993. Edtlh Planet. Sci. Lefi.,l2O:409-430. KauimaD et al..1996, Geol. MaB.. 133:509 533. Gorokhov etal.,l995, Srrdryraphf an l C.oh!:ic Coftelatian, 3(l):l-28.Gorokhov er al.. 1996, in BottrcU et al (Eds.),Ptut. 4th ltuemat. Sthtp. Geochitn. Eofih !Srdz.c, Ilkley, UK. Land-Hydrospherc Inreractions (Themc 5):71:l 717. Kuznersov ctal.,1997. Daklerlr Russion A.dtl. SLi. (EarthSci. Sect. ), 353(2):249,254. Semikharov el al.,

800 850ceotogicatage, Ma

FcuRE is? Variation of rhe 315r/365r Etio inmdine limesmnes of thc Late Prcteroruic(Neoproterozoic) Shaler Croup on Vicioria lslandin the Northwest Teritori€s of Canada. Dataiion Asmcrom er al. (1991).

1998, Dokladr Rrsia, Aud. Sci. (Ed h Sti.Se.r.), 360:488-492. Jacobsen and Kaufinan,1999. Chen. G?a\.. 16l:31 57 -

19.2b Snowball Earth claciations

During tbe Late Prcterozoic (or Neopfoterozoic)Em. the Earth expericnced two episodes of globalglacialion known as lhe Varangian al about 600 Maand fte Stunian at aboul 700-760 Ma. Each oftlrese glacialions occured in two pulses which werefbllowed by lhe deposition of limesrones that aredeplered in LrC by about 0.5% relarive to normalmdine limeslone (Jacobscn and Kaufman. 1999).

The occurerce of glacial diamicrites atsealevel close to rhe equitor suggests thdl the entireEtuth was covered by ice in a condilion known as''Snowball Earth" (Hoffman et al., 1998). Ar thesctimes, the 375./365r ratios of scawater should havedecreased because the fluxes of S. derived from theconlinents were reduced to zero ard only Sr dis-chfiged bt hotsprings along hidocean ddges coulde.ter ihe o.ems. Althou8h the facrs are not jn doubt,

450 19. The Oteahs

their interpretation in tcrms of global glaciations(SDowball Elllh) has been queslioned by Kennedyet al. (200h. b).

Jacobsen and Kaufman (1999) demonstraledftar the 37sl6sr ratios of limestones deposiledimmediarely after the Vdangian and Stunianglaciations jn Fjgure 19.8 did not decrease rsexpected. However. the d€pletion of the so-calledcap limestones in ''C is specraculd. In t|e dehanotalion thar ii used to express the isotope composnion of C, N. O. and S. the drrC valuesof postglacial limcstones declined from about+8%o ro 5loo coftpared to values ned +2.0%d(Figure 27.7) for nomal marine llmeslones ofPhanerozoic age. Kennedy ei a!. (2001b) presenredcvidence lhat the decrease of lhe 6'rC values ofthe cap cdbonares was caused by the decomposi-rion of ma.ine methue hydrale deposjts followingthc global glaciations. Altematively, Jacobsen andKauiman (1999) suggesred ihat the 6'rC valuesdeclined becaDse the C ori8inated primdily fromthe rnantle, whose 6rrc value is -5.5%o. Jacobsenand Kaufnan (1999) also demonstrated by numer-ical nodeting thal the 37Sr/36Sf ratio of seawaterdid not decrease becausc Sr has a longer oceanicfesidence timc than C (i.e., 106 yeds for Sr and105 years lbr C). The 3tsl6sr ratio of seawaternould have decreased from about 0.707 to lessthan 0.7032 if each of the Snowblll Claciadonshad lasted as long as l0 x 106 yedrs. Therefore,the absence of a significant dccrease of the 375r/365r

ratio in thc cap limestones implies fiat each oftheseglaciations lastod less than abo l t 106 yetus.

The indease of tbe 3tSr/36S. ratio of seasaterfrom 0.7056 ar 830 Ma in Figure 19.8 (Asneromet al., 1991) to 0.7093 a1 513 Ma (Middlef-aECambrian, Monhnez et al., 1996) has ben attribured 1o an increase in the flux ofcrustal Sr causedby upljft and increased erosion durine the PanAftcan conrinental collision.

19.2c Early Proterozoic and ArcheanCarbonates

The rise ofthe 3rsr/36sr mtio ofseawater during theLate Prcterozoic Era is part of a ldger trend thalbcgan in Lae Archean ine (veizer and Compston.lu/o vei rer . lo8o ' The i \o .oprc evoluron o l sr

Late Prcrercalc Seawaier

\Nlvi

v----

6sr

7080o.

0.7474

0.7060

500 800

Geo ogicalage, Ma

Fcus F.s vdiarion of rhe tsrl6sr ratio ofseawatef during the Late Proterozoic(Neoprobrozoic) Era. The cune was drawnfree-hand to include the Iowest values of the3?Sr/a6sr ratios. Vl, V2 : Vtrangian SlaciationrSl, 52 : Sturtian glaciation both of which wereglobal in scope and caused the Snowball Eannphenomenon. Adapted from Jacobsen mdKaufman (1999) and based on data compiled byrhem from Derry er al. (1989, 1992, 1994),Asmerom et al. (1991). and Kaufnan er al. (1993,1996) for cdbonate rccks from Siberia, Namibia,Svalbard. ud Cuada.

in seawater during ihe prc-Neoproterozoic historyof the Earth is dimcult to reconstruct becausethe ages of the avaiiable caJbonate rocks de notwell constrained {Veize. et al., 1983) and becruseof the alteralion of these rocks during diage.esisand during slrxclural deformation and regional

veizer et al. (1982) domonstrared ftat A.cheancalcites and dolomi@s have higher concentrationsof Sr, Ba. Mn. and Fe thm cdbonate f@ks ofPbanerozoic age but de depleted in r3O and Na.lnaddition.lheir 3?Sl6Srmtios are sinilarto thos€ ofmantle derived volcdic lock of thal line. Veizerer al. (1989b) .eported thar hydrorhermal caibon-ales within volcano-sedimenla.y complexes ofArchean age in Norlh America, South Affica,and Australia have low 3?5165r ratios of abour0.7020+0.0008. In a subsequent paper, Veizeret al. (1989a) demonstrared that carbonare rocks ofsedimentary origin associated wjrh lnte Archeangreensione belts in Canada md Zinbabwe haveaverage initial 375r/365r rados of 0.7025 i0.0015al 2.8+0.2Ga. In addition, a suite of Early

Naotlrniun in the O.eahs 451

Archcan tero.n dolomiles, sideritcs, a.d ankeriies'n South Aiiica. AustraliA. and India yielded minitial 375165r ratio of 0.7011 + 0.0008 at 3.5 +0.1 G!. Both values are indistinguishable from rhe375165r .atios of hydrolhemal cdbonares andiiom mlntlc derlved volcanic rocks of Afchean age.However. in somc cases, the Early Archean cdbo.ales aere sigoilicandy enriched in radiogenic 3?Sr

during postdeposirional alrerarion. Fo. exmple,cdbonales of the Onle.Bacht crcup in Swazilandhave measured 375./365r ratios bctween 0.7:10 and0.760 from which the isotope composition ofSr jnEdly Ar.hea! seawater cannot be recovered.

Subsequenr sludies by Veizer er al. (i990.1992a, b). Deb e1al. (1991), Mirora and Veizer(1994), Zachdlah (1998), and Ray er al.. (2002)prcvidcd additionil infomation conceming lrace-element concentralions and the isotopc comlosilions of Sr, C, and O of Precambrian carbonatercks. For example, Vcizcr c1al. (1992a) demon-strurcd rhat fte Bruce Limesronc Member (2.35 +0.10 Ga) of the Espmola Formation in the Hu.o-nian Superyroup exposed along thc north shoreoi Lake Hufon in Ontario is lacustrine in oriein becausc its lorcst 375r/365r mtio ar less than1000 ppm Mn is 0.71128.

Nevenheles, fte availablc data conplled inFigurc 19.9 by Shields and Veizer (2001) md Rayd al. (2002) dcmoDsrrate thai rhe NTSr/r6Sr fatiooi seawater inc.cascd ma.kedly from near-mantlevalues (0.702) dr about 2.5 +0.3 ca lo about 0.705

10Ga. Thls rise ol the 315165r ralios ofthe oceans mosl likely rcco.ds a decrease in rheintensity of volcaric acrlvity and an incease i.the inpuL of Sf by rive6 dralning .ocks of rhcIrowing continental crust wbere mdiogenic 37Sr

had accumulared bt decay of 37Rb.

I 9 . 3 N E O D Y M I U MI N T H E O C E A N S

The concenrGtion of Nd in seawater is exceed-ingly Iow prima.lly becausc the tivaleDt Nd ionis slronglt sorbed lo the sudaccs of colloidil pr,tides and because Nd is incoryomted into biogenicphosphate such as fish eeth. The low concentra-non of Nd in solutio. in seawaler poscs analyticalproblems that h.ve made it diflicDh to measurc ils

1 2 3ceo ogi€lage, Ga

Hcuru Fe hotope conposition of Sr in seawalerduring thc Archean and Proterozoic Eons. The375r/365r ratios of marine cdbonare rocks ofArchean age de similar to thosc of mmtle S.rcpresented by cHUR-sr. The cuNe was adapledfrom Ray et al. (2002) and Shields andveizer (2001).

isotope composirion directly. For this reason, thestudy of the isotope geochemistry of Nd ln theoceans is supplemented wirh analyses of smplesof steletal calcium cubonate and phosphalo. feFromanganese nodulcs, and heavy,metal sedincntdeposited by hotsprirgs along midocean ridges. Theresulh of such studies indicate thar, in contrasr toSf, the isolope conposition of Nd in the presen!day oceds is nor consrant but vdics rcgionallydep€nding on inluts by rlves draining rocks olthe continental crust (lowrasNd/rdNd) dd manrle-derived volcanic rocks (hish rBNdy'4Nd).

19.3a Continental Runoff

The concenlrations of Nd in river water and therole of solption in rhe transporl of rhis e1€mont insreams have been investig.ted by

Martinetal., 1976, .r- G?rpbr. Rer., 8l:31 19,3124. Keaslcr and Lovelmd, 1982, €ardrPknet. S.i. Lett..61:68 72. coldstein eral..1941, Earth Planet. St:i. I?t.,70:221 236.

6'st0.708

0.706

o.704

4.702

0.700

Proterozoic/Archean Seawater

Prore.ozoc I Archesn-Lale I Middle Ear y I Llte Mdd e

452 19. The Oceans

Stordal and Waserburg, t986. Eafth Plonet.sci. Lefi.,1'11259 272. Goldstein and Jacob-sen, l9a'7 , chetL c.aL ( lsotoPe Geosci. SecI ) '66:245 272. Goldstein and Jacobsen. 1988Eanh Plaflet. Sci. 12tt..81t249 265. Elde.field et al., 1990, Ger.lim. Cosma.hin A.t1.54:971 991. Andersson eral.,1992, EdrthPlaaet. Sci- Lett., 113::159-472 Anderssonet ̂ 1..2$1, Geochin. cosmochin Acta,65:521 527. All€8re et^1.,1996. Chen. Geot.,lil:93.-.112. sholkovitz, 19a9, chem. Geol ,77:47 51. Sholkovitz. 1992.Edtth Pl.!n.t Sci.Izu., 114:71-84. Sholkolitz, 1995, A4ldrGeoched., 1:1-31. Sholkovitz eta1.. 1994.Gea.hin. Cosnochin. Acta. 58:1567 1579.

The presentation that follows is based legelvon the work of Coldslein and Jacobsen (1987) on

lhe geochemistry of Nd and Sr in riveNater. The

results of their study demonstrare lhat the tlansportof Nd by riaers is strongly controlled bv the PHof tbe water. which determines the Polaritv of

sudace charges of colloidal panicles. A1 low pH,

most of tho surface charges de posjlive, causingNdr+ and the catiors of olher REES to be insolution in the water. Therefore, Iihered samplesof acidic waters can have high concentrations of

Nd dd other REES- As lhe pH of lhe wal€r nses,the chalges of surface sites on coloidd panicles

become negatile because of desoation of H*

ions. Consequendy. lhe couoidal panicles attacl

and hold an increasing number of Nd3+ ions with

increasing pH, causing the concentation of Nd

in ionic solution in ihe $ater ro decrease Thisphenomenon is illustrated by Flgure 19 10, which

shows that lhe Nd concent€tions of filtered wate!in North Amerjcan dvers decrease steeplv wioiDcreasing pH. Evidently, filtratlon of lvater havinCned neulial PH rernoves most of the Nd f.om the,) . rem Md Lause\ ,he r i l r ra le ro ha\e Io$ Nt.oncenralron'. Most ol rhe Nd tt"nsponed by I ver

ar ned nculrdl pH r. rofted Lo colloid.l panicle\

rhdr are depo.ired $hen rne 'rcrm' oi\chu8-" Lherr$drer inlo e.ruarres along rhc (od\r' Consequenrl)onl ) lhe.mi l l l rac_ion o. Nd thar r \ in t rue

ionic solution in riveB is acually incorPorared

1 0ph

FrcN 1e.ro The pH dependence of theconcentration of Nd in nltered water (0 2 pm) ofriveB in Noirh Anenca. The average Ndconcentration (weighted by the discharge) of thefive largest rive$ is 16.0 x l0 ''� g/g, which isequivalent to picogranrs per gram or pans per

lrillion (!p0. Data fron Goldstein and

The concentrations of Nd in filtered samPles(0.2 Fm) of surface water on the North AmeFcan conlinent anal)zed by Goldstein and Jacobscn(1987).dse from 5.30 x l0-r'� s/s (picosmms peigmrn or pads per tritlion, ppt) in Lake Huron {pH:8.35) to 3150 ppt in the Potomac tuver (PH:4.80). The average Nd concentation of the liveldgest nve6 of North America (MississipPi, Mis_souri. St. Lawrence, col mbia, and Ohio) weightedby their discharges is 16.0 ppt These rive6 draiian aeaof 5.0 x 106 km'�, which is 83% of the lotalarea drained b) Lhe Nonh Americdn iver Includelin the study of Goldsiein and Jncobsen (198?)

Ihe r r ' \d / {Nd ra l ro" ot rhe Nodh Amer icalflve^ ,e\pre+ed relarre ro L'.512618 ior th.p 'e.enr \a lue of CHUR-\d ' rdnge f iom 0 5 l l58b

!S. Lours R ver ' lo 0.51248J 'Colurbra River)The Sl. Loui. Ri\er dtJin\ rocl\: oi Precambianage In de Supe o recronrL Prov'nce of CJ.rd.sherer . rhc Co.umbir Rr \er dmin. manr le_denvedbac" (, ot teni$) agc tn Ore8on dnJ wrsh'n8ron

3.0

_9

North Amerlcan Rivers

. i

The average '$tlol'ratlo and 37sl6sf ranos oflhe five l gest rivers (listcd abovc and wcightedby their discharye) de 0.512202 and 0.71048,rcspeclively. All of tbe five6 of North Americaincludcd in thc srudy of Goldstein and Jacobsen(1987) plot in quadrant lv of Figure 19.ll, aserpected for rivers draining a variety of rocks of

The average seighled concenlrations and lso-topc ratios of Nd enterinS tbe ocerns (prior tosorption of Nd in estlaies) in Table 19.3 dclcDdprimdily on tbe rges and hhologies ot the .dja-cent continents. For example. the riven discharg-in8 warer inlo the Atldtic Oced have rn averaBe weighted rarNd/r{Nd ratio of0.5ll9l (Nd:55.? ppt), whc.eas lhe rivers draining inlo thePacitic Ocean havc rarNd/'aaNd - 0.512489 (Nd :2?.8 ppo. The lowrlrNd/'4rNd isotope ratio of Ndenterine lhe Arlanric Ocein reflects the prevalenceof Precmbrl.n and descendent Phmerozoic sed-incntdy rocks in the continenls lhat border lhisocean. In contrast, thc Nd that ente6 fie Pacific

North American Fivere

t l l v

,

N.odyniun in th. Oc.dns ,153

Tabl€ 19.3. Averag€ Weighted Concent..tionsand Isotope Ratios of Nd in River Wate. Priorro Loss€s due to Sorption in Estuaries

lmr/y Nd, PF '4rNd!4Ndi

20,323

t3.123

4,878

4. l l5

42_439

55.? 0 51199t

27.8 0,5 L24ri9

26.6 0.512191

2t .6 0.5r l3 l9

40.5 0.5 330

0.700 0.710

EcmE le ri Isotope railos of Nd and Sr insolution iD lillered water of rives in North

S,,n?r Coldicin rnd Jarobsen. 1937!Rcl!lvc ro 05126:13 for cHUR-Nd.

Occan has a conpdativcly high 'arNd/raNd raliobecause il originares primarily from mantle-derivedvolcanic rocks that prelail on the islands and con-tinenls alo.g the borde.s of the Pacific basin.

19.3b Mixing of Nd in the Baltic Sea

The mixing ofNd and Sr ln conlinental runoff wilhseawater is well illustrated by astudy by Andc6sonel al. (1992) of lhe Ballic sea in Figure 19.12. Thewaler in this basin has low salinities ranging fron2.4(fr ro 11.11'7%a rnd nsing to 14.2777m jn rheKattegalt, wbich forms the outler of the Baltic Seabelween Dennuk and Sweden. The rive6 enterlngthe Baltic Sea fron the north drin tbe Precambrian.ocks of the Baltic Shield. The souftem rivere aUdrain primarily sedimerttry rocks of Phanerozoicagc. Thcrcfore, thc isotopc ratios of Nd dd Sr inthe walcr ofthe Baltic S€a de the rcsult ofmjxirgof selwater und continental draiDagc in varying

The .oncentmtions of Sm and Nd in unlil'tered water at localities A and C in the BalticSea increase with depd as a resull of desorptionof lhese elements from p{rticles that are sinhnglhrough the water column- For example, the con-centralions of Nd in water srmples collecled atlocality C in Fieure 19.13 increase lrom 5.13 pptat5 m !o 23.65 ppr at 225 m. ln general, the Nd con'cenralions of waler in the Balfc Sea are up to about

143Nd

0.5126

0 5122

0.5118

0 5 1 1 4

A,herica. The rlrNd/'44Nd ratios q€rc conected to0.512638 for CHUR-Nd and the 3tSt36Sr rarios@ relalive to 0.71025 for NBS 987. Tbe aste.iskEpresents lhe average isotope ratios of Nd and Srii the live ldgest rive6 of North America(wcightcd by dischuge): 0.512202 ed 0.71048,receplilely. Dlta Irom Goldslein and

45:l

FrcLrRE rrr? Map of the Baltic Sea. The pointslabeled A, B, C, D, and E de collecting silesrefered to in the text. Adapled from Anderssonet al. (1992).

Central Basln, Balllc Sea

ten times higher thm the concentralions of Nd inunfiltercd seawater in the open ocean (Piepgras md'Wasserburg,

1982).The isotope rados of Nd Dd Sr of the water

in the Baltic Sea both vary egionally dd wilhdepth in ditrerent parts of the basin. The loweslr43Ndr"4Nd rarios (0-5ll5l4 0.511674) occur arpoint A in the brackish water of the Gulfof Bo$'nia. having salinities betweer' 2.460 and 3.1767*,cobpaed to 35-289% in the North Sea al pointE netr the mouth of the Skagemk (Ande6sonet al, 1992). The low larNdraNd mtios of thewater a1 poin! A are alributable to rhe fact thatthe major riven discharging into the GulfofBothnia drain fte Precmbrim rocks of the BalticShield. Aberg and Wickman (i98?) reported thatttre 37sr/6sr ratios of 44 rivers which enter thispart of rhe Baltic Sea fron Slveden and Finlandrange from 0-71273 to 0.73664 with an Dnweighteddithmetic med of 0.72855 40.00150 (N = 48,2r). The isotope geochemistry of Sr in differentpans of the Baltic Sea was presented by Litfvendahler al. (1990).

Tne average isotope ratios of Nd and Sr inwaler of the Gulf of Bolbnia, in the central basin,md ln the outflow chdnel ofthe Baltic Sea definean isotopic mixins hyp€rbola in Figure 19.14. The37srr6sr ratio of water in the Kattega( (poinlD, 0.709202) approaches tbat of seawarer at themouth of tbe Skagedak (point E, 0.709168).However, the ra3Nd/'aNd.atios range widely tion0.51151 (point A, Culf of Bothnia) to 0.512125(poini D. Kattegatt) rel{live to 0.512638 for thepresentrarNd,/raNd mtio of CHUR'Nd- Thesresults demonstiate that the ra3Nd/raaNd ratio!of water in the Baltic Sea vary in response kmixilg of water masses having different isotopi.

The 3?5165r ntios of water in tbe Baltic Seaare well corelared in Fieue 19-15 witb the Biprocals of the Sr concentration- This relationshitcotrlirms that Sr is a conservative eiement in lhrBaltic Sea and elsewhere in the global @eans. Ifcontrast to Sr. tbe isotope ratios and the eciprocalconcentratlons ofNd scatter widely iD Figure l9.l(and demonstrate the strongly ronconservative gcochemical behavior of this elemetrt in the Baldc SeaAlnroush dre geochemical properlies of Nd and S'in oceans are clearly differenl, even Sr is not .

E

200

0 1 0 2 0 3 0Sm and Nd, ppl

FrcuRE re tr Concentrations of Sm and Nd inunfilrered wate. of rhe central b6in of the BalticSea colected ar poinr B (57"20'0"N and20'03'Y'E) fiom depths between 5 ard 225 m.The units of concentration aJe l0-r2 C/8equilalenr to picograms per gram a.d lalts perlrillion (ppt). Datr from ADdessor et al. (1992).

lllxlng, Baltic Sea

\

"-'"-

'!*"':-,,"""""r43Nd

0.5124

o,5122

0.5120

0,5118

0.5116

0.5114

Neodtniuft in the Ocears 455

ttst

0.7096

0.7094

o,7092

0.70900 40 80 120 160

(1/Sd x 10'�, ppm 1

Fcuc D.$ Evidence for the conservalivecharacter ofSr in the Baltic Sea. The collectingsites are idendfied in Fig@ 19.12 dd the375./365r ratios have been adjusbd ro 0.71025 forNBS 987. Data from Andersson et al. (1992).

c.,/)'a

ED

0.709037sra6sr

ocuRE 's ,a Two-component isotopic mixing ofNd dd Sr in seawaler and river waten in the culfof Bothnia and the centlil basin of rbe Ballic Sea.IlE isotope .atios of botl elements varydependinS on the proportions of mixing. Thetr3N.VtraNd rarios de relarive to 0.512638 forCH[IR-Nd, whereas the 375r/365r raiios wercadjusted ro 0.71025 for NBS 987. The averageisorope ratios of Nd and Sr in $e water of theCulf of Bolhnia md of the central basin wereweighted by ihe concentraiions ofNd and Sr,r€spe.tivety. The collecliDg sites de labeled A, B.C, md D as in Figure 19.13. Datafrom Andersson et al. (1992).

perfectl) conservative element, as indicated by thesnall deviations of the data points in Figure 19.15ftom tbe nixing line.

19.3c Present-DaySeawater

The concentrations of Sm dd Nd originallyreporled by Piepsras er a]. (19?9) and Piepgras andWasserburg (1980. 1982) for seawater from lheAdantic .lnd Pacific O@ds and from the DrakePassage between Antarclica md South America

AM

0.5120

0 . 5 1 1 8

0 . 5 1 1 6

0.51140 40 80 120 160 200

(l/Nd) x 1oP ppt l

FrcuE D b Evidence for the st.onglynonconseruadve properties of Nd in the BalticSea. The collecdng sit€s @ identified inFigu.e 19.12 and fterarNd./'aaNd rarios have beenadjusred ro 0.512638 for CHUR-Nd at the prcsentlime. Data from Andesson et al. (1992).

vary widely but have the following average values:

Sm = 0.55 +0.04 ppt Nd:2.6 +0.2 pp!

The concentralions of both elements increase withdeplh. For exmlle, the concentration of Nd ar

" j/o*, *^*

456 19. The Ouans

Station 315 in thc Drake Passage increases from1.85lpt a1 a depth of 50rn ro 4.21 ppi at3600 m (Piepgrd and Wasserburg. 1982). Simildincreases of the Nd conccntration wi$ deplh havebeen reported lbr water in rhc Nodh Adanlic andlhe South Pacilic Oceans. The concenlration ofNd in scawater is about six 10 seven o.de6 ofmagnitude lower Lb.n its concent.ation in silicatcrocks. which implies that this elcmcnt has alow oceanic esidence time of aboul 300 teds.omparcd to about l0o years ior S.. Consequendy.rhc isoLopic conposition of Nd in the oceans isno1 consrant bur vdies depending on thc agesand Sm/Nd ratios of the sourccs on the conlinentsand in the ocean basins from qhich it is denled.Therefbre. Nd is a ,?rt?r isotopic tracer in theoceans than Sr, whose isotopic composition ishomogcnizod by the c;cnlation of seawaler.

The LarNd!*Nd ratios of scawater mealuredby Piep8ras and wasserburs (1980) rrnge from0.511936 10 0.512077 in lhe Atlantic Ocean andiiom 0.5124.12 to 0.51253 in the Pacinc oced relativc to 0.512638 ior CHUR'Nd. The r4Nd/'{Nd

ratios of Atlantic seawater in Figurc l9-17 arclowcr than those of the Pacilic OceaD bccauserhe Nd in rhe Atllntic is derived prinrilyfrom sialic rocks of the conlinental crusr ofthc adjacent conrinenls, whereas fte Nd in thePacific Ocean o.iginates predominmlly iiom youngmantle'derived volcanic rocks oD occanic islandsrnd on island dcs th.t suround the Pacific basin(Tablc 19.3). Alblradc and Goldsein (1992) pDblished a map showing the parcd of vdiation of€(Nd) values of feromanSrnese deposils in thc

The rsNd/ulNd ntios of ftnonanganeseDodulcs, rcd clay. and netalLiferous sediment inFigure 19.17 arc indistinguishable from tbose ofthc Nd in solurion in seawaler at each sire, whichconfirms the hypoftesls lhat ihese materials inco!porared Nd from the ambient scawatcr and therebypreserved irs isolopic composition. Consequently,ibrcmanganese nodules and metalliferous sedimcntlhat hlve accumulaled on the @ean floor co.lain arecord of changes in the isompe conposition otNd in seawater it each siie ot' dclosition. Suchchanges nay occur for a variety ofrcsons. rangingfrom L@al volcanic activity 10 global realiSnments

. imE

€ { o

r€Nd/r(Nd

FrcuRE rw compnnson of larNdreNd ratios ofseawater, Mn nodules, and metalliferous scdimentdeposired in the oceans. The isotope composilionofNd in seawater vdes both within and amongthc ocean basins. However, the rarNd,/i4Nd ratiosof Mn nodules and metalliferous sediment appedto be compalible with those of seawatcr in theocean in shich they werc precipilalcd. AllrarNdraNd ratios have beer coroctcd for isotopefractionation ofra6Nd/r{aNd = 0.7219. Darafrom Piepgras ct al. (1979), Piepgras indwasscrblrg (1980. 1982). o'Nions et al. (1978),and Mcculloch and wasse.burs (1978).

The diftbrence in the isotope composition ofNd in ihe waters ofthe Adantic and Pacific oceuswas used by PiepFas and wasserburg (1982) tosludy mixing of the warer in the Drake Passagc.

6 Fd.hy

a b^drt"

iad

612 P^"irt'

Neo.bniun h the Oceaas 45'7

Table 19.4. Ave.age Nd Concenarations ofMarine Ferronangan€se Nodul6 and Othe.Typ€s of D€posits

Nunber Averageofsamplcs Nd.

Oc€an at Sne ppln R*g"

5 114 129 225rl 173 59.4 2805 t?t 90j-262

l8 173 59.,1 280

1 6 3

0.5126

4.5124

o.5122

0.5120

0.5114a o.2 0.4 0.6 0,8

1/Nd, ppfr

EGURE re 13 lsolope composiiion and reciprocalconcentrations of Nd in seawater of tbe DmkePassage where water from the Pacific and AtlanticOceans mix. Dala from Piepgras et al. (1979)and Pieperas and wasserburg (1980, 1982)corected to '4rNdy'{Nd:0.512618 forprcsent day CHUR Nd.

qhich connects these two oceans. The resultsin Figure 19.18 clearly separate lhe Nd of thelhfte regions such that ihe LarN.VraNd raiios ofseawater in the Drake Passage are iniermediatebelween Nd in the Pacific and Admlic Ocans. TheIarNd/raNd ratios of water in the Drake Passageare clustered above ald below 0.51220, whereaslbe Nd concentations range widely form 1.19 to4.21 ppt depending on the depth from which the

19.3d Ferromanganese Nodules and Crusts

Nodules and crusts of fenonanganese oxyhydroxides thar fom on the botrom of the oceans andin some lakes de stongly etriched in Sm andNd relative to the concentations of tlrese elemenlsin solulion in the water. The average Nd concent adons of fcromansanese nodules jn the majoroceans Iisted in Table 19.4 are virtually conslantal about 173 ppm (28.0-59.4 ppn), which yieldsan enrichment facto. oi aboul 6 x t07 relative io

2,05

Metalliferous Sediment3 t?.6 15.7-20.1

Red CIay| 33.0

2 2.1 x 10 ' (2.2 3.2) x l0 "

Mn Nodules (Lacusrine)

r 3,40

Srrrdr Pietges er 31. 1979.

the Nd concentralion of seawaler (2.7 x l0 6 ppm,o. 2.7 ppo in fte Pacific Oces. Manganese nod-ules in the Antarctic Oced and in tbe Scotia Serhave lower Nd concetrtrations of about 65 ppm(63-67 ppm). Hydrothermal fenomansanese crustshave still lower concenhations of Nd of about3.2 ppm (2.05-4.45 ppm). whereas metalliferoussediment and deep-sea clay contain 17.6 and33.0 ppm Nd, respectively. All of the materials inTable 19.4 de edched in Nd comlared to seawaterard to river water in Table 19.3.

The r$Ndr'reNd .atios of the fe.romanganesenodules in Figure 19.19, analyzed by Piepgnset al. (1979). vary regionally, mucb like tberl3Ndr'reNd ratios of seawater (Figurc 19.1?).This simildity of the tasNd'{Nd ratios of nodules and seawater supports the assumltion that

458

Ferromangan4e Nodules

Paciiic

Atlantic

lndran

Pacitic

Antarctic

Scolia

LakeOneida<- crust Manite

143Nd

o.5124

o.5122

0.51203isr

0.710

6 8 0.5120 2 4 6143Ndy'44Nd

ncr:rr u r Range of 'asNdraNd ratios offenomangdese nodules in the oceds of lheworld and in Lake Oneida, New York.Hydrothermal crusls dd mehlliferous sedimentwere excluded. The 'a3NdlraNd ratios wereadjusted to be compatible wilhllrNd/l4Nd: 0.512638 for CHUR-Nd at thepresent time. Dara from Piepgras ei aI. (1979).

the Nd in feromanganese nodules and sinilarmarerjals deposited in the oceds orieinated fiomrhe mbient seawater at the sire of deposition.The sources of Nd in Mn nodules were alsoevaluated by o'Nions et al. (1978). Golds0ein ando'Nions (1981). and other investigator identifiedby Pietgras et al. (1979).

The evidence tbat ferromanganese rodules mdcrusts record the isotope composition of Nd dis-solved in the seawater where they formed has moti-vated efforts to use them to detect dd interpretlocal chmges in the r4rNd/14Nd rano of seawa'te. in the geological past- For example, Palmermd Elderfield (1985, 1986) analyzed the ferromm-gmese coatings of fordiniferal shells ranging inage ftom 60 to 0 Ma in a long sediment core recov-ered on the Rio Grande Rise in the South AdanticOced (DSDP rr€ J57). The ra'\d/ "Nd ntio. olthese feromdgmese coatings in Figure 19.20varysystematically with time in spile of evidence, dts-cussed by Palmer and Elderneld (1985, 1986), thalthe abudances of lhe REES werc allered durine

Age, Ma

ncw rq.a variadon of 'a3Nd/'{Nd and37sl6sr ratios of fomminiferal tests composed ofcalcile coated with ferommgdese oxyhy&oxidein a 47?-m core fom the Rio Grande Rise in lheSoulh Admdc ocem (DSP 35?). The r43Nd/'4Nd

ralios were conected for isotope fractiomdon 10'6Nd/'{Nd = 0 7219 and de relarive to0.512638 for the preseft value ofCHUR-Nd. The37s/36sr ratio of NBs 987 reponed by the autboNwas 0.71025. Dala from Palmer and Elderfield(1985, 1986).

diagenesis. The 375r/365r ratios of the forminiferalshells in Figure 19.20 are consistent with the vari-atiotr of this ratio in seawater during the CenomkLra rn Figu,es lq . l dd lq . r .burrheydororcomtare with lhe ra3N{v'{Nd Etios. which rise steeplybetween 60 dd 50 Ma and then decline ireguldly

The varialion of the l4rNdtaNd ratio of seaqarer ar rh is s i re rn rhe soulh Ar lanr ic rccordschanges in prcportions of Nd derived from conrinenul dnd \olcdi, 'ources. The increa.e of rher!'Nd/'4Nd rduo tiar \railed ar 60 Ma qds cau.edb) Innu\ oi Nd which odgrnaled from mtnrledenved \olcdnic rcck . The gradual de.line ot rherr lNd/r4\d €Uo ol seawaler r rh i ( . i re ber$ee!

0.704

0.70640 60

South Atlantic Ocean

\ , - - " - \

about 50 Ma and the present records a shift towddcontinenhl sources of Nd. Theefore. Pilmer andElde.lleld (1986) suggested that the xrNd/r{Nd

E io. Jr DSDP \i,e l-< re, ' ,d rhe decl,ne of vo,canic aclivity following the formarion of the RioOrande Risc pior to Late Cenorcic time. Thercsults of this study strcngthened the hypothesisthat feronanganere oxlhydroxides havc prcscrvcda recoil ofrhe vdation otrlrNduNd rarios in thcmean Hotre\er . rh. re.ord ef lec, . / , . . , /evenr .in contrast to Sr, whosc isolopc composiiion in the@eads is slobal in scope.

Feromanganese cnsls are sell suited forplleo o.erlogrJphn .rudie' ber cu.e rhey J e dero' ic ' l cr \cD nos r . , .c \ be.qeen 1.4 dd 64 mmper nillionyears. ConscqucDtly, a crust havinS ad' i , \ae- ut .n l ) l<0 mr mu) (onrarn r rccord\Drnni l t 6n mi l ion yeJ, . t , . i , re n l dep^. r ,onr a \ l . m t u ' y .at diftlrent depths in the oceans and therefore canbe urd ro dpre,r i re ,he lou(c ' u t Nd . r f lc rcnrlevels within a given ocean baxin.

For lhese reasons, Lins etal. (1997) uiedfercnanglnese crusts to study the effecl of theclosurc of the Panama Sateway on the lsolopecomposiion of Nd and Pb in thc watd of thePacinc Ocean. The crusts originated from differentdeplhs on the tops oi scamounls in the equrtorial

Dl1 l :depth 1.8 km, l l '38.9 'N, l6 l '40.5 'ECD29 2r deptb 2.3 km, 16",12.4'N, 168'14.2'WVA l3/2: depth 4.8 km, 9"18'N, 146"03'W

Ihc basal portions of CD29 2 and Dll I werepanly replaced by C! phosphate. which mayhale changed the r$Nd/'{Nd ranos of tle crustsdeposited prior ro 26 Ma (CD29-2) and 20 Ma(Dl1 l). The VAll/2 crusr was not phosphalized.

The'4rNd/r4Nd r.tios of two crusts ecovereddepths of 1.8 km (D11 l) and 4.8 km (VA13/2)

in Figu.e 19.21 increusc bctween about 20Ma&d the presenr. The pronle of cnNt CD29-2 issinila! to thal ofDll I and therefore is not shownin Figurc 19.21. The increase of the 'arNdraNd

ndos implies that the abundance of volcanogcnicNd in the waler increased ineguhrly with time.In lwo of the crusrs (Dl1-l and CD29-2) thelrNd/laNd ratios decreased slighlly during the

Neo.l\miu,i in the Oceans 459

Equatori.l P.<ifi < o.e.n

n\

, \

\ , , '1 ' - ' '

\ I er.' \ , : - -" ' \

"\ 4.Bkm

\ r , \ " 1 -\ /

. D r i l

TeE

o

I

FrcuRE Leri Varialion ofrasNd/raaNd ratios oflwoleromanSarese crusts (Dll 1 and VA 13/2)collected !t dilioront dcpths ( 1.8 and 4.8 km,respectivel)) in lhe ceniral Pacific Ocean. Theisorope ratios de relaiive to 0.511858 ior lheLaJolla Nd standdd and have a reproducibility ofl0 x l0 6. The lowe. pal1 of Dll,l, deposiredpdor to 20 Ma. was phosphatized. Data from Lingc1al . (1997).

p.rlt 3-4 million ye!n. Howcver, lhc decreaseir not elidenl in the VAl3/2 crust. which wasrecovered fiom the deepest water (4.8 km) and hasconsistenrly lower rarNd/r{Nd ctios thm the orhcrlwo cnsts recovered from shallower wabr (e.g.,L8 km for Dl I ' l ) .

The differences iD the 'a3Nd/raaNd ranos ofrhe crusts in Figure 19.21 de consistent with thewcll-known decrelsc of this ratio wilh dcpth infte oceans exemplilied by the data jn Table 19.5.conpiled by Ling et al. (1997) from data in thelile.aure. VAl3,2 in Figure i9.2i Inay hale beendeposiled in no.thward-spreading Antarctic bottomwarer, which ls known 1o have a tow iarlt,Lrr{t,ld

,""Nd

0.51250

460 19. The Ocears

Tabte 19.5. variation of the r'r3Nd/t4rNd Ratiowith DeDth ir the North P.ciffc Oc€an

>3.6

14Nd-iENd

o.51244

0.51242

0.51240

0.51238

0.51236

0.5t2633 0.5124120.5I24840.512407 0.512330

.t"rro?: Line €r al., 199?."Relative ro 0.512638 for CHUR Nd ai ihe prcsent line.

rilio (0.512176). Ling et al. (1997) suggested thatthe incfcase of rhe ra:Nd/r{Nd mtios of seawater.which staded at aboul 20 Ma, was caused bya decrease of the amounl of wnter thal flowedthrough the Panama gateway from the Atlantic inrothe cent.al Pacilic OceaD. The compdadvely lowr{rNd'aNd ratio of seawater in the Atlantic Oceanis well illustraled in Figures 19.17 19.20.

Feroman8mese crust YA I 3/2 from tbe centralPaclnc Ocean was also analyzed bl Abouchmiet al. (1997). who recovered two suites of samplesby means of a high precision driU ratber than by tbemethod of scraping used by Ling e1 al. (1997) andother investigators. As a result. Abouchami et al.11997) were able to oblain 75 samples weiShinebetween abou10.l and 1.0 n]g from a depth intenalof olly 1.5 mm represeniing the prsl 400.000 years.which implies an average time esolulion of about5000 years per sample. In additioD. Abouchamier al ( as7) drilled . \e.ond ,cr oI \ample\ thalerrended ro l0 MJ. A8e dctemina,ion\ bd\ed orrhe decJ) or un.Lpponed : oTh refetred ro b) thraurhor , rndiLJre rhrr ,he Browr l ' nre of VAl l / :rD.re, .eJ r 86 2 kx 186.000 )ear i ago\ l ion,r 0 - 0 l m m 1 0 6 ) ' J u o - l l r o S o l 2 k a r r o6 . . 1 + 0 . 1 m m x l 0 6 y ( 8 6 + 2 t o 0 k a ) .

The ! rNd/r4 l \d rcuo. or ,he h eh.re\o lur ionsamples ofVA l3/2 in Figure 19.22. vdied signifii .dr l ) $ Ih r ime dur inp rhe pa{ 400.000 yedr Inaddir ion. 'he Nd ronLenlraron\ ro Frgu,e 1s.22,dalhed $om rbour 2o5 ppm (106 ro 116 La) ror b o u l 2 l 2 p p m r t T k a ' A b o u c h d m r e r a l . , l o 0 7 ,con.ide'ed qherher rhe high re.olurion noropic'ecord 'n f rgure 1o.22 ref lecr . chanSes in th.

P|o\enance of o,mo\pher ic du{ reg. . A. id orSourh America) or changes in climatic condi.r ion. {e.9. . g lJ . ia l or rnrerg a i r l i They u.ed rhe

0 100 2oa 300 400Age, ka

(a)

Nd,ppm280

260

240

220

2000 100 200 300 400

Age, ka

{b)

FcuRE re.,, (d) Va.irlion or B3Nd/laNd rados ofenomansmese cnst vA l3l2 (,1830 m) @overein the central Pacitic Ocean at 9"18'N, 146'03'w(same as VA l3l2 in Fis(e 19.21). The isotoperatios de relative to 0.511874 for the LaJolla Ndstanddd and have a reproducibility of +17 xl0 6. The ages of the samples were dcnved frotrac.umllation rates measured by fte '�roThP'�Th

merhod (section 20. la). The smples reprcsent tlruppermost lf5 mn of the crust ed were depositeduring the past 400,000 yeds. (b) systematicincr€asc of the Nd con€nlrations with increasintage of the feromanganese oxyhydroxide inVAl3/2. Drta from Abouchani el al. (1997).

Ferromanganese CrustVA 1 3/2 Paclllc Ocean

fl,\r1 ,,'\\ / r

high resoludon samples to demonstrate that thevdiaNe of the average '�ftPb/rePb ralio during!wam" intcfglacial ages is significanrly larger thanduing lool glacjal ages. In addition, they cileda study by Chuey el al. (1987) who reponed thatduring interglacials the atmosphere ovcr thc PacificOced conrained more dlsr than it did duringglacialepochs. However. the rarNd/raNd ratios oft])e high fesolutioD samples of VAl3/2 do nor cor-relate wirh thc rccipfocal Nd concenliations (nolshown). Therefore, the isotope comlosilion of Ndin this feromanganese crust is nol explainable as aset oftwo component mixlures of aftoslheric dlstderiled ftom di erent sources.

Similar sludics of the isolope composition ofSi, Nd, and Pb havc bccn crried out on fereman-Sanese sirmples in thc Adlnlic and Indian Oceansl

Abouchami er al.. I 999. Geo. hi,n. Cosna.hih.Ackt .63:1489 1505. Frant and O'Nions,1998. Eanh Planet. S.i a40., 158r12l-130.Bufion cra1..1999. Eaflh Planet. Sci. Lett.,17l :149 156. O Niors et^1. ,1998. Ear thPldnet. Sci- kn. 155:15 28. Christensener al., t991. Sckrce,217:913 9la.

19.3e wllter Rock IDreEction (Ophiolites)

The isotope ratios ol Sr and Nd ot'igneous rccks inthe oceans are altered by intcractions wilh seawater. For exmple, Jacobsen and Wasserbufg (1979)reporlcd ihat rhe 3?sr/36sr ratios of rocks of theBry of lilands opliolite complex ir Newfoundlandnnge widel) from 0.702541o 0.7080,+, whereas ther43N.Vr{Nd nlios olthese rocks vary o.ly slighrly.As a rcsult, the data points representing these sam-plesdeviare systemarically fron lhe rnanrle anay inFigure 19.23. Mcctrlloch er al. (1980, l98l) sub-sequendy rcpolted simild results for rocks fromthe Samail ophiolitc of Oman. I. addition, sam'ples of ahered bisalt analyzed by O Nions et al.(1978) also scatrer widely in Figure 19.23. The disbbution of data points ln Figure 19.23 inplies lhar$e alteration of maiic igneous rocks by seawatercaused significanr increases of 3?5r/365r ralios buroily snall decreases of ihe rlrNd/rdNd r-tios.

The alteration of the isotopc compositlons ofS!and Nd ofrocks erposed to seawdcr is governed

by an equation derived by Mccuiloch et al. (1980)based on the asumplion that the cftcct of isolopiccxchan8e betwen seawater and rocks dcpcnds onfte magnitudc of the sater rock ratio and on rheconcentrations of tbc clements of interest in rhewater and in fte rocks.

Ler € be an isotope ralio ofclemcnt X. and W,n be weights olseawater and rocks fiat paniciparein thc isotope excbange, €', €t are the isotope ratioin thc jniiiai slaSe and nnal stare. and I,, X,, arcthe concentralions of clcment X in the rocks andin the water. Inilially. fte isotopc ralio of X in tbewater (ei) dltrers f.om thar in the rocks (61). Inthc linal srage of fte exchange, the €-lalues of thcr@ks approacb tbose of seawater. Thereforc, thc'solopic balance is expressed by rhe equation

E / x , R + s / x " , w : . t , x , R + E i x " , w ( 1 9 . 1 2 )

Solving fof lhc water fock ratio yields

Neodthtiuht iii the Otean: ,161

+:(H)t*l ( 1 9 . 1 r )

Alte.natively, equarion 19.12 can bc solved ibfthe E-paramcler of the rocks after the excbange

(19. i4)

Dlvldlng the numentor and denomiDatof of theriglt side of equation 19.14 bl R yiclds

t e',x, +.i , ,x,,{w/R)' i : x , + xJwR)( rq . r5 )

This cquation can be used to calculate lhe finalisotope ratio 6 of any eleDent in rocks ftar haveexchanged isotopes with seawatd (e.9.. Sr. Nd. Pb).The numerical value oithe final €-value of the rccksvaries as the water rock ratio increases.

in thc derivation oi equation 19.15. the iso-lope composition of thc selected element can beexpressed by irs E-value relativc to CHUR. as indicaled above, by the measurcd isotope frtios (e.9..375165r). or by the d-paramete. used to cxprcsthe jsolope compositions of oxygen, .arbon, sulfur.

462 19. The oceans

2

0 5 D 0

3

u I

I)

R = I

Basalt-seawaterinte€ction

o =U4Fr

T - -I o s u

IIIt

z 6

z ,

2

0 5 t 2 0

l0 12 14 16 l8 0.7108?Sr/87Sr

Fc,c o., Lqdene of dllerarion ot ophioliric rcck: and b.sJll by seawrrer. The 3'5r/365-r ralios de

affecred much more Lhd lhe ts'\d/jsNd rstio. until the warer rock ratio (a) exceed' l0' Complere

rcsetting of lhe isotole ratios of Sr and Nd h the rocks occurs onlv wh€n ,R > 10! Data f'om Jacobsen

dd Wasserbuis (1979). Mccunoch e! al. (1980). and O'Nions et al. (1978).

ff w/a : I x 10'�= 100, equation 19.15 yields

0.51300 x 8.00+0.51245 x 26 x l0 6 x 100= ---0+16

" lo-u t loo

4 . 1 0 4 + 1 . 3 3 2 x 1 0 4 4 1 0 4 ^ - . ^ ^ ^: - 8 n + r s , t o -

:

\*s./.0.70300 x 120 +0.70918 x 8.0 x 100

= - l20+ s"0. 100

8.-1----ifi---------t-

and other elements whose isotope composinons areaffected by massdependeni isotope fractionation(Section 26.2). The isolope ratios of sr atd Ndcalculated by means of equatio, 19 15 e the coordinates of points on the mixiry hlperbola definedby the injtial ud nnal isotope mtios of rocks thatinteract with increasing amounB of $'ater expressedby the water .ock mtio.

The calculation is illustrated by the follow-

C{Nd/''Nd)' = o.5l3ooNd. = 8.0 PPm

d?s#6sr)i = o.703ooSr. - 12O PPm

C4Nd/rqNd)'� = 0.51245N d y = 2 . 6 x 1 0 6 p p n

e?srr6s.),. = 0.?0918Sr. = 8.0 ppo

84..16 + 567 344 651 70 ^ -^---120 + 800 920

Ledd in the Oceans 463

Ihese rcsulls demonstraE lhal for W/a:ldthe r4jNdleNd ratio of the rocks changes onlyinpercepribly. whereas the 37sr/36sr ratio increasessignifi.anlly. The isorope ratio ofNd in the basalt isnot sensitive to alteration by seawater because theNd concentation of seawaler is very low (2.6 x10-6 ppm).

The etrech of waier r@k inte.actior causeihe isorope rados of Sr and Nd in Figore 19.23to nove along a hyperbola as the water rocklario i.creases. The 3?Sl6Sf ralio of the basaltchanges from 0.70300 to 0.70918, whereas the!3Ndy'aNd ralio remains virtually uDchug€d astbe waler rock ratio rises to l0a. The calculationsindicate that the FrNdy'4Nd ratio of lhe basaltb€gins to chmge only when the water rock ratioeaches 105. Under these condirions. the jsotopc

composition of Sr has been completely resel to tbe3?Sr/36sr ntio of seawater. The !3Nd/'aNd ratioofthe rock linally approaches ftat of seatlater whenl,tE wller rock rdtio is l0!.

The inreraction belween seawater and maficiSneous rocks is facilitated by the convection ofhotseawater through the oceanic cruslas it foms alongthe midocean ridSes (Bickle ud Teagle, 1992). Asa result, the mass of warer to which the rocks are€xposed increascs with time and the waier-rockratio can rcach large vdlues sufilcient lo raise theinitial uSr/36Sr ratios of the uhramahc rocks ofthe Samail ophiolite of Oman to 0.7071 relativeto 0.7i025 for NBs 987 (Lanpherc et al.. 1981).

The isotope mtios of Sr and Nd of ophioliteselsewhere in lhe world lary similarlyl

Troodos Ophiolite, Cyprus: Pelerman et al.,\971. U.S. Geol. Sutr. Prof. Pdpe\ 75OD:157 - l6l . Spooner et al., 1977,Cea.hih. Casnathin. Acta,41:873 890. Mcculloch andCameron. 1983, Gzol.,s), ll:727 731.

Kings River Ophiolit€, California: Shawer Al.. 1981, Contib. MineraL. P.ttol.,96:281 290.

Inlerprerations of the isotope ratios of Pb in ophiolites and of asociated sulfide beding sedimentaryrocks hale been reported by

Chen and Pallister. 1981, .1. Geophls. Res..86:2699 2708. Tilton et al., 1981, I Geo-prls. ner.. 86(84)12763-2715. Doe. 1982,Can. J. Eatth S.i, 19:1720 1723. Spoonefnd Gale, 1942, Nature.296:239 242. Hare-lin et al., 1984, Earth Plarct. Sci. lztt., 6'7.351 366. Hamelin et al., 1988. Chetu Geol.,68:229 238- Mukasa and Ludden, 1987,Geolosr, 151825-828. t Huray et al., 1988,Geolosy, 16:362-365. Benoit el al., 1996,Chen. Geol., 134:199 2l4.Booijet a1..2001,Geochin. Acta, 64.3559 -3569.

1 9 . 4 L E A D I N T H E O C E A N S

The concenlratiotr of Pb in solution in streamsand in the ocenns is strongly aiTected by sorptionof Pb'�+ to the eleclrically chdged surfaces ofcolloidal panicles and by the low solubilities ofthe salts that Pb tbms with natrrallf occurringacids (e.s.. PbCOr, PbSOa, PbS). The soAlion ofPbz+ is controlled by the activity of H+ ions ardhence by fte pH of the water. As the envnonmentalpH rises fron acidic to near-neutal conditions, thefraction of Pb'+ that is sorbed increases. wherasrhe concentration of Pb'�+ remaining in solutiondecredes to low values expressed in picogrms perglam or pans per trillion. The average concentrationof Pb'�+ in the waler of streams and lakes issignincantly higher than thar of seawater becausefte water in stems may be acidic in some caseswhereas seawater is stightly basic itr virlually allcases. ln addition. the elevated concenhtions ofcdbonate. sulfate, and bisulnde ions (in rcducingenvi.onments) of seawater limlt the concentrationsof Pbr+ that can c@xist in equilibrjum with theconesponding Pb salts.

The sorpdon of Pb'�+ ions by colloidal particlesdoes notsig.ilicandy alter the isotopic compositionof Pb remaining in solulion be.ause its isotopesare not fractionaled appreciably by tbis proces.However. rhe isotope ratios of the sorbed Pb differfiom those of the Pb contained within suspendedmineral panicles. as demonstru|ed in Section 18.2a.

The average concenlration of Pbr+ in stremsk abour I x l0 '�elg Inanograms per snm, orparls per billion (ppb)]. The avemse Pb concentration of seawater is nedly tbree ordes of magnitude

464 19. The Oceahs

lowe! at 2 x 10 r'�glg. Thc concentration of Pbin the oceans varies inegulmll and is no! relatedto saliniry, depth, or gmgraphic factore. The meaneedic reside.ce time of Pb in the oceans is onlyabout 50 yeds (Taylor and Mcl-enoan. 1985).

Lead is ioxic to plants, animals. and humanswhen present at elevated concentradons. The effectof environmental Pb on human heallh was discuss€d by Faure (1998), Nriasu (1978a, b, 1983a,b), Paite6on (1965), Tatsumoto and Patterson(1963a). dd othe6 refened to by $em.

19.4a Sorption of Pbz+ by OxyhydroxideParticles

The sorption of Pb'+ on oxyhydJoxide pecipitates of Fe, Al. and Mn was demonstrated by Le€el al. (2002), who neutralized acid mjne-emuentcollected in lhe former mining disticl at Duck-

increasing pH from the waler of Davis Mill Creek(pH 2.2, Pb : 8 ppb) in Figue 19.24 aplroacbed100% at about pH 4. ln this case, the renovalof Pbz+ from solution was conaolled both by thefomalion of oxybydroxjde precipiiates of Fe mdAl as well as by the chmge in poldity of electical surface chdges on tbese prccilitates tompredomindtly positive ro predoninandy negativ€.. rhe pH was rai.ed. In car. where rhe water

So.plion ofPb:+at Riting pH

contains insDfficient Fe jn solutjon to peci?itatoferic oxyhydrcxide at low pH. the removal of Pts+is delayed urtil oxyhydrcxide precipitates ofFe. AI,or Mn fbrm at near-neutral pH.

The renoval of Pb'�+ from solulion is accon-pnied by a complemen@f increase of the concenlration of Pb in the oxyhydroxide precipilatesthat form natxrally in streams contminated by acidmine drainage. For exmple, Munk et al. (2002)reported tbai Al hydroxysulfate pEcipitates thltfom downstream of the confluence of lhe Snaketuver (pH 3.0) \rilh Deer Creek (pH 6.3) in Sum-mit County, Colorado, are ennched in Pb elalive tolhe vater by more tban four ordeis of magniiude.

The data in Figure 19.25 show thar the concen'trations of Pb in lhe Al-hydroxysDlfate precipitatesftat form in fte Snate River increase to nedly350 ppm as the pH of the water rises ro aboul6.3. Fartber downstrem, tbe Pb concentrations oftbe precipitales decrcase lo aboul 100 ppm becausemost of lhe Pb in solution ras removed by sorplion

300

200

100

0

100

3ao2A

4030l 0

FrcoRr rcx sorpron or PDMill Creek, DDcktown, Tennesee, d the pH wasraised in the laboratory from 2.2 to aboul 8.0. Thesorbent was composed of the oxyhydroxides of Fedd A1. which precipitated ftom the water atincrealing pH. Adapted ftom Lee et al. (2002)-

Distance, m

Fc@ F.r Variation of the concentration of Pbin Al-hydroxy sulfate precipitaEs that form in theSnake River of Snnnit County, Colorado, inresponse to an in.rease in the pH caused bymixing of the water of the Snake River with thewater in Dee. Creek- Data ftom Munket al. (2002).

4 5 6 1 3

Snake Rlver, Colorado

'/

. / .

Lead in the Oc.ahs 465

upstearn. ln olher words, the water is pDrined bythe renolal of Pb and olher trace netah by sorp-tion on precipitates that fo.m as the water of tieSnake River is neDtralized by mixing with warer of

Thc results oflee eral. (2002) and Munket al. (2002) illustrale the impo.lance of sorptionfof the concentation oi Pb in streams. Al lowpH, Pb'�+ sd the cations ol orher tracc merals dein solurion in rhe waler. As the pH rises, Pb,+and lhe cations of other tace netals arc removedfrom solution by sorption to pdticles of chemical precip'tatcs, organic matrer, and nineral grainssuspended in thc wrter. Consequendy, the coDcettrations of Pb jn streams on the continenrs varywidely dcpcnding on the pH dd the presenc€ ofcolloidal paniclcs. These pa.licles are ultimatclydeposiied in reservojrs, lakes. md esrDries. Con-sequenily, tbe water that is disclrarSed inio rheoceans has been purified by the prior removalof Pb'�+ and cations of othcr trace metals. Thesorption of anions and cations from aqueous solu-tions by particles ol various kinds is atrected bynoy environmental parameteF, includlng the pHof the wa(er, lhe poldriry a.d maeniiude of elec-tlic.l cha.ges of the ions. the ionic strength andcnperature of the solurion, and the chcmical composilions. grain sizes, and surfacc chdacteristicso[ thc softent p.Jticles. These matters have beeDdhcused by Balistieriand Munay (1982), Dzombal and Morel, (1990), Stulnn (1992), Stunm andMolsan (1996). LangmDir (1997). No.dst.om andAlpers (1999). Schemel elal. {2000). and others

lg.,lb Aerosols and Eolian Dust

Atnosphcric deposition of eolian dDsl and aerosolpartcles is a signiJicant source ot Pb in thcoceans (Jones et al., 2000). These kinds of mareri-als have surp.isjngly high concenr.ations of Pb witha range ofisolopic coDpositions depeDding on theirsources. Aeiosol panjcles originate primdily fromaulomobile exhuusl. smehng of Pb ores, combus-tiod of coal. and indust.ial ennssions. The Ne oftetraethyl lead as an antiknock additive ln easolinecaused sidespread contmination of rhe suface of

thc Earth. including rhe confnents a.d the surface

Chow dd Johnstone. 1965. Science,147.502503. Chow and Ead. 1972, Scien.e.116.51051 | . GhAzi. 1994. Appl. Ceocheh.,9.62'7 -636.Boutron elal.. 1994. Geochift. Cosho.hin.4. ,a,58:3217-3225. Ri tson cr a l - . l994,Geo-chift. Costuothih. A.ta, 58:3297 3305. WuAnd Boyle,1997, Geochin. C.snochnn. A.ta,61:3279-3283.

The chemical composition of eolian dust andaerosol parlicles in Foshm ud other municipali-iies in fte Pearl Riler delta, cuangdong Province.Soulh Chlna, was deiermined by Zhu et al. (200i).They coilccted eolid dusl by placing PVC cylinders rvith a diameter of 20 cm on rhe roofs ofbuildi.es ln Foshan for onc month. Aerosol srm-ples were couected aI the same sites by pumping aiithrough a 10 cm lilter for four hours. The chemical compositions of the eolia. dust and lerosolswere determined by x ray fluorescence using amul-

The concentralions of Pb in eoliar dustcollccted at Foshan vary seasonally from 1.53%in Janudy to 0.397. in July with a nean of0.48%. Aerosol parlicles likcwise have vriable PbcoDcentrations between 2.65 and 0.207., av€raging1.2570 (Zhu et al., 2001). The same auftos alsoreportcd Pb - 0.09% for aerosol panicles in ao.al mea nearFoshan, whercas autonobile exhaustcollected at the tail pipes of iwo trucks and twobuscs contal.ed up to 31.94% of Pb accompaniedby high concentralions of SOr of up !o 35.58,/,.

The e.lian dust &d aercsol panlcles collecledby Zhu el al. (2001) conrain varying mounts ofsilicrte minenls composed of SiOr, Alror, FeO,KrO, MgO, md CaO. In addnion, these particlescontain suliur. expre$ed by the aulhors as SOj.The sDm of the concentralions of Cu. Pb, andZn in lhe eolian dust in Figure 19.26 is positivelyconelalcd wilh the concentmtions of SOr. TheEfore, the eolian dust probably contains panicles ofCu-Pb-Zn sullide or suuhte. The concentradonsof Cu-Pb-Zn in aerosols in Figure 19.26 rangeonly fron 0.41 to 3.587. (wnh one cxception at8.227,) and have low SOr concentrations between0.12 tnd z.o"/a

Sao

25

20

1 5

Foshan, P€rrl-Flver Defta,

o f f i 1 0Cu +Pb+Zn, PPm

FrcuRE rc26 Concennations of Cu, Pb' and Zn in

eolim dust and aerosols collected near Foshan

Pdl River della, Gumgdong Province' South

China, in relation to fte concentrations of Sor'

Data from Zhu eI a1 (2001).

Zhu €t al. (2001) point€d out that the silicate

fraction of the aerosol paJticles have higher con-

centrations of SiOr, Alzo3. M8O. and Cao than

rhe @lian dust and resemble basaltic andesite in

composrrion. li so. then fte Cu Pb. and Zn of fie

aerosols erther rcsde wrthin lolcdic dDsr pamcles

o. are sorbed to silicale rulerals (e g. clay niner-

als) or both. ln addition, Zhu et al. (2001) denon

strated ftat the chemical compositions of silicate

minerals in the eolid dust and in the aerosol at ros

han drfter lron those ol Chine'e loes rliu la85

Morioka md Yashiro, 1991) and of soil in Gums

done Pro\rnce iliu. Iogl' Therefore the eolan

dusiand aero.ols apper ro be of locrl or Fstoadl

oriein in the Pedl River deltaThe botope rario' ol Pb 'n eol''n du{ dd

aerosols in Table 19 6 are resolvable into several

source componenr. defined in iigure 19 27 b) theit,opbltopL ed zo3pbl0aPb 6rio. There rarioc

were chosen b€caus lhev represent the radrogenic

Pb lormed b) U fld Th. re\Pec'ivelv Thetefoe'

Table 19.6. lsotope Rarios of Pb in Eoli'n Dust

Aemsol Particles, and Acid-IxachableFractions of Soil near [ochan, Pearl River

Delta, Guangdong Province' South Chi'a

,6rb/'uPb zoPb/ePb r3PbAPb

38.660

38,?93

18.097

18.620

18.382

18.574+0.006718.5.16

i8,6r1

15.571

t5.579

15.690

15.685 38.91'�7

75.612 38.6?9

15.5t8 38.?41

Pb smples in multicomponent mixtures can be

distinguished on lhe basis of both rhe ages and-

the U/Th rarios of rhe; sources The samples or

eolian dlst and aerosols in the Foshan dea of China

contain a mixnne of Pb Present in the local soil. in

automobile emissions, in the Fa*ou Pb-Zn mine,

and in meteoric precjpitalion ln addition' sone

eolian dust and aerosol samples conlain Pb whose'�osPblMPb rato exceeds those of fte Pb sources

listed above. The provenance of this thorogenic Pb

was nor identified by Zhu et al. (2001).

Tbe Pb concentratiots of aerosols were also

measured by Bollhdfer and Rosnan (2000' 2001,

2002) based on analvses of air fillers (Bollhitf€r

et al., 1999). The results for sites in the nodhem

hemisphere revesl a wide range of Pb concenrtr

hons in unrrs ot nMogrm' ot Pb percubic nererof

air. Large citi$ ryoicall) hdve highaero'ol Pb'oF

centrations compared to small towns ntd rular Nas

The\ererulrs md $oie otother in ve(rj galor\ d€ mon

sEale rhat eolian dusr and ae'osols conlaminare

'orl, 'noq. rnd rhe surfs.e oflhe oces' rS;monerri

e! al.,2000: Ketterer et a1..2001).

19.4c Seawater and Snow

The studies of Pb in lhe oceans by Titsumoto and

Parre6on (l9ol. b) deno.'lraled lhal ushlle'ed

ilEb-

39.0

38.8

38.6

3 8 4

38.2

38.0

37.6

/t' / l/ _ .

1 8 0 18.2 1a.4 14.6 18.4

467

y?;'I

I

I

Ecuru rr.:7 Isotope ratios ofPb;n eolian dust,aercsols, nereo.ic precipjlation, a.dacidleachable iractions of soil nerr Foshan in thePearl Riler deha oi Guangdong Province, SouthChina. TIe atmospheric Pb in this region is anuldcomponcDt nnxture of Pb deriled from thelocal soil, fron the Fankou Pb-Zn mine, udliom automobile exbausl. Some of the samplcscontain Pb ot unknown origin having hi8hrsPblilPb nrios. Tbe average isotope ratios ofte principal sources of Pb dc listcd inTable 19.6. Data lion Zhu et al. (2001).

but acidified surface waLer above a depth of 1000 mii fie Picinc Ocean and in the Meditenanean seais contaminated wirh Pb. The Pb concentrationsof sudace waler in the Pacinc Oceu dd in theMediteFanean Sea in Figure 19-28 convergc belowa deprh of 1000 m to an average valuc of about

o 0.1 0.2 0.3 04Pb, mgr'L

Fc@E 1s:s Concentralion profiles of Pb in lhePaciiic Oc€ln and Lhe Medileranean Sea. Notethat a concentarion of 0.02 tre,/L is cqual to20 ! 10 r'�glml- or 10 19.6 x l0 ''� g/g based ona density of seawater of 1.02 g/ml-. Adapled frcmTanumoto and Patleson (1963a. b).

0.02 [g/L (:20 x 10 '�ElL = 20 x l0 'r g/ftL).Deep water at folr sites in lhe Atlantic oce.naralyzed by Tatsumoto and Patrenon (1963b) .lsoapproached Pb concenlrations of 0.02 FB,{-

Tne same authors reponed that snow in theLassen Voicanic Nalionai Park of Caiifoinia conhined an average Pb concentralion of 1.6 pg/kg.This value is 80 dmes higher than the averagc Pbconcentation of uncontaminated seawater, whichimplies that ihc snow and the surface sater of theoceans were cont.rminated by deposition of atmospheric aerosoh. The conjecrure is conlinncd bythc fact thdt the isorcpe ratios of Pb in srcw atLasen Park in Table 19.7 de simild to those ol'Pb in gasoline sold in souihem Califomia duringfte 1960s and to aerosols collccted in Los Ange-les (Chow ard Johnslone, i965).

The co.centration of Pb in a core of ice mdfim drilled al Camp Century in northwestem Green-land records ihe hismry of anlhropoge.ic cmi$sions of Pb in the northern bemispbere. Murozumiet al. {1969) reponed that the conccntrations ol Pbincreased Siadually at 6Bt from about 0.011 [g/kgin 1750 AD to 0.068 rLg&g in 1945. Subsequendy.

Terraerhyl

468 19. The Ocedns

Table 19.7. Isotope Ratio of Pb in Snow atLsssen Volcanic National Park and GasorineSold in Southem C.lifornia in the 1960s

Mareriat r06pbl04pbro7pb/:@pbro3pbF0lpb10 12 g/q

28

1980 a4 88 92 96

RGURE D.D Decrease of the average annualconcentration of Pb in surface waler of thenorthwest Adantic Oced ned Bermuda in unitsof l0-'e^g or l0 L

s/s (ppo. Data from Wuand Boyle (1997).

As a rerult, lhe concent.ations of Pb in snow iGreenland (Bouton et al., 1994) ud in the surfacwater of the Addtic Ocean (Wu and Boyle, l99thave both declined dlamatically.

The decline of the Pb concentrations of sqrfacwaler in lhe Nonh Arlantic netr Be.muda betwee1980 and 1997 is recorded in Fi8ure 19.29 base.d othe data of Wu and Boyle (1997). Their measDftments indicate thal the average annual Pb concerlration ofsudace waterin the northwelrem ArlanriOcean declined from about 34 x l0 r'� g/g in 198to just over l0 x t0 rr gg ir tWZ. ttre contamnml Pb is now being transponed into deeper wat(in the oceans by the sinking of larticles to whicthe Pb is sorbed. Decreascs of the concentratioiof Pb in surface water of th€ MediteruDean Sea iresponse to pollution abalement were also reponeby Nicolas et al. (1994). Additional contributioito tbe sludy of aDtbropogenjc Pb contaninalion cseawater are found elsewhere:

Schaule and Patterson. 1981, Earth Planet.Sci. Let., 51:97-116. Schaule and Pauer-son, 1983, in wons eral. (Eds.), r/ac?

11.9218.0,{

r8 .69

1 8 . 0 1

15.6515.63

15.52

15.74

31.9038.01

38.30

38.40

S,,/.?r Ta$umoto and Paterso., l963bi Ch.w and

the concentration of Pb increased sharply to morerhJr 0.16 t reAe In lob0. lne rurho^ arr r ibuteLlhe slow increase of the Pb concentmtions berween1750 and 1945 to emissions trom Pb sneltersand from rhe combustion of coal. The dramalicincrcase in Pb concenlrations in the most recenlpJr I l ' )4 . lvo0. r . . cor . .equen, e ̂ t $e use ofrp 'ae h l l i (ad In grsobne. ILre col r .minrr ion o,snow and fim in Antarctica by othJopogenlc Pbhas been documented by

Ng and Patrerson, 1981, Geo.hitu. Cost]1chin. Act1,15:2109 2121. Bourron, 1980,J. Geopbr. Res.. 85:7426 7432. Bouiron,1942, Ahnvh. Enynor.. 16:2451 2159.Bout.on dd Palterson. 1983, Geachift. Cosnochin. A.ta, 1'71355 1368.

The use of tetraethyl lead as an additjve togasoline in the United States staned in i923 andpeaked in Lhe 1970s. whcn up to 250,000 merrictons of leaded gasoline were consumed annually inthe United Slates and in wesrem European countriesalone (Wu and Boyle, 1997). Subsequenlly, the useoi leaded gasoline declined pecipitonsly in NorthAmcrjca because the additioD of relraethyl lead togasoline was banned. By the yed 1990, the use ofleaded gasoline had declinedto about 20,000 melrictons per yed, most of which was sold in Europc.

Metak ih S?o\|atea pp. 487-504. Plenum.New York. Seltle and Parte6on, 1982, J.Gbphrx. Res.. 87:8857-8869. Flegal andPlrteson, 1983, Eafth Planet. Sci. Left..64:19 32. Hmelin elal., 1989, J. Ccoprrs.Res-,94:16243 16250. Helmes et al., 1990.ltlanne Pollution Bllt, 2l:515 518. Veronet al.. 1993.J. Geoph.r'i i.r..98: 18269 18276.

19.4d Feromanganese Crusts

The low concenlrations of Pb in seawater are anobstacle to thc srudy of the isotopic compositionsof this elcmcnt in the oceans. Therefore, mdineienonanganese crusts and metalliferous sediment,which are enriched in Pb relativc to scawater. harebeen used to determine the provenance of thiselenent in different paits of the oed from itsisotopic composilion (c.9., O Nions et .1., l9?8).

The concent.ations of Pb in lenomangarcsecrusts and sed:menhry Pb orcs iD Table 19.8tuge sidely from several hundrcd up to nedly4000 ppm (Ling et al., 1997r Abouchami et al.,1997). Even madne clays as well as calcareousmd radioldian oozes conrain berween 20 and120 ppm of Pb (Wcdcpohl, 1974). The highest Pbconcenralions occr,r in ore minerals of sedinentary Mn deposiis (e.g., 15.200 ppm in pyrolusitemd 59,875 ppm in hollandite) anaiyzed by Doeer al. (1996).

Tbe isotope ratios of Pb of sutJace scrapings(<1.0 Im) of feronanganese deposits in different pafts of the lndian Ocean fom rwo componenlmixing lines (Vlastelic et al., 2001). For example,tbe r6Pb/'�ePb and '�o3Pb/'ePb rarios of fenomo-ganese deposits in the .orthern Indian Ocenn jnFigure 19.30 deline a stmight line thar includesPb in samples of MORB from the Indian Ocean.The other component of Pb in these femman'ganese dcposits is cnriched in ladiogenic '�6Pb

dd '�o3Pb dd lberefore originaEd from conti-nenral sources. vlastelic et al. (2001) conside.edwbethd lhe radiogenic Pb compo.ent enters iheNonh Indian Occan in the lbm ofeolim dDst ftomnortheaslem Africa and from the Arabian perinsulaor by the discharge of .ivers draining the tlimalayas(e.g.. the Indus. Ganges-Brahmaputra). Allhoughthese rlves have formed large deposits of sedime

I?a.! ih the Oceant 469

Table 19.8. Average Concentratio6 of Pb inMarine Fe.romanganese Crusts, Calca.€ousOoze, and Sedimentary Mn Ores

Matendl Pb, ppm' Reierences/'

1716 (5)99'�7 3607928 (53)683-1333l2t 12)

48 (552)

18 (238)

25 (5)

59,875

"The numbr of smples included in ech average isndrcated rn pmnrheses.'LLinEcr 21., 1991,EaihPkt\et. Sci. L ..146: I 12,_2. Abouchami e1al., 199?, Gc..hin. Cos'norhih. A.ta,6r r3957 3974. 3. wedepohl, 1971, Hondbook oJceo-.n€,,isra-, 82K-6, Sprinec.venag. Heidelberg. 4. Doed al., 1996, in Basu and llart (Eds.),Ge.phls, M.aos.,.95 i39l 408.

at theif mouths, lhey contribule very lilde soluble Pb to the Indian Oced. Nevertheless, Vlaslelicet al. (2001) cited isotope ranos of Pb in sedinenlsuspended in lhe water at the mouih of the GmgesBrahnapulra Rive. (G. B.):

,06Ph 203ph

r01Ph " ?oaPh _ -

These values detine a poirt jn Figure 19.30 thatis colinee with the isotope mtios of Pb inienomanganese delosits of the northem tndianOcean. Accordingly. $e isotole ralios ol feromanganese deposits of the northem Indian Ocean cmbe inte.preted as rwo-componen( nixrures of Pbderived from Indim Ocean MORBS and Pb sorbedto sediment suspended in the water at the mouthof the GaneesBrahnaputra Riler. Howeve., Frank

470

38.0 L14.0

,o4Pb

Northern hdian Oc€tnand calef, 1998. Mineral. MaB., 624:1 2.von Blanckenbui8 nd lget, 1999, EaxhPlanet. Sci. Lett.. 169:l 13-128. von Bleck-enburg and Niigler, 2001. Paleoceonogruplry,16:424 434. Albddde el al., 199'1. Geochin.Casnochin. Acta,61.121'7 -129I- Ling et al.,1997. Eanh Pldhet. Sci. Lett., 146:1 12.Abouchami er al.. I 997. Ceo chim. Cosnochin.Ada. 6I:3957-3974. AboDchami et a1., 1999.G.ochitu. Coshlochin. Acta, 63: 1489- 1505.

I 9 , 5 O S M I U MI N C O N T I N E N T A L R U N O F F

Osnioln is a siderophile tace element whose concentiations in rerestrial silicate rocks are gener-ally less thd 3 ppb, whereas extmterrestrial r€kshaae high Os concentrations ranging from 605 ppbin ca.bonaceous chondrites to 15,260 ppb in ircnmeborites {Table 13.2). In addition, certain minerals (e.9.. Cu Ni PGE sulfides dd chronite) con-tain 554 ed 50.7 ppb Os, .especlively. The elementis rcleased into solution in surface water by chenical weathering of rcks and mine.als on the continents md is translorted into the oceans by streams.

The isotopic composition of Os in crustalrocks changes continuously by the decay of lone'lived I37Re \lith a halflife of 41.60 x 10e yeds(1.: 1.666 + 0.006 x 10 "y '). As a result, the'37ov'36os ratios of rocks and ninerals incre3sewjth time at a mte thar depends on then Re/osmtjos. Altematively, the isotopic composilion ofos can also be €xpressed as rhe reosy'sos dtio,which is oblained by multiplying the r37os/r36os

.atio by 0.11986 (equation 13.4).Certain ace$ory minerals ud rocks listed in

Table 19.9 have elevated Re/Os mtios and thereforecan become enricbed in radiogenic !37os compdedto the conlnon mirtrals and rocks. The sources oiradiogenic '37os include the sunde ninerals ofMo.Cu. Ni, and Fe as well as mafic votcanic rocks,black shale, and grmite gneisses of Precanbrianage. The Re-Os method of dating is the subject otChalter l3-

19.5a Rivers

The concentation of Os in rive6 was mea-sured by Shanna and wasserbu.g (1997), who used

99.0

18.5 19.02a6Pbt2a4Pb

EGURE D r Two-componenl isotopic mixing lircdefiDed by the '�sPbluPb and '�o3PblsPb ratiosof feFomanganese deposits in the rorthem IndimOcean. including the Somali and Mascdene b6ins(\rest and the cenlral Indid bastu in the east. Thestaight line was fitted to 13 fenomanganese datasets by lqst+qu@s regression and is interpretedas a mixing line of Pb in Irdiai Oced MoRBsand in sediment suspended jn $e water of tbeGanses-Brahmaputra River (G.B.). Datafrom vlastelic et al. (2ool).

and O'Nions (1998) questioned whether the isotopecomposition of Pb in fermmanganese deposits inthe lndian Ocean actually records the erosiotr ofthe Himalayas.

The work of Vlastelic et al. (2001) in theIndifl oceatr md that ofLing et al. (1997)dd Abouchami et al. (1997) in the Pacinc ocemdemonstates thal the Pb in feromdgdese deposilsis advected by de€p currents fiom distant sources,except in the vicinjly of submarine hot springs. Thedependence of the isotope conposnion of Pb andNd in mdine fenommganese nodules and crusts onthe circDlation of deep water has been documented:

Abouchani md Goldstein. 1995. Geo.htn.cosftochin- Acta. 59.lg09 - 1820. Aboucbrmi

Chalcopynte

Pyrite

Shale

Tabb r9.9. AYerage Re/Os Ratis ofTerrcstrial Minerals and Rocks lbat Ar€Enriched in Re and DeDl€ted in Of

Mineral Rc, ppb Os, ppb Re/Os

63.8601371461 6 8

2.3210_83_68

0.840.34

100232.3

Table 19.10. ConcentIrtioni and 137os/r.Os

Ratios of Water in Larg€ Rivers

kg 8/c 13?Os/'36osc

5.2

2.0

6.0

5.7

7.02

8.57t0.162.85

2.59+0.178.40+0.23

8. i1(10.4)

9.8 (10.4)+0.211.8 (14.4)

7.1 (8.8)

9.9 (tO.1)

osmiuh in Coatircntal Runalf 471

Iiltered rhrcugh 0 4) p m filre^ dd $ere rhen dcidrlicd wirh ulrdpu'e HCI Lo pH Lb2 pnor ro dalysis. Therefbre, the results penain p.imdily to Osin ionic solution (OsOt) rather than ro the totalmount of os present in the unfillered waie..

The concentrations of Os in the riven analyzedby Shdma and Wasse.burs (1997) vary morewidety thm expecled ftom ualyiical erors. lnplnjculd. rhe mea.ured O\ concen|JJrion\ olthe Columbia and Connecticu( tuveB de aboDrthree times lower than those of tbe Mississippiand Vislula Rilers. The analysts suggested thatthe low Os conceDtralions of the Columbia andConnecucur Rrre^ tue $e re.J l t o t .oTr ion o lOsOt to rhe walls of lhe conlaineB becausethese two samples pere nltered and acidified aflerthe samples arrived in the labomtory. whereasthe water sanples of the Mississippi and VistulaRivers were filtefed dd acidified at the rime theywere collected. Therefore, Sharma and Wasserburg(1997) concluded rhat the mosr reliable value ofthe Os concentration of river water is about 8.6 xl0 15 g/8 based on the 6.0-kg sample of warer ollhe Mississippi River.

The ''osy'eos ratios (coaected for jncom-plete srmple-spike equilibradon) range frcn 8.8(Connecticut River) to 14.4 (Columbia River).which presumably renects differences in the Re/Osratios of the rccks ard minerals thar release Osinto solution during wealhering. For example. the'37OVI36Os ratio or water ln the Mississippi River(10.4, Trble 19.10) is simild to thar of loes inthe upler drainage basin of this rjver (10.3,10.9)reponed by E$er and Turekian (1993a).

Pegram er a]. (1994) provided infomationabout ihe concentrations dd the ilotope rarios ofOs itr bulk sediment and of Os that is leachablewith acid hydrogen peroxide frcm this sediment_Their resutts in Figure 19.31 for ihe t.ibuiaJies ofLake Oneida in upstate New York indicare tha!about 4470 of Os in bulk sedihen! is extractablewith this eagenr and that rhe conc€nrmtion ofOs in the bulk sedimenr ranges from 9.5 + 0.5 ro40.2 t l0 ''� g/g (ppo of dry sediment wnh a medof 30 pp!. This value is about 3480 rimes ltuger thanthe concentration of nltered river \rater reportedby shdba and walserburs (199?).

h addnion. Pegram et al. (1994) demonstraledthat the Os leached from sedimenl of rhe tibDtaries

9r2 '�70

6.2t 229.10 t6

1 1 . 1 1 50_385 60.510 2l0.182 20

<0.03 >28<0.02 >17

5.90 r71.7t'�7 132

Rivef

Misissippi

Misi$ippi

S,ur.?r Shama dnd Waserburg, 1997.qNunben in p@ntheses de corrccred lor inconpleteequilibdtion of thc sanpb wnh $e Os spilc.

a procedure that included copfecipitation of Oswith ilnic hydroxide followed by isotope dilu-tion analysis (Sbtuma er al.. 1997). The resDlts inTable 19.10 include both the Os concentration andlbe sto"/'$Os ratios. Tbe warer samples wde

472 19. The oc?ahs

-1*o"

20

1 5

1 0

5

Lake On€lda and Tribularies,

Mn'

a"\

o 2a 4a 60 80 100 120

Os concsntalion, 10 1'� g/g

FrcuM rer Concenlrations and hotope ralios ofOs in Lake Oneida and its tributdies jn upstateNew York. The average '37oyr36os ratio of 10.5of the continenldl crirsr is from Esse. andTurekiu (1993a). The data for Lako oneida arefron Pegram elal. (1994).

of Lake Oneida h.s higher 13?OV136OS ratiosthan lhe Os of the bulk sediment. These Esultstheeforc suggest that radioeenic r37os may bereleased preferenlially by selective dissolution ofcenain minerals baving high Re/Os ratios divorthat lhe sorbed Os in lhe sediment originated from

Muganese nodules in LaIe Orcida includedin Figurc 19.31 have a high average os concentta-tioD (110 pp, an.l a high average '37OV136OS ralio(17.0) comped to the average value of this ratio(10.5) in the continental crust exposed to wealher-in8 @sser and Turckian, 1993a).

osnium leached by Pesram e1al. (1994)from sedimenr in ri!e6 that drain a variety ofcrustal rocks in Ll1e USA also ha! compaiativelyhigh r37os/r36os ratios ranging from 19.29+0.11(Susquehuna, PA) to 10.08+0.01 (James. vA)with a mean of about 16 (weighted by dischaige).This value was contimed by the r3?Ov136os ratiosof the leachable Os in the sediment of the GangesRive. of India (15.37 +0.03 dd 16.93 f0.09)md of the Rio Maipo in Chile (18.00t0.04).These rcsults snppon the conclusion of Pegranet al. (1994) that the leachable Os being deliveredlo fie o.ean. by rhe ri\eF ot lhe world ht\ J h'8her

r3?oy'36os ratio lhan tha( of os that exisls in theweathering crust on tbe continents (10.5). How-ever. the leachable os in th.ee streams dralningultamafic ophiolites in Califomia md Oregon haslow '37os/'36os tutios (1.4-7.1). These and otherriv€6 draining mantle-derived malic dd ullramaficrocks reduce the 137os/r36os ratio of the oceansfron about 10.5 (Esser md Turekian, 1993a) to8.7 (shama er al.. 1997).

19.5b Soils

The suggestio, by Pegrm et al. (1994) thal lheOs leached from river sediment originates prcfer€ntjally ftom ce.tain minerals ftat have highRe/Os rarios was supponed by Peucke. Ehrenbnnkand Bium (1998). These authos used dilute HCIto leach vrious fractions of soil smples fommoraines that were deposited between about 0.4md 138 ka on hecambrian gneisses in the WindRiver Range of Wyoming. The results revealed sev-eral interesting aspects of ihe way soils rclease Osduring chemical weathering:

1� Most of the Os in gianitic gneisses resjdesin maSnetite, which has a compantively low'37Oslr36os ratio because ofits low Re/os ratio.

2. Biolite (or mineral incluslons) has a higbRe/os Grio dnd lhereloe conrans mosr ofrne 'diogeni r3'o'. dependinS on Ihe age ol

3. When size fraciions of soil are leached witb coldd.ture Hct. llle r3-o,/r360\ rlio. ot $e resutUn!.oluuons d?c.?a 11 wirh increasing conrenuduorof lhe acid ftom 0.05 to 0.5 N.

4. Thel37os/r36os ratios ofos leached from young.oi s (. 5 \x' m ,i8,e/ rhm rho"e ol Lhe Os

5. the Os leached liom old .orl (tt5 ka) ha'similar'novrsos ratios as the bulk soil.

o The bu \ soil o! rne B horizon :s eirlhed 'n Osrelative to the A and C horizons.

The bigh t37ov136os ratios of leachates ofyoung ,or l , , . r0 5 kal in rhe Wind Ri \er Rdgede rhe lesul otoxrdarion olbioli'e. Soils thal havebeen e\po.ed ro wearherinS Io' more rhan "boul5000 )e. . 10 loneer.onran biol,le 6nd thertoe

osniuh ih Contin.ntal Rsnolf 473

release Os whose isotopc composition h similar tothat of the remaining minc.als (i.e., phgioclase andquanz. K feldspd. residual nugnetite). Osniumleleased by soils of any age by leacling withdilure HCI (0.05 N) is more radiogenic than Osrcleascd by leaching with more concenlrared HCI(0.5 N) because lhe morc conccntrared acid auacksmindrls having low Rc/Os fatios (e.g., magnerire.plagioclase. and K'feldspar).

h nr Iinal analysis. the isotopc compositionolos released by chcmical weathering depends onlhe Re/Os rarios of thc mincrals and on their sus-ceplibility io weathe.ing undcr a given ser ofenvi-ronmcntal conditions. As a result, rhe r3tOVi36Os

fltios of Os released by weathcing chmges wilhnne but apprcaches the lalle of ftis rario in themost O!-rich and/or most abundanr minerals.

19.5c LacustrineFerromanganeseDeposirs

Fenonanganese nodules of lacust.ine origin con-lain high concenlrarions of Os. which they sorbliln the atnbienr water in which they form. There-fore, such deposirs record thc isotope compositionofos that was released by chenical wcatbering oftlre rocks and soil in thejr drainage basin.

Pegram er al. (t994) li6t repo.ted r3?ovL36os

6tios of leromaneanese nodulcs tiom tile Oneidain upstate New York. This lake feceives drainagefmm shales, siltstones, and sandslones of Ordovi-cian ro Dcvoniln a8e tbar are covcrcd by Pleist@ene till containinS mineral grains derived fromthe iSneotrs and mctlmoryhic rocks oflhe Precambian Shield of Canada. Thc results in Figure I 9.31indicatc that the LsTOVrr6Os ratios of ihe mdules(15.0-18.1) de consistenr with thc isotope ratiosolOs in thc bulk sediment in lhe tributary rive6 ofLake Oncida (6.49 19.5).

The diifcrence in lhe isotope compositonof Os released by wcathering of Precanbrianand Plnnerozoic rocks was clcafly demonstraredby PelckcrEh.erbrink and Ravizza (1996). whomalyrcd fcromanganese deposits in the Baltic Seamd in its suroundiDg drainage area. Thc rcsultsin Figre 19.32 demoDstrate the wide rangc ofwosy'sos rurios of Os rcleased by searheringof Phaneiozoic and Pfecmbdu r@ks. The feno-nangaDese noddes thal fomcd within the Ballic

0 2 0 4 0 6 0i37ovr360s

.'cuN F r Isolope composition of Os intenomangaDese deposih in the drainagc dea oflhe Baltic Sea which includes rocks ofPhanerozoic and Precambrian ages. Darafron Peucker-Ehrcnbdnk ud Ravizza (1996).

Table l9.ll. Isotope Ratios of Os ofVarious Components Ent€ring th€ BalticS€a

.t !u.r Pcu.ter Ehrenbnnk and Ravizza. 1996.

Sea itsclf have intemediate '37OVr33Os ratiosbecause ftoy contain a mixlure ofOs derived tiomboth conlinental sources and from seawater of theNoirh Sea.

Peucker-Ehrenbrink and Ravizza (1996) developed m isolopic mixing model for Os i. the BakicSea based in pda on lhe ffsumptions llsted inTabte 19.I L The autbon noted ftar thc wosy'eos

l 0l522.58.6

1 7 . 4

411 19 Th. Otqts

ratios ol lenomang.nese deposits i. ders underlain by sednncnla{, rocks of PhaDcrozolc age southJl l l ,c Br l , ic ( ( r runee on y r ro, I r .b ro t t .0 insp'te ofbcnrg covered by till derived from Precmbrian rccks of rhe Baltic Shietd. The) concludedr h " r , T . u m p : . r r \ e y l o $ I ' O . * ' O . r r o " olcnor . ' rs"nc\c depo\ i r . i r lh i . dca inply lhar lhcmrnerals contajniDg radiogenic Os were removedhr $c: . rhp- 'S. i ( r l l ,c I l w1, depuircd oerweer10 and 20 ka.

AJJrr .unr l mec.ur enr . ot r3 O. / R6O, rauorof lacusrine iirondeanese dcposil by PeuckerEhrcrbr ink rnd B.Lm Llaq8, . .nhnn rhe d f ferenc(bctwcen the i3ros/'36os ratios of Os released b)Preclmbri.n rnd Phlncrozoic rocksl

Pbanero^ic: 12 3 (1.35-20.68)fteca'nbrianr 102.5 881

Fe.rcmaDganese nodulcs in both geological se!tings had an average Os concenrrition of 102 xl0 i r g /g (1.7 x r0 i r -262.5 x l0 r? g/g) .

19.5d AnthropogenicContamination

Osnnun is used ir Dedical facilirics as a fixarive inclcctron microscopy lnd as an oxidanl. Accordingto Reesc (1996), rhe Uniled Stares imported 55 kgof Os in 199:1. mosr of which o.iginated from oredeposits associated with the nantle-derived maficrocks ol lhe Bushveld Conrplex (Sourh Africa) andiio'n lbe Ni{u ores at Noril sk (Russir), bothof whicl have rsosleos ratios close ro 1.0. Int?tct. Mlnnr (1991) proposed a value of ]Dlo forthe Lnos/LsO" rario ofrhe average silicale Earlhbased on analyses of mande-dcrived manc andultmmafi c igncous rocks.

The releasc of commercial Os having anr3royL3r'Os rario close ro 1.0 inro rivcN can sis-nificurly aher rhe isotope cohposition of Osthat enters irc ocean. Examplcs of such anthro-pogenrc conramination of Os in coasral sedi-mcnt were repo.tcd by Esser and Turekjan (1993b)and Ravizza .nd Bothnc. (1996).

Subsequendy, Williams er al. (1997) demonnrdted that rhe rsrOV'36Os ratios and lhe Os concenlrations of bulk sedirnent wirhin Long lslandSound in Figurc 1933 lary systcmaticrlly wilh

I.'curc D.r Localions of smples ol sediDentcolle€ted in kng hldd Sound for a study ofisotope compositions md conccntmtions of Os.Adapted from Williams et al. (1997).

dislance f}om New York City and from orhemunicipalilies located ar rhe wesr end of rhe SouncThei.datain Figufes 19.34a and b demonstrate rhrthe '37oyr36os ratios of tbe upper 2.0 cm of sedinetrt increase from about 5.1 close to the sewagoulthll to 8.2 about 60 lm ro the oast, wbereas thOs concentrations decrease in rhe same distancfiom 127 x l0-'z S/g close to New York Ciry r2l x l0 ''� g/g. In addition. the dnta of Willimer al. (1997) in Figure t9_3:1. show rha! the oconcentratioDs of the sedinent de positively co!relaled wirh the concentralions oI organic carborThis reiation means that Os eirher is sorbed to particles of organic matte. or is bonded to organimolecxles in lhe sediment or borh.

The day of data presenled by Williams et a(1997) in Figures 19.34d . demonsfates that Ohaving a low '37ovr36os ratio is being dischargsinto Long lsland Sound al irs westen end. Thconlaminanl Os is sorbed by organic mater d,ninefal panicles in the sedimenl. causing itconcentratlon nea. the sewage outfalls to risabove nornal valucs of about 20x l0 ', g/€In addition, mixing of the contminant Os wilcrustal Os causes a localized isoropic anonal.in the sediment in the wesr end of Long hlan,

The Os thal enters Long Island Sound borh iiwaslewater md in rive6 is at leasr partly reraine,by the sediment deposited in this estuary. Althougithe data ol Williams et al. (1997) do not pemia quantitative evaluadon. rhcy clerly demonstranthar Os is strongly sorbed !o sediment padicles

4ls N connecticul

f

8.0p

B u.o

ot 80

a.i;-:-'-'-7',\a)

(b)

\ . , (c )

- ' (

\ . ' / '' . - . - : 2 /

Osniuh tn the O,adns 41t

The princ'prl sources ol Os dd rhen app'owdreisotope compositions me

L Precanbrian basement rocks cortaidng bioriteK-feld5par. dd olhr Re-rich and Ospoo'minerah th Rlea,e Os hsviDg b8h 13 OvrbosBrios of about J5 bur onging lo values grerrerthd I00i

2. Phane.ozoic redimenrrry 'ockJ Ghale, s,trstone. "ddsrone) whose r3-o\/r36os rario!approach 10;

3. Mafic, nande derived igneous rocks of Tertiaryro Recenr a8e har hsve r37oyr36os rarios of

I. Micromeleorir€ du\r composed ot siticare min-erals, which alio hr. n rFOs/ sos .ario of

In conuar ro S'. fie isoropic compo(iuon o"Or 'n tie oceds i! nol buffered b) cdbonate rocks.which causes dF r3'OVr36O< rarro of seawater robe rnsrlive ro irpd,s ol adiogenic 3'O. fion.Precambrian granitoids.

In addition, Os ditrers fmm Nd and Pb byhrvine d subrtanLially longer resrdence rime orabour ld - 105 yetus. Consequenrly. rhe crnlariorot waler in Lte oreans is bener able ro homogenizede uoLopic composilion of O\ Lhd rhose ofNd dd Pb, although not as well as tha! of Sr,whose residence time is of the order of ld years.Therefore. the isotopic composirion of Os in th€oceans has vdied .egionally with time and hasprirnarily recorded chmges in tbe flux of radiogenicr3?Os derived fiom h@mbrid Shield arcas

19.6a Seawater

The Os concentration of seawater is very iowod has been diincut to determioe (e.g.. Koideel al-. 1996). Consequendy. the di@t determina-tion of the '37os/r36os rario in seawater hasalso be€n dimcult. The analytical problems wereovercome only re@ndy by Shanna er al. (1997),who reponed that a suite of samples collecredat different depths from 25 to 3000 m ir rheAtlantic Ocean ned Bemuda have vinually con-stmt os concentmtions sd r37oyl36os mtios-

0.00 2 0 4 0 6 0

Distancekm

scm 1r:!a (a) Variation of the rTOs/'sOs ratiosof bulk sediment in Long hland Sound along awesl to'east profile shown in Figure 19.32. Thesediment smples all represent the upper 2-cmlayer recovered by a box corer (b) vdiation ofthe Os concentations of rhe bulk sedimetrtsamples in Dnits of l0 rz

e/c- (4 Concennadonof carbon in organic matter in the bulk sedimenr.Data from winiams e1al. (1997).

which implies that only a fmction ofthe Os releasedby weathering dd by mlhropological uses on thecontinenis enlers the ocean.

1 9 . 6 O S M I U M I N T H E O C E A N S

The isoiope compositiotr of Os in the oceans. likethose of Sr, Nd. and Pb, is detemired by inputsfron difiefent sources and by its Esidence time.

176 19. The Ocea\

This result suggests that Os is a consenative ele-ment in lhe oceans. Three aliquots of a smple taken from a deprh of 3000 n near Bemudayielded an avemgo '37oyr36os ratio of 8.7 +0.2.Another set of drce aliquors from a depth of3000 m collected in ihe Pacinc Ocean ned Hawaiiyielded an idenrical '37os/r36os ratio of 8.7+0.3 (Shma er al.. 1997). However, seawater col-lected at a deprh of 1764 m along the Jud deFuca Ridge ln the Pacinc OceaD seems to havea lower rnOVr6Os ratio of 6.9 + 0.4. which rheauthors attributed to the input of mantlederlved Os('37oy'36os - 1.0) by hydrothermal solurions (or1o unexplained analytical p.oblemt. The most reliable measuremenr ofthe Os concent.ation ofunfil-lered seawater repoded by Sharma et al. (1997) is1.6 x 10-i5 S/S.

The Os concentrations and 'uosy'6os ratiosof seawater from the Indian Ocee de alsoinvarimt with depth and have average values of10.86+0.0? x 10-15 g/g and 8.80+ 0.07, respeclvely (Levasseur et al., 1998). The samples forthis study were collected ar rwo siles aiong theSoulhwest Indian Ridge from a maximum depthof 4560 m witholt detecling a chmge in rhereos/'sos ntio caused by inplt of nantle-derived Os-

The concenrarion of Os in seawater inthe IDdian Ocem (10.86 x l0 15

s/g) deterninedby kvasseuret a]. (1998) is rhree tmes higher ihdthat reported by Sharma et al. (1997) for seawaterin theAdantic md Pacific Oceans- This discrepancyis probably an difact of the analytical procedureused by Sbdma el al. (1997), which may nor haverecovered all of the Os p.esent in the smples. Theprocedures usedby Levasseuret rl. (1998) does no!include a prcconcenkalion step bu1 instead relieson the oxidalion of Os to OsO4 and the simultane,ous destruction of dissolved organic natler during48 h incubation inan oven a!90"C. Levasseuret al.(1998) suSSesled that a certain iiaction of the Osin seawaler foms a stable organometallic complexthat prevents jt from equilibrating with the Os spjkein tbe procedure Dsed by Sharma et al. {1997). Themeasurement ofrher3TOyr36Os ratio of seawarer isnot afi@ted by the ditrerent analylical proceduresprovided that all ionic and molecular foms of Osin seawater have the sam€ isotopic compositlon.

Estimates of the rcsidence time of Os inthe oceans e atrected by the nncedainty ofthe Os concentrations of watcr in the world\rive6 and in seawater. Nevertheles, all of theeslimates converge to values between loa atrd 105yeds. Levasseur et al. (1998) a$umed thft nvcrsprovide only 80% of the toral inpul of Os ro theoceans and bracketed its residence tine ber$een1.6 x 10+ and 6.5 x loa yetus.

As in the case of Sr, thc different isotopicvdieties of Os released by wealhering on thc connnents mix during t.anspofl by rivers beforc tncyenrer fte oceans. Therefore, the i37osy'36o. ratioof water in the Mississippi River (10.4) represenbOs denved by weathcring in a ldge part of theNonh Ame.ican continent, including rocks of Pre,cambrim and Phanerozoic ages. The continental Osmixes in the oceds witb Os rleased by meteoriticdust (r37os/r36os) = 1.05) md with Os derivedfrom weathering of mande derived mafic lolcdi.rccks (137os/r36os = l.l0) extruded along mido,cean ridges and on ocednic islands. By combiningmeteoritic and mande-derivod Os (r37os/'36os :1.075), the isotopic composition ofOs in the oceanscan be treated as a two-component mixturc:

r s r o s r l l s o s r / r t o s \

(19.16)eherc lhe subsc.ipts are defined as sw = seawater,n = meleoritic and mmlle-derived. and v = riverwatcr. ID addition. this equrlion describes nixjngof lhe element in pure lbm withour regard ions concen(ations in thc media iD which i( isdelive.ed to the oceans. Subrtituting values for thesTosy'sos ratio. md solving equalion 19.16 forJ yield

8.8: 1.075/ + 10.4( t / )

. f = 0 t 7

This selected data ser indicaEs tlar approximately17% of the Os in the oc€ans originates frommeteoritic dusl and from mande-derived rocks in

Equation 19.16 also demonstates lhat ther3?Os/136Os ratio of seawater is a linear funclionof the r37o.y'36os ratio of river warer. assuming

Inar rhe numcrilJl vatues ot the Os rsoropc ralroso metedric ds' md manrte_dcnved rocks aremvrnanr wrth hme, dr leisr dunng the phonerozoicBon, and thafthe fractior ot' Or derived from thesesources remajns constant (i.e., f - O.tl):

r ' t o ' r( s ,o i / . :0 .182 +08i ( f f i )^ r ro rzr

Thereforc,_ if the '37oyr36os ralio of river water'nma.e\ due,v t r increo\c :n rhe rdre or {e1rhe,_ng of bioritc bearing precambrian basement rocks(e.9., atrer an orogeny), rhc rrosy'so. rario ofsejwdrer J\o r,\e\. rur e\dnp.e. ir rhe d!erJsc'o." 'O. . r r o incrcc.e. pro id l l ) r o ? 2 5 ' T l b t e t o . | I : p e u . k e , _ t h , e n b r i J ] n dR-v i / /c . laa6, . Ine o Ov.oos rd o of .ea$JLc,

l G; : l = 0 182 +0.81 22.5 : 18.8

The -ppdenr rn,

\.r) d lhe .soroF cornpoluonoI ur .n rhe ocecns .u cfrnSp\ In rhc r. oJ suosmrio o l O. r r n \p w] ler hc. rorrvatcd !h(study of Os in ma.ine fcromarganese and orhersed'inenlary deposirs of Ccrozoic age.

19.6b Mereoriric Dusr

The prernce u. dn e\Urrene. t r id t c^mponenr i rterestnct \edimcnt wis tirst demon\rrated bv I u.kand T\r rck iJn r t98l ) in Creraceous Tenrry iounOd4 c'r)\ bd.cd on rhe r tos i3 ov 'oo\

rdfio. olr . re r 66 comp.red ro an ivcmBe of? 57In \even

fodules Subsequenrt), Esqer aidluek'rn (1988) e\fimatcd Lhc Jclrerion rarc oiexuaterre\ra.t parti.tcs from rir r36Ovri?Os ranoqor peragrc ctry and ieromangcnese nodutes in rherrcrn. ucean I hey .^ocluded thdr rhe flux ot crr-Donaceous mereorite dust to the Eanh,s surface is4.9 x l0/ kg/y and tbar abour 20% ofthar mare.ialdissolvcs in seawarer

. More .ecently. Sharma et ai. (1997) rctered

ro rsotopi( evjdence lhal u! to 90d. of cen neremenr\ rMe O. Ni. trd C

e!.rpomre trcmco(mrc \pherulcs dunnE rheir pisqse rhrouah thetrmo,phere They su88e{ed r t r , . " "n" in r* . - "

Osmiuh in the Oeans 471

oi O. In rhse.pherute\ a j .o e\sporures .nd Fut tmdlety d i . \o t \ed In .eawJrer . Tbe pre$n.r off i \ "co.mi .

O\ rn.eaware ' lower. i r . r r /Os/r&O.r . r i o j r m J 0 4 r o 8 . 7 E U . t . a s r p o a e d b \ S h r , . r lcr a l (19s7r and LevJseur er at . ( t998) .

19.6c FenomanganeseDeposits

The slo\' rare of deposirion of ferommqanesccmsl3 conbined wirh their enrichment in Oicorrpared ro seawarer nakes rhem an ideal nediuin rosludy rime-dependent vdiations of the reOsy'vosmuo or fJsartr Tle comprdri\et) tonS oceatucr lde1le r imc ot O. d. tou\ \ i .oroprc compo\ron ro be homogen. /ed a lmorr d\ $e, t a . Lhar ot Srdd ru. h moe r rhar rho\e ur \d dd pb. Therr-rore. u. \olope rat.o. of .rromdAdnc\e (ru\ I j andorher \ed'menrtu) deposrr, re.o,J Seotogi.at even...on " 6/obr l ra le. t ike lhe sorope idr ios or Sr Inmanne carbonares and phosphates.

The O\ col,enrrat|ons oi Je.omr'nedesecrusr\ .eponed by Buron er ct. , tuaq, ranae frou,I . l .0 l4 ro J .61)r . I0 '8 /B

ppor and Loninn rhdrrney de .nongly en,rched In O\ retaLve -o \edwarr,r10.86 . t0 'g lg, .

t re O\ rn fercmdp"ne\<deposits is sorbed from seawateL and is asiumec1o have rtre same r37OV'36Os rario as fte Os ;,seawaler, excepr for possible in sjtu decay of '37Reand the presence of mantte-derived Os neu hor\Danf s atong mid_oLean,dge\ . Houe\cr . nc, rherrn . r tu deldy ot 'Re nor pro( i r ,0 ro submdinenor spmgs appear lo bave affected the r3rov136os- , l ro. -uf rncrJt t ' tcrods \edrmenr mnSing In cEc"orn r lJ , to 0.1n,1 Mr rn, orc, recorercd i rurn rh(Hacrhc oceJn rPeucteLbhrenbrink cr ar , too5J b\De Deep ser Drttrlg prosr,m ,nd O.eJr Driltin;Progmrn (DSDp/ODp)

_ _ Flowekr. Bunon er rt (1999) reDoned tos" OVriiOs rafio! tor hydrorhenn.j fedomxn-gm\e crusrs. r rhe Cibr tuo f rar lure zone (6.8| ] :EUU09) In Lhe Nofih Artdnr|c dd on the IJrdcas secmounr, Mar iJnr k t rnds t8 t7 l +0.028). rothe Weqr Prcrhc O!c.n In .ddrrion. B!no. er rl(1999) obsrvcd a decrease of ttre 137os/rs6osraros rr I redomrntsanele crulr (t27 KD. VA ll/2_9"18 N. L4oo3 Wt hom 8.b9 ar 2.08 Md ro4.159ar 004 Ma. The de.rease oi rhe r3rOV R"Os .arroswas probably caused by thc Iocalized addirion of

478 19. Thz Oceans

utradiogenic Os in the form of meteoritic dDst orof panicles of mantle-derived rocks. Two episodicdedeases of '37OVrsOs ratios from 8.6 to 8. I at 20and 160 ka were aho reportcd by Oxbush (1998)in each of two closely spaced sediment cores collected along the Edt Pacific Rise al about 17"00 S,114"00'W. These episodes coincide with lhe terminatioos oflhe last rwo continenlal glaciadons ofthe Pleistocene Epoch.

The rsosy'eos ratios of r€cenlly depositedhydogenetic ferromanSinesc crusts in the Adantic,Pacific, ud lndian Oceans in Table t 9. 12 vary onlybetween nalrow limiis except for local anomaltesThe dara of Burton et al. ( 1999) in Table 19. 12 suggest that the average '3?os/s6os ratio of ferromanganese crusts in lhe Atlantic Ocea. (8.78 + 0 07)is dighlly higher ihan those of the Pacific (8.59 +6.05) and Indiar (8.58 + 0.10) oceans. The authorsarributed the apparent 137Os enrichment of Os jn

lhe Atlanlic Ocean to the discharge of najor nvesdiainirg the adjacent continents (e.8-. Amazon,Zaire, Mississippi. Orinoco). Neverlheless. excePtfor local momalies discovered by Burlon et al.(1999) and Oxbursh (1998) and for small region.lditrerences (e.s., in the Baltic Sea). ihe'37os/rtuOsratios of the major oceans apped to be constui ona global scale at the present time.

Table 19.12. Averase 137ov1ffiOs Ratios andOs CoocentratioN of Surface Layers ofHydrog€netic Fe.ronanganese Crusts

Number ofSamples in

Each Os.ocean Average l0 c

dg '3?os/'36osd

19.6d Isotopic Evotution duringCenozoic Era

The varialion of RTosy'so! ratios of the ocedsduring the Cenorcic Era has been documented iDseveral different ways: (1) by leachi.g Os Aoma long core cornposed of pelagic clay depositedin the North Pacific (Pegram e( al., 1992). (2) bydalysis of Os in melalliferous sedimcnt of differcntages in L\e Pacific Ocem (Peucker-Ehrenbrinker al., 1995), and (3) by a study of metalliferouscdbonates deposited clole to the Eas! PrcificRise (Ravizza, 1993).

Pegrm et al. (1992) used acidified hydrogenperoxide (69d lo leach Os sorbed to clay mineralsin a 24 m piston core (LL4-GPC3) taken in lheNo.th Pacific Ocee at 30'19.9'N md 157"49 9'waboul 200 kr noflh or lhe H.wdrfl Irld.d:. Th(core intersects lhe Crctaceous-Tenialy boundarysh;ch $di ide.rified by a peal in Lhe Ir concenustion at a depth of 20.5 m below the top of tbe coreIn addiuon. Pegrm el .l i 1092, dcmonnmted $alure 3-Oi/rR6o\ rduor ol dre leachare\ ar hieher Ina1l cases thatr those of the bulk clay samples.

The 'eosy'sos ratios of the leachates of sedimenl in core LI-44 CPC3 derease with deptbtiom 8.24? .t0.009 (0.32 0.36 n) to 270910004 L l?.15 l / .40 m' . The dme-dependent ! " r j -

ar ion o l rhe r" 'OJr36Os rdr io. o l rhe lea.h"re 'r simlu ro rl'e \sial'on ot rhe 3 srl6Sr rauoIn I'igure lo.1 bur drlTers in delail. Pegrar er al.

L I9a2) \d i \ned themselves lhar the 'Os/ ' *O'

a l ,o5 o l rhe leachate\ f f 'u lhc ienr ly . imlat torhose oi sedwarer ro pro\ide . t.|d record of tbee\olution of Lhe Fotope con po\iuon of Os in theNorlh Pacific Occan. Howeve!, they acknowledgedrhr' r\e acrdrned hydrogen peroxide md) alr""lun,denl ried detrirdl Os b€a'ing mineral phr\e\.cru,ing Ltrc r8'OJr*6o' ratios of le.charrs ro n\e b)Lb% depending on !h. concenlJ.uon ofrhe hydr'gen peroxide be$een 2.7 and 307,.

Pegam e' ar . , loq2, a l r ibdred the l imrdepenocnr r i .e o l rhe lsro- / rR6o\ ra l ro o l q$"

2.

l � m increase of the mual irput to the oceans ofos having elevated 137oyr36os ratios,an incaase of the rmosy'$os ratios of lhecontinental Os component because of enhanced

1.91

2.22+0.281.48+0.35

8.78

8.59+0.058.58+0.10

20

J,,r.i Bunon er al., 1999.'ft€ rsoJisos ratios were coftected for hotopefracr'onat on ro N'osrrsos = 1.08271 and conlerted to'3?oti36os rarios by mulriplyi.g them by rsrotr$os

0, r 19969

wearhering of Re rich black shales dd relaredgraniloids or because of fte exposure ro wea_lhenng of older Precamb.ian rocks on tne

3. a decrease of the flux of mantle_de.ived ormeteorilic Os entering lhe ocems, or

4. a combinanon of two or more of the fac1o6

subsequenr measurements of r37ovr36os iarios ofnetalliferous cdbonates by Raviza (1993) andof metalliferous sediment by peuckecEbEnbrinket al. (i995) have estabthhed a body of data thatwas used to construct the isotoDic prcnle inFiBUre 19.35 tor Os In rhe ocerns d;rjnsihe ce""zoic Er.. In addrrion ro Lhe v.Jue of rhr,-Os iqoroDeprofile as a reco.d of globat geolo8jcal erlvlty. itcan also be used for datin! nonfossiliferous marinerccks of LrLe Cenozorc lNeogene) a8e (Ravizza,l99t) .

o 2 0 4 0 6 o a oA9e,Ma

EGURE D.i Veiarion of the '37Oy136Os ralio ofseawater dunng rhe Cenorcic Era:K = Cretaceous. P: Pal@gene. E: Eocene,O = otigocene. M - Miocene, pl = pliocen€.Adapted from PeuckeFEhrenbrink er al. (1995)ed based o. data by pegram el at. (1992),Ravizza (1993), and peucker-Ehrenbrinket al. (1995).

Oshiun in the Oceans 479

The Incrcase or rhe r"'O"/r3.O. rar;o ot ,e,warer dunng rhe past 15 mi ion years may havebeen .du.ed in pr( b) rle wedhennA of btach\hrle. rn the HimJayr\ drained by rhe Crnges6rdnmrputra Jtd Indu. river .ys.ems (pesrarner a l . . loa2: PeuckeFEhrenbnnr, e l , I , loo5i ie i . -bcrB e l d l . . raal r . Ho$ever. Aufon et a l . I too,po:nreo our rhar rhe rvemge rs ovreo. rarroof feromangmese deposirs in the Indid Oced(8.58+0.10, Table 19.12) does no! support rhepostulated inpur of exc€ss radiogenic r3?Os fron

Amlyses ot O. 'n btdck shJte In rhe Nffa)didra inage in the Hrma a)a\ by pre^on-wickmd.,erd l . (2002, ord y ietd h igh r ' /OJrbor mnL. .betw€en 42.8 ud 49.0 wift a mdinum valucof 86.9 in one of several soit profiles (Mo 601,20.50cn I However. rhe Os drar wa. rete i fdf iom rhe b lact shr te qas coprecip i rored t r i rhienlc oxyhydioxide at tbe site of weatheriDg,whertu fe Re e\caped jn ,o ution. Therelo,.lhe Os rlea:ed by wealhenng or t'r.* .rr.r. .the Himalayas appees ro be scavenged by fericoxyhydroxide and is uttimarely deposired wirh thesedrment in rhe Indian Ocean. Consequentlv. rherddioeeni( 3-os i. pre'enred from ;ec,i; rhcrsorope ,omoojuon ot O, in ,..".'., or ,t " t"oi",

The sedimenr rmnsporled by rhe NdryaniRrver ente6 rhe cdger Bnimapurn riler D.tem and 's depo.ired rn Lhe BenAa, Fd Sedi_menl samples in two cores of the Bengal Fanwere leached by Reisberg et al. (1997) using|ne H:O -H?SO1 reagenr of pegran er dt. 1,9q21994). The 37Os/rMO! 6rios ot rhe .erchares17.82-ll.q4) exceed rho,e of \eaware n Frsurrl9 J5 dur ing rhe, ime r l rervat f rom t5.Jro0.a-Ma.when rhe redirenr sd! depo.ired. the presenceol excess \orbed 3 O. retrr ve ro Or rn.eaq"Lermay be'htrrdensric ot scd:menl trd.poaed by !h,Gdges-Brahmaputra river systen. For exampte,andy.e, of ledchares b) pegrm er at. (,a9.2, orse.hent rn the Ganges River yielded '37oy'36osralios of 15.3? and 16.93. Alremarively. Reisberger al. (1997) propoqed rhar rhe eKess rxdiosenrr37Os oriSinared from sedinenr upslope of rhe coGing site eirher by desorprion during atrer.rron or by

OrDluh i. th.O<ani

P r l , M , l o l E . l q F - K

,180 19. The O.eans

In any ca!e, a large fraction of the Os that

is relcased by weathering of cdbon'iich shales in

the Himalnyas aPpeds lo be srbed to sedimentplnrcles, leavrnS only a smrll amount rn solunon

Thereforc. the Him.llva( s probablv not the

dominant source of radiogenic r?os in seawater

The study of wealhering of black shale in

tbe Himalalas by Pierson-wickmam et al. (2002)

revealed (he close associxtiot ol both Os ud

Re wirh organic matter' This propedy of Os

was used by Ralizza and Turctjan (1992) to

measure the rr?oVr36os rdtios of bulk samlles

o 'carbon nch mMne .edrmelr tn lhe Pdcinc mt

Ar h nul Ocecn\. Thc O . in lher sample\ onginate!

ldEel\ lrom seau et \7s l00cd h)drogeneric) andqr, a ' .c iaaa q i ln oreJnic mlr ler ' l l r ' 60%t

The resultinS sTos/rsos .atios (8.15-892) of

rhe bLlL .edrmenr .dnples ) ielded an a\enge

oL 8.54-0.12, wl ' ich \ indb ' ingu'hab'e f rom

rhe a\er"ge " Os/r360" rrlro\ of h)drcgenerrc

fenom.ngane.c cru ' r ' in lao le lo l2 Eponed

by Buton et al. (1999).

1 9 . 7 H A F N I U MI N T H E O C E A N S

Thc \oropic omPo\irion ol Hf in lhe cru\l anri

mJnrle or lhe L]nh chdnge\ silh rime becausr

or the de.dy o ' long ' r 'ed | 6Lu ro s lable LroHl

with a halflife of 35.7 x l0' vears, which vi€lds a

de.ay constanr of 1.94 x 10 " v ' (Chapbr l2)'

lFe hrpneJ Hl .oncenrrr r roni Inred in table l2 '

occur in cenain accessorv minerals, including

/ i rcon (15. '7 l ppm,. brddclev i re ( l l l40ppm)

and eudialyte {1736 ppm). all of which resist

chemical weathe.ing (zircon and baddelevite) or

Gcur rdre l ) 'cudrJ lv te, The Hl concenrra l ions or

igneous rocks range hom I 14 ppm (puidotite) to

12.84 ppm (rhyolite) Srony meteonbs nave row

Hf concentations of about 0-2 ppm in chondrites

od 0 7 r ppn In ("i ch a.hondnre. (Boswell and

Elderneld, 1998).The

-oH7 Hi rJuo' or mo"r ro ' \ ' dd

minerals vary only between nanow limits, which

requires thar they must be measured with great pre

cision. For rhis reason, il is conlenient 1o exlress

176Hf/r?7Hf ratios by means of the €0_noiaiion

relative to CHUR_Hf defined bv equation 12 3:

^ [ ( ?6HV ?'Hni,, . l .- .r " (Hn= | - : : : : - ; . - - - '

I ' NL '

" ' / 'CHUR I

where (lftHfltzHt8mR = 0.28286 (Patchett.

l98 l r or ( r - ' HUr7-Hn0RR - 0 282712 rBl i 'hen

Ton er at . , t9a/r The r"Nd/ '4 \d ra l ios can be

expressed similally by equation 9 6 in S4tion 9 2b'

where

t , - _ : | = 0 5 1 2 6 1 8\'4Nd,/.,.,r

Sample\ navin8 po'iu\e r0 value' for tt dd \d

ha\ ; h isher r -oH0l

than th; corresponding values of CHIIR for

Hf and Nd.During partial m.lting of ultramafic rocks

in the lithospheric or asthenospheric mande' Hf

and Nd preferentially enter the melt phase md

are rheRby temo\ed frcm rhe mantle by rhe

uowdd mo\emenl ot btuet ma8ru' A' a esull.

the continenral crusr has Io$er a\erage Lu/Ht

and Sm,/Nd ratios ihan the ultramafic rocks

of the mantle The ditrerence in these ralios

ha cau'ed crusral Hl and Nd lo ha\e lowet

DresenGday -"Ht'7Hf

and 'Nd/'o"Nd ratio'

thm young mantle-derived volcanic Mks Conse-

iruently, crustal rocks @ chdrclenzed by neganve

;o-values of Hf and Nd. whereas matde deriven

\olcdic rocks in mosr.a\es hdve posrlrve or near

19.7a Teresrial Hf-Nd Array

The similaf,ty of the geochemical !rcperties of

lhe Lu-Hf and Sm-\d couple\ oullined above

causes the 176Hf/'77Hf and r43Nd/'eNd ralios of

crustai rocks to be positiaelv correlated This rela

rionship ma) be disrurbed b) chemi'al we8'her_

ing, which affects Hf-rich minerals (eg. ztrconl

diffeEntly than Nd-rich ninerals (biolile. mon-

azite. amphiboletThe sqvalues of Hf and Nd of sediment in dif'

ferent geological enviJonnents reported bv Vervoort

et al. (1999) become increasinglv negative wirh

€o (Nd)

+20

-20

terrig.nou. Ht-Nd An.y

2

-60 -40 20 0 +20eo{Hf)

Ecuro Dr Corclanon of so values of Hf ddNd in sedimenr ranging in age from Recent toArchean: 1- Pelagic sediment. 0 cai 2. Deep{eaturbidites. 0 Ga; 3. Canadian Cordillera,0.14-0.56 cai 4. Palcozoic rurbidiles.0.30-0.45 cai 5. Proterozoic sedimenr,1.85 1.88 Ga; 6. Biririe sedimenr, 2.10 ca! 7.Archean shale, 2.66 2.90 ca: 8. Fluvial orsballow-water sediment, 0.21-2.20 ca. Thesu values were calculated relative ro ('76Hfl|77H03HUR=0282772and

(rarNd/'{Nd)3!uR = o 512638. Datafiom Verloort et al. (1999).

mcreasing age from Rccent to Arched. In addi-lion. the average 60-values of Hf and Nd of thesesamples in Figure 19.36 de strongly corelated anddefine thc terestrialHf Nd array" reprcsented by

e"(Nd) = -3.6r .18 +0.5737.o(Hf) (19.18)

Allhough ftis equition represenrs the isotope com-losition of Hf and Nd in rhe crusr expressedjn the eu-noration, il does not pass trcughtbe origin as exp&ted from rhe definition ofhe chondritic unifom reservoir- For example,i f so(Ho:0, cquar ion 19.18 y ie lds €o(Nd)-

Hahitm ia the Oceans 481

1.6148. Vervoort et al. (1999) attribored thjs dis-depaDcy 1o an rnconsistency in the definitionof CHUR.

The average Hf concentralions of differentrypes of nuiDe sedimenr mnge from 2.8 ro6-7 lpm depending primarily on the presence ofdetrital zircon grains. The average Hf and Ndconcentratons of all sedimenrs included in rh€srudy of verv@rt et al. (1999) de

Hf= 5.0 ppm Nd = 25 ppm

19.7b Rivers and Seawarer

The occunence of Hf in wearhering{esistdt min-erals (e.9., zircon md baddeleyire) conrributes ioits low concenlralions in solutioD in rivers and intne oceans. Godfrey et al. (1996) reporled Hf con,centrarions rdglng fiom 0.57 x t0 ', ro 4.53 x10-r'� g/g in rivers of South America and Siberiasampled by J- M. Edmond. These vatues are nedly40 limes hiSher than the concenhations of Hf inseawater D the northeast Atlandc Oced determined by Codfrey et a1. (1996).

The Hf coDcentrations of tle surface layerof the northeastem Adantic Ocean adjacent ro theCeltic Sea decrease from 432 x 10 i5 g/g netr theedge of the shelf ro 73.2 x t0 '5 g/g in the openocean.In addirion, codfrey er ai. (196) confimedthal fte Hf concentrarions of seawarer increasewitn depth similar ro tbose of micronurrients.Howeve!, the concentrarion of Hf stols iising ata depth of about 3000 m, presuftably because ofsorption of Hf to suspended parricles in the warercolumn- The dara of codfrey €t al. ( 1996) yietd aJI)average Hf coDcent.a(ion of 135 x 10 's g/g forfive smples of seawater collected from deprhs ofgreater thar 3000 m.

Using the concentialions of Hf in riverwater (3.57 x l0-'2 s/s) and seawarer (143 xl0 Ltr g/g). codfrey et al. (1996) estjmated tnar th€ocedic residence time of Hf is 1.5 x 103 yearsand therefore is significddy longer than that ofNd, wbich is only 0.3 x 10r yeds (Section 19.3c).Consequendy, the isotole composirion of Hf inseawaier is expccted to vary less rhan that ofNd (McKelvey and orids. 1998).

482 19. The O.edn!

19.7c Recent Fenomanganese Nodules

The concentrations of lif in hydrogenelic fenoman-sdese nodules in Table 19.13 reponed by whneet al. (1986) range from 4.93 lo 8-76 ppm dd havea mean of 6.7 ppm, which is aboui eight ord€rs ofmagnitude higher rhan lhe Hf concertration oI sea-water. A1l other types ofclay'ricb sedimert iisted inTrble 19.13 have slighdy lower Hf concenlrationsbetwee! i.62 ppm in siliceous ooze to 4.0 ppm in

The tr6Hfy''Hf ratios of bulk sanples ofluine sediment in Table 19.13 range widelydepending on the prorortions of coniinental mdmande HJ they contain (Pettke et al., 1998). v&ious types of deep{ea clay have e'(Hl) vatuesbelw@n +4.5 (DSDP 452, Pacific) to -17.9 (siltysand, Admtic). The six feflomanganese nodllesanalyzed by wbire et al. (1986) have a nean €u(Hovalue of +3.6, which suggests ihat a ldge liacfonof Hf dissolved in seawaler originates from mantle-derived volcanic lock and from hydrothemal

Trble 19.13. H.fnium Concentrations andeo(Ht Values of Ferromansanese and Otherrypes of Deposits in the oceaN

fluids dischtrged by hot springs along mido-

subsequenl srudies by Godfrey et al. (1997),Albarede er al. (1998), and David et aI. (2001)have confirmed ihat hydro8enetic feromanganesenodules ;n the major oceans have distinctive iso-tope compositions of Hl Their data in Table 19.14yielded averase e0(H0 values of +1.9 for theAtlantic Oces, +5.3 for the Pacinc Oceai, and+4.3 for tbe Indian Ocean. In addiiion. bydrolhermal fenomanganese deposirs malyzed by Godfreyet al. (1997) have hisher eo(Hf) values than hydro-genetic nodules in the Atlantic md Paciiic Oceans.In general, tbe isotope composilions of Hf inTable 19.14 indicate that feromangnnese depositsin the Adantic Ocean conlain a larger proponion oicontiDental Hf than those of the Paclfic Ocean. Theaverage e01H0 values of feromansanese nodulesin the Indim Ocem tre intermediate betw€cn thoseof the Adutic md Pacifrc Oceans.

The isolopic compositions of Hf and Nd ofhydrogenetic fercmdganese deposits of all threeoceans jtr Figure 19.37 are positively conelated by

€0(Nd) = r3.8099 1.5288e0(H0 (19.19)

This line is displa@d from the teEestrial Hl-Ndanay (equation 19.3), but the two lines intersecta( aboDt €o(Ho: +10.6 md eo(Nd) = +2.5. Boih

Table 19.14. Average e"(Hf) Values ofHydmg€netic (A) sd Hydrctheml(B) I€rronansan€s Nod'rles of the Major

€"(HoB

DeposirHf.ppm e'(Hf)

Composite, DSDP 452

Red clay

Sandy clay

Silty smd

6.7 (1)1.0 (2)|.62 (4)3.67 (1)2.77 (1)

3.07 (1)

2.80 (1)

+4.5 ( l )+r.6 (6)+4.0 (2)

0.80 (4)2 . 1 ( 1 )

-5.3 (2)-8.3 (1)-9.8 (1)-17.0 (1)

17.9 ( l )

lrdian

+1.9 (53)+5.3 (2r)+4.3 (4)

+3.1 (s)+8.8 (12)

,torr?i white er al., 1986.N,r.r All dara repr€senl complete dissolutions. Itet6tlfl''Hf rdio of JMc 4?5 was 0.282161 aDd ro(Htvalues wde ecalculated relative lo ('?6Hfl'??HttHUR =O.2an12. The slmples originalcd fron the Adantic,Pacinc, dd Indim Ocoans. The nunbe. of smples isindicaled in prenrh€ses. Addilional Hf concentratlods oiferonmSanese nodules wde reponed by Godfrey et al.(199?) dd Albande er al. (1998).

N,rcr Dara conpiled fron white erd. (1986), God-fiey er rl. (1997). Dalid eral, (2001). and Arbeedeer.l. (1998). All .'(H0 !alu$ ee ELadve ro, ' ^ H r , ' H n l @ - o ) 8 2 7 7 2 r d a R t y ' - H r - 0 2 8 2 t 7

or 0.28216 for JMC-475. The dara ior the brdrcthermalnodules de by codftey et al. ( 1997). The nunb€r of sm-ples included in ea.h rv€rage is indicaled in pmnlheses.

eo(Nd)

+2

0

€

-12

HdJriun tu the Ocea8 483

bdr shich rersrs $eaLhering. Therelore. Lhe Hlthat gcs imo solurion on ihe conrinents and isullinat€iy sorbed by feronanganese rodul€s in rheeems is enriched in ladioSenic '76Hf, causing itss0(H0 value to be less negarive than that of theHl rhar eri\L' in lhe unweahered rorks. In ofterwords. chemical wealhering of Hf bedlng mineralson the continents causes isotope liactionatioo of th€Hf lhat go€s into solution and is tansported into

Tbe eo(Ho value of the Hf in solurion onthe continents can be calculated by subsrirutinCr'(Nd) of Archean shale, based on the asumprionthat Nd is not fractionated isotopically by chemical weathering. Therefoe. if 60(Nd) = -29.5.equation 19.19 yields

-29.5 + 13.8099eo(HO = : lO.2

- 4 0 + 4 + 8

"o(rDFcuM i,r7 Epsilon-values for Hf and Nd ofhydrogenetic feromanganese nodules in theoceans: solid circles : Atlantici opencircles = Pei6c: crosses = Indian Ocean. Allru-values were calculated .elative b('76Hflt71HD9cNR = 0.282ii2 and (r43Nd/lsNd)BHUR :0.512638. ln addinob, allmeasured rftHfy'rHf

ratios are relative ro 0.28216 foi lhe JMC 475 Hfisotope stdded. The tenigenous Hf-Nd anaywas reploned from equation 19.18 based onse.l'ment smples analyzed by Vervoort et al.(1999). Data from Albdcde et al. (1998)ed David et al. (2001).

quasilined days may be the result of mixingof rwo components ln the Hf-Nd isoropic plane.The point of inteNeciion of the two lines inFigure 19.37 represenrs the Hf and Nd rhat originated from sources in the mantle. The tercstrialHf Nd eay in Figure 19.36 can be viewed asa series of mlxtures of this mande componentwjth varying mounts of Archean share (point 7.Fisure 19.36) whose coordinares are r0(H0:-43.5 dd€o(Nd) = -29.5 (Veroon et jl., 1999).

The component of continental Hf present in theferomd8anese nodulesjn Fi8ure 19.37 is enrichedin Ediogenic r76Hf compared to Hf in Archeanshale because of the preferential weathering ofm'nerah having higher LulHf ratios thm zircon,which conlains most of fte Hf in crustal Nks

1.5288

The explmation of the €o-values of Hf andNd in hydrogenetic fenomanganese nodules inFigure 19.38 considers ftem ro be mixturcs oftwocomponents that arc prese.t in solution in s@water:

l� Mantle derived cornponent ,o(Ho : +10.6.€"(Nd) = +2.5

2. Crustal componert .u(Hf) = l0 2,€o(Nd) = 29.5

Consequendy. the ru value of either Hf or Ndh feromanganese noduies can be expressed byequanons of tbe fo.m

s0(nodule) : '0(mdtle)/ + '0(crusr)(l ,f)

09.20)where / is the abundance of the mmtle component.

For exa6ple, a nodule in the Pacific Oceanlhat is rcpresented in Figure 19.38 by a point whose

eo6rj - 16-g eo(Na) = -: o

cootains about 827o of the mdtle componenr basedon equation 19.16: For €o(H0: +6.8,

+6.8: +10.6/ 10.2( l f )

484 19. The Ocedns

€o(Nd)

+24

0

20

40

Mixing ol Hf and Nd in Tberefore, the isotopic mixing model 'nFigure 19-38 connrms the conclusion originallyexpresed by Wlite ct a1. (1986) that the Hfin feFromangdese nodules is dominantly derived fromsources in fte mantle by chemical weathering ofvolcanic rocks etupted within the ocean basins. TheHf and Nd of mantle derived volcanic rocks areboth released into solution wirhout isotopic fmc-tioration because insufncient tine has passed aftererupiion for the radiogenic isoiopes to accumulateand because zircon is les abundant inbasaltic rocksin rhe ocean basins ihan it is in the granilic rocksot rhe . onrnenrxl crusr. In .onllu.ron. the 5rudy olhydJogenetic l€Fomdganese nodules suggests thalthe isotope composition of Hf in solution in seawa-1er vdies re8ionaliy and is dominated by Hf thaloriginaled by weathenng of mdtle-de.ived malic

19.7d SecularVariations

The (eresirial Hf Nd uay also nanifests itself;nthe closely corelatEd secular variation of e"(Hf)and su(Nd) of feronanganese crusts. Time seriesof so(Hf) values recovered fiom welt-dated feromaiganese crusts have been published by a num.be ol re,effch groups. InclLding Lee et al. rlaqoind horrowsb er al !2000r In addition. DJvi-er .1 . \20{1, dcmon,rra led ,h" 'h" ' - " r r ' ' - "

ratios of fccomanganese nodules are negativelycorelaied sith '�o6Pb/'sPb nlios.

Tte -6Hi / ' -Hf

i , r ios. .orecred for deca) orr '61 u ond erpre*ed a. ' rHl . o f a lenomdng.ne.ecrust (BM 1969) fron fte San Pablo seamoDnt alJ9"0 N, o0"r / W in rhe nonhwe\r Alhnlic Oceandehne lhe prof le in FBure lo .Ja. $hrcn e\ rendsfrom the present to ihe Late cretaceous at about60 M" lPiouow.li er rl.. 2000). The i'rHl) vaiuesdr fii5 lie remained \inucll) consrdl dr cbuut I lfrom rhe Lrre crelaceoJs lo the O',eo.ene Epoch.AIer declrrung lo aborr ' 1.0 during 0re middleMro.ene, the . ' (Hf) \J lue rerumed lo -J a( rheend ol rhar eporh bur dec rned 'h"rpl) duing rhePlio/Pleistocene to about -1.0 at the present time

The er(Nd) values of the same crust alsormrined con'lant dr 10.5 un.il eul} OliEoleneud ,tfled a 'lo$ declne unLil ,he end of (hcMrocene !po,h. wher rhe ' ( \d) va lue\ decl ined

40 20 0 +20 +40c'(H0

EGUc rr$ Relation of the €"-values of Hf andNd ofhydfogenetic feromanganese rodules in lheAilmtic, Pacific, and Indian Oceans !o theterigenous Hf-Nd anay. The diagmmdemonsrates the enrichment of fedomanganesedeposirs in adioSenic '76Hf relarive to sedimenton fte condnents. The s0(H0 value of thecontinental Hfcomponent in the oceans wasderived by shiftiDg the Hf of Archean shale fromthe Hf Nd may to the Fe-Mn anay. The isolopecomposilion of Hf in seawaier is the resuli ofmixing of Hf thrt originaled by wcathering ofmantle'derived igneous rocks and of crustal rocks,represented hcrc by Archean shale. Thetenigenous Hf Nd may was plotted fromequanon 19.18 (Fisure 19.36) and the FeMnanny is from Figu.e 19.37. based on data fromAlbarade er al. (1998) dd Davjd et al. (2001).

Simlldly, for E'(Nd) = 3.0,

_ 3 0 = 2 . s / 2 9 . 5 ( 1 / )

f - 0 8 2

Ar the other end of the compositional spectrum,a feromanganese nodulc in lhe Atlantic Ocean ischdacterized by a loint whose coordinales de

,o(no = +o.zo "n(No) : -rz q

The abuMance of the mande componenl in this

Ell/l1969. NW Arlantic

10 20 30 40 50 E0i|-,tr.

+2

0

2o(Nd)

- 1 2

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0Age, tMa

oalRE rr.! Compeison of the seculd vdationsof Eu(Hf) and 6u(Nd) in a hydrogeneliclenomangdese crust (8M1969) fron San Pabloseamounr (39'0' N, 60"57' W) in the norrhweslAtlantic Oced. Adapted ftom Piotrowskiet al. (2t[0).

shaply to i3. Thereforc, there aplears to be asmall mismatch between the Hf and Nd isotopeproiiles at lhis sire during rhe Oligocene andMiocene Epochs followed in boih cases by adecline of the sLr'lalues during the past four million

Piorrowski et al. (2000) considered rhat rhechanges in fte €! values of Hf and Nd h thenorthwest Atlantic Ocean could have been caus€deithef by changes in the deep wrter circulariono! by the conrinental glaciation of the northemhemisphere. The glaciation of Norlh Ame.ica andScandinavia exposed Precambrian Sranitoids rowelhenng tnar would have released unradiogenicff dd Nd into the @eus. The de.oupLing of theisotopic evolution of Hf and Nd, which is nostevident bctween 15 dd 4 Ma. occuFed becauselhe du-lalues of Hf entering the oceans were risingwhereas the ro-values of Nd were decreasing. TheElson, noted beforc, is the "zircon etrect" (i.e.,the Esistance to weatheriDg of zncon preventsuDndiogenic Hf fron going into solution). Theunradiogenic Hf of zircons was released only

Hafniun in the o.eans 485

when large volumes of finely ground ftecmbrianemnitoids became exposed to weatherinS as aconsequence of full-sc.le continenral glaciation.This phenomenor does not @cur ln lhe Sm-Ndsystem because tie Nd-bearin8 minerals in igneousand metmoQhic rocks have sinild susceptibiliriesto chemical weatbering. Therefore, Nd is norfractionated isolopically by rhe selective weatherine

The 8r(Hf) prcfile of a fenomangmese ctust(VAl3/2) colected in rhe Cenrral Pacific Ocean(9"18'N, 146'03'W) is conpded in Figure 19.40to a profle of,06Pb/,sPb rarios (Davidet al.. 2001).The negative correlation of the isotope compositions of Hf and Pb in this cnst is not wellexpressed. Although 6'(Ht declined with lime whiie

E' (Hr)

+6

1 0 20

YE!

+2

18.6

14.5

0 1 0 2 0 3 0Age, lMa

FIaURE re 40 Secultr isotope variations of Hl ddPb in a hydrogenetic ferrcmmganese crusr (VA132) in the central Pacific Oced Ge€ alsoFigure 19-21). Dala from David et al. (2001).

486 19. The Oceant

rhe r06Pb/'�01Pb ratio;ncreased, the two lrofiles dif'

fcr subshntially in detail. A compdison of ihe

6,(Hf) pronle of crusl VA13/2 in Figure 19.40

lo tbe proule of rl3Nd/'{Nd ratios of the sa,ne

crust in Fiaure 19.22 (Lin8 el al , 1997) indicates

thar lhe isotope composition of Hf is better coF

relJred { , rh rhc Nd/r4\d ra io ' rhan wr lh th '

1 9 , 8 S U M M A R Y

The srudy of 1he isotope conrpositions _of. Sr.

Nd, Pb. Os, and Hf iD the @eans rcveaN now

geo.l'emr.al proce scs on thc curld.e ol the

Earth lffecl thcse elements The elements enter

rl-e gcu(hemlJl rln\por s)'tea when rhe) de

releJ\ed inru .olulrun bJ the chemicrl wea'hel

ing Jf rhe ninerJl. i4 trhrch rhev ̂ ccur' va4

ins fraclions of the resulling ions de sorbed to

tti surraccs of suspended mineral dd organic

paiticles at nea.neutral pH and therebv become

\ulnerrh le Iu depo.r l ;on rn h l (e ' dnd e ' Iudes

The concentmtions of ions remaining in sotu-

uon uc ign riclnrl) reJu.ed bv 'hi' process'

Nhich diminishes fte flux of these elements to

Thc conccntrations of the elemen$ in sea-qorer . r - ane.reJ b) the balan.e berween rhei r

annual inputs and outpuls froft $e ocems Tl,"

re.u l r rnc mrtn i rude. of 'he mean ocedic rer i_

Jenle ure ' L le eminc rhe c\ re1r lo $hlh rhe

elemenn are jsolopically homogenizcd bv the cir

culadon of the occans on a dmescale of about

10r yea6. Elements with lonS residence limes(e.s.. Sr and Os) have constant isotope compo-

sitions lbrougbout fte ocems of lhe wodd. EIe_

ments h.ling shon residence times (e.g Nd' Pb.

Hi) have isotopic compositions that val]' regionally

lnd reflect the ages rnd prent daugbter rarios or

Strontium a.d, to a lesser extent, Os recordpast changes in their isorope compositions that de

Dresened in marine carbonates (Sr) and redoman

;aiese deposits (os) The isoropic evolution of Sr

ii the oceans is a recold of slobal geological acuv_

,1\ durn! the Phsnerc/oic dd Prorero/oi. Eons In

addrron,- rhe r r re dependelr 's r iJuon o l3 Sr /sSr

ratlos permits precise age deterninations of marine

cdbonates of Cenorcic ageThe regional lanability of the isotope ratios of

Nd. Pb, and Hf in the oceans makes these elemenls

useful traceB in the study of lhe circuladon of the

oceans. h addidon, lhese elements can be used

to identlfy their sources on lhe continents andwirhin the ocem balins. ln all cases, the notoPrc

composftions of Sr, Nd, Pb, os. and Hf aie the

result of mixing of two or more lsotoprc vdeues

of these elemerts that originate kom different

The study of fte migralion oi eleftents hav_

ing stable radiogenic isotopes iuuminates sev'

eral aspects of this Process that might otherwrse

1. The release of these elemerts into solution dur_

ing chemical weathoring of lhejr host rninerals jn

rocks, rcgolith, soil. md sediment can cause $o

topic ftactionation because of differences in the

susceltibilities to weathenng of these hosl nin-

e-rls \e.e . Pb and Ht in z'rcon. Sr In mdsco\ ite'

Nd in gdne!, Os in magnetite)-.7. Tenestrial and exttdlenesnia' dusr and altro

spheric aerosol particles conlribute siStrificmdvto r t r in 'enrone' o 'cenoln e lemenl ' rn 'o luuon

in the oceans (e-s., Pb dd Os).3. The con.enrdrionr of ttace elemen'' in aque

oLi soluuon\ se strongly dependenr on sorption

oi "arion' (od anion., Io eratri.dl) chdged

sites on the surfaces of snspended mineral and

orgdic p.flrcles dd ro. olloid.l pmrcles qhoY

d,adeters are lcss thd 0.r5 pm Con'equenu)the environmenlal pH. the presence ol vdtous

\inds ot.orb€m", and lhe collecrion and 'ubs(_

quent treatment of water samples prior to analy

n. all ha\e a proiound elTect on the concenlr"

lions that are measued4. Lakes, resenoirs, and estudies collect sedimem

in \uspen.ion tn smdn( fd fierb) Prewnr lhesorbed flactiotrs of all elements trom entenng

5. In some cases, the isoroPic composilions and

, oncenrnuon" of cerr,jn elemenl\ /e g Pb arld

Ot In 'oluuon in "eaqarer or \orbed ro pelagic

clay and feronangdese oxvhvdroxides de

enecti\e rscets of nrtuopogenjc conrdmin$r\

sol0pes

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