The origin of carbonaceous matter in pre-3.0 Ga greenstone terrains: A review and new evidence from...

6 (2006) 259–300www.elsevier.com/locate/earscirev

Earth-Science Reviews 7

The origin of carbonaceous matter in pre-3.0 Ga greenstone terrains:A review and new evidence from the 3.42 Ga Buck Reef Chert

Michael M. Tice ⁎, Donald R. Lowe 1

Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA

Received 4 October 2005; accepted 17 March 2006Available online 11 May 2006

Abstract

The geological record of carbonaceousmatter from at least 3.5Ga to the end of the Precambrian is fundamentally continuous in termsof carbonaceous matter structure, composition, environments of deposition/preservation, and abundance in host rocks. No abioticprocesses are currently known to be capable of producing continuity in all four of these properties. Although this broad view of thegeological record does not prove that life had arisen by 3.5 Ga, the end of the early Archean, it suggests a working hypothesis: most if notall carbonaceous matter present in rocks older than 3.0 Ga was produced by living organisms. This hypothesis must be tested by studiesof specific early geological units designed to explore the form, distribution, and origin of enclosed carbonaceous matter.

The carbonaceous, environmentally diverse 3416 Ma Buck Reef Chert (BRC) of the Barberton greenstone belt, South Africa,provides an opportunity for such a study. Upward facies progressions in the BRC reflect deposition in environments ranging fromshallow marine evaporitic brine ponds to a storm- and wave-active shelf to a deep, low-energy basinal setting below storm wave base.Abundances and ratios of Al2O3, Zr, TiO2, and Cr track inputs of various types of volcaniclastic and terrigenous clastic materials. Inparticular, Zr/Al2O3 and Zr serve as proxies for concentration of windblown dust and, indirectly, as proxies for sedimentation rate. Cu,Zn, Ni, and FeO were concentrated in the most slowly deposited transitional and basinal sediments, inconsistent with a hydrothermalsetting but consistent with a normal marine setting. The distribution of microfacies defined by associations and layering of clastic,ferruginous, and carbonaceous grains correlates with facies transitions. Fine carbonaceous laminations, which occur only in shallowplatform settings, represent photosynthetic microbial mats. These were ripped up and the debris widely redistributed in shallow anddeep water by waves and storms. The isotopic composition of carbonaceous matter ranges from −35‰ to −30‰ in shallow-watersettings and to −20‰ in deep-water units. The heavier δ13C in deep-water carbonaceous matter is thought to reflect microbialprocessing, possibly by fermentation and methanogenesis, of organic matter originally produced in shallow water.

Hydrothermal origins for BRC carbonaceous matter are clearly excluded by the inferred depositional setting of the rocks as a whole,an inference supported by field, petrographic, and geochemical analysis. We suggest that the biological model proposed here for BRCcarbonaceous matter is the best currently available. The hypothesis that “at least some carbonaceous matter present in rocks older than3.0 Ga was produced by living organisms” should be regarded as likely until extraordinary contradictory evidence is presented.© 2006 Elsevier B.V. All rights reserved.

Keywords: carbonaceous matter; Archean; photosynthesis; microbial mat; chert

⁎ Corresponding author. Current address: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California91125, USA. Fax: +1 626 683 0621.

E-mail addresses: [email protected] (M.M. Tice), [email protected] (D.R. Lowe).1 Fax: +1 650 725 0979.

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260 M.M. Tice, D.R. Lowe / Earth-Science Reviews 76 (2006) 259–300

1. Introduction

Recent re-evaluation of the geologic record of theearliest life on Earth has led to suggestions that some ofthe oldest putative microfossils (Schopf and Packer,1987) and carbonaceous matter formed through abiotichydrothermal processes (Brasier et al., 2002; Garcia-Ruiz et al., 2003). Similarly, many early Archean chertshave been re-interpreted as hydrothermal exhalitesrather than products of normal marine sedimentaryprocesses (Paris et al., 1985; Westall et al., 2001; Brasieret al., 2002). This controversy, together with newquestions about the biogenicity of isotopically lightcarbon in ∼3.8 Ga Isua rocks (van Zuilen et al., 2002),has cast a haze on the earliest history of life.

The difficulty at the root of the problem of ancientlife detection in general is that there is no “vital force,”i.e. there is, in principle, no biological product whichcannot be produced abiotically. Therefore, there is no setof measurements which could definitively distinguishbiological from abiological materials. While thistheoretical statement is stretched to the point of breakingwhen applied to well-preserved metazoan fossils orcomplex organic materials (e.g. ribosomes), it takes onparticular force when considering relatively homoge-neous carbonaceous matter (CM) in metamorphicterrains or hypothetical steps in the transition fromprebiotic to biotic systems.

There have been three major recent approaches inidentifying biological carbonaceous matter in ancientrocks. (1) Researchers looked for CM having an isoto-pic composition less than about −15‰ vs. PDB (the“isotopic” approach). Such fractionation was believed toreflect a kinetic isotope effect associated with enzymaticprocessing of carbon. (2) Researchers sought to testcarbonaceous matter or associated deposits againstpredetermined lists of biogenicity criteria (the “list ofcriteria approach”). Each criterion was designed to eitheridentify features likely to be produced by living orga-nisms or unlikely to be produced by abiotic processes.(3) Most recently, Brasier et al. (2002, 2004) have sug-gested that the search for early life would best proceed bysystematically testing competing abiotic hypotheses (the“falsification” approach).

The “isotopic” approach, represented in the work ofSchidlowski (1988, 2001) and Mojzsis et al. (1996),finds greatest prominence in cases where intensemetamorphism and deformation have erased potentialtextural and morphological evidence. Sufficiently largedepletions of 13C are identified with not only a biologicalorigin, but with specific enzymes associated with knowncarbon fixation pathways. Discovery of abiotic process-

es leading to similar isotopic fractionations (Horita andBerndt, 1999; van Zuilen et al., 2002) has called the moststraightforward applications of this approach intoquestion.

The “list of criteria” approach is exemplified bySchopf and Walter (1983) and Buick (1984). Thebiogenicity criteria for microfossils proposed by Schopfand Walter (1983) are here analyzed as representative ofthis approach. Each criterion is classified as either apositive test (one which some or all true microfossilsshould pass), a negative test (one which some or all falsemicrofossils should fail), or both.

1. True microfossils should “be of relatively abundantoccurrence” and “be members of a multi-componentbiologic assemblage.” This criterion is a positive testof biogenicity; Schopf and Walter (1983) make animplicit comparison to modern microbial populationsand apply some assumptions about preservation tomake a prediction about fossil microbes. Someabiotic products could pass this test and some truemicrofossils could fail it, but most true microfossilsshould pass it.

2. True microfossils should “be of carbonaceouscomposition or, if mineralic, be a result of biolog-ically mediated mineral encrustation or a product ofmineral replacement.” This criterion is anotherpositive test of biogenicity. It is potentially morestringent than the first criterion: although someabiotic processes produce CM, all microbial fossilsshould start out as CM.

3. True microfossils should “exhibit biological mor-phology—be characterized by a range of variability,including life-cycle variants, comparable to thatexhibited by morphologically similar modern and/or fossil microorganisms.” The nature of thiscriterion depends on the structure analyzed. A sphereis a biological morphology, for instance, but it is alsoan extremely simple shape that could potentiallyresult from a host of abiological processes. In thissense, this criterion is a stringent positive test thatnearly all microbial fossils should pass but whichmany potential abiotic products could also pass. Onthe other hand, internal membranous structures suchas nuclei characterize only a subset of knownmicrobes, but are unlikely to be produced in abioticstructures. In the case of this biological morphology,this criterion functions as both a positive and anegative test.

4. True microfossils should “occur in a geologicallyplausible context.” This criterion functions mostly asa negative test. It eliminates, for instance, misleading

261M.M. Tice, D.R. Lowe / Earth-Science Reviews 76 (2006) 259–300

structures in highly metamorphosed rocks or carbo-naceous linings in cross-cutting hydrothermal veins.

5. True microfossils should, “to the extent feasible(depending on existing data), fit within a well-established evolutionary context.” This criterionfunctions as a caution against apparent microfossilssignificantly more complex than known microfossilsof the same age.

6. True microfossils should “be dissimilar from poten-tially coexisting abiological organic bodies.” Thiscriterion functions explicitly as a negative test. Carbo-naceous products of known abiotic processes fail thistest, whereas not all true microfossils would pass it.

The “list of criteria” approach thus applies both posi-tive and negative tests of varying strength to the problemof biogenicity. Structures satisfying all criteria arelabeled “probable microfossils,” and structures satisfy-ing most criteria are labeled “possible microfossils.”

The “list of criteria” approach is fundamentally de-signed to filter a small number of convincingly biolo-gical structures from a large number of potentiallymisleading abiological structures. As such, it is likely tobe helpful in the analysis of material from geologicterrains in which diagenetic and metamorphic alterationis minor enough to allow for preservation of abundantfine-scale carbonaceous structures, and which representdepositional environments likely to allow taphonomicpreservation of pristine fossils. Unfortunately, suchterrains become increasingly sparse toward the earlypart of the preserved geologic record, and are exceed-ingly rare in the critical early-to-middle Archean. Thegreat bulk of carbonaceous material in this interval isrelatively structureless, and candidate structures for the“list of criteria” approach are correspondingly rare. Thisdoes not imply that the search for evidence of early life isdestined to fail in N3.0 Ga metamorphic terrains, nor thatthe “list of criteria” approach has no value for analyzingputative microfossils, but that another approach must beused to analyze the most ancient available material.

The “falsification” approach proposed by Brasier etal. (2004) is less an independent approach than a critiqueof the “list of criteria” approach. The criteria approach iscriticized as proceeding primarily by deduction andinappropriate comparison to modern organisms, withoutserious consideration of alternative abiotic hypotheses.Brasier et al. (2004) suggest that a more falsificationistapproach would be appropriate, and that investigationmust proceed by testing the null hypothesis of abiolo-gical origins for relevant structures and material. Yet the“list of criteria” approach explicitly includes negativetests designed to falsify known abiotic hypotheses. Thus,

the falsificationist critique is probably best viewed as avaluable re-evaluation of the actual practice of investi-gators following the “list of criteria” approach and of thebreadth of abiotic hypotheses tested.

In this sense, the “falsification” approach is subject toits own criticism of the “list of criteria” approach. Thispoint is best seen when it is realized that the “nullhypothesis” of abiological origins is effectively an infi-nite set of hypotheses. No criteria have been proposed bywhich these endlessly possible hypotheses can benarrowed down to finite sets of practically testable hy-potheses, so it is not clear that testing of any number ofspecific “null hypotheses” will ever be enough to clearlyestablish the past existence of life from geologicalevidence. In fact, it is generally true of historical hypo-theses that the number of possible explanations forinteresting geological phenomena is limited only by theimaginations of the investigators. It is for this reason thatgeologists and other historical scientists typicallyproceed by searching for “smoking guns,” pieces ofevidence so characteristic of one particular hypothesis asto make invocation of other hypotheses superfluous(Cleland, 2001). In the case of testing for early life, thisapproach would amount to searching for a uniquefingerprint of life in the early geologic record.

Unfortunately, no such smoking gun or fingerprint iscurrently known. As already discussed, carbon isotopicfractionation is not unique to life (Horita and Berndt,1999; van Zuilen et al., 2002). Despite recent sugges-tions (Schopf et al., 2002), Raman scattering spectra arenot useful for unique identification of biologicallyproduced CM (Pasteris and Wopenka, 2003). Identifi-cation of carbonaceous filaments is not necessarilysufficient for the identification of microfossils (Garcia-Ruiz et al., 2003). Multiple supporting lines of evidencemust therefore be employed, each one incrementallydecreasing the likelihood of abiotic hypotheses andincreasing the likelihood of a biotic hypothesis. This isessentially the procedure embodied by the “list of cri-teria” approach, although such lists as of yet have hadonly limited applicability (spectacularly preserved mi-crofossils and stromatolites, both exceedingly rare priorto 3.0 Ga). It is less clear how to proceed in investigatingthe CM found abundantly in N3.0 Ga rocks. It is evenless obvious how to treat evidence that is necessarily lesscompelling than the idealized “smoking gun” in light ofcurrent debates.

1.1. Reframing the debate

We must have a way of approaching the problem ofearly life that respects the nature of historical science,


respects the current lack of any single smoking gun, andis more generally applicable to the sparse materialsavailable for study N3.0 Ga. Recognizing the need for anew approach does not require that we disregard pre-vious results, however. On the contrary, we must takeaccount of what is already known about the very earlygeologic record to place the debate in context.

Detection of past life on Earth is frequently seen as ananalogous problem to detection of past life on Mars, afair comparison since Martian paleobiologists will workwith many of the same materials as terrestrial paleobiol-ogists (e.g., McKay et al., 1996). But, like all analogies,it has its limits. It has been suggested, for instance, thatwe should be as skeptical of evidence for early Archeanterrestrial life as of evidence for ancient Martian life(Brasier et al., 2004). It is a scientific truism that “extra-ordinary hypotheses require extraordinary evidence.”Given our current state of knowledge, is the hypothesisof early Archean life on Earth really as extraordinary asthe hypothesis of early life on Mars? Or put in thelanguage of Bayesian analysis (see Jefferys and Berger,1992, for a readable discussion of Bayesian analysis),should the hypotheses of early Archean life on Earth andearly Martian life be assigned similar prior probabilities,a measure of relative confidence in a hypothesis givenknown data, relative to competing abiotic hypotheses?

At a very basic level, the answer has to be no. Life isknown to have evolved on Earth; it is yet to be deter-mined if life ever evolved on Mars. On this basis alone,the prior probability that life was present on Earth atnearly any given point in the past must be consideredgreater than the prior probability that life was present onMars at any given point in its history. Just how muchgreater for specific points in time, such as the earlyArchean, remains to be seen. In other words, on Earth itis legitimate to rephrase the life-detection question as“How far into the past does the record of life extend?”Such a question would be meaningless on Mars.

How should what is known about geological CM setthe stage for discussions of specific new data relevant tothe detection of early Archean life? We suggest that therecord of CM may be evaluated for continuity ordiscontinuity in four properties: (1) CM molecular orcrystalline structure; (2) CM elemental and isotopiccomposition; (3) CM distribution in rocks formed underdifferent conditions; and (4) CM abundance in rocks. Iflife had originated at some point in time represented inthe geologic record, we might expect to see some sort ofbasic shift in the record of geologic CM. For instance,since prebiotic processes of CM formation are unlikelyto have been as productive as later biological processes,it is possible that less CM would be found in ancient

rocks overall than in younger rocks. Since such prebioticprocesses may have been primarily associated withcertain environments, prebiotic CM might have beenenvironmentally restricted in ways not found in youngerrocks. Prebiotic processes might be recorded in CMhaving differing molecular structure or compositionfrom later biological CM. In contrast, if a globe-encompassing biota was present during deposition ofthe entire geologic record, it seems likely that many ofthese properties would exhibit continuity over time.

1.1.1. Carbonaceous matter structureLaser Raman spectroscopy and XRD studies (Hayes

et al., 1983; Wedeking and Hayes, 1983; Brasier et al.,2002; Schopf et al., 2002; Tice et al., 2004) indicate thatearly Archean CM belongs to a structural class of car-bonaceous compounds termed “graphite-like carbon” byPasteris and Wopenka (2003). This classification isbased on the predominance of sp2 C–C bonds, distin-guishing this material from other insoluble carbonaceousmatter dominated by sp3 C–C bonds (“diamond-likecarbon”). Graphite-like carbon can exhibit a range ofstructural order, from disordered CM to fully orderedgraphite (Pasteris and Wopenka, 2003). Continuing withthe terminology of Pasteris and Wopenka (2003), CMfrom the 3.5–3.2 Ga Barberton greenstone belt and the3.5–3.3 Ga Pilbara Block is classified as “transitional tographite,” a level of order structurally intermediate tothese two endmembers. Since graphite-like carbon canbe produced by abiological processes as well as thermalalteration of biological materials, structural informationcurrently available for early Archean CM alone is notuseful for determining its origins.

The degree of structural ordering in graphite-likecarbon as reflected in its Raman scattering spectra ispotentially informative about the thermal history of thematerial, however. In particular, disordered CM hostedin rocks metamorphosed to prehnite–pumpellyite faciesor higher undergoes a characteristic loss of non-carbonatoms (e.g. hydrogen) and organization of aromaticcomponents into increasingly large graphitic domains(Wedeking and Hayes, 1983), all reflected in Ramanspectral characteristics (e.g. Wopenka and Pasteris,1993; Yui et al., 1996). It is therefore significant thatCM hosted by cherts of the Barberton greenstone beltyield spectra consistent with lower greenschist grademetamorphism (Tice et al., 2004), in agreement withchlorite geothermometry of associated volcanic rocks(Xie et al., 1997). In a more qualitative sense, Buseck etal. (1988) used HRTEM (High Resolution TransmissionElectron Microscopy) to demonstrate that PrecambrianCM exhibits a continuum of structural order. The least

Fig. 1. Precambrian CM N/C ratios. (A) All samples from Hayes et al.(1983) and Strauss and Moore (1992). There is an apparent increase inN/C beginning at about 2.0 Ga. (B) Samples from (A) with H/C b0.3,controlling for thermal alteration. Except for one carbonate-hostedsample near 0.6 Ga, N/C is similar for samples of all ages.


crystalline material studied was essentially structurelessCM from relatively unmetamorphosed Neoproterozoicand Mesoproterozoic terrains, followed by transitionalCM from the greenschist-metamorphosed Barbertongreenstone belt. The most crystalline material was fullyordered graphite from the amphibolite-metamorphosedearly Archean Isua Sequence.

While this continuity does not rule out most abioticorigins for early Archean CM, it does preclude thoseprocesses that deposit fully crystalline graphite or anyCM of significantly higher structural order. It would alsobe remarkably coincidental if all early Archean CM wasproduced by abiotic processes with direct productshaving crystallinity similar to greenschist CM, such asprecipitation from high-temperature methane-rich fluids(Pasteris and Chou, 1998). It is most likely that at leastsome and probably most early Archean CM, like CM inyounger greenschist terrains, originated as less orderedmaterial.

1.1.2. Carbonaceous matter compositionCM stored in sedimentary rocks 3.5 Ga and younger

has carbon isotopic compositions almost universallybetween −15‰ and −35‰ vs. PDB (Schidlowski,1988, 2001). Schidlowski (2001) even suggests that theaverage isotopic composition of sedimentary CM hasvaried by no more than about 5‰ over the last3.5 billion years. Although it is now recognized thatsuch carbon isotopic fractionation can be produced bypurely abiotic processes (Horita and Berndt, 1999; vanZuilen et al., 2002), the apparent continuity of the earlyArchean record with later times when CMwas producedprimarily by biological processes is impressive.

CMN/C ratios show an apparent increase beginning atabout 2.1 Ga (Fig. 1). However, almost all of this increaseis probably due to better preservation of young CM.Comparing only CMwith H/Cb0.3 to control for thermalalteration reveals almost no significant variation in N/Cduring the Precambrian. It is most likely that CM depo-sited N2.1 Ga originally had N/C ratios higher than arecurrently preserved. Again, while it would be naïve tosuggest that significant primary quantities of nitrogen inancient CM implies a biological origin, the apparentcontinuity of the compositional record must ultimatelyplace constraints on any abiological hypotheses proposedas an explanation for early Archean sedimentary CM.

1.1.3. Carbonaceous matter distributionOne of the most basic observations that can be made

about 3.5–3.0-Ga CM is that it, like nearly all youngerCM, is found almost exclusively in sedimentary rocks.CM is rare in igneous rocks. This is true even in

thoroughly serpentinized ultramafic rocks, where oxi-dation of olivine to magnetite would have provided themost likely driver for the Fischer–Tropsch-type synthe-sis reactions favored by Brasier et al. (2002) as the sourcefor their hypothesized hydrothermal organic matter.

Within 3.5–3.0 Ga sedimentary rocks, CM occurs infacies deposited in paleoenvironments including shallowevaporitic lagoons (Barley et al., 1979; Lowe, 1983;Buick and Dunlop, 1990; Lowe and Fisher Worrell,1999), current-active platform settings (Lowe, 1999),and basin settings below storm wave base (Lowe, 1999).CM-rich sediments were deposited atop felsic, mafic,and ultramafic volcanic rocks (Lowe, 1999). In general,CM appears to have been a ubiquitous component ofclastic-poor marine sediments, much as it was in youngersedimentary sequences.

1.1.4. Carbonaceous matter abundancePerhaps the most notable characteristic of the distri-

bution of CM abundance in early Archean sedimentary


rocks is how unremarkable it appears in comparison toyounger distributions (Fig. 2). Even without controllingfor metamorphic alteration or lithology, early ArcheanCM abundances fall well within the range of abundancesobserved in younger rocks. Average CM abundancesin rocks of all lithologies N3.0 and b3.0 Ga havingH/Cb0.3 are statistically indistinguishable. The geolog-ic record of CM abundance therefore exhibits funda-mental continuity at least as far back in time as ∼3.5 Gaand possibly as far as 3.7 Ga.

1.1.5. Continuity in the carbonaceous matter recordAlthough the continuity of the geologic CM record is

not a strong evidence for the emergence of life by3.5 Ga, it is at least striking that a broad view of therecord provides no compelling motivation to considerabiological origins. Indeed, while the record permitsabiological hypotheses, it is difficult to conceive ofabiotic processes capable of generating a record

Fig. 2. Precambrian CM abundance in sedimentary rocks. (A) Allsamples from Strauss and Moore (1992), this study, and Rosing(1999). Samples N3.0 Ga have similar abundances to samplesb3.0 Ga. (B) Samples from (A) with H/C b0.3, controlling forthermal alteration. Abundances in samples N3.0 Ga are statisticallyindistinguishable from abundances in samples b3.0 Ga.

essentially identical to that formed by biologicalprocesses in younger strata. For instance, it is not atall clear that a primarily hydrothermal source could haveproduced CM in the quantities and distribution found inthe early record, or that the CM produced wouldconsistently have isotopic compositions in the rangeobserved. It is more plausible that a global atmosphericphotochemical source in an atmosphere with a high C/Oratio could have replicated the quantities and distribu-tion of CM in the early record (Tian et al., 2005), but it isnot yet known if the isotopic record would be replicated.Moreover, the same photochemical source would haveproduced a dense hydrocarbon haze resulting in a stronganti-greenhouse effect and a cold early Earth (Pavlov etal., 2001b), inconsistent with evidence for a hot climatebetween 3.5 and 3.2 Ga (Knauth and Lowe, 2003). Atpresent there is no better explanation for the early CMrecord than that life had emerged by at least 3.5 Ga.

Such reasoning from the geologic record provides noproof that like had evolved by 3.5 Ga, nor is it intendedto. Instead, we suggest that such reasoning about thegeologic record of CM in general must frame necessarydebates over the origin of particular pieces of N3.0 GaCM. In particular, we propose that the best workinghypothesis based on knowledge currently available isthat most if not all carbonaceous matter present in rocksolder than 3.0 Ga was produced by living organisms. Wejudge this hypothesis to be more likely than null hypo-theses postulating an abiotic origin for all CM older than3.0 Ga. The emergence of life before 3.0 Ga thereforeshould not be regarded as an extraordinary hypothesis,and at the least should not be considered as of similarprobability to the hypothesis that life existed on Mars atsome point in its history.

1.2. A geological approach

In light of what is currently known about the earlygeologic record, we suggest that future studies focusmore generally on developing models describing theorigins of CM in particular geologic units. Because thereis not currently a “smoking gun” associated with CM bywhich we can definitively determine biogenicity, suchmodels must ultimately be judged by how coherentlythey account for all CM in the study material in terms ofprocesses operating in the inferred depositional envir-onments of the host rocks. Environmental reconstruc-tion is key: given the large number of possibleexplanations for ancient CM, comprehensive deposi-tional models must be used to eliminate physicallyimplausible hypotheses from a number of physicallypossible mechanisms of formation.


Two prominent critiques of previous Archean paleo-biological work have gained significant support in partthrough re-evaluations of depositional models. Thereinterpretation of Apex Chert “microfossils” as abiotichydrothermal precipitates is supported in part by theinference that the host rocks are part of a hydrothermalvein (Brasier et al., 2002, 2005). Isotopically lightgraphite grains in the Isua Sequence (Mojzsis et al.,1996; Schidlowski, 2001) are of questionable biologicalorigin because the enclosing rocks appear to be metas-omatically altered volcanic rocks rather than sediments(Rosing et al., 1996).

Unfortunately, much recent discussion of earlyArchean rocks has been clouded by testing betweendepositional models that are only implicitly stated andrepresent an inadequate range of alternatives for des-cribing the likely complexity of actual surface environ-ments on the early Archean earth. For instance,hydrothermal origins for early Archean cherts havebeen inferred from their geochemical similarity to mo-dern hydrothermally deposited sediments or hydrother-mal fluids, such as a slightly positive europium anomaly,the absolute abundances and relative ratios of heavymetals, or correlations between heavy metals and ironabundances (Sugitani, 1992; Kato and Nakamura, 2003).However, these similarities could also have resulted fromprecipitation in a normal marine setting, physically farremoved from any local hydrothermal source, in anocean compositionally controlled by hydrothermal input(Veizer et al., 1989). It has also been suggested thatpervasive early silicification of sediments requiredhydrothermal fluids as a silica source (Westall et al.,2001). This suggestion ignores the possibility thatnormal marine water was saturated with respect toamorphous silica in the Precambrian (Siever, 1992;Lowe, 1999). While observations such as these areinformative about the composition of fluids involved inprecipitation and diagenesis of these rocks, they contri-bute little to discrimination between hydrothermalsettings and normal sedimentary marine environmentsin an ocean compositionally similar to hydrothermalfluids.

In this study, “hydrothermal system” will be used torefer specifically to an environment in which sedimen-tation and early diagenesis are controlled by precipita-tion from emerging, subsurface hydrothermal fluids dueto chemical saturation induced by decreasing tempera-ture or pressure, or by mixing with ambient surfacefluids to form insoluble precipitates (ex. ferric hydro-xides, barite, sulfides, etc.). Once hydrothermal fluidshave mixed significantly with marine fluids and pre-cipitation is no longer controlled by these processes,

deposition is considered to occur in a marine sedimen-tary, not hydrothermal, environment. A modern analogillustrates the need for such a distinction. While mostmarine dissolved calcium is derived from continentalweathering, carbonate reefs are not generally classifiedas “continental deposits.” Such a classification would beonly minimally informative as to the physical andchemical environments in which reefs actually form.

In the stratigraphic record, this division betweenmarine and hydrothermal systems may be expressed in anumber of ways. Mixing of hydrothermal and marinefluids would result in geochemical trends identifiable insuites of precipitated materials. Mixing of hydrothermaland marine fluids and accompanying mineralization ge-nerally occurs within a short distance of the hydro-thermal source, resulting in deposition of vent stocks,chimneys, and mounds (Hannington et al., 1995).Deposits of limited aerial extent (hundreds to a fewthousands of meters) result from this restriction and thegeologically brief periods of typical vent activity(Hannington et al., 1995). Deposits would be expectedto interfinger with and grade into normal marine or non-marine sediments. Internal facies changes would reflectprogradation of mounded deposits and/or debris apronsof hydrothermal precipitates. Interpreting rocks as hy-drothermal deposits requires identification of featuressuch as these consistent with precipitation from mixing,cooling, or depressurizing fluids.

2. The Buck Reef Chert as a test case

2.1. Suitability of the Buck Reef Chert

The 3416 Ma Buck Reef Chert (BRC) is the basalmember of the Kromberg Formation in the OnverwachtGroup of the Swaziland Supergroup, South Africa(Fig. 3). It consists of 250–400 m of carbonaceous andferruginous chert exposed continuously along N30 km ofstrike in the west limb of the Onverwacht anticline,discontinuously in the east limb of the Onverwachtanticline and in the Kromberg syncline, and locally about50 km to the northeast in Swaziland (Lowe and FisherWorrell, 1999). At its base, the BRC interfingers with thefelsic volcaniclastic sandstone of the underlying memberH6 of the Hooggenoeg Formation. In the central part ofthe west limb of the Onverwacht anticline, this sandstonehas been interpreted as coastal and braidplan deposits(Lowe and Fisher Worrell, 1999). The lowest 0–80 m ofthe BRC, including lenses of chert interbedded withfelsic volcaniclastic sediments of the top of H6, containsilicified evaporites (Lowe and Fisher Worrell, 1999).The overlying 200–300 m of carbonaceous and

Fig. 3. Location maps. (A) General map of South Africa showing location of Barberton greenstone belt. (B) Map of the southern part of the Barbertongreenstone belt showing outcrops of the Buck Reef Chert (BRC). Principal outcrops lie around the Onverwacht anticline (OA) and the Krombergsyncline (KS). (C) Simplified stratigraphy of the Onverwacht Group (dark gray) and Fig Tree Group (light gray) in the southern domain of theBarberton greenstone belt. Section height above the base of the Komati Formation indicated on the left. Note scale change above KrombergFormation. BRC is the basal unit of the Kromberg Formation. (D) Map of the BRC in the central part of the west limb of the Onverwacht anticline.Measured sections (Fig. 4) are indicated by thick lines at A and B. Qc = Quaternary cover; fi = felsic intrusive rock; ev = evaporite and black chertfacies; bwc = black-and-white banded chert facies (both contorted and laminated); bfc = banded ferruginous chert facies.


ferruginous cherts of the BRC shows a progressive up-ward transition from current-worked, particulate carbo-naceous detritus into finely and continuously laminatedunits, suggesting a transition to deeper water. Carbona-ceous cherts from unspecified locations in the BRC have

yielded possible microfossils and preserved microbialbiofilms (Westall et al., 2001). The abundance of poten-tially biological carbonaceous material, together withorthochemical deposits and features suggesting well-developed transitions from evaporitic to shallow


platformal to deep-water depositional environmentsmakes the BRC an ideal unit for examining the relat-ionship between carbonaceous matter abundance andmorphology and depositional conditions and environ-ment, and for possibly establishing the root origins andcontrols on the distribution of CM in these ancient rocks.

2.2. Geologic setting

The stratigraphy of the Barberton greenstone belt hasbeen summarized by Lowe and Byerly (1999). The

Fig. 4. Measured sections through the Buck Reef Chert. See Fig. 3 for locatiosilicified evaporites, evaporite solution collapse features, and wave ripples; cnferruginous banded chert; bwsf = interstratified black-and-white banded cherevaporite and black chert facies; LBW = lower black-and-white banded chebanded ferruginous chert facies. Dark gray layers are mafic intrusive rocks. T

Swaziland Supergroup is divided into the basal, predo-minantly volcanic Onverwacht Group and the succeed-ing sedimentary Fig Tree and Moodies Groups. Aroundthe Onverwacht anticline, the two lowest units of theOnverwacht Group, the Theespruit and Sandspruit For-mations (Viljoen and Viljoen, 1969), are in fault contactwith the rest of the group or occur only as isolatedxenoliths in surrounding plutons, respectively. The otherfour formations of the Onverwacht Group (Komati,Hooggenoeg, Kromberg, and Mendon Formations) andthe Fig Tree Group form a continuous stratigraphic

ns. ss = current deposited felsic sandstone of H6; ev = black chert withg = conglomerate; bwc = black-and-white banded chert; sfbc = slightlyt and slightly ferruginous chert; bfc = banded ferruginous chert. EV =rt facies; UBW = upper black-and-white banded chert facies; BFC =hin horizontal lines along left of columns indicate laminated intervals.


sequence (Fig. 3C). The Komati Formation is a 3.7-km-thick accumulation of komatiitic volcanic rocks with nomajor sedimentary units. The Hooggenoeg Formation,3.8 km thick, consists predominantly of units of basalticand komatiitic volcanic rocks capped by thin sedimen-tary units. The formation is capped by member H6, acomplex of shallow dacitic intrusions, flow rocks, andvolcaniclastic units that was emplaced and erupted at3445±3 Ma (Kröner et al., 1991).

The overlying Kromberg Formation includes∼1.7 km of mostly mafic volcanic and volcaniclasticrocks with the BRC at its base. A thin detrital layer at thebase of the BRC has yielded a single zircon age date of3416±5 Ma (Kröner et al., 1991). A felsic tuff in theFootbridge Chert at the top of the Kromberg Formation,1.3 km above the BRC, has been dated at 3334±3 Ma(Byerly et al., 1993, 1996). The overlying MendonFormation, about 0.3–1 km thick, is composed of cyclesof komatiitic volcanic rocks capped by thin chertysedimentary units.

2.3. Materials and methods

A 220-m-thick section of the BRC was measured onthe central west limb of the Onverwacht anticline (Fig.4). A total of 46 samples was collected for slabbing andthin-sectioning. 22 of these samples, along with 13supplementary samples collected in a smaller section1.4 km to the west, were analyzed for major and traceelement abundances by X-ray fluorescence at theWashington State Geoanalytical Laboratory, Pullman,Washington. 19 samples were analyzed for total organiccarbon and δ13CCM at the Stanford University StableIsotope Laboratory. Photomicrographs of more than 400carbonaceous and mineral grains were collected andused to establish a morphological and compositionalclassification scheme of grain types for point-counting.Thin sections of 38 relatively unweathered samples werepoint-counted, including five samples from the evapo-ritic facies, 18 samples from the carbonaceous cherts ofthe lower BRC, and 15 samples from visibly ferruginouscherts of the upper BRC. Principal component analysisof point-count data (Wackernagel, 1995) was used todefine distinct groups of grain and texture assemblages,or microfacies.

In order to identify opaque materials and mineralgrains too small to identify optically and to distinguishbetween different carbonate minerals, mainly calcite,dolomite, and siderite, Raman spectra were collected insitu from polished thin sections. The instrument usedwas a Kaiser Hololab D5000 Raman microscopeequipped with a 785 nm diode laser oriented normal

to the sample. This instrument had a spot size of ∼1 μmwhen focused through a 100× objective lens, aneffective 4000 channels, and 4 cm−1 resolution. Anaverage power of ∼40 mW was applied at the samplesurface. Spectra were typically collected for 100 s orlonger to obtain acceptable signal-to-noise ratios.Spectral features were interpreted by comparison withknown reference materials including disordered carbo-naceous matter, quartz, calcite, dolomite, magnesite,ankerite, and siderite.

2.4. Lithofacies of the Buck Reef Chert

Along the west limb of the Onverwacht anticline,the BRC includes four main lithofacies (Fig. 4): (1) abasal silicified evaporite and black chert facies 0–80 mthick that interfingers with the underlying felsicsandstone of the Hooggenoeg Formation (Lowe andFisher Worrell, 1999); (2) an overlying lower black-and-white banded chert facies up to 60 m thick; (3) anupper black-and-white banded chert to slightly ferrugi-nous chert facies about 100 m thick; and (4) an upperbanded ferruginous chert facies 50–100 m thick. Acapping unit, up to 60 m thick, of black-and-whitebanded chert was not studied.

2.4.1. Evaporitic facies

2.4.1.1. Description. The silicified evaporite facies ofthe BRC was described by Lowe and Fisher Worrell(1999). It is composed of laminated and wave rippledchert (Fig. 5A), silicified evaporitic layers originallycomposed of nahcolite (NaHCO3, Fig. 5B) andevaporite solution and solution collapse layers. Largesolution cavities may be filled with megaquartz, massiveblack chert, or locally cave-type formations, includingsilicified geopedal soda straws (Fig. 5C). Wave rippleshave small, ∼20 cm, wavelengths, indicating formationin shallow water. They are defined by interlayered thin,lenticular black-and-white layers (Fig. 5A).

2.4.1.2. Interpretation. Lowe and Fisher Worrell(1999) interpret the volcaniclastic sands of H6 asbraidplan and coastal deposits, and the evaporite faciesas the deposits of shallow protected coastal lagoons andevaporitic brine ponds. Evaporite crystals grew duringwetting and drying cycles. Wave ripples with wave-lengths as short as those observed in this facies typicallyindicate deposition under less than a meter of waterdepth (Evans, 1942). Evaporite solution features reflecta period of exposure and evaporite dissolution. Sodastraws representing hollow stalactites also reflect

Fig. 6. Soft-sediment deformation features in black-and-white bandedchert of lower black-and-white banded chert facies. (A) White bandsshowing periodic disruption and soft-sediment foundering in a matrixof deformed laminated black chert. (B) White chert plate breccia in amatrix of black chert. Pens are 15 cm long.

Fig. 5. Evaporite and black chert facies. (A) Wave ripples (arrows) insilicified sediments of the evaporite facies. (B) Upward-radiatingsilica-replaced evaporite crystals (a) cutting across and draped bylaminated chert (b). C) Quartz-filled soda straw structures developedduring evaporite solution events.


exposure diagenesis in the vadose zone (Esteban andKlappa, 1983).

2.4.2. Lower black-and-white banded chert facies

2.4.2.1. Description. The contact between the evapo-rite and the overlying black-and-white banded chertfacies is marked by a thin, 50–100-cm-thick, regionally

developed clast-supported conglomerate composed ofclasts of silicified komatiite, black-and-white bandedchert, silicified felsic volcaniclastic sandstone, cleartranslucent silica, and cavity-fill quartz in a matrix ofmicroquartz. The base of the conglomerate is locallyscoured.

The black-and-white banded chert facies crops out forN50 km along strike and is composed largely of bands ofblack carbonaceous chert b1 to∼15 cm thick alternatingwith bands of pure, white-weathering, translucent chertfrom 1 mm to 10 cm thick (Fig. 6A, B). Black and whitebands form subequal parts of the rock. Slightly weath-ered black bands display massive to crudely laminatedlayers of sand and granule size particles. In the lower∼60 m of this facies, major disrupted units of black-and-white banded chert are interbedded with intact layers. Inthe disrupted units, white bands are disrupted to formrounded or contorted masses (Fig. 6A) or angular plates


(Fig 6B). These masses and plates float in a black chertmatrix. Some white bands show plastic deformation anddisruption but little overall displacement. Other massesare thoroughly mixed. Black material flowed plasticallyaround disrupted chunks of white chert precursor. Roundand contorted masses of white chert are most common inthe lower part of this zone, while plates are more com-mon in the upper part.

Coarse megaquartz-filled cavities are widely devel-oped in the lower black-and-white banded chert facies.In undisrupted units, cavities are stratiform and mostunderlie white bands. In disrupted units, cavities aretypically lenticular, bounded above and on the sides bywhite chert plates or masses (Fig. 7).

2.4.2.2. Interpretation. The regional extent of thebasal conglomerate, its erosive contact with the under-lying evaporite unit, and the lack of similar conglom-erates throughout the rest of the unit suggest that itmarks an unconformity. It is most likely a transgressivelag formed in the high-energy wave-active zone andstranded during marine flooding. The scoured base mayhave formed during a period of exposure, possiblyduring the time that evaporite solution collapse featuresand related structures developed in the underlyingevaporitic unit.

Scour, cross-bedding, and other evidence of high-energy current activity are absent in the overlyingblack-and-white banded chert. Black bands containabundant carbonaceous grains up to 3–5 mm indiameter, but no sand-sized detrital volcaniclastic

Fig. 7. Geopedal megaquartz-filled cavity (a) underlying a deformedwhite plate (b). Druzy quartz fills cavities formed by escape of buoyantfluids, probably water, from still fluid black chert. Rising fluid waslocally trapped beneath impermeable layers and plates of white chert.Hammer handle is 20 cm long.

grains. The absence of hydraulically coarse sedimentmakes it unlikely that this environment was subject toany vigorous wave or current activity, which wouldhave suspended and transported the sand-sized, low-density carbonaceous material. Deposition was outsideof the high-energy beach or near-shore environmentthat might be predicted at this point in stratigraphy byrelationship to the underlying evaporitic facies.Instead, any high-energy near-shore environmentsare probably represented by the underlying unconfor-mity and conglomerate. Water depth was probablyN∼15–20 m, the depth to which average wavesgenerate cross-bedding and scour in the modern ocean(Allen, 1970).

Most banded sediment was disrupted by early soft-sediment flowage and deformation to form brecciasoriginally composed of rigid plates to irregular softplastically deformed masses of white chert within a fluidmatrix of black chert. Soft-sediment disruption is inter-preted to reflect the effects of storm events, which set upinternal stresses and mixing within the still soft, gela-tinous silica-and organic-rich bottom materials (Lowe,1999). Modern storm waves can mobilize sediment toabout 200 m water depth (Komar et al., 1972); it is likelythat the lower black-and-white banded chert facies wasdeposited on a shelf under water depths between about15–200 m.

The consistent location of megaquartz-filled cavitiesbelow white chert bands and masses suggests that theseare geopedal features formed by fluid escape, either gasor water, after the white chert precursor was solid butbefore lithification of the black bands.

2.4.3. Upper black-and-white banded chert facies

2.4.3.1. Description. In the upper black-and-whitebanded chert and slightly ferruginous banded chertfacies, black bands are finely and evenly laminated andparticulate layers are rare (Fig. 8). Black and white bandsare b1 to 3 cm thick. Toward the top of this zone, blackbands take on a dull, slightly ferruginous appearance inoutcrop. White band disruption and brecciation andmegaquartz-filled cavities are less common than in thelower black-and-white chert facies.

2.4.3.2. Interpretation. The near absence of particulatelayers and soft-sediment disruption and brecciation inthe upper black-and-white banded chert facies reflectsdeposition in a very low-energy environment only rarelyaffected by currents, waves, or storms. The setting re-presented by this facies was near or just below stormwave base.

Fig. 9. Banded ferruginous chert. Note even banding and fine,continuous laminations. Pen is ∼10 cm long.

Fig. 8. Black-and-white banded chert of the upper black-and-whitebanded chert facies in which black bands are finely laminated andsome white bands consist of several thin, distinct layers or laminations.Black bands are slightly ferruginous. Hammer is 40 cm long.


2.4.4. Banded ferruginous chert facies

2.4.4.1. Description. The overlying banded ferrugi-nous chert facies is composed of alternating bands ofrelatively pure white-weathering chert, 1 mm to 2 cmthick, and dark rust-colored, iron-oxide-rich material,b2 cm thick (Fig. 9). The dark ferruginous bands arehighly weathered, and in places are completely replacedby boxwork masses of goethite or hydrous ferric oxide. Inless weathered examples, dark bands are finely laminatedand contain siderite. Subsurface samples of bandedferruginous chert contain siderite and no ferric minerals.Primary goethite is unlikely to have been preserved at the∼300 °C peak metamorphic temperatures experienced bythe BRC and throughout the rest of the Barbertongreenstone belt (Xie et al., 1997; Tice et al., 2004).Instead, primary goethite would today be represented byhematite, which is absent. The primary ferruginousmineral was most likely siderite that has now beenoxidized bymodernweathering (Lowe and Byerly, 2003).Band disruption and brecciation are rare to absent.

2.4.4.2. Interpretation. Like the upper black-and-white banded chert facies, the banded ferruginouschert facies was deposited in an extremely low-energyenvironment. The near absence of band disruption andparticulate layers and the ubiquity of fine laminationsimply deposition well below storm wave base in a deepbasinal setting. Sedimentation was by gentle settling offine material from the overlying water column.

2.5. Carbonaceous matter and other microfacieselements

Carbonaceous matter in the BRC is composed of sub-micron inclusions in a chert matrix. Raman spectralcharacteristics of BRC CM (Tice et al., 2004) areconsistent with organization into graphite crystalliteswith in-plane diameters of a few nanometers (Wopenkaand Pasteris, 1993). Each inclusion thus representsdisordered clumps of hundreds of millions of crystallites.

Inclusions are organized into micron to millimeterscale regions of concentrated CM and intergrown chert.At this scale, BRC CM occurs as discrete masses,laminations, networks, and diffuse masses. Walsh andLowe (1999) classified CM from throughout theBarberton greenstone belt and found that CM mor-phology correlates with depositional environment.BRC CM was reclassified for this study into fourmajor morphological groups (Fig. 10): carbonaceousgrains, laminations, networks and diffuse masses, andcavity fill CM.

2.5.1. Carbonaceous grains

2.5.1.1. Definitions. Four types of discrete carbona-ceous grains were identified in the current study: (Kgf)wispy grains with aspect ratios N10 (Fig. 11A), (Kgs)simple grains, (Kgl) grains composed of contortedcarbonaceous laminations, and (Kgc) compound grains.Kgf, Kgs, and Kgc grains correspond to grain types ofthe same names of Walsh and Lowe (1999). Kgs grainsare composed of one or two domains of concentratedCM (Fig. 11B), Kgc grains are composed of three or

Fig. 10. Flow chart for classifying Buck Reef Chert carbonaceous matter (CM) by morphology.


more domains of concentrated CM (Fig. 11C), and Kglgrains are composed of contorted carbonaceous lamina-tions (Fig. 11D).

2.5.1.2. Descriptions. Kgs, Kgl, and Kgc grains occurin massive and graded layers, generally mixed withdetrital, sand-sized volcaniclastic or silica grains. Theyrepresent detrital particles composed of organic matter.

Kgc grains show substantial variation in structure.Some are clearly recognizable as ripped up chunks ofcarbonaceous sediment, such as grains composed ofKlr network. Others are composed of multiplesmaller carbonaceous and silica grains bound byisopachous rims of silica. This subclass correspondsmost closely to the “lobate compound” class ofWalsh and Lowe (1999). Still other Kgc grains are

Fig. 11. Carbonaceous grains. (A) Kgf post-depositionally compacted carbonaceous grains (arrows) with high aspect ratios. Scale bar is 0.2 mm. (B)Kgs simple carbonaceous grain (arrow) with b3 internal clots or chunks of denser CM. Scale bar is 0.1 mm. (C) Kgc compound carbonaceous grainwith N3 internal zones of concentrated carbonaceous matter. Scale bar is 0.5 mm. (D) Kgl complex carbonaceous grain with N3 internal zones ofconcentrated carbonaceous matter and composed of contorted laminations. Scale bar is 4 mm.


composed of tightly packed smaller carbonaceousand silica grains bound by a diffuse carbonaceousmatrix, similar to material composing layers in theevaporite facies.

Kgl grains are composed of laminations or networkinterpreted below to represent microbial mats whenfound in situ. Kgf grains form layers in which their longaxes are aligned parallel to bedding.

2.5.1.3. Interpretations. Kgs, Kgl, and Kgc grains allappear to represent ripped up carbonaceous and sili-ceous sediment. Kgl grains most likely represent rippedup microbial mats. Kgc grains were ripped up from avariety of sediments, including microbial mats andpartially silicified detrital layers. Kgs grains aregenerally smaller than the other grain types with simplermorphologies that make their origins more difficult toinfer, but they may represent transported and brokenlarger rip up grains.

Kgf grains appear to represent soft carbonaceousgrains compacted by burial (Walsh and Lowe, 1999).

2.5.2. Carbonate, silicified carbonate, and carbonate/CM grains

2.5.2.1. Definitions. Four types of carbonate and re-placed carbonate grains are recognized in the presentstudy. Cp grains are small, ∼10 μm grains of siderite(Fig. 12A). Crh grains are larger, 10–200 μm well-formed rhombic siderite grains (Fig. 12B). Cp and Crhgrains are commonly replaced by goethite or hydrousferric oxide in surface samples as a result of modernsurface oxidation (Lowe and Byerly, 2003). Cg grainsare 1–2 mm, silica-replaced rhombic minerals (Fig.12C). KF grains are composed of siderite and diffuse CM(Fig. 12D). While KF grains are thus composite carbo-nate and CM grains, siderite is the major component sothey are here classified with carbonate grains.

2.5.2.2. Descriptions. Cp and Crh grains occur iso-lated within a chert matrix and form thin flat laminations,layers, and lenses. Particularly in the lower black-and-white banded chert facies, where carbonate grains are

Fig. 12. Carbonate, silicified carbonate, and carbonate/CM grains. (A) Fine Cp siderite grains. Scale bar is 30 μm. (B) Large Crh rhombic sideritegrain. These grains are frequently oxidized, forming goethite-filled rhombic cavities after siderite. Scale bar is 0.2 mm. (C) Cg grains. Quartz-filledrhombs after twinned dolomite(?). Scale bar is 1 mm. (D) KF grains (arrows). Silt-sized grains composed of disseminated siderite crystals (now in partoxidized to goethite and hydrous ferric oxide) and diffuse carbonaceous matter. Scale bar is 70 μm.


only a trace constituent of the rock, Cp and Crh grainstend to occur in thin laminae without associated carbo-naceous grains. In the upper black-and-white bandedchert and the banded ferruginous chert facies, wherecarbonate is commonly a major constituent of the rock,Cp and Crh grains are typically mixed with fine carbo-naceous grains although thin layers composed only ofcarbonate grains still occur. Cp grains are usually locallyof very uniform size. Neither type of grain was observedto have displaced or distorted neighboring carbonaceousgrains. Crh crystal margins are commonly corroded oretched. No crystallographic twinning was observed inCrh grains.

In contrast, Cg grains observed in this study havedense, black borders, probably representing CM dis-placed during crystal growth. One of four examplesobserved possessed a crystallographic twin, suggestingthat twinning was not uncommon in the replacedmineral.

2.5.2.3. Interpretations. Occurrences of Cp and Crhgrains isolated from CM indicate that the siderite was

formed by direct precipitation rather than by reductionof ferric oxides by organic matter. Moreover, thepaucity of clastic material throughout most of the BRCimplies that reduced iron was not supplied bymobilization within the sediment. Instead, the overly-ing water column must have been saturated withsiderite. There is no evidence that crystal growth withinthe sediment displaced CM, and etched faces on Crhgrains may actually suggest some degree of localundersaturation. It seems likely, therefore, that at leastsome siderite formed within the water column andconstituted part of a background hemipelagic rain.Concentration of siderite grains in thin laminae thatlack sand-sized detrital carbonaceous grains that arecommon in the lower black-and-white banded chertfacies could indicate that deposition of siderite wasslow, and that detectable abundances accumulated inshallow-water environments only during breaks in CMsedimentation. In contrast, it is not clear if siderite inKF grains was precipitated in a mobile or suspendedcarbonaceous grain, or if precipitation occurred in thesediment.

Fig. 13. Silica grains. (A) Sa grain composed of nearly puremicroquartz. Scale bar is 0.2 mm. (B) Sd grain. Silica graincontaining b50% concentrated carbonaceous matter. Scale bar is0.2 mm.

Fig. 14. Silica grains. (A) Sa grain with nearly pure silica core andlarge rim containing diffuse carbonaceous matter. Scale bar is 0.1 mm.(B) Very coarse Sa grain showing highly irregular, possibly corrodedboundary. Scale bar is 0.4 mm.


Cg grains are far less common than Cp, Crh, orKF grains. While no relict carbonate is present todirectly determine the original composition, it is likelythat Cg grains represent silica-replaced dolomite.Twinning is uncommon in siderite but common indolomite. Displacement of surrounding CM suggeststhat these grains precipitated diagenetically. If dis-solved calcium ultimately limited calcite and dolomiteprecipitation in the early Archean oceans (Grotzingerand Kasting, 1993; Lowe and Fisher Worrell, 1999),then formation of Cg grains may have been aresponse to transient local enhancement of calciumin pore fluids. Calcium depletion during later burialcould have resulted in dissolution and subsequentreplacement by silica.

2.5.3. Silica grains

2.5.3.1. Definitions. Two types of silica grains aredistinguished. Sa grains are composed almost entirely of

microquartz, although some contain extremely diffuseCM (Fig. 13A). Sd grains are microquartz grainscontaining regions of concentrated CM which compriseb50% of the grain (Fig. 13B).

2.5.3.2. Descriptions. Sa grains display a limitedrange of morphologies and compositions. Well-rounded grains are common and widespread. Theyoccur individually or associated with detrital carbo-naceous grains and commonly in graded layers.Others occur compacted in layers with Kgf grains.Rarer Sa grains display cores or rims containing CM(Fig. 14A), or have complex, wandering boundaries(Fig. 14B).

Sd grains are commonly associated with complexcarbonaceous grains (Kgl and Kgc) and are much lesscommon than Sa grains.

2.5.3.3. Interpretations. Sa grains represent relativelysoft, possibly gelatinous detrital siliceous sediment.

Fig. 15. Other grains. (A) P grain composed of disseminatedcarbonaceous matter and very fine phyllosilicates. Scale bar is0.2 mm. (B) Lv grain composed of very fine phyllosilicates, probablyafter feldspar or a volcaniclastic particle. Scale bar is 0.3 mm.


Some probably represent grains of silica gel eroded frompartially silicified sediment, although composite grainssuggest that carbonaceous matter accretion and silicaprecipitation occurred at least occasionally at thesediment surface. The few examples of Sa grains withwandering boundaries that have been identified wereassociated with Kn mat-like laminations, oftenappearing to rest at unstable angles on top of matsurfaces (Fig. 14B). It is possible that they wereoriginally formed as siliceous concretions withinmicrobial mats, and that their complex boundariesresult from aggregation in a diffusion-limited envi-ronment. It is also possible that they represent silicagrains deposited on mat surfaces which weresubsequently corroded.

Sd grains most likely represent ripped up chunks ofpartially silicified sediment. The rarity of this grain typerelative to Sa grains (nearly pure silica) and Kgc grains(mostly carbonaceous matter) suggests that segregationof predominantly carbonaceous and predominantlysilica sediment, possibly within black-and-white“proto-bands”, occurred at very shallow depths in thesediment column.

2.5.4. Other grains

2.5.4.1. Definitions. Four types of other grains weredistinguished in this study. P grains are aggregates ofdiffuse carbonaceous matter, silica, and very fine phyl-losilicates (Fig. 15A). Lv grains are micromosaics ofmicroquartz and phyllosilicates, probably sericite (Fig.15B), H grains are chlorite clots, and R grains arepyrite.

2.5.4.2. Descriptions and interpretations. Micas in Pgrains are typically aligned, suggesting that thesegrains represent chips of carbonaceous mud. Lv grainsrepresent altered dacitic volcaniclastic material derivedfrom the underlying felsic sands of member H6 of theHooggenoeg Formation (Lowe and Fisher Worrell,1999). No chlorite grains preserve detrital shapes, andmost probably represent alteration products of detritalgrains eroded from komatiitic or basaltic volcanicrocks.

2.5.5. 2-D carbonaceous laminations

2.5.5.1. Definitions. Three types of carbonaceouslaminations have been identified in the BRC, termedKlb, Klm, and Klr. Klb laminations are simpleundivided carbonaceous layers separated by thinlayers of pure chert (Fig. 16A). In contrast, both

Klm and Klr laminations anastomose and bifurcate.Klm laminations have constant intra-lamination thick-ness and bifurcate around lenses of pure chert andaround carbonaceous grains (Fig. 16B). The thicknessof Klr laminations varies laterally over very shortdistances (Fig. 16C). These laminations bifurcatearound lenses of pure chert, but not around carbona-ceous grains.

2.5.5.2. Descriptions. Outsized detrital carbonaceousgrains (typically Kgc) are ubiquitous in Klm lamina-tions. Laminations drape large grains, forming tent-likeor “open eyelet” structures that tend to subdueunderlying topography. When eroded, they occasionallyproduced roll-up structures, or folded chips of lamina-tions (see Kgl grains above).

Individual Klb laminations are only 1–5 μm thickand separated by chert laminations 1–10 μm thick.They wrap tightly around detrital grains and othertopographic elements rather than draping them and donot form the large “open eyelet” structures around the

Fig. 16. Carbonaceous laminations. (A) Klb laminations showing fine,undivided layering. Scale bar is 0.5 mm. (B) Klm laminations showinganastomosing and bifurcating habit and constant intra-laminationthickness. Scale bar is 1 mm. (C) Klr laminations showinganastomosing and bifurcating habit and varying intra-laminationthickness. Scale bar is 0.2 mm.

Fig. 17. Kn network composed of a web of very fine strands ofcarbonaceous matter. Top of network is a smooth, dense surface. Scalebar is 0.2 mm.


sides of carbonaceous particles characteristic of Klmlaminations.

Klr laminations are crenulated and highly irregular,varying substantially in darkness and thickness. Largerirregular carbonaceous grains are distributed randomlythroughout layers of Klr laminations, but never withinbifurcations.

2.5.5.3. Interpretations. Klb laminations correspondto the “fine carbonaceous laminations” of Walsh and

Lowe (1999), who interpreted them as fossil microbialmats on the basis of their morphological similarity tomodern mats and their tendency to form roll-upstructures when eroded. Because they are thinner andmodify underlying topography less than other mat-likefeatures identified in this study, they are interpreted hereto represent microbial biofilms.

The ubiquitous presence of outsized carbonaceousdetrital grains but not smaller grains approaching thethickness of individual laminae makes it unlikely thatKlm laminations originated as very fine carbonaceousgrains. The bifurcating habit of Klm laminations alsoindicates that they were not formed by settling offine carbonaceous grains out of suspension or bycurrent deposition. They formed roll-up structures(see Kgl grains above), implying cohesive strength ator near the sediment surface. It is significant thatonly Klb and Klm laminations and Kn networks(definition follows) formed roll-up structures, sug-gesting that the necessary cohesive strength was aproperty of these particular carbonaceous laminationsand networks rather than the encasing silica. Theircarbonaceous composition and cohesiveness suggestthat Klm laminations represent microbial mats(Simonson et al., 1993; Sumner, 1997; Walsh andLowe, 1999).

The crenulated, irregular, bifurcating habit of Klrlaminations likely has its origin by a differentmechanism. Darker regions of these laminations occurpreferentially below clear spaces. Where these spacesare less common or locally absent, CM forms a lessdifferentiated, diffuse matrix. It is likely that Klr

Fig. 18. Two Kn networks. Top network forms laminations which drape an underlying coarse detrital layer (a) and show internal anastomosingcharacter (b). The bottom network (gray band at d) grew around detrital grains resting on its surface (c) and down into the interstices between detritalgrains (d). Two well-sorted layers of CM and silica detritus (e and f) separated by a thin and discontinuous layer of carbonaceous network (g). Theupper layer is composed of very coarse sand- to granule-sized Kgc, Sa, and Sc grains stacked only a few grains thick (e). The lower layer is composedof medium to coarse sand-sized Kgc and Sa grains (f). Scale bar is 1 mm. From microfacies III, 30 m in section (Fig. 4).


Fig. 19. Roll-up structures in CM. Rolled up segments of mat-likelaminations demonstrate that these laminations possessed cohesivestrength at the sediment surface. (A) Multiply folded example from thelower disrupted black-and-white banded chert facies. (B) Two matsegments almost enclosing multiple carbonaceous and silica grainsfrom the upper evaporite and black chert facies.

Fig. 20. Diffuse CM. Kd finely dispersed, structureless carbonaceousmatter with isolated simple carbonaceous grains (dark). Scale bar is0.2 mm.


laminations represent a matrix of fine carbonaceousmaterial compacted between harder silica grains.

2.5.6. 3-D carbonaceous networks

2.5.6.1. Definition. Kn is composed of very finestrands that interconnect to form a web-like network(Fig. 17).

2.5.6.2. Description. Kn layers commonly includetwo network structures: (1) open, three-dimensionallattices of carbonaceous strands that fill intersticesbetween grains; and (2) fine, dense laminations that caplayers, drape detrital grains or other bottom irregular-ities, and form discontinuous flat-to-concave-upwardlaminations (Fig. 18). Lattices and laminations gradeinto one another, and laminations probably representcompacted or collapsed network. Openings in thenetwork lattice do not correspond to individual quartzcrystals or optical domains, and optical domains

commonly cut across carbonaceous laminations, indi-cating that the lattice structure was not formed inresponse to displacement of carbonaceous matter duringquartz crystallization. Indeed, the uniqueness of thisstructure considered relative to other types of Buck ReefChert carbonaceous material strongly suggests thatnetworks were not formed as a result of any stage ofsilica crystallization or precipitation since silicificationwas ubiquitous. Chunks of network have been locallyripped up and deformed plastically, indicating that theywere cohesive (Fig. 19). Kn grew around and drapeddetrital grains deposited on underlying network sur-faces. Open, 3-D network often extends downwardbetween the uppermost grains in detrital layers (Fig. 18).

2.5.6.3. Interpretation. The carbonaceous composi-tion, draping habit, and cohesiveness suggest that theselamination-forming networks represent microbial mats.Growth of mats to only shallow depths in underlyingdetrital layers suggests that the sediment surface was theoptimal growth location for the constructing microbes,potentially because of access to nutrients or light.

2.5.7. Diffuse carbonaceous matter

2.5.7.1. Definition. Kd is extremely fine, diffuse,massive CM with variations in concentration and afew outsized particles (Fig. 20).

2.5.7.2. Description and interpretation. Kd CMtypically contains isolated simple carbonaceous grainsand forms massive to crudely laminated layers. It isinterpreted to represent a well-mixed, soft organic andsiliceous ooze.

Fig. 21. Carbonaceous cavity fill. (A) Kcv cavity fill carbonaceous matter. Scale bar is 0.1 mm. (B) Sm grain. Silica grain with internal cavity filled bycarbonaceous matter and silica microspheres. Scale bar is 0.5 mm.


2.5.8. Carbonaceous cavity fill

2.5.8.1. Definition. Kcv fills or lines cavities, many ofwhich show an initial stage of filling by silica as lepi-spheres (Fig. 21A). Sc grains are ripped up chunks ofCM-cavity-filled silica (Fig. 21B).

2.5.8.2. Description and interpretation. Kcv liningsfrequently form isopachous layers around all sides ofcavities, indicating that the CM precursor was fluidrather than particulate. Occurrence with diagenetic silicaphases suggests that this fluid was also diagenetic,possibly early hydrocarbons. The same type of origin

Table 1Point-count data and microfacies assignments

TSA5-1

TSA5-2

TSA5-3

SAF475-10

SAF475-11

SAF475-12

TSA5-24

SAF475-13

TSA4

Position(m)

3 4 9 14 15 16 16 17 19

Klb 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0Klm 0.0 0.0 0.0 0.8 0.0 0.0 0.0 0.0 49.Klr 0.0 0.0 0.0 0.8 0.0 0.7 17.9 0.0 0.0Kd 0.0 0.0 80.3 74.2 19.3 17.1 10.7 80.8 2.0Kn 0.0 0.0 0.0 0.0 0.0 0.0 5.4 0.0 0.0Kcv 5.3 2.7 0.0 0.0 0.9 0.7 5.4 0.0 0.0Kgf 0.0 0.0 0.0 0.0 0.0 0.0 7.1 3.0 0.0Kgs 3.5 73.2 19.1 16.7 44.0 42.1 12.5 12.1 4.1Kgl 0.0 0.0 0.0 2.5 5.5 0.0 0.0 0.0 22.Kgc 0.0 0.0 0.0 2.5 10.1 16.4 16.1 2.0 20.Cp 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0Crh 0.0 0.0 0.0 0.0 0.0 0.0 5.4 0.0 0.0Cg 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0KF 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0Sa 0.0 0.9 0.0 0.8 14.7 20.7 19.6 2.0 2.0Sd 0.0 0.0 0.0 1.7 0.9 2.1 0.0 0.0 0.0Sc 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0P 87.6 2.7 0.0 0.0 2.8 0.0 0.0 0.0 0.0Lv 3.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0H 0.0 19.6 0.6 0.0 1.8 0.0 0.0 0.0 0.0R 0.0 0.9 0.0 0.0 0.0 0.0 0.0 0.0 0.0Microfacies I II II II II II II II III

All data are reported as percentages normalized to 100. Number of grains counted was typically 80–

has been suggested for other Archean pore- and fracture-filling CM (Buick et al., 1998; England et al., 2002;Rasmussen, 2005).

2.6. Microfacies

The results of point-counting of CM types and asso-ciated grains in black bands (Table 1) were analyzedusing principal component analysis to identify groups ofsimilar grain, lamination, and network associations.These groupings were used to define microfacies. For amore complete discussion of principal component anal-ysis see Wackernagel (1995).

5- SAF475-14

TSA5-6

SAF475-15

TSA5-10

TSA5-26

TSA5-7

TSA5-27

TSA5-28

TSA5-29

21 21 24.5 30 30.5 41 42 46 47

0.0 0.0 0.0 0.0 16.6 49.8 40.0 0.0 0.00 0.0 11.1 0.0 1.1 0.0 0.0 2.9 0.0 0.0

0.0 0.0 0.7 0.0 13.4 0.0 2.9 0.0 0.029.9 0.0 27.7 9.7 0.0 11.5 2.9 12.0 0.00.0 0.0 0.0 4.3 0.0 0.0 0.0 0.0 0.00.0 0.0 1.5 0.0 0.0 0.0 0.0 0.0 0.02.3 0.0 0.7 0.0 43.9 1.6 0.0 1.3 0.033.3 13.9 30.7 10.8 16.3 19.0 21.8 17.3 1.4

4 1.1 5.6 0.0 2.2 0.0 1.6 0.0 1.3 0.04 5.7 61.1 13.9 34.4 3.7 14.8 26.5 65.3 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 35.20.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 60.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 3.424.1 5.6 21.9 31.2 6.1 0.0 2.9 1.3 0.03.4 2.8 2.9 3.2 0.0 1.6 0.0 1.3 0.00.0 0.0 0.0 3.2 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0II III II III II III III III V

120.


2.6.1. Principal component analysisPrincipal component analysis is a data-transforma-

tion technique used to convert sets of correlated vari-ables (x1, x2, x3, …, xN, where N is the number ofvariables) into equivalent sets of uncorrelated principalcomponents (PC1, PC2, PC3, …, PCN). Each componentis a linear combination of the original variables, andthere are as many components as variables.

PCi ¼ Rjaijxj ð1ÞHere, i and j vary from 1 to N, and aij are weightings

which convert correlated variables into uncorrelatedcomponents. These weightings are scaled such that Eq.(2) holds.

RiðaijÞ2 ¼ 1 ð2ÞThese weightings, and through them the principal

components, are the output of principal componentanalysis.

By convention, components are listed in order ofdecreasing significance, i.e. each succeeding componentaccounts for less of the total variance of the set ofmeasurements than the component before it. Bydiscarding the components that represent very smallamounts of the total variance, a large initial set ofvariables can be converted into a smaller set ofcomponents that captures the key information of theoriginal system.

Principal component analysis also provides auseful technique for visualizing correlations between

TSA5-37

TSA5-30

TSA5-9

TSA5-8

TSA5-38

TSA5-11

TSA5-40

TSA5-12

TSA5-22

TSA5-31

TSA13

48 50.5 52 61 71 80 83 87 89.5 90 91

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 0.9 0.0 0.0 0.0 0.015.6 9.4 17.1 0.0 4.1 0.0 41.2 2.3 2.3 48.9 59.50.0 0.0 6.1 2.9 0.0 1.9 8.8 0.0 4.7 0.0 12.20.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 3.1 0.0 2.9 1.0 0.0 0.0 0.0 3.9 0.0 1.52.2 12.5 0.0 0.0 2.1 31.2 30.7 0.0 55.0 8.9 9.90.0 20.3 12.2 13.2 21.6 11.5 7.9 2.3 27.1 0.0 16.00.0 0.0 4.9 1.5 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 12.5 56.1 77.9 55.7 0.0 0.9 0.0 0.8 0.0 0.071.1 28.1 0.0 0.0 5.2 0.0 0.0 83.7 0.0 8.9 0.02.2 12.5 0.0 0.0 0.0 29.3 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.08.9 0.0 0.0 0.0 1.0 12.7 8.8 11.6 0.0 33.3 0.00.0 1.6 2.4 0.0 8.2 12.1 0.9 0.0 6.2 0.0 0.80.0 0.0 1.2 1.5 1.0 0.6 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.6 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0V IV III III III IV IV V IV IV IV

variables. Eq. (2) implies that any pair of weightingsfor a single variable (e.g. amj, anj where m≠n) plotswithin the unit circle. As a consequence of the wayin which the weightings are derived, such plots placeweighting pairs for correlated variables close togeth-er, those for uncorrelated variables 90° apart, andthose for anticorrelated variables on opposite sides ofthe circle. For instance, if x1 was highly correlatedwith x2, then a plot of the weightings for the firsttwo principal components applied to x1 and x2, (a11,a21) and (a12, a22), would consist of two points veryclose to each other near the edge of the unit circle.Plots of weighting pairs along with a unit circle arecalled circle of correlation diagrams, and are usefulfor visualizing correlations and for determining whatdata behavior is captured by sets of principlecomponents.

Point-count data are included in Table 1. Forprincipal component analysis, some classes of CMand other grain types were combined. Since theytended to occur together, Kgf and Klr were combinedto define Kf. Kgl, Kgc, and Sd were combined todefine Kc, a class of complex, ripped up grains. Klb,Klm, and Kn were combined to define Km, a class ofbiofilm- and mat-like laminations and networks. P, Lv,and H were combined to define Cl, a class of clasticgrains. KF, Crh, and Cp were combined to define Fe, aclass of sideritic grains. These classes, together withKgs (simple carbonaceous grains), Sa (silica grains),and Kd (diffuse CM), composed more than 80% of thecarbonaceous matter and associated grain types of each

5- TSA5-14

TSA5-15

TSA5-16

TSA5-17

TSA5-33

TSA5-18

TSA5-34

TSA5-36

TSA5-23

99 111 120 138 140 143 153 173 182

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.7 9.0 0.0 0.0 37.7 0.7 7.0 0.0 0.04.2 0.0 0.0 0.0 3.3 3.5 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.8 19.1 0.8 0.0 0.083.1 2.2 0.0 0.0 14.8 67.4 46.9 0.0 0.010.6 0.0 0.0 0.0 12.3 6.4 0.8 0.0 0.00.0 0.0 0.0 0.0 0.8 0.0 0.0 0.0 0.00.7 0.0 0.0 0.0 19.7 1.4 0.0 0.0 0.00.0 10.1 0.0 29.0 0.8 0.0 5.5 0.0 0.00.0 9.0 0.0 0.0 0.0 0.0 38.3 64.2 100.00.0 0.0 0.0 0.0 0.0 0.0 0.8 0.0 0.00.0 69.7 100.0 71.0 4.1 0.0 0.0 35.8 0.00.7 0.0 0.0 0.0 3.3 1.4 0.0 0.0 0.00.0 0.0 0.0 0.0 2.5 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0IV V V V IV IV IV V V

Fig. 23. First three principal components (PC1, PC2, and PC3). Thesecomponents were used to divide point-counted samples into fivemicrofacies, labeled I through V. Microfacies II clusters in thesouthwest quadrant of the plot of the first and second principalcomponents. Likewise, microfacies III clusters in the northwestquadrant, microfacies IV clusters in the southeast quadrant, andmicrofacies V clusters in the northeast quadrant. Microfacies I isdistinguished by an extremely negative third principal component, aconsequence of high clastic grain abundances.


sample, and typically more than 96%. The set ofcompiled data (Kgs, Kc, Km, Kf, Fe, Sa, Cl, Kd) wasrenormalized to sum to 100%.

So that variations in categories that are never morethan minor components of any sample would not beobscured by variations in major components, this reno-rmalized data set was transformed by renormalizingeach category by the category mean and standarddeviation to produce a set of eight variables havingmeans of 0 and standard deviations of 1. Principalcomponent analysis was applied to the transformed set.The eight derived principal components accounted for27.0%, 17.0%, 14.7%, 13.5%, 11.7%, 9.0%, 7.0%, and0.0% of the total data variance, respectively. Of thesecomponents, the first three captured 58.7% of thevariance. Circle of correlation diagrams (Fig. 22)illustrate the weightings of the first three principalcomponent, a1j, a2j, and a3j. The first principalcomponent discriminates between grain assemblageswith abundant ferruginous grains and those withabundant simple carbonaceous grains, silica grains,and complex carbonaceous grains. The second principalcomponent emphasizes assemblages with abundantcomplex grains and microbial structures. The thirdprincipal component discriminates between assem-blages with abundant clastic grains and those withabundant carbonaceous compacted features. These three

Fig. 22. Circle of correlation diagrams. Weightings for the first threeprincipal components (a1, a2, and a3) are plotted against each other.Unit circles are also plotted. Features with closely correlateddistributions plot next to each other in these diagrams. For instance,Kc (complex carbonaceous grains) and Km (mats) plot close to eachother because they tend to occur together.

components were used to divide counted samples intofive distinct assemblages or microfacies (Fig. 23): (1)microfacies I (represented by one sample) has PC1N0,PC2b0, and PC3b0; (2) microfacies II has PC1b0.2PC2

and PC2b0; (3) microfacies III has PC1b0 and PC2N0;(4) microfacies IV has PC1N0.2PC2, PC2b0, andPC3N0; and (5) microfacies V has PC1N0 and PC2N0.

2.6.2. Microfacies I

2.6.2.1. Description. Microfacies I is characterized byits high clastic component. This component is primarilyP grains, or amalgamations of carbonaceous matter andphyllosilicates. Microfacies I is also the only micro-facies containing Lv felsic volcaniclastic grains. Carbo-naceous grains are simple in morphology. No visiblesiderite grains are present. The only sample composedof microfacies I is from the base of the evaporite facies(Fig. 24).

2.6.2.2. Interpretation. Microfacies I is an association,in order of abundance, of mud chips, felsic volcaniclasticsand grains, and simple carbonaceous grains. Thisassemblage suggests deposition in an environmentsubject to currents that mixed nearby clastic material

Fig. 24. Microfacies, elemental and CM abundances, elemental ratios, and CM isotopic composition vs. stratigraphic position in samples collectedfrom the Buck Reef Chert.


with carbonaceous grains. The same currents could havebeen responsible for ripping up microbial mats toproduce the carbonaceous grains. This interpretation is

consistent with its known depositional setting of shallowbrine ponds developed on a distal alluvial planeconstructed largely of felsic volcaniclastic debris.


2.6.3. Microfacies II

2.6.3.1. Description. Microfacies II is an associationof simple carbonaceous grains, silica grains, clasticgrains, and complex carbonaceous grains, althoughmost samples contain little or no clastic component.Simple carbonaceous grains (Kgs) and silica grains (Sa)are much more abundant than complex carbonaceousgrains. Kd diffuse carbonaceous matter is particularlyabundant in this microfacies. No visible siderite grainsare present.

Layering in microfacies II sediments occurs in threemodes: (1) thin, b1 mm, layers of simple carbonaceousgrains alternating with pure silica layers of subequalthickness; (2) relatively thick, 1 to N5 cm, massivelayers of simple carbonaceous grains and silica grains;and (3) thick, N5 cm, massive to crudely laminatedlayers of diffuse carbonaceous matter (Kd) and simplecarbonaceous grains.

Microfacies II sediments comprise most of theevaporitic facies and much of the lower half of thelower black-and-white banded chert facies (Fig. 24).

2.6.3.2. Interpretation. The low abundance of com-plex carbonaceous grains, such as in microfacies III, issuggestive of a current-active environment that tendedto break apart carbonaceous particles and regularlydisturb the sediment surface. Ripped up chips of Kncarbonaceous laminations (Fig. 19) indicate the pres-ence of microbial mats, although none are preserved inplace. This is also consistent with a current-activesetting for deposition of microfacies II sediments.Thicker massive, unsorted layers reflect depositionduring the waning stages of energetic events, probablystorms.

Thin alternating layers of carbonaceous grains andchert could have formed as laminations of carbona-ceous matter and particulate silica hydraulicallyseparated by alternating currents, as laminations formedin an environment in which silica and carbonaceousmatter were alternately and rhythmically deposited, orthrough an early diagenetic separation. Stacks of 20 ormore of these layers of relatively uniform thicknesssuggest one of the latter two alternatives. Layers of thistype are found in association with silicified evaporites,and may have formed as a result of wetting and dryingcycles.

The preservation of complex carbonaceous grainsand equant simple carbonaceous grains against sedi-ment compaction implies silicification at shallowdepths in the sediment column (Walsh and Lowe,1999).

2.6.4. Microfacies III

2.6.4.1. Description. Microfacies III is an associationof complex carbonaceous grains, simple carbonaceousgrains, silica grains, and mat-like laminations. It isdistinguished by a high abundance of complex carbo-naceous grains relative to simple carbonaceous grains,silica grains, and compacted carbonaceous features. It isthe only microfacies with samples containing abundantmicrobial structures (Klb, Klm, and Kn). Some samplescontain trace abundances of siderite grains. Clasticterrigenous grains are absent.

Four primary types of layers occur in microfacies IIIsediments: (1) thin (generally b2 cm) massive tonormally graded layers of complex carbonaceous grainsand silica grains (Figs. 18 and 26); (2) layers of Klmlaminations with trapped simple and complex carbona-ceous grains (Fig. 25); (3) layers of Kn networkintergrown with or draping layers of simple andcomplex carbonaceous grains and silica grains (Fig.18); and (4) layers of Klb laminations with poorly sorteddetrital carbonaceous grains (Fig. 26). Microfacies IIIsediments comprise much of the lower black-and-whitebanded chert facies (Fig. 24).

2.6.4.2. Interpretation. Preservation of in-place mi-crobial mats suggests that, unlike the environment thatfavored deposition of microfacies II, the microfacies IIIsetting was not frequently subjected to extreme currentor wave activity. The presence of ripped up chunks ofcarbonaceous sediment suggests that the sedimentsurface was only episodically disturbed, leaving sedi-ment time to partially silicify and consolidate in place.As in the microfacies II environment, silicificationoccurred early relative to burial and compaction.

Two particularly thick and well-preserved examplesof mat-like laminations allow examination of theprocesses involved in mat construction. Fig. 18 showsan example of a mat composed primarily of Kn network-constructed laminations draped over a detrital layer. Fig.25 shows a thick stack of Klm laminations. In bothcases, anastomosing, lenticular, or cuspate elements aremost frequently present in areas of topographic relief,typically generated by the presence of detrital grains.This suggests that, instead of being formed by bubbles,diagenetic silica precipitation, or recrystallization, thesevoids formed during mat growth. Rather than creatingadditional relief, these elements subdued it, creatingrelatively flatter surfaces. It is possible that thesestructures result from local biological responses thatmaximized mat surface exposure to sunlight. Structuressimilar in geometry but larger in scale have been

Fig. 25. Klm laminations and detrital layers. Klm laminations anastomose and bifurcate (a) around large detrital carbonaceous grains (b). Two distinctlayers of detrital carbonaceous grains (c, d) overly the basal Klm layer. The lower detrital layer is matrix-supported at its base (c). Grains in (c) displaya continuous range of shapes from rolled up mat chips (Kgl) to complex grains (Kgc) to simple grains (Kgs). Grains in (d) are predominantly verycoarse mat chunks. A slightly disrupted layer of Klm laminations overlies the detrital layers. Scale bar is 5 mm. From microfacies III, 19 m in section(Fig. 4).


Fig. 26. Layer composed of Klb laminations and poorly sorted detrital carbonaceous grains. Laminae drape grains without significantly modifyingtopography (a). Layer was disrupted (b) when sediment was partially silicified, and resulting stratiform cavities were infilled by pure silica. Laterquartz vein (c) cuts early silicified CM layers and early disruptive silica. Scale bar is 1 mm. From microfacies III, 41 m in section (Fig. 4).


observed in late Archean cuspate microbialites (Sumner,1997, 2000), where they frequently formed attachmentson the sides of vertical carbonaceous supports.

The presence of isolated outsized detrital grainssuggests that relatively thick mat growth (up to severalmillimeters) occurred during intervals of low currentactivity. Particulate detritus was carried in by occasionalmore energetic events, but more frequent backgroundcurrents kept the mat surface swept clear of finer, low-density material. Mats responded to the presence ofdetrital grains locally, either by draping resultingtopography or by developing low-density cuspatestructures that allowed them to quickly reestablish aflat surface. Currents may have locally ripped up matchunks, but they were generally not of sufficientstrength to obliterate or bury entire mats.

Klm and Kn microbial mats were thin and not relief-forming, similar to Synechococcus–Chloroflexus mats

occurring in 60–73 °C regions of modern Yellowstonehot springs (Walter, 1976; Lowe et al., 2001). In contrastto later stromatolite-dominated platforms (Beukes,1987), the Buck Reef Chert shallow seafloor wasstructured primarily by abiotic physical and chemicalprocesses despite the ubiquitous presence of bioticallyproduced organic matter and mats. Unlike mats growingin lower-temperature regions, Synechococcus–Chloro-flexus mats are not known to silicify (Walter, 1976;Lowe et al., 2001). BRC mats may also not havesilicified as rapidly as surrounding sediments. This mayexplain the scarcity of in-place mat deposits and thickmat accumulations relative to the abundant erodeddetrital carbonaceous grains. It is possible that BRCorganisms even produced organic acids or ligands thatlocally lowered silica activity in mat pore fluids(Bennett and Siegel, 1987) as a mechanism to preventsilicification during mat growth.

Fig. 27. Microfacies IV. Layers are composed of fine-to medium-grained carbonaceous grains showing strongly contrasting amounts of compaction.Bases (a, c, e) are composed primarily of carbonaceous and silica grains compacted in place, although rare uncompacted grains are also present. Topsare less compacted (b, d). Compacted grains are typically optically lighter than uncompacted grains, either because CM was concentrated duringcompaction, grains silicified prior to burial resisted compaction, or both. Less compacted tops in rhythmic layers such as these strongly suggestsilicification by interaction with overlying marine fluids. Scale bar is 1 mm. From 89.5 m in section (Fig. 4).


Table 2Unnormalized bulk elemental compositions and carbon compositions of dark bands

TSA5-1 TSA5-3 TSA5-7 TSA5-13

TSA5-17

TSA5-20

TSA5-4 SAF475-11

SAF475-12

TSA5-9 TSA5-18

TSA5-11

TSA5-34

TSA5-29b

TSA5-29r

TSA5-32

TSA5-35

SiO2 95.76 96.48 96.57 97.47 96.83 45.56 98.79 98.51 98.76 98.67 97.33 92.34 87.22 97.69 76.42 98.78 98.58Al2O3 2.22 0.98 0.20 0.26 0.20 0.22 0.17 0.48 0.32 0.15 0.19 0.19 0.17 0.18 0.15 0.16 0.13TiO2 0.076 0.035 0.001 0.005 0.000 0.001 0.000 0.006 0.006 0.000 0.000 0.004 0.000 0.000 0.000 0.000 0.000FeO⁎ 0.288 0.820 0.037 0.072 1.048 49.247 0.051 0.389 0.250 0.043 1.889 6.044 9.962 0.661 18.377 0.175 0.086MnO 0.000 0.006 0.000 0.001 0.009 0.083 0.000 0.004 0.003 0.000 0.049 0.132 0.663 0.038 0.804 0.004 0.001CaO 0.05 0.04 0.02 0.04 0.03 0.06 0.01 0.01 0.00 0.00 0.01 0.00 0.01 0.01 0.04 0.01 0.00MgO 0.06 0.17 0.00 0.02 0.04 0.05 0.00 0.06 0.05 0.01 0.03 0.04 0.19 0.15 0.73 0.00 0.00K2O 0.61 0.07 0.02 0.02 0.01 0.01 0.02 0.04 0.02 0.01 0.01 0.00 0.01 0.01 0.01 0.01 0.01Na2O 0.03 0.00 0.00 0.00 0.02 0.05 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00P2O5 0.010 0.007 0.005 0.007 0.008 0.022 0.003 0.003 0.004 0.003 0.004 0.011 0.007 0.002 0.047 0.006 0.005Ni 10 75 17 10 10 69 19 35 19 12 54 43 67 13 32 13 11Cr 23 20 0 3 2 4 0 10 3 0 2 17 22 0 38 0 0Sc 3 1 6 2 9 2 2 3 0 5 6 4 5 8 6 4 2V 5 4 0 0 0 0 0 9 0 0 0 12 7 0 0 9 0Ba 92 9 6 0 15 0 13 17 6 25 7 9 2 8 6 7 3Rb 17 3 2 2 2 0 3 2 3 1 1 2 0 1 0 1 1Sr 4 4 2 3 3 5 2 4 3 4 3 6 6 2 7 1 2Zr 21 10 4 4 3 13 5 7 5 4 5 7 7 4 7 4 4Y 3 4 2 4 11 17 1 3 2 1 8 4 4 2 7 6 7Nb 2.6 0.8 2 0.3 1 3.8 1.3 2.5 1.7 2.2 2 2.8 2.9 3.4 3 3.7 1.7Ga 2 2 1 0 1 4 2 4 0 0 2 2 1 0 3 3 0Cu 5 17 3 1 3 44 0 12 5 2 3 7 6 0 12 2 0Zn 3 4 0 1 4 19 3 88 1 0 0 0 7 0 8 0 0TOC 0.12 0.44 0.11 0.16 0.11 n.d.† 0.09 0.19 0.16 0.07 0.17 0.19 0.34 0.05 0.27 0.07 0.03δ13CCM −30.61 −35.87 −36.87 −34.21 −20.05 n.d. −31.83 −32.73 −32.12 −30.90 −29.90 −27.32 −25.73 −28.13 −23.24 −28.37 −28.51

Units are wt.% (oxides and TOC), ppm (Ni–Zn), and per mil (δ13CCM).*Iron abundances are reported as equivalents of FeO.†n.d. = not determined.


The association of Klb laminations with poorlysorted, hydraulically fine grains (e.g. Fig. 26) suggestsdeposition in very low-energy environments. The factthat these laminations do not modify topography, incontrast to Klm and Kn mats, may reflect constructionby different organisms, e.g. by coccoids rather thanfilaments or by non-phototactic microbes, but could alsobe an effect of growth of the same microbes underlower-energy conditions.

A matrix-supported base for a detrital layer above aset of Klm laminations (Fig. 25) implies either thatthere was a fine particulate silica phase present at thetime of deposition of that layer, possibly like on thefloors of modern Yellowstone hot springs (Lowe andBraunstein, 2003), or that carbonaceous grains weregenerally coated by thin coatings of silica. Periodicdeposition of layers of a fine particulate silica phasemay account for thin silica layers in Klm and Klblaminations.

2.6.5. Microfacies IV

2.6.5.1. Description. Microfacies IV is an associationof compacted carbonaceous matter, silica grains, andvariable amounts of ferruginous grains. Typical assem-blages contain abundant Kgf carbonaceous grains, Klflaminations, and Klr network and a low proportion of

complex carbonaceous grains, simple carbonaceousgrains, silica grains, and ferruginous grains. Terrigenousand volcaniclastic grains are absent. Less commonuncompacted carbonaceous grains are fine to very fineand simple in shape. Some samples contain abundantferruginous grains.

Layers are thin, b3–5 mm, and almost everywherelack post-depositional soft-sediment disturbance. Whilemost carbonaceous grains are compacted, many layershave uncompacted tops (Fig. 27) and a few uncom-pacted particles scattered among more compactedgrains.

Microfacies IV is most common in the upper black-and-white banded chert facies (Fig. 24), and is only aminor component of the lower black-and-white bandedchert facies.

2.6.5.2. Interpretation. As in the microfacies IIIenvironment there is no evidence for wave or currentactivity during deposition of microfacies IV. Currentstructures and scour are absent. Deposition was out ofsuspension, but in an even lower-energy setting thanmicrofacies III subject to essentially no wave or currentactivity. Sedimentation of carbonaceous grains, silicagrains, and siderite was most likely by hemipelagicsettling. There is no evidence for the growth of in situmicrobial mats.

TSA5-8 TSA5-2 TSA5-10

TSA12-16

TSA12-6

TSA12-7

TSA12-15

TSA12-17b

TSA12-17s

TSA12-10

SAF491-1

TSA12-13r

TSA12-8

TSA5-23

TSA12-13b

SAF131-5

BRCd TSA12-1

99.48 96.65 98.82 86.93 98.41 99.64 97.56 99.19 92.71 99.52 62.31 92.32 95.82 41.54 98.35 96.48 62.81 87.060.14 1.36 0.21 0.25 0.89 0.17 0.24 0.23 0.38 0.22 0.21 0.19 0.18 0.19 0.12 0.15 0.35 9.120.000 0.065 0.000 0.002 0.027 0.004 0.004 0.005 0.001 0.000 0.000 0.007 0.000 0.000 0.003 0.004 0.000 0.2370.020 1.176 0.041 10.840 0.191 0.120 1.339 0.224 5.835 0.034 33.155 6.188 3.626 55.509 1.378 2.736 34.253 0.3510.000 0.007 0.000 0.407 0.003 0.000 0.085 0.011 0.133 0.000 0.530 0.017 0.013 0.087 0.003 0.015 0.204 0.0000.00 0.01 0.00 0.06 0.04 0.03 0.02 0.01 0.01 0.02 0.03 0.03 0.02 0.04 0.01 0.04 0.00 0.010.00 0.17 0.00 0.18 0.10 0.06 0.11 0.05 0.10 0.05 0.04 0.06 0.07 0.06 0.07 0.10 0.17 0.350.01 0.02 0.01 0.02 0.21 0.03 0.04 0.04 0.05 0.06 0.04 0.03 0.01 0.02 0.02 0.02 0.04 2.770.00 0.00 0.00 0.06 0.07 0.05 0.05 0.05 0.06 0.05 0.01 0.06 0.05 0.04 0.05 0.07 0.05 0.090.004 0.003 0.003 0.006 0.009 0.003 0.002 0.002 0.002 0.001 0.477 0.008 0.010 0.057 0.004 0.008 0.020 0.02811 47 18 22 14 12 17 11 21 6 73 9 10 93 11 17 61 90 61 0 26 43 6 6 14 33 0 66 16 8 93 2 2 69 123 4 0 1 2 1 0 4 1 3 5 0 2 4 0 2 4 00 10 0 1 12 0 0 8 2 0 1 4 1 8 0 0 3 21

12 3 7 14 56 15 30 33 14 15 11 10 6 0 8 8 2 5162 1 2 1 5 2 2 2 1 3 0 2 0 0 1 2 0 564 4 5 4 4 5 4 2 7 3 5 2 3 7 3 5 4 194 17 5 6 10 5 5 5 5 3 10 6 5 15 5 3 13 702 4 2 1 1 2 21 2 3 1 28 3 3 20 3 2 9 103.8 2.8 1 1.7 0.5 0.8 0.8 0.9 1.3 0.7 2.7 1.8 0.4 4.7 1.7 1.2 2.9 6.30 1 3 1 2 0 1 1 3 0 4 2 0 6 2 1 3 91 23 4 47 22 22 15 21 24 18 55 30 22 105 13 31 76 320 2 0 29 14 11 10 13 19 12 91 20 14 79 13 20 61 180.18 0.31 0.09 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

−27.07 −31.94 −32.59 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.


Silicification of carbonaceous layerswas not as early inmicrofacies IV sediments as in those of microfacies I, II,or III. Uncompacted grains are intermixed with com-pacted grains throughout microfacies IV layers, mostlikely indicating that carbonaceous grains were already invarious stages of silicification prior to deposition or thatthey exhibited varying resistance to compaction beforesilicification. This inference is consistent with theobservation that uncompacted grains typically appearless optically dense, although apparent density variationscould also result fromvarying degrees of compaction. Thesediment source was probably composed of heteroge-neously silicified carbonaceous material. In many cases,uncompacted grains are particularly concentrated near thetops of layers, and compacted grains are rare. Silicifica-tion thus appears to have preferentially preserved the topsof thin layers against compaction, a pattern of diagenesisstrongly suggesting rapid cementation of the uppermostfew millimeters of layers and the overlying water columnas the source of dissolved silica.

2.6.6. Microfacies V

2.6.6.1. Description. Microfacies V (banded ferrugi-nous chert) is characterized by a greater abundance ofsiderite grains (Crh and Cp) and siderite/carbonaceousmatter aggregates (KF) than any other grain types. Somesamples contain a small amount of simple carbonaceousgrains. Terrigenous and volcaniclastic grains are absent.

Layers are very thin, b1–2 mm, and contain variableamounts of carbonaceous matter that is typicallycompacted.

Because all banded ferruginous chert (BFC) sampleswere extensively weathered, none was point-counted.However, relatively unweathered enclaves in a few BFCsamples preserve grain assemblages composed predom-inantly of tightly packed KF grains, suggesting thatbanded ferruginous chert was originally composed ofmicrofacies V.

Microfacies V sediments are most common in theupper black-and-white banded chert facies and thebanded ferruginous chert facies (Fig. 24).

2.6.6.2. Interpretation. While some euhedral rhombicsiderite is likely to have formed diagenetically, theabundance of siderite inferred for the original microfaciesV sediments suggests that it was amajor primary sediment.Although siderite is present in composite grains withcarbonaceous matter, it is widely developed in layers andlenses lacking CM. It is thus unlikely to have formed byreduction of iron oxyhydroxides. Siderite in this micro-facies is thought to have precipitated in the water column.

The detrital population in microfacies V is composedof extremely fine, hydraulically light grains andprecipitative mineral grains. Sedimentation was domi-nated by hemipelagic settling of very fine siderite andcarbonaceous grains. Silicification was slow relative tocompaction.


2.7. Clastic-derived elements (Al2O3, Zr, TiO2, Cr)

2.7.1. DescriptionWhile bulk compositions of rocks from throughout

the Barberton greenstone belt have been severely alteredby metasomatism, relative abundances of comparativelyimmobile elements have frequently been preserved(Duchac and Hanor, 1987; Hanor and Duchac, 1990;Lahaye et al., 1995; Byerly, 1999; Lowe, 1999).Relative ratios of Al2O3, Zr, TiO2, and Cr, in particular,have proven useful for identifying the original composi-tions of highly silicified ashes (Lowe, 1999).

In the area of the Buck Reef Chert studied, Al2O3, Zr,Cr, and TiO2 abundances (Table 2) vary systematicallywith lithofacies and section height (Fig. 24). Abun-dances are moderate in the evaporitic facies, decreasingupward into the lower black-and-white banded chertfacies. Abundances are lowest in the lower and upperblack-and-white banded chert facies, with Zr and Crbecoming slightly more concentrated toward the top ofthe upper black-and-white banded chert facies. WhileAl2O3 and TiO2 abundances remain low in the bandedferruginous chert facies, Cr and Zr are present in levelsthat approach or even exceed those of the evaporiticfacies. TiO2 levels are below detection limits in nearlyall samples except those from the evaporitic facies.

Ratios of Al2O3, Zr, and Cr also vary systematicallywith lithofacies and section height (Fig. 24). Evaporiticfacies cherts have compositions similar to that of daciteor dacitic ash, although some have komatiitic ashaffinities. Zr/Al2O3 ratios are 9–15, and increase upwardinto the platform facies. Cr/Zr ratios are 1–4. Platformand transitional facies cherts have compositions similarto dacite, but enriched in Zr. A few cherts have slightkomatiitic affinities. Zr/Al2O3 ratios are 15–45. Cr/Zrratios are generally 0–0.6, with isolated examples ashigh as 5.4. Basin facies cherts are most enriched in Zrand Cr, with Zr/Al2O3 ratios between 35 and 80 and Cr/Zr between 0.3 and 6.2.

2.7.2. InterpretationThe similarity in immobile element ratios between

evaporitic facies cherts and dacite and dacitic ash isconsistent with the presence of volcaniclastic andterrigenous material visible in samples from that facies.The overall upward decreasing abundances of Al2O3,Zr, TiO2, and Cr in this facies reflect the upwarddecreasing content of terrigenous clastic and volcani-clastic material. Immobile element abundances withinthe evaporitic facies thus reflect mixing of volcaniclasticdetritus derived from the coastal system represented bythe underlying felsic sands of H6 into locally-produced

carbonaceous and siliceous sediment. Decreased abun-dances of Al2O3, Zr, TiO2, and Cr within the lower andupper black-and-white banded chert facies relative to theevaporitic facies reflect, in part, negligible mixing ofterrigenous and volcaniclastic detritus into the shallowand deep shelfal environments, most likely due toerosion, subsidence, and submergence of the underlyingvolcanic complex.

Zr is typically enriched relative to the other immobileelements in the lower and upper black-and-white bandedchert facies and in the banded ferruginous chert facies,with Zr/Al2O3 ratios generally greater than those foundin any primary source rock, including felsic volcanicand volcaniclastic rocks. Similar Zr enrichment isobserved in loess deposits due to concentration ofzircons along with other heavy minerals during aeoliantransport (Taylor et al., 1983; Gallet et al., 1998;McLennan, 2001). High Zr/Al2O3 in Black Seasediments has been used to infer relative input ofwindblown silt (Martinez-Ruiz et al., 2000). Zr/Al2O3

greater than about 20 in BRC cherts thus most likelyreflects a primarily windblown source of clasticsediment. Rare cherts in lower and upper black-and-white banded chert facies and in the banded ferruginouschert facies with Cr/Zr greater than about 2 containwindblown sediment derived from a source terrain withat least some komatiitic component.

Enrichment of Zr and Cr in the banded ferrugi-nous chert facies is best explained by concentrationof windblown sediment in slowly deposited basi-nal sediments. A slow rate of deposition is consis-tent with evidence for sedimentation by hemipelagicsettling of very fine material in microfacies IV and V.

2.8. Heavy Metals (FeO⁎, Cu, Zn, Ni)

2.8.1. DescriptionNo attempt was made to chemically determine

relative amounts of FeO and Fe2O3 in this study, so alliron abundances are reported as equivalents of FeO(FeO⁎). Nearly every chert examined for this study hasmeasurable abundances of FeO⁎, Cu, Zn, and Ni (Table2). Abundances vary systematically between lithofaciesand with section height (Fig. 24). Abundances aremoderate in the evaporitic facies (0.2bFeO⁎b1.2 wt.%;5bCub25 ppm; 2bZnb90 ppm; 10bNib80 ppm), lowin the lower black-and-white banded chert facies andmost of the upper black-and-white banded chert facies(0.03bFeO⁎b0.3 wt.%; 0bCub5 ppm; 0bZnb3 ppm;10bNib20 ppm), and high in the banded ferruginouschert facies (0.2bFeO⁎b60; 0bCub110; 0bZnb80 ppm; 10bNib100 ppm).


2.8.2. InterpretationBroadly, heavy metal distributions within the BRC

are similar to those of the clastic-derived immobileelements, especially Zr and Cr. This observationsuggests a statistical test for correlations betweenmetal and clastic abundances. However, simple regres-sion of metal abundance against Zr and Cr abundancedoes not suffice to explore the relationship betweenmetals and clastics since Cr is highly correlated with Zr(P=9×10− 6 for the regression coefficient). Thisrelationship tends to mask significant correlationsbetween abundances of either clastic element and theabundance of any other element. Moreover, clasticelements in the upper BRC derive from a windblown,zircon-enriched source while those in the lower BRCderive from varying mixes of dacitic volcaniclasticmaterial and komatiitic ash.

For these reasons, cherts were divided into twogroups, one with Zr/Al2O3b20 and one with Zr/Al2O3≥20. For each group, Cr was regressed againstZr and the resulting regression relationship was used tocalculate a new quantity, ΔCr=Cr−Cr⁎(Zr), whereCr⁎(Zr) is the Cr abundance predicted by the Zrabundance. By definition, ΔCr is not correlated withZr, making it a suitable substitute for Cr in multipleregressions. For each group, metal abundances wereregressed against Zr and ΔCr (Figs. 28−31).

Fig. 28. Metal/clastic correlations: FeO*. Results of t-tests for significance ofin each panel. Dashed lines show range of expected FeO* if supplied by terrigand Fegley (1998).

In nearly every case, regressions for the Zr/Al2O3b20group of samples, which contain moderate clasticmaterial, yielded no significant correlations (PN5% fort-tests on regression coefficients). The only exception isa marginally significant relationship between Cu andΔCr (Fig. 29). However, because no significantrelationship was detected between Cu and Zr, andbecause a correlation with Cr would be detected in thiscase by a correlation with both Zr and ΔCr, thiscorrelation is likely to be coincidental. These resultsfor samples with Zr/Al2O3b20 could suggest that therewas no direct relationship between clastic sedimentationand deposition of metals in the BRC, i.e. that Fe, Cu, Ni,and Zn were not primarily deposited as constituents ofdacitic or komatiitic material, or they could suggest thatat least two materials with very different metal/Zr ratioswere mixed to varying degrees. For instance, felsic rockstypically have FeO/Zr ratios of about 0.01 wt.%/ppm,whereas ultramafic rocks have ratios of about 0.5 wt.%/ppm (Lodders and Fegley, 1998). The second possibilitygains support from petrographic observations of bothdacitic and basaltic-to-komatiitic material in evaporiticfacies rocks, and from the fact that nearly all samplesfrom the Zr/Al2O3b20 group have metal abundanceswithin the range for terrestrial materials with equal Zrabundances (Figs. 28−31). It seems likely, therefore, thatin BRC rocks containing a significant component of

coefficients in multiple regression of FeO* on Zr and ΔCr are indicatedenous material using ultramafic and granite compositions from Lodders

Fig. 29. Metal/clastic correlations: Cu. Results of t-tests for significance of coefficients in multiple regression of Cu on Zr and ΔCr are indicated ineach panel. Dashed lines show range of expected Cu if supplied by terrigenous material using ultramafic and granite compositions from Lodders andFegley (1998).


terrigenous or volcaniclastic material, the metals Fe, Cu,Ni, and Zn also have a clastic source.

In contrast, regressions in the Zr/Al2O3≥20 groupyield highly significant correlations. Correlations of allmetals with Zr are particularly strong, with confidence

Fig. 30. Metal/clastic correlations: Ni. Results of t-tests for significance of ceach panel. Dashed lines show range of expected Ni if supplied by terrigenouFegley (1998).

levels all b7×10−6. It is not as clear if any significantrelationships exist with ΔCr, however. Both Cu and Znexhibit statistically significant correlations with ΔCr,and plots of FeO⁎ and Ni vs. ΔCr seem to suggest asignificant relationship if only one sample with

oefficients in multiple regression of Ni on Zr and ΔCr are indicated ins material using ultramafic and granite compositions from Lodders and

Fig. 31. Metal/clastic correlations: Zn. Results of t-tests for significance of coefficients in multiple regression of Zn on Zr and ΔCr are indicated ineach panel. Dashed lines show range of expected Zn if supplied by terrigenous material using ultramafic and granite compositions from Lodders andFegley (1998).


anomalously low ΔCr (TSA5-20) is excluded. To testthis possibility, the regression analysis was repeatedwithout including data from TSA5-20 (results notillustrated here). After this exclusion, a significantrelationship was detected between FeO⁎ and ΔCr(P=5×10−4), but the apparent correlations for Cu andZn were no longer significant (P=0.10 and 0.18,respectively). The most likely explanation for thesensitive dependence of these apparent correlations onthe inclusion of one sample is that none of the metals isdirectly related to Cr, but rather to an underlyingvariable correlated with both Zr and Cr.

What is the underlying variable controlling metalabundances? In the group of samples for whichsignificant correlation exists between metal and clasti-cally-derived element abundances, Zr/Al2O3 is greaterthan 20 implying that windblown dust is the primarysource of Zr and Cr. Assuming a relatively constant rateof supply of windblown material, high Zr and Crabundances in these rocks correspond to concentrationof dust in slowly deposited sediments; Zr and Cr wouldthus correlate inversely with overall sedimentation rate.The correlation of metal abundances with Zr would thenimply that metals were also concentrated in slowlydeposited sediments.

Concentration of metals in slowly deposited sedi-ments is not consistent with a proximal hydrothermalsource of metals. Indeed, if the BRC represented theexhalative deposits of a hydrothermal vent, metal abun-

dances would be highest in proximal vent deposits leastenriched in windblown dust and lowest in backgroundmarine deposits most enriched in windblown dust, andan inverse correlation between metal and clastic abun-dances would be observed.Metal enrichment is thereforenot an indicator of a hydrothermal origin for the BRC.

Instead, metal enrichments are likely to reflect abackground “rain” of precipitated minerals in an earlymetal-rich ocean. If average surface temperatures were70±15 °C (Knauth and Lowe, 2003) and the earlyEarth's surface was anoxic (e.g. Rasmussen and Buick,1999; Canfield et al., 2000), then hydrothermally-derived metals would have been substantially moremobile than in the modern oceans. In this case, theprimary process removing metals from the ocean wouldhave been precipitation of metal sulfides and carbonates.This conclusion is consistent with suggestions thatpositive europium anomalies in Archean chemicalsediments reflect an early ocean composition controlledby high-temperature hydrothermal inputs (e.g. Derryand Jacobsen, 1990; Kamber and Webb, 2001; Tice andLowe, 2006).

2.9. Carbonaceous matter abundance and isotopiccomposition

2.9.1. DescriptionCM preserved in the evaporitic and lower black-and-

white banded chert facies (Table 2) has a mean carbon

Fig. 32. Correlation of CM isotopic composition and Zr/Al2O3.δ13CCM is positively correlated with Zr/Al2O3 (P=0.02).


isotopic composition of −31.9±2.9‰ (S.D.) relative toPDB, while CM preserved in the upper black-and-whitebanded chert and banded ferruginous chert facies isisotopically heavier with a mean composition of −27.2±4.3‰ (Fig. 24). Total CM abundance is highest in theevaporitic and upper black-and-white banded chertfacies and lowest in the lower black-and-white bandedchert and banded ferruginous chert facies (Fig. 24).

2.9.2. Interpretation13C enrichment in organic matter deposited in deep-

water settings could be explained by primary isotopiccompositions that varied between environments, pref-erential metamorphic alteration of carbonaceous matterin basin facies rocks, or diagenetic, possibly microbial,alteration of carbonaceous matter deposited in differentsettings.

CM in upper black-and-white banded chert andbanded ferruginous chert facies rocks is present asdetrital grains that have undergone varying degrees ofcompaction. Carbonaceous grains become simpler inmorphology and finer in size with the transition fromshallow platform settings to deep platform and basinsettings, suggesting that deep-water carbonaceousmatter is detrital in origin and had a shallow watersource. It is therefore likely that all BRC carbonaceousmatter had the same initial isotopic composition.

Although carbon loss during metamorphism tends topreferentially remove 12C (McKirdy and Powell, 1974;Des Marais et al., 1992), differential heating is unlikelyto have produced the variation observed here. One of themost 13C-depleted samples (−35.9‰) from the base ofthe section is located next to an igneous intrusion. Theisotopic composition of carbonaceous matter (δ13CCM)does not correlate with distance from intrusive dikes andsills. Partial equilibration with isotopically heavy sideriteduring metamorphismwould have resulted in correlationbetween δ13CCM and iron abundance or iron-to-organiccarbon ratio independent of depositional setting. Instead,δ13CCM does not correlate with either parameter in upperblack-and-white banded chert or banded ferruginouschert facies rocks (P=27.1% and 22.8%, respectively)although iron abundance varies over nearly the samerange as in the BRC as a whole. Similar shallow- to deep-water 13C enrichments in carbonaceousmatter have beenobserved in 2.5–2.3-Gyr-old sequences (Beukes et al.,1990). Such trends are unlikely to be explained bypreferential metamorphic isotopic resetting of rocksdeposited under deep water. Instead, 13C enrichment indeep-water carbonaceous matter most likely reflectsdifferences in composition prior to deep burial andmetamorphism.

Deep-water sediments contain elevated levels ofwindblown dust as indicated by high bulk Zr/Al2O3,reflecting slow sedimentation and silicification rateswhich would have resulted in slow burial of depositedmaterial. δ13CCM is positively correlated with Zr/Al2O3

(Fig. 32), suggesting that organic matter was preferen-tially 13C-enriched in slowly buried sediments. Thisrelationship between δ13CCM and sedimentation ratereinforces the conclusion that variations in δ13CCM wereset before deep burial. The combination of greater trans-port distance and lower burial rates in the basin settingwould have subjected deep-water organic matter tolonger periods of near-surface biological degradationthan material deposited in shallow water. The magnitudeof enrichment in 13C associated with slow burial indi-cates preferential loss of 12C, probably by generation ofmethane by methanogenesis. Typical biogenic metha-nogenesis today results in kinetic fractionation effectsleading to methane δ13C values as much as 20–25‰depleted relative to source acetate (Gelwicks et al.,1994). Methane loss by a combination of fermentationand methanogenesis would leave substantially lessdepleted residual simple organics such as acetate (Blairand Carter, 1992). Loss of methane would proceed untilthe sediment was silicified, drastically reducing perme-ability and effectively producing a closed isotopicsystem.

Some sulfate reducers produce biomass similarly lessdepleted relative to substrate composition in closedsystems (Londry and Des Marais, 2003), but the lack ofabundant pyrite, even in iron-rich deep-water sedimentswhere fractionation is most extreme, suggests thatsulfate reduction was not significant.


2.10. Discussion of Buck Reef Chert results

2.10.1. Depositional environmentsSedimentary structures in the evaporitic facies and

grain associations and layering styles in the microfacies Iand II sediments that compose it are consistent withdeposition in shallow coastal lagoons dominated bywetting and drying cycles and periodic storms assuggested by Lowe and Fisher Worrell (1999). Shallowwaves and occasional storms ripped up local microbialmats and repeatedly reworked the sediment surface,preventing formation of complex carbonaceous grains orchert banding. Instead, black cherts were depositedconsisting of mixtures of clastic material and simplecarbonaceous grains (microfacies I), or simple carbona-ceous grains, evaporitic, and silica grains (microfaciesII). White silica laminations developed in wave ripplesappear to represent silica grains and possibly evaporiticgrains hydraulically separated from carbonaceousmatter.

Pervasive soft-sediment deformation and the abun-dance of microfacies II and III sediments in the lowerblack-and-white banded chert facies suggests depositionunder the influence of weak waves or currents. Theunit's wide extent, abundance of hydraulically finedetrital grains, and absence of coarser particles and high-energy current structures indicate deposition on an openmarine, wave- and current-active shelf. Disruption ofpartially consolidated sediment by weak waves andcurrents and by periodic storms allowed formation ofabundant complex carbonaceous grains, but was not sofrequent that microbial mats could not be preserved in-place (microfacies III). There is probably a widespreadunconformity between the evaporite facies and thisshelfal facies that would represent shallower shelf andshoreface settings. These areas had little or no coarsevolcaniclastic sediments available and were subject tocurrent and wave activity that eroded and removed anyCM deposited here between high-energy events.

The rarity of soft-sediment disruption and brecciationand abundance of microfacies IVand V sedimentation inthe laminated black-and-white banded chert faciesreflect subsidence of the volcanic platform to a depthnear or below storm wave base. Waves, currents, andlarger-scale storm activity that affected the bottom wereinfrequent. This facies represents a transitional environ-ment between the underlying moderate-energy platformfacies and the overlying no-energy banded ferruginouschert facies.

Virtually all CM appears to have formed originallywithin benthic microbial mats. Many complex carbo-naceous grains preserve structures reflecting origins as

Klm or Kn microbial mats. The only non-detritalcarbonaceous structures observed were laminationsand networks representing microbial mats, suggestingthat most BRC CM was ultimately derived by current,wave, and storm erosion of shallow-water benthicmicrobial communities.

2.10.2. Causes of silicificationMultiple generations of silica are evident in the BRC,

ranging from early white bands that lithified near thesediment surface to late cross-cutting quartz veins.Several lines of evidence suggest that the earliestgenerations of silica precipitated from normal marinewater (Lowe, 1999; Knauth and Lowe, 2003) rather thanhydrothermal fluids (de Wit et al., 1982; Paris et al.,1985; Brasier et al., 2002). (1) Silicification in the BRCoccurred in sediments deposited along at least 50 km ofstrike, and in marine environments ranging fromshallow-water evaporating ponds to a quiet, deep-waterbasin. Such persistence in space and depositionalenvironment is unlikely for a hydrothermal system. (2)In shallow water, silicification occurred at extremelyshallow sediment depths and may have been syndeposi-tional. Regionally uniform silicification of shallowsediments by fluids flowing up through or along alreadysilicified sediments is unlikely. (3) In sedimentsdeposited at intermediate depths, probably ∼200 m,thinly stacked layers are silicified preferentially alongtheir tops (Fig. 27). This pattern indicates that fluidsphysically above the sediment surface were the source ofdissolved silica rather than fluids seeping up through thesediment column. (4) No preserved vent stocks ormounds have been identified in the BRC; in fact, no largecross-cutting silica-rich features have been identified.No facies relationships in the BRC suggest the existenceof local vent mounds or breccias. (5) Most of the BRC,which has an age of b3416±5 Ma (Kröner et al., 1991),was deposited nearly 30 million years after emplacementand eruption of the underlying felsic volcanic complex,which has an age of 3445±3 Ma (Kröner et al., 1991).While that event was associated with wide scaletonalite–trondhjemite–granodiorite intrusion that droveregional hydrothermal activity (Knauth and Lowe, 2003;Tice et al., 2004), there was no clear heat source availableduring BRC time to drive widespread hydrothermal fluidflow. The BRC is singularly lacking in volcanic orvolcaniclastic components. (6)Metals were concentratedin sediments which also concentrated windblown dust,i.e. those likely to have been deposited most slowly.This pattern of accumulation is inconsistent with ahydrothermal metal source and most consistent with anormal marine setting. (7) Rare earth element (REE)

Fig. 33. Isopachous silica coatings in carbonaceous sediment. Theselayers represent the earliest generations of silica in black bands. Scalebar is 0.5 mm.


distributions in BRC rocks display a nearly constantheavy REE enrichment and slight positive europiumanomaly regardless of depositional environment (Ticeand Lowe, 2006). This constancy is inconsistent withmixing between marine and locally-derived hydrother-mal fluids.

The first generation of silica to form in the BRC wasmost likely particulate silica sediment which mixed withdetrital carbonaceous grains to form mixed carbona-ceous/siliceous oozes, such as the partially matrix-supported detrital layer in Fig. 25. As in modernYellowstone hot springs (Lowe and Braunstein, 2003),this silica probably precipitated directly from the watercolumn. In this case, however, there was no evident localsource of supersaturated dissolved silica other thanseawater. It is possible that supersaturationwas enhancedby evaporation in lagoons and shallow platform settings.Such enhancement would have persisted during intervalsbetween storms, which would have tended to partiallymix these slightly evaporitic masses with ambientseawater.

One of the next generations of silica formed theprecursor for white chert bands, which exhibited bothplastic and brittle deformation when black bandprecursor was still soft and fluid. Lowe (1999) makestwo arguments for an early diagenetic separation of blackand white bands in the Barberton greenstone belt thatapply directly to the BRC. (1) White bands throughoutthe Barberton greenstone belt are uniformly less thanabout 15 cm thick. If white bands are depositionalfeatures, it is highly unlikely that conditions necessary toform the thousands of white bands found in the BRC,across tens of kilometers of the ocean floor, would nothave persisted long enough in some environment to formthicker deposits. (2) With the exception of chemicalprecipitates such as siderite, white bands are pure chert.However rapidly white band precursor could have beendeposited, it is unlikely that carbonaceous grains wouldhave never been mixed in. These arguments pointcompellingly to an early diagenetic origin for most BRCwhite bands, which may explain a further observationspecific to the BRC. While black and white bands in thelower and upper black-and-white banded chert facies andthe banded ferruginous chert facies are subequal inthickness, maximum band thickness decreases system-atically from about 15 cm in the lower black-and-whitebanded chert facies to about 1 cm in the bandedferruginous chert facies. This thickness change couldreflect lower permeabilities in fine-grained, laminateddeep-water sediments than in coarse-grained shallow-water sediments, and a consequent shorter characteristictransport length for silica-depositing pore fluids.

Another very early generation of silica formedisopachous rims around carbonaceous grains withinthe sediment column (Fig. 33). This generation is mostextensively developed in microfacies III shallowplatform sediments, and was probably responsible forpreserving equant complex carbonaceous grains againstcompaction. Conversely, its absence in sedimentsformed below storm wave base allowed significantcompaction of CM in deep-water settings. Its formationprobably reflects both high silica concentration in theoverlying water column and the likely high permeabilityof relatively coarse microfacies III sediments. Subse-quent generations of silica filled remaining open porespace in shallow-water sediments and prevented furthercompaction in deep-water sediments.

2.10.3. Source of carbonaceous matterMat-like laminations and networks are preserved in-

place almost exclusively in microfacies III sediments inthe lower black-and-white banded chert facies depositedin a shallow platform setting. The near absence of matsin microfacies IV sediments of the upper black-and-white banded chert facies deposited in a deep platformto basin setting, even in relatively uncompactedenclaves, suggests that BRC mat-constructing commu-nities were restricted to water depths b200 m. Restric-tion of these laminations to shallow water probablyreflects confinement to the euphotic zone, whichgenerally corresponds to depths of b150 m (Lalli andParsons, 1997). While UV-polymerization of simpleorganics may also have been able to form carbonaceousfeatures restricted to shallow-water environments,radiation in much of the UV spectrum should havebeen rapidly attenuated in the uppermost 10–15 m of the


water column (Kappler et al., 2005). Thus, UV-dependent processes are unlikely to account for featuresinferred to have been formed abundantly under waterdepths 15–200 m. An ecological restriction is moreplausible, and the depth restriction is most consistentwith a biological origin. The lack of nearby hydrother-mal inputs precludes the high-temperature fluid/metalinteractions commonly proposed for hydrothermalabiotic formation of reduced carbon compounds(Huber and Wächtershäuser, 1997; Horita and Berndt,1999). It is possible that methane haze formation in anatmosphere with CH4/CO2∼1 has resulted in depositionof abundant carbonaceous matter not directly related tolocal biological carbon fixation later in Earth history(Pavlov et al., 2001b). Such haze would have alsoresulted in a strong anti-greenhouse effect (Pavlov et al.,2001a), inconsistent with evidence that surface tem-peratures during deposition of the BRC were high(Knauth and Lowe, 2003; Lowe and Tice, 2004).Evidence for a hot early Earth thus argues for anatmosphere with CH4/CO2≪1 and against a hazeorigin for BRC CM.

The isotopic carbon composition of bulk carbona-ceous matter associated with mats is −35‰ to −30‰compared to PDB, consistent with fixation by organismsemploying the Calvin cycle (Schidlowski, 2000).Organisms with a variety of physiologies use thispathway, including some types of oxygenic andanoxygenic photosynthesizers, and many chemoauto-trophs such as sulfide, iron, and hydrogen oxidizers(Madigan et al., 1997). The absence of ferric oxides inthe platform facies implies that carbon was not fixedpredominantly by iron oxidation. Sulfide and hydrogenoxidation both require free O2. The presence of sideriteand absence of ferric oxides and the lack of primarycerium anomalies throughout the BRC suggests that thepartial pressure of O2 was very low (Tice and Lowe,2006), making both of these metabolisms unlikely asprimary carbon fixation pathways. The restriction ofmats to the euphotic zone, the isotopic composition ofBRC CM, and the widespread distribution of sideriteand lack of hematite together suggest that BRC matcommunities were photosynthetic and anoxygenic.

3. Conclusions

The Buck Reef Chert was deposited under progres-sively increasing water depths in environments thatranged from shallow coastal lagoons to an open marinewave- and storm-dominated platform to a deep basin. Itwas cut off from sources of terrigenous and volcani-clastic sediment for most of its history, resulting in

sedimentation that was dominated by biological andchemical processes. The result was the accumulation ofan enormous thickness of carbonaceous and ferruginouschert. There is no evidence that deposition wasinfluenced by local hydrothermal systems.

The morphology of carbonaceous matter variessystematically with depositional environment. Carbona-ceous grains and mats were generally weak and easilyeroded by even low-energy waves and currents. Matgrowth was restricted to shallow-water environments,probably the euphotic zone. This distribution and thecarbon isotopic composition of −35‰ to −30‰ suggestsphotosynthetic fixation. Detrital carbonaceous grainsformed by erosion of microbial mats were distributedthroughout shallow- and deep-water environments.

Thus, a close field, petrographic, and geochemicalinvestigation of perhaps the largest accumulation ofcarbonaceous chert in the geologic record supports theworking hypothesis developed through a broad exam-ination of the geological CM record: the bulk of CM inthe BRC and rocks older than 3.0 Ga was produced byliving organisms. Ultimately the strength of this supportderives not from identification of microfossils, nor fromany single conclusive piece of evidence or “smokinggun”, but from the degree to which the model proposedhere satisfactorily accounts for all CM in the BRCwithin the context of the rocks themselves. We suggestthat future studies focus more generally on all CM foundwithin ancient geological units subject to the constraintof detailed paleoenvironmental reconstructions, and lesson restrictive analyses of exceedingly rare features likepossible microfossils and stromatolites.

Acknowledgments

This research was supported by NASA ExobiologyProgram grants NCC2-721, NAG5-9842, NAG5-13442, and NNG04GM43G to DRL, and by grants toDRL from the UCLA Center for Astrobiology. MMTwas also supported by a William R. and Sara HartKimball Stanford Graduate Fellowship and by a HarveyFellowship. The authors are grateful to the MpumalangaParks Board and especially Louis Loock (RegionalManager), Johan Eksteen, and Mark Stalmans, forallowing access to the Songimvelo Game Reserve. Wewould also like to thank Sappi Limited and J.M.L. vanRensburg, Forestry Manager, for permission to accessprivate forest roads and many key areas during thisstudy and Mr. Collin Willie for permission to accessoutcrops on Farm Schoongezicht. This manuscriptbenefited from comments by Martin Brasier and JohnHayes.


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