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Geochimica et Cosmochimica Acta 74 (2010) 6655–6668

Molybdenum isotope evidence for mildenvironmental oxygenation before the Great Oxidation Event

Yun Duan a,⇑, Ariel D. Anbar b, Gail L. Arnold a,1, Timothy W. Lyons c,Gwyneth W. Gordon a, Brian Kendall a

a School of Earth & Space Exploration, Arizona State University, Tempe, AZ 85287, USAb School of Earth & Space Exploration, and Department of Chemistry & Biochemistry, Arizona State University, Tempe, AZ 85287, USA

c Department of Earth Sciences, University of California, Riverside, CA 92521, USA

Received 12 April 2010; accepted in revised form 25 August 2010; available online 11 October 2010

Abstract

We use the molybdenum isotope paleoredox proxy to look for evidence of small amounts of O2 in the environment�50 Ma before the Great Oxidation Event (GOE) in a high resolution profile from the �2.5 Ga Mt. McRae Shale. Themolybdenum isotope compositions (d98/95Mo) from samples throughout the sequence span a range from 0.99& to 1.86&.All samples have heavier d98/95Mo compared to average upper continental crust. In addition, the upper (S1) and lower(S2) black shale units within the Mt. McRae Shale exhibit systematic differences in average isotopic compositions and distinctpatterns of d98/95Mo variation. Heavier d98/95Mo values occur in the S1 unit, where d98/95Mo correlates with Mo enrichments.In the S2 unit, d98/95Mo is not as heavy and is relatively invariant.

Based on sedimentary Fe proxies we infer that S1 sediments were deposited under euxinic conditions, so that Mo removalfrom bottom waters was likely quantitative. Thus, d98/95Mo in this interval likely records coeval seawater. The lighter d98/

95Mo values in the S2 unit may indicate a less fractionated ocean Mo inventory relative to the S1 unit. However, sedimentaryFe proxies suggest that S2 sediments accumulated under a water column that was ferruginous rather than euxinic, raising thepossibility of non-quantitative Mo scavenging and hence an expressed d98/95Mo fractionation relative to coeval seawater.Because any associated fractionations during this process would have favored the light isotope in sediments, the lighterd98/95Mo values in the S2 unit represent a lower limit on the value in contemporaneous seawater.

After evaluating a range of hypotheses, we conclude that the isotopically heavy d98/95Mo values seen throughout the Mt.McRae Shale likely reflect the effects of oxidative weathering and adsorption of Mo to oxide mineral surfaces on land or insurface oceans. The extent of environmental oxygenation in either unit is difficult to assess due to uncertainties in the globalMo isotope budget. Because of the small ocean Mo inventory in the Late Archean, documented by low concentrations of Moand low Mo/TOC, the extent of oxygenation required to account for the observed fractionations is much smaller than in mod-ern oceans. However, when juxtaposed against the record of d98/95Mo through time, our findings provide further evidence ofthe onset of environmental oxygenation before the GOE.� 2010 Elsevier Ltd. All rights reserved.

0016-7037/$ - see front matter � 2010 Elsevier Ltd. All rights reserved.

doi:10.1016/j.gca.2010.08.035

⇑ Corresponding author. Tel.: +1 253 2545128; fax: +1 4809658102.

E-mail address: yun.duan@asu.edu (Y. Duan).1 Present address: Max Planck Institute for Marine Microbiol-

ogy, 28359 Bremen, Germany.

1. INTRODUCTION

The oxidation state of the atmosphere and oceans priorto the “Great Oxidation Event” (GOE; between 2.45 and2.22 billion years [Ga] before present; Bekker et al., 2004;Canfield, 2005) is an area of active research. Recently, theabundances of redox-sensitive metals were investigated inthe Mt. McRae Shale, deposited �2.5 Ga in the Hamersley

6656 Y. Duan et al. / Geochimica et Cosmochimica Acta 74 (2010) 6655–6668

Basin, Western Australia (Anbar et al., 2007). Significantand anomalous molybdenum (Mo) and rhenium (Re)enrichments were found in a pyritic black shale unit withinthe Mt. McRae Shale. The Mo and Re concentrationsreach �40 ppm and �40 ppb, respectively, relative to aver-age upper continental crustal abundances of 1.5 ppm and1 ppb, respectively. These enrichments are thought to beauthigenic, based on strong positive correlations with totalorganic carbon (TOC) contents, indicating that Mo and Rewere present in seawater at appreciable levels >50 millionyears (My) before the GOE. Based on these data as wellas sulfur isotope relationships (Kaufman et al., 2007; Rein-hard et al., 2009), it was hypothesized that low but possiblypersistent quantities of O2 existed in some surface environ-ments before the GOE. This O2 resulted in mobilization ofMo, Re and S during oxidative weathering of crustal sulfideminerals in subaerial and/or submarine settings, followedby transport and accumulation in Late Archean oceans(Anbar et al., 2007; Reinhard et al., 2009). Contemporane-ous nitrogen isotope data suggest coupled nitrification anddenitrification in Late Archean surface oceans, consistentwith the presence of O2 in surface seawater (Garvin et al.,2009; Godfrey and Falkowski, 2009).

Molybdenum isotope variations in sedimentary rocksmay provide additional information about paleoredox con-ditions. The chemistry of Mo in natural waters and its longocean residence time (�800,000 years today; Morford andEmerson, 1999) make the Mo isotope system particularlyvaluable for tracing global ocean redox conditions (e.g.,Siebert et al., 2003; Anbar, 2004; Arnold et al., 2004; Willeet al., 2007; Pearce et al., 2008b). Fractionation of Mo iso-topes in the oceans today is driven largely by preferentialremoval of light Mo isotopes to manganese (Mn) oxide-richsediments (Barling et al., 2001; Barling and Anbar, 2004;Siebert et al., 2003) and suboxic sediments (“suboxic” refersto bottom water conditions in which dissolved O2 is low butH2S is absent in the water column; “suboxic sediments”

then refer to sediments deposited beneath a suboxic watercolumn) (Poulson et al., 2006; Siebert et al., 2006; Poul-son-Brucker et al., 2009). These two sediment types repre-sent the major Mo sinks in the oceans today (Bertine andTurekian, 1973; Morford and Emerson, 1999; McManuset al., 2006). Light Mo isotopes may also be preferentiallyretained in soils during weathering, possibly due to adsorp-tion to oxyhydroxides, generating isotopically heavy Mo inriver waters delivered to the ocean (average d98/95Mo of0.7&; Archer and Vance, 2008). Combined, these processesleave modern seawater with a heavy d98/95Mo of �2.3& rel-ative to Mo in the continental crust (average d98/95Mo of�0&; Barling et al., 2001; Siebert et al., 2003). Molybde-num is also removed from solution in basins with high[H2S]aq and restricted circulation (Helz et al., 1996; Erick-son and Helz, 2000; Algeo and Lyons, 2006). In such set-tings, Mo can be removed quantitatively from bottomwaters, or nearly so, such that the Mo isotope compositionin sediments can mimic that of seawater (Barling et al.,2001; Siebert et al., 2003; Arnold et al., 2004; Neubertet al., 2008; Dahl et al., 2010). It is therefore possible totrack the Mo isotope composition of ancient seawater– and hence the global redox conditions in ancient environ-

ments – by examining Mo isotopes in organic-rich shalesdeposited beneath euxinic water columns (Arnold et al.,2004; Pearce et al., 2008b; Gordon et al., 2009; Kendallet al., 2009).

In this study, we examine Mo isotope compositions in asubset of the Mt. McRae Shale samples studied by Anbaret al. (2007) to test the hypothesis of mild Late Archeanenvironmental oxygenation. Even weakly oxidizing condi-tions in the atmosphere or surface oceans could lead toMo isotope fractionation during weathering, transportand sediment removal, generating a heavy seawater Mo iso-tope signature in the Late Archean. We therefore predictthat the isotope composition of Mo in the Mo-enrichedzone of the Mt. McRae Shale should be heavier than thatof the upper continental crust. Previous studies of Late Ar-chean sediments from South Africa (�2.65–2.5 Ga) usedMo isotopes in this way to argue for increasingly oxidizedconditions (Wille et al., 2007). The goal of the present studyis to determine if such signatures exist in the Mt. McRaeShale at a time when other proxies point to increasing oxi-dation potential at the Earth’s surface.

2. SAMPLES AND METHODS

The sedimentary rocks analyzed in this study come fromthe Mt. McRae Shale, which comprises the upper �85 m ofa �1000 m continuous drill core (ABDP-9) recovered fromthe Hamersley Basin in Western Australia as part of the Ar-chean Biosphere Drilling Project of the NASA Astrobiol-ogy Institute. This stratigraphic sequence consists ofcomparatively shallow carbonate facies and deeper-waterblack shale and banded iron formation (BIF) deposited inrelatively deep water below wave base (Krapez et al.,2003; Barley et al., 2005; Anbar et al., 2007). The rockshave experienced only low grade metamorphism (preh-nite–pumpellyite facies to <300 �C) and minimal deforma-tion (gentle folding dips <5�) (Anbar et al., 2007). TheMt. McRae Shale includes carbonate/marl interbeds under-lain by two black shale units, referred to as the S1 (upper)and the S2 (lower) units. The black shale units are separatedby �20 m of siderite BIF (Fig. 2). Rhenium–osmium dataprovide a depositional age of 2501.1 ± 8.2 Ma for the S1unit (Anbar et al., 2007), which is consistent with a previousU–Pb zircon age of 2504 ± 5 Ma from a tuffaceous bedwithin the Mt. McRae Shale (Rasmussen et al., 2005). Thisstrong age agreement argues against resetting of the Re–Ossystem, thus indicating that primary geochemical signalshave likely been preserved since the time of deposition.Radiometric age constraints suggest an integrated sedimen-tation rate of �4.5 m/Ma during Mt. McRae Shale deposi-tion (Trendall et al., 2004). The sedimentation rate of BIFsis usually much higher than that of black shales, thus the S1and S2 intervals were probably deposited much less thanseveral million years apart.

Thirty-five bulk samples were selected at sampling inter-vals ranging up to a few meters. Sample spacings of �1 mwere typically achieved for the S1 black shale unit, theinterval associated with the highest and most variable Moabundances, to obtain higher-resolution profiles. Based onthe previously measured Mo abundances, 0.05–0.08 g of

Table 1Mo isotope measurements on bulk SDO-1 samples and the elutions.

Mo (lg) Cumulative Mo (lg) Cumulative % Mo eluted Cumulative d98/95Mo (&) 2-SD (&) Analysis replicate #

Bulk analysis n.d.a n.d.a n.d.a 1.10 0.13 16b

ElutionsElution 1 5.6 5.6 29% 2.69 0.06 4Elution 2 4 9.6 51% 1.51 0.06 3Elution 3 3.4 13 68% 0.85 0.05 3Elution 4 4.1 17.1 90% �0.06 0.09 4Elution 5 1.9 19 100% �1.21 0.01 3

a n.d., not determined.b Sixteen analysis replicates are the total of multiple MC-ICP-MS measurements on solutions of 6 sample powder of SDO-1, a USGS black

shale reference material. The elution curve was tested on one sample and multiple ICP-MS measurements were conducted on each eluted part.The trendline for these 5 d98/95Mo values could be used to indicate the offset from the bulk isotope value. Mo cumulation at 95% correspondsto an offset of 0.13&.

Mo isotope evidence of oxygenation before GOE 6657

powder were subsampled from each interval to guarantee aminimum amount of 200 lg Mo for isotopic analysis.

Sample powders were dissolved completely by standardacid digestion (HCl–HNO3–HF). The Mo-bearing solu-tions were passed through an anion resin column (BioRadAG1-X8) to remove most of the major elements/ions(e.g., Na, Mg, Ca, Zr, Ru, NO3�, SO4

2�) followed by a cat-ion resin column (BioRad AG50W-X8) specifically to re-move Fe. Several drops of 0.3 M hydrofluoric acid wereadded to each dissolved sample after resin chemistry to en-sure that Mo remained in solution. Aliquots taken beforeand after ion chromatography were analyzed with a quad-rupole inductively coupled plasma mass spectrometer(Thermo Scientific X-Series) to quantify Mo recovery andto assess sample purity after chromatography. Molybde-num concentrations were determined using an isotope dilu-tion method. Iron was monitored, and the samples withhigh Fe/Mo ratios were rejected to avoid potential isobaricinterference from 56Fe40Ar+. The absence of Zr in purifiedsamples was verified, since this element is added subse-quently to correct for instrument bias during high precision

Fig. 1. Molybdenum isotope compositions measured on the bulk sampleson 16 MC-ICP-MS measurements of 6 powder splits. The gray bar represyield reaches 95%, which demonstrates an isotopic fractionation of 0.13&

isotopic analysis (Anbar et al., 2001). We rejected any sam-ples with Mo yields less than 95% to ensure that isotopicartifacts due to chromatographic effects do not exceed0.13& (Table 1 and Fig. 1). A full procedural blank wasprocessed and analyzed along with each batch of samplesto make sure that the Mo blank contribution was less than2% of typical sample size.

Molybdenum isotopes were measured using a multiplecollector inductively coupled plasma mass spectrometer(MC-ICP-MS; Thermo Scientific Neptune) with an Ele-mental Scientific, Inc., Apex inlet system. We used auxiliarynitrogen gas to introduce �25 ng Mo for each measurement(�0.1 ml/min at 0.025 ppm), followed by a �1 min rinse atthe end of each run. Samples were analyzed in duplicate ortriplicate on the same solution. Data are reported relativeto our in-house Mo standard (Johnson Matthey Chemical,Specpure lot #702499I) using the standard d notation withreproducibility typically better than 0.15& (2-SD):

d98=95Mo ¼ð98Mo=95MoÞsample

ð98Mo=95MoÞstandard

� 1

" #� 103:

and the elutions of the SDO-1 standard. The average value is basedents the 2-SD reproducibility. The 95% line is marked where the Mo

due to chromatographic effects.

6658 Y. Duan et al. / Geochimica et Cosmochimica Acta 74 (2010) 6655–6668

For each instrument run, at least three quality control stan-dards were measured to ensure data integrity. These stan-dards include a Mo solution purified from the USGSDevonian Ohio Shale standard (SDO-1) and two gravimet-ric standards prepared in our laboratory by mixing a 95Moisotope spike solution (96.47% 95Mo) with Mo purifiedfrom SDO-1 (GravSDO) and the Specpure Mo standard(GravMo) in carefully weighed amounts to produce Moisotope variations known to high accuracy. The typicalexternal precision is provided by the SDO-1 standard,which has an average d98/95Mo of 1.16 ± 0.16& (2-SD,n = 238) based on measurements of five separate powderdigestions over 18 months. The long-term reproducibilityis also illustrated by GravSDO with an average d98/95Mo

Table 2Mo isotope data for the Mt. McRae Shale.

Depth (m) [Mo]a (ppm) d98Mo (&) 2-SD (&)

Organic-rich marl interval

109.00 8.04 1.24 0.07112.52 6.79 1.02 0.13113.46 11.46 1.45 0.05117.31 2.83 1.27 0.11121.39 3.70 1.21 0.01

Upper black shale unit (S1)126.15 7.36 1.07 0.18131.60 13.78 1.61 0.07133.97 19.40 1.38 0.11135.58 8.35 1.14 0.10138.74 16.77 1.23 0.04139.01 18.98 1.35 0.09139.97 25.01 1.26 0.12140.25 22.59 1.35 0.05140.50 29.07 1.33 0.08140.95 30.50 1.44 0.10141.47 21.94 1.45 0.09141.72 18.64 1.15 0.07142.08 19.05 0.99 0.16143.45 41.35 1.43 0.11145.61 40.05 1.86 0.15146.45 40.30 1.63 0.05147.30 27.09 1.48 0.10148.27 20.18 1.39 0.13149.30 37.08 1.80 0.05150.24 16.53 1.47 0.15153.08 8.65 1.42 0.12

Sideritic BIFs interval

157.80 7.99 1.25 0.18163.95 5.06 1.09 0.09168.36 3.35 1.00 0.17

Lower black shale unit (S2)174.67175.51 3.26 0.99 0.09177.10 3.15 1.00 0.11178.83 3.44 1.07 0.17180.33 2.94 1.12 0.10182.50 2.52 1.19 0.11187.46 3.17 0.99 0.08

a Mo abundance data were originally reported by Anbar et al. (2007).b Analysis replicate refers to multiple ICP-MS analyses on the same sac FeHR/FeT and FePY/FeHR data are from Reinhard et al. (2009).

of 0.18 ± 0.13& (2-SD, n = 241). Excellent accuracy isdemonstrated by the agreement of the measured offset be-tween GravMo and our in-house standard, with an averaged98/95Mo value of �0.98 ± 0.13& (2-SD, n = 327) and theoffset value of �0.95& ± 0.08& calculated from the gravi-metric weights. In addition, the measured offset between thedaily average SDO and GravSDO is �0.95 ± 0.07& (2-SD,n = 66), which compares very well with the offset of�0.95 ± 0.08& calculated from the gravimetric weights.

3. RESULTS

In the Mt. McRae Shale, d98/95Mo spans a large range ofvalues from 0.99& to 1.86& (Table 2 and Fig. 2). Black

Analysis replicate #b FeHRFeTc FePYFeHR

c

23323

3 0.22 0.3735 0.16 0.623 0.75 0.953 0.68 0.973 0.81 0.973 0.90 0.853 0.85 0.983 0.92 0.913 0.95 0.883 0.82 0.973 0.83 0.983 0.80 0.983 0.74 0.923 0.28 0.723 0.61 0.944 0.46 0.653 0.35 0.853 0.64 0.943 0.41 0.873 0.37

4 0.42 0.5132 0.44 0.09

3 0.35 0.473 0.37 0.6043 0.32 0.634 0.29 0.663

mple solution.

Fig. 2. Stratigraphic and geochemical profiles of the Mt. McRae Shale. d98/95Mo: this study; [Mo], [Mn], [Fe], Mn/Carbonate: Anbar et al.(2007); Fe speciation data (FeHR/FeT and FePY/FeHR): Reinhard et al. (2009). The dash lines in the two right panels are thresholdsinterpreted to reflect euxinia (Raiswell and Canfield, 1998; Reinhard et al., 2009).

Mo isotope evidence of oxygenation before GOE 6659

shales from the S2 unit have uniform d98/95Mo values, aver-aging 1.06 ± 0.17& (2-SD). In contrast, black shales fromthe S1 unit exhibit considerable variability in d98/95Mo, rang-ing from 0.99& to 1.86&, with an average of 1.39 ± 0.44&

(2-SD). Overall, values in S1 are heavier than those from S2.A pronounced upward increasing trend in d98/95Mo beginsright above S2 at the bottom of the siderite BIF layer (173–153 m) and extends smoothly into S1, reaching its peak valueat �145 m depth. After the peak, d98/95Mo generallydecreases up-section but with large variation within a 0.5&

range over the remaining portion of the S1 shale. In the over-lying carbonate/marl unit, d98/95Mo varies over a range from�1& to �1.5&. The Mo isotope trend across the coregenerally parallels the Mo abundance profile (Fig. 2).

4. DISCUSSION

4.1. Fidelity of the d98/95Mo record in Archean black shales

as a proxy for seawater

4.1.1. S1 unit

Mean and maximum d98/95Mo values in the S1 unit(1.39& and 1.86&, respectively) are closer to the value formodern seawater (�2.3&) than to average crustal Mo(�0&), suggesting oxidizing conditions. However, beforeusing any sedimentary Mo isotope record to infer paleore-dox information, it is important to evaluate its integrity as

a proxy for the d98/95Mo of coeval seawater. Molybdenumisotopes in black shales can record seawater d98/95Mo ifdepositional conditions are euxinic, with [H2S]aq >11 lM,conducive to quantitative conversion of molybdate to parti-cle-reactive tetrathiomolybdate (MoS4

2�) and removal ofMo to sediments. Quantitative transfer to sediments maynot be necessary, since Mo isotope fractionation duringtransfer of tetrathiomolybdate to particles or sedimentshas not yet been reported.

However, Mo isotope fractionation is well-known to oc-cur during Mo removal under suboxic conditions, wheredissolved O2 concentrations are low or negligible, or underweakly euxinic sedimentary environments where [H2S]aq isbelow 11 lM (Erickson and Helz, 2000; Nagler et al.,2005; Poulson et al., 2006; Siebert et al., 2006; Neubertet al., 2008; Poulson-Brucker et al., 2009). In such settings,light Mo isotopes are preferentially transferred to sedi-ments, producing black shales with d98/95Mo lighter thanthat of contemporaneous seawater. Therefore, a first stepin applying the Mo isotope paleoredox proxy in blackshales is to evaluate independently the redox state of the lo-cal water column (Gordon et al., 2009).

Geochemical data (e.g., TOC and pyrite sulfur concen-trations; degree of pyritization [DOP]; and ratios of totalFe to Al [FeT/Al], highly reactive Fe to total Fe [FeHR/FeT], and pyrite Fe to highly reactive Fe [FePY/FeHR]) areused commonly to constrain ancient water column redox

[Mo] (ppm)0 10 20 30 40

δ98M

o (‰

)

0.8

1.0

1.2

1.4

1.6

1.8

2.0

y = 0.017x + 0.962R2 = 0.545

Mo/TOC1 2 3 4

y = 0.285x + 0.813R2 = 0.590

a b

Fig. 3. (a) Correlation between d98/95Mo and [Mo] for samples from 135 to 150 m depth which have overall high FePY/FeHR; (b) correlationbetween d98/95Mo and [Mo]/TOC for the same samples.

6660 Y. Duan et al. / Geochimica et Cosmochimica Acta 74 (2010) 6655–6668

conditions (Berner, 1970; Raiswell et al., 1988; Raiswell andCanfield, 1998; Wilkin and Arthur, 2001; Lyons and Sever-mann, 2006). In the S1 unit, elevated values for TOC (aver-age = 9.22%; Anbar et al., 2007), FeT/Al (average = 1.1),FeHR/FeT (average = 0.58) and FePY/FeHR (aver-age = 0.86; see Reinhard et al., 2009 for details) are consis-tent with deposition in a dominantly if not persistentlyeuxinic water column. In particular, for samples from depth135 to 150 m, FeHR/FeT and FePY/FeHR values are high,which unambiguously indicates deposition under euxinicwaters (Raiswell and Canfield, 1998; Sageman and Lyons,2003; Reinhard et al., 2009). Although the exact sulfide le-vel is unknown, these conditions are most conducive to cap-turing the seawater d98/95Mo in sediments, especially thosedeposited in ancient basins when ocean Mo concentrationswere low (Lyons et al., 2009).

At this same depth in the core, we see a weak positivecorrelation between d98/95Mo and Mo abundance(R2 = 0.545) (Fig. 3a). This observation could be taken toindicate non-quantitative scavenging of Mo even in theseeuxinic sediments. Non-quantitative Mo scavenging undersuch conditions has been observed in most modern euxinicsettings (Algeo and Lyons, 2006). First, fractionation mayoccur because water column [H2S]aq does not exceed the11 lM necessary for quantitative conversion of molybdateto particle-reactive tetrathiomolybdate (MoS4

2�; Helzet al., 1996; Erickson and Helz, 2000). In such cases, sedi-mentary d98/95Mo may be offset from dissolved d98/95Mobecause of an isotope fractionation between molybdateand thiomolybdates scavenged onto particles in the watercolumn or sediment (Tossell, 2005; Neubert et al., 2008;Dahl et al., 2010). Fractionation might also occur if the rateof in-mixing of oxygenated, Mo-rich seawater exceeds therate at which Mo is converted quantitatively to tetrathio-molybdate and is removed into the sediments. If the Mt.McRae Shale represents one of these scenarios, the heaviestd98/95Mo of 1.86& reflects a lower limit on the isotope com-position of Mo in late Archean seawaters.

However, non-quantitative Mo scavenging predicts aninverse correlation between d98/95Mo and Mo/TOC values.

This expectation follows because the extent of fractionationexpressed during incomplete removal to sediments – whichshould appear as a fractionation toward lighter d98/95Mo inthe sediments – should increase as the amount of Mo in theoverlying water column increases, due to mass balance con-siderations. Mo concentrations in the water column arebest tracked by Mo/TOC (Algeo and Lyons, 2006; Scottet al., 2008). Suppressed enrichment reflects a lower inven-tory and therefore greater likelihood of quantitative Mouptake, thus muting any light offset relative to the seawaterd98/95Mo value. Contrary to this expectation, we see apositive correlation between d98/95Mo and Mo/TOC(R2 = 0.590; Fig. 3b). Therefore, the trends seen in S1 morelikely reflect real variations in seawater d98/95Mo. Thisinterpretation is consistent with the relatively low Mo/TOC values of 2–3 ppm/wt% for the S1 unit in the Mt.McRae Shale (Anbar et al., 2007), which suggest that de-spite the Mo enrichment in these sediments the oceanicMo reservoir was extremely low compared to today, facili-tating quantitative transfer of Mo to sediments. SeawaterMo concentrations were likely similar to, or even lowerthan, the Mo-depleted, sulfidic deep waters of the modernBlack Sea (sediment Mo/TOC �4.5 ppm/wt%; Algeo andLyons, 2006; Scott et al., 2008), where Mo is quantitativelyremoved to sediments (Arnold et al., 2004; Neubert et al.,2008). Therefore, quantitative Mo removal during S1 depo-sition was most likely, particularly during deposition of the135–150 m interval, which exhibits the persistently highFePY/FeHR ratios that point to euxinia (Reinhard et al.,2009), leading us to conclude that d98/95Mo values in thesesediments are a proxy for contemporaneous seawater. Thisconclusion compels us to explore alternative explanationsfor the correlation of d98/95Mo with Mo abundance, dis-cussed further below (Section 4.3).

4.1.2. S2 unit

The S2 unit displays relatively uniform d98/95Mo valuesof 1.06 ± 0.17& (2-SD). These values are heavier thanthose of average upper crust but lower than mostd98/95Mo values in the S1 unit (Fig. 2). Although Mo

Mo isotope evidence of oxygenation before GOE 6661

abundances are much lower in S2 (average = 3.2 ppm) thanin S1 (average = 18.5 ppm), Mo enrichment factors in S2are typically 2–4 and thus are significantly above the detri-tal baseline. Therefore, Mo in the S2 unit includes a non-trivial authigenic component that is isotopicallyfractionated.

Unlike the S1 unit, however, there is good reason to be-lieve that the Mo isotope compositions in S2 do not accu-rately reflect contemporaneous seawater. The FeHR/FeT

values from this unit are elevated above siliciclastic back-ground, suggestive of Fe transport and scavenging underan anoxic water column (Reinhard et al., 2009). However,low values of FePY/FeHR (an average of 0.51) indicate thewater column was sulfide-limited, suggesting the S2 unitwas deposited under a ferruginous water column that wasanoxic but not sulfidic. If so, water column [H2S] was al-most certainly too low for quantitative conversion ofmolybdate to thiomolybdates (Reinhard et al., 2009), whichwould have reduced the efficiency of Mo removal to sedi-ments, thus allowing fractionation during transfer fromwater to sediments. No significant correlation is seen be-tween d98/95Mo and [Mo] or Mo/TOC in S1, but this isprobably because of the very low Mo concentrations andrelatively uniform isotope compositions compared to S1.

Studies of Mo isotopes in modern anoxic sediments,although not exact analogs, provide clues to the isotopefractionation during Mo scavenging into Late Archean fer-ruginous sediments. Specifically, sediments from three an-oxic Mexican margin sites demonstrate an invariantaverage d98/95Mo of 1.6 ± 0.1&, suggesting an anoxic frac-tionation of �0.7& from the present-day seawater value of�2.3& (Poulson et al., 2006; Poulson-Brucker et al., 2009).Further, unlike modern anoxic sediments, as suggested byFe speciation data, the S2 unit may have deposited undera ferruginous water column that generated appreciable con-centrations of Fe oxides (Reinhard et al., 2009). Hence, theMo isotope signatures may have affected by fractionationsduring Mo adsorption to these oxide minerals. Althoughthese isotope fractionation factors remain uncertain, recentlaboratory studies on Mo isotope fractionation duringadsorption to ferrihydrite, as an analog to diverse Fe oxy-hydroxides, demonstrate that light Mo isotopes preferen-tially adsorb to ferrihydrite, as they do to Mn oxides, butwith a smaller fractionation factor (D98/95Moaq-ferrihydrite

�1.24 ± 0.35&, Wasylenki et al., 2008a,b; and�1.11 ± 0.15&, Goldberg et al., 2009). These results sug-gest a significant offset between the Mo isotope compositionof Late Archean seawater and ferruginous sediments, suchthat the Mo isotope data from S2 only provide a lower limiton the value in contemporaneous seawater.

4.2. Potential cause(s) of heavy d98/95Mo signatures

The d98/95Mo values observed in the S1 and S2 units re-flect a strikingly heavy seawater Mo isotope signature whencompared with average crustal Mo (d98/95Mo �0&). Anumber of potential causes can be considered. These in-clude: (1) weathering of isotopically heavy Mo from molyb-denites, known to have large variability in d98/95Mo of upto 1.9& (Hannah et al., 2007); (2) influx of Mo from low

temperature hydrothermal systems, which are isotopicallyheavy compared to average crust (McManus et al., 2002);(3) oxidative weathering on land followed by riverine trans-port, which supplies isotopically heavy Mo to the ocean to-day (Archer and Vance, 2008); and (4) preferential uptakeof light Mo isotopes by oxic and/or suboxic ocean sedi-ments (Barling et al., 2001; Siebert et al., 2003, 2006; Poul-son et al., 2006). These possibilities are each consideredbelow.

4.2.1. Weathering of isotopically heavy crustal materials

It is conceivable that the Mo isotope composition of theArchean crustal rocks weathered to provide Mo to theHamersley Basin was heavier than the assumed averagecontinental crust value. This possibility arises because re-cent Mo isotope measurements of 20 molybdenite samplesfrom a variety of settings and ages reveal a total variationof �1.9& in d98/95Mo (Hannah et al., 2007). Althoughthese data were not calibrated against a known referencestandard and so cannot be easily compared to other surfacereservoirs, this discovery suggests that Mo isotope variabil-ity in crustal materials is larger than previously supposed,even though the mean value remains �0&. Given the smallocean Mo inventory in the Late Archean, the local Mo iso-tope composition in a basin could have been strongly im-pacted by d98/95Mo of the local weathering environment.

The relative contributions of various Mo crustal sourcesto the Hamersley Basin are difficult to quantify. However,the isotopic composition of these sources can be estimatedfrom measurements in the S2 unit. Much lower Mo concen-trations and enrichments, as well as the weak correlationbetween Mo and Al (Anbar et al., 2007), suggest a signifi-cant detrital component in the Mo budget in S2. DetritalMo constitutes 30–45% of the bulk Mo for the majorityof samples, estimated from Mo and Al contents in the sam-ples and assuming a detrital Mo/Al ratio equal to that ofthe average continental crust. Therefore, the Mo isotopecompositions in S2 should reflect a mixture of detrital Moand scavenged seawater Mo. The seawater Mo componentis presumably isotopically heavier than detrital componentsbecause known Mo isotope fractionation processes prefer-entially remove light isotopes from solution (Barlinget al., 2001; Siebert et al., 2003; Poulson et al., 2006; Was-ylenki et al., 2008a,b; Goldberg et al., 2009). Thus the aver-age S2 value, 1.1&, places a firm upper limit on d98/95Mo ofdetrital – and, hence, crustal – Mo entering the basin. If weassume the Mo isotopic composition of the crustal materi-als delivered to the basin were broadly similar during depo-sition of S1 and S2, given that they were likely depositedmuch less than several million years apart as discussed ear-lier, then a crustal source heavy enough to account for theS1 values is precluded.

4.2.2. Isotopically heavy hydrothermal Mo

Another possibility is that the Mo isotope signature ofS1 was imprinted by hydrothermally derived Mo with aheavy isotopic composition. However, Re/Os geochronol-ogy reveals no post-depositional disturbances of Re or Os(Anbar et al., 2007), suggesting substantial post-deposi-tional enrichment of Mo is unlikely because such a process

[Mn] (wt%)0.0 0.1 0.2 0.3 0.4 0.5

δ98M

o (‰

)

0.8

1.0

1.2

1.4

1.6

1.8

2.0

[FeHR] (wt%)0 2 4 6 8 10

MnFeHR

Fig. 4. d98/95Mo vs. [FeHR] and d98/95Mo vs. [Mn] demonstratepoor correlations for all measured samples from the Mt. McRaeShale. We use [FeHR] instead of [Fe] because the former is a betterrepresentations for enrichments of Fe.

6662 Y. Duan et al. / Geochimica et Cosmochimica Acta 74 (2010) 6655–6668

would have affected the distribution of Re (Kendall et al.,2009). Syngenetic hydrothermal Mo input to the basin alsowas probably not a factor because of the lack of co-varia-tion of enrichments of Fe and Mn with Mo isotopes(Fig. 4); Fe and Mn are usually released in large quantitiesin association with hydrothermal sources (Tribovillardet al., 2006). Additionally, a syndepositional hydrothermalhypothesis requires d98/95Mo as heavy as 1.86& in Mo fromhydrothermal sources to the basin or to the sediments.Available data on Mo isotopes in modern low temperaturehydrothermal fluids indicate a d98/95Mo of �0.8& (McM-anus et al., 2002), or even lower (Pearce et al., 2008a).Molybdenum isotope compositions as heavy as those seenin S1 are unknown in hydrothermally derived Mo. Also,high-temperature hydrothermal system most likely acts assinks instead of sources for Mo due to the low solubilityof Mo sulfides (Anbar et al., 2007). Based on all these con-siderations, a hydrothermal Mo contribution to theHamersley Basin is an unlikely explanation for the observedMo isotope values.

4.2.3. Oxidative weathering on land followed by riverine

transport

If the isotopic composition of Archean riverine Mo washeavier than the estimated value for modern rivers, wemight expect an isotopically heavy Mo source to the ocean.Recent measurements from modern rivers yield a total-Mo-weighted d98/95Mo value of �0.7&, but with a wide rangeof isotopic compositions spanning from 0.2& to 2.3& (Ar-cher and Vance, 2008). These isotopically heavy values areinterpreted to be the result of Mo isotope fractionation dur-ing oxidative weathering due to preferential retention oflight Mo in soils by adsorption onto Mn–Fe oxyhydroxides(Archer and Vance, 2008). It may be hypothesized then,that in late Archean terrestrial weathering conditions,

which were characterized by very low partial pressure ofatmospheric O2 and the absence of major biological influ-ences, the soil horizon would have been much thinner andoxide minerals would have been much scarcer at the surfacecompared to today, thus creating less opportunity foradsorption and fractionation of Mo isotopes.

If the dissolved Mo pool was much smaller, however,the extent of fractionation could have been greater even ifthe abundance of oxide surfaces was much lower than to-day’s. For example, if only 10% of dissolved Mo was ad-sorbed to oxide minerals in soils with a fractionationfactor (D98/95Modissolved-oxide) of 3.0& (Barling et al.,2001), and assuming an initial Mo pool with d98/95Mo�0&, the residual dissolved Mo pool would have ad98/95Mo value of �0.3&. On the other hand, if thepercentage of Mo adsorbed increased to 50% because theinitial dissolved Mo pool was extremely small, the residualMo pool would have a d98/95Mo value of �1.5&. There-fore, we cannot exclude the possibility that the Mo riverineflux in the late Archean was isotopically equivalent to, oreven heavier than, the modern riverine value, and thus con-stituted a heavy source of Mo to seawater during that time.

Importantly, this mechanism requires the presence ofoxidized mineral surfaces to drive Mo isotope fraction-ation. The suggestion therefore is that the heavy d98/95Movalues in S1 are a consequence of at least a mildly oxidizingsurface environment. The dissolved O2 concentration ofrivers may have been sufficiently elevated to enable trans-port of dissolved molybdate to the Late Archean ocean.Regardless, this model is consistent with the overarchingconclusions of our paper – i.e., a mildly oxidizing surfaceenvironment.

4.2.4. Preferential uptake of light Mo isotopes by ocean

sediments

Heavy Mo isotopic compositions in seawater could haveresulted from the removal of Mo into oxic and/or suboxicsettings in the Hamersley Basin due to fractionation duringMo removal with Mn and Fe oxides to sediments (Barlinget al., 2001; Siebert et al., 2003; Poulson et al., 2006; Poul-son-Brucker et al., 2009). Such a model may be captured byMn-to-total carbonate ratios (Mn/Carbonate) in our sam-ples (Fig. 2), which shows overall higher normalized Mncontents in S2 (average Mn/Carbonate = 0.010) comparedto S1 (average Mn/Carbonate = 0.005). This drop is alsorecorded in the siderite BIF layer between the two blackshale units, as marked by Mn contents that decrease stea-dily upward. Assuming that Mn in these rocks is mainlyassociated with carbonate phases and that the Mn contentin the carbonates scales in proportion to the concentrationof Mn in the ocean, our drop in Mn/Carbonate may reflectdepletion in the dissolved Mn inventory in seawater. Thisrelationship would be expected if increasing environmentaloxygenation led to a general drawdown of Mn due to theprecipitation of Mn oxides in shallower parts of the basin.Based on the best estimates available, deep sea oxygenationdid not develop until the Late Neoproterozoic era or evenlater (Canfield, 1998; Canfield et al., 2007, 2008; Shenet al., 2008; Lyons et al., 2009). Therefore, Mn oxide forma-tion during deposition of the Mt. McRae Shale was most

Mo isotope evidence of oxygenation before GOE 6663

likely restricted to mildly oxic shallow waters on continen-tal shelves rather than deep basins.

Similar to the Mo sink associated with Mn oxides, theexistence of a significant expanse of suboxic seafloor couldalso have driven seawater Mo isotopically heavy owing tothe fractionation that occurs in such environments (Poulsonet al., 2006; Siebert et al., 2006). Suboxic sedimentary envi-ronments such as continental margins are recognized asimportant Mo sinks today (McManus et al., 2006). In LateArchean oceans, the extent of bottom-water oxygenationand associated Mn oxide formation was presumably small,and so Late Archean suboxic sediments could have consti-tuted a relatively more important sink for Mo compared totheir percent contribution in the modern mass balance.

An additional consideration is the potential influence ofFe oxide minerals on Mo isotope fractionation. Bandediron formations were widespread in Late Archean to Paleo-proterozoic sedimentary settings, reflecting Fe2+-rich deepoceans. As previously mentioned, light Mo isotopes prefer-entially adsorb to ferrihydrite but with a smaller fraction-ation factor than that associated with Mo adsorption toMn oxides (Wasylenki et al., 2008a,b; Goldberg et al.,2009). Multiple mechanisms have been proposed for Fe oxi-dation to ferric oxyhydroxide in the photic zone, includingdirect oxidation of Fe2+ by O2 derived from oxygenic pho-tosynthesis, photo-oxidation (Cloud, 1973; Cairns-Smith,1978; Braterman et al., 1983; Anbar and Holland; 1992),and anoxygenic photosynthesis using Fe2+ as an electrondonor (Walker, 1987; Widdel et al., 1993; Ehrenreich andWiddel, 1994; Kappler et al., 2005). The latter two mecha-nisms do not require free O2 and may have been responsiblefor BIF deposition at some locations and times during theArchean. However, nitrogen and sulfur isotope data indi-cate that dissolved O2 abundances in the Late Archean sur-face ocean were sufficient to support coupled nitrificationand denitrification and an oxidative sulfur cycle (Kaufmanet al., 2007; Garvin et al., 2009; Reinhard et al., 2009).Therefore, while anoxic production of ferric oxides mayhave played a role in BIF formation, such oxides werenot the sole mechanism for the total production of Fe oxi-des in the ocean and the associated fractionation of Mo iso-topes. Thus, Mo adsorption to Fe oxides in mildly oxic andsuboxic shallow water environments, like Mn, could haveplayed a major role in driving seawater Mo to isotopicallyheavy values.

4.3. Synthesis: preferred interpretation

4.3.1. Heavy d98/95Mo values in the S1 unit

After considering these possible explanations for theheavy seawater Mo isotope composition in S1, it seemsmost plausible that Mo isotope fractionation in the S1 unitwas driven by Mo adsorption onto oxide minerals formedby oxidation in subaerial weathering environments, in thesurface oceans, or both. Heavy d98/95Mo values of up to1.86& suggest that the Mo isotope composition of seawaterduring the deposition of S1 could have approached themodern value. However, unlike today, the deep oceans werelikely anoxic during that time (Isley, 1995; Canfield, 2005).Removal of Mo to oxic or suboxic sediments was most

likely restricted to shallow waters, associated with Mn–Feoxides deposited on continental shelves. We propose thatseawater d98/95Mo was highly sensitive to the adsorptionof even small amounts of isotopically light Mo to oxideminerals because of the very low Mo inventory in Late Ar-chean oceans, as inferred from low Mo/TOC ratios. Assuch, the observed heavy Mo isotope compositions do notrequire extensive deep ocean oxygenation.

We can invoke a simplified model to assess the extent ofoxygenation required to produce the observed heavy d98/

95Mo in Late Archean oceans (see Appendix A). Despiteuncertainties associated with the model, such as the relativeimportance of Mo isotope fractionation occurring in sub-aerial weathering environments vs. in the oceans and thecomplexities of a system with multiple sinks that can frac-tionate Mo isotopes to differing extents, important insightscan be gained from such a model. Assuming that Mo inLate Archean rivers had an average isotopic compositionsimilar to today, the model confirms that the flux associatedwith the oxic sink does not need to be very large to generatethe heavy Mo isotope compositions observed in the Mt.McRae Shale. For example, if we assume that the riverineMo input was 1% of today’s, a seawater d98/95Mo of�1.39& can be generated with a Mo oxic output flux thatis no more than 0.35% of the riverine Mo flux into modernoceans, as compared to �35% today (Fig. A1; Scott et al.,2008). This solution is not unique and includes manyassumptions, but it demonstrates that the heavy Mo isotopesignatures in the Mt. McRae Shale do not require extensiveoxygenation of the Late Archean oceans.

4.3.2. Isotopic variation within the S1 unit

Non-quantitative Mo removal was debated as a possibleinterpretation of the weak correlation between Mo isotopevariation and Mo abundance (Section 4.1.1 and Fig. 3a).However, as discussed earlier, euxinic conditions (as shownby high FePY/FeHR) and the inferred small Mo oceanicinventory favor quantitative Mo removal to sediments.Therefore, the Mo isotope variation in S1 more likely re-cords real variation in seawater values. One possible expla-nation is a linkage between d98Mo, Mo concentrations andthe amount of O2 present in the environment. For example,increased O2 would lead to a higher Mo influx to the oceansas well as an enhanced opportunity for Mo isotope fraction-ation during oxidative weathering and the deposition in oxicand suboxic sedimentary environments. Hence, a conse-quence of oxygenation could be higher Mo concentrationsgoing hand-in-hand with heavier isotopic compositions.

Alternatively, sedimentary Fe speciation evidence ofwater column euxinia during S1 deposition (Reinhardet al., 2009) suggests the co-variation in Mo and d98/95Momay reflect drawdown of the oceanic Mo inventory viaMo scavenging in euxinic environments. In this interpreta-tion, the onset of persistent water column euxinia, as re-corded by uniformly high FePY/FeHR >0.8 in the lowerpart of S1 (�146 m) coincides with the heaviest inferredseawater d98/95Mo (1.9&), which reflects a balance betweenMo removal to oxide minerals and euxinic sediments in thebasin. If water column euxinia expanded in response to ris-ing oceanic sulfate concentrations caused by oxidative

6664 Y. Duan et al. / Geochimica et Cosmochimica Acta 74 (2010) 6655–6668

weathering of continental sulfide minerals (Reinhard et al.,2009), the removal rate of Mo to euxinic sediments mayhave outpaced Mo resupply from oxidative weathering tothe basin, driving down the ocean Mo inventory. A propor-tionally higher fraction of Mo removed to euxinic sedi-ments relative to the input flux and/or relative to removalwith oxide minerals could have driven seawater d98/95Molower, towards the riverine input d98/95Mo.

These two explanations lead to different implicationsabout redox conditions reflected in the declining Mo con-centrations and d98Mo in the upper portion of S1. The for-mer implies a decreasing oxidizing condition while the lattersuggests an expansion in euxinia. A full evaluation of thelate Archean Mo isotope budget and the significance ofthe S1 trends require further investigation.

4.3.3. S2 unit

As we discussed in Section 4.1.2, if Mo scavenging wasnot quantitative during deposition of S2, the Mo isotopecomposition could have been offset from the seawater valuedue to preferential uptake of light Mo – expressed due toincomplete transfer of Mo from ferruginous seawater tosediments. Therefore, the d98/95Mo recorded in S2 sedi-ments presumably represents a lower limit (1.1&) for coe-val seawater, which implies similar oxidizing conditionsduring deposition of S2 compared with S1.

Age0.0 0.5 1.0 1.5

98M

o (‰

)

0.0

0.5

1.0

1.5

2.0

2.5

Mo

(ppm

)

0

50

100

150

200

250

300

Phan Proterozo

δ

Fig. 5. The lower panel: Mo isotope record in black shales and other orgthis study and Wille et al. (2007); solid circle = samples of all other ages (et al., 2007; Neubert et al., 2008; Pearce et al., 2008b; Gordon et al., 20enrichments in black shales (Scott et al., 2008). Two dashed lines separaGreat Oxidation Event as defined by the multiple sulfur isotope record (Boceanic Mo reservoir from Mo enrichments and Mo/TOC ratios (Scott

4.4. Mo isotope records through time

Our d98/95Mo measurements in the 2.5 Ga Mt. McRaeShale are consistent with those reported from �2.65 to2.5 Ga Ghaap Group (Transvaal Supergroup, KaapvaalCraton) of South Africa, in which variable d98/95Mo valuesrise as high as �1.72& together with mildly elevated Moconcentrations (up to 8.3 ppm; Wille et al., 2007). The Gha-ap Group findings were also interpreted as evidence ofocean oxygenation in the Late Archean (Wille et al.,2007). It has been postulated that the Kaapvaal Cratonand the Pilbara Craton were components of a single conti-nent, “Vaalbara” (Cheney, 1996). Therefore, the similaritiesin their Mo isotope patterns imply oxygenation at least on aregional scale.

Nhe d98/95Mo data from the Mt. McRae Shale can becompared with published data from other black shalesand organic-rich sedimentary rocks to examine temporaltrends and the potential link with environmental oxygena-tion (Fig. 5). Setting aside for a moment samples fromthe Late Archean (reported in this study and in Wille etal. (2007)), variations in d98/95Mo in a general sense mirrorMo abundances, which are tied strongly to the size of theoceanic Mo reservoir (Scott et al., 2008). Mid-Archeanblack shales exhibit small Mo isotope fractionationsrelative to the crustal baseline, suggesting a sedimentary

(Ga)2.0 2.5 3.0 3.5

ic Archean

GOE

anic-rich rocks through time. Open circle = Archean samples fromArnold et al., 2004; Siebert et al., 2005; Lehmann et al., 2007; Wille09; Kendall et al., 2009). The upper panel: temporal trend in Mote Phanerozoic, Proterozoic and Archean. The grey bar marks theekker et al., 2004). The solid curve in the upper panel is the inferredet al., 2008).

Fig. A1. The estimated Mo flux into the oxic sink. It is expressed asa fraction relative to the Mo riverine input flux into the modernocean and plotted vs. fE, the fraction of the Mo euxinic flux. A y-axisvalue of 1 corresponds to the modern ocean Mo riverine flux. Eachcurve represents a riverine flux assumed as a fraction relative to thepresent-day value. The assumed fractions vary from 10% to 0.001%.

Mo isotope evidence of oxygenation before GOE 6665

Mo budget that was dominated by detrital inputs (Siebertet al., 2005; Wille et al., 2007). This interpretation is consis-tent with the low Mo abundances in these samples(1–2 ppm) – similar to average continental crust (Siebertet al., 2005; Wille et al., 2007). Mid-Proterozoic black shalesexhibit intermediate d98/95Mo values (Arnold et al., 2004;Kendall et al., 2009), consistent with increased Mo enrich-ments resulting from a persistent oxidative weatheringsource and strong oceanic sinks, specifically widespreadeuxinia (Canfield, 1998; Scott et al., 2008; Lyons et al.,2009). Heavy d98/95Mo values are typical of the Phanerozo-ic black shales (Neubert et al., 2008; Pearce et al., 2008b;Gordon et al., 2009). Together with extreme Mo enrich-ments, these Phanerozoic values reflect a greatly expandedoceanic Mo reservoir with pervasive deep ocean oxygena-tion and correspondingly limited euxinia (Scott et al., 2008).

Set against this context, the d98/95Mo from the LateArchean reported in this study and in Wille et al. (2007) con-stitute a spike toward heavy values superimposed on anotherwise stepwise temporal Mo isotope trend (Fig. 5).These variable and relatively heavy isotopic values suggestthat the fraction of dissolved Mo in Late Archean oceansthat was affected by oxidative processes increased sharply,to be proportionally comparable to today. As discussed pre-viously, the much smaller size of the Mo oceanic inventory atthat time, implied by low Mo enrichments and low Mo/TOCratios, means that much less extensive oxygenation is re-quired to explain the heavy values of the Late Archean(Fig. A1). This Late Archean Mo isotope spike is thereforereadily interpreted to reflect the advent of mildly oxidizingsurface environments, well in advance of the GOE. Afterthe GOE, enhanced atmospheric oxygenation led to an in-crease in the riverine Mo flux and the ocean Mo inventory,which may have outpaced the increase in the extent of oceanoxygenation. This, and the increasing proportion of Mouptake under euxinic conditions, may have caused thed98/95Mo to shift back to less fractionated values.

5. CONCLUSIONS AND IMPLICATIONS

The Mo isotope profile of the 2.5 Ga Mt. McRae Shalereveals large isotope fractionations of up to 1.86& in d98/

95Mo relative to the average continental crust value. Theupper (S1) and lower (S2) pyritic black shale units in theABDP-9 core are characterized by variable d98/95Mo values(1.39 ± 0.44&) and relatively lower but uniform d98/95Movalues (1.06 ± 0.17&), respectively.

Sedimentary Fe proxies provide independent evidencefor the presence of euxinic water column conditions duringthe deposition of the S1 unit (Reinhard et al., 2009), fromwhich we infer that Mo removal from bottom waters wasquantitative. Sedimentary Fe proxies for the S2 unit suggestferruginous water column conditions and likely non-quan-titative behavior (Reinhard et al., 2009), hence the Mo iso-tope data from S2 represents a lower limit for the d98/95Movalue of contemporaneous seawater because any associatedfractionations would have favored the light isotope. Over-all, despite distinct patterns in the two black shale units,the Mo isotope record throughout the Mt. McRae Shale re-flects heavier seawater Mo isotope values relative to aver-age continental crust.

These heavy values are not readily explained by Mo ofdetrital or hydrothermal origin. Instead, they are most likelythe result of isotope fractionation driven by Mo adsorptiononto oxide minerals formed by oxidation in subaerial weath-ering environments, in the surface oceans, or both. The extentof oxygenation is difficult to quantify, but mass balance con-siderations indicate that only a relatively small Mo flux asso-ciated with oxic sink was required to generate the observedd98/95Mo values in a �2.5 Ga ocean with a much lower Mocontent compared to today. When viewed from a broad per-spective, including that provided by previous work (Anbaret al., 2007; Kaufman et al., 2007; Garvin et al., 2009;Reinhard et al., 2009), the Mo isotope record in the Mt.McRae Shale provides further evidence for mild oxidizingconditions at the Earth’s surface during the Late Archean.

ACKNOWLEDGMENTS

We thank the NASA Exobiology Program, the NASA Astrobi-ology Institute and the NSF Low Temperature Geochemistry andGeobiology Program for financial support. Comments by DerekVance, Simon Poulton, Andrey Bekker and an anonymousreviewer improved the manuscript.

APPENDIX A

A.1. A quantitative examination of the Mo isotope budget

To quantitatively explore the extent of oxygenation, weconstruct a mass balance model of the Mo isotope budgetfor the late Archean ocean, modified from that initially pro-posed by Arnold et al. (2004) and updated by McManuset al. (2006):

dRFR þ dHFH ¼ dOFO þ dEFE þ dMFM: ð1Þ

Here, “F” and “d” refer to the Mo flux and Mo isotopecomposition for each of the components involved. This

6666 Y. Duan et al. / Geochimica et Cosmochimica Acta 74 (2010) 6655–6668

model includes two Mo inputs, from rivers (R) and fromlow temperature hydrothermal systems (H). Three typesof Mo sinks involved are: (1) oxic sink (O), manganeseoxide sediments deposited in oxic waters; (2) euxinic sink(E), with overlying water column H2Saq >11 lM, which isconducive to quantitative Mo conversion to tetrathiomo-lybdate; and (3) an intermediate sink (M) including anoxic,suboxic, weakly euxinic, and BIF sedimentary sinks. Theanoxic sink refers to the sink in which Mo is deposited withH2S present in sedimentary pore fluids but not in the over-lying water column. The suboxic sink is associated with sed-iments deposited under water columns where H2S is absentand O2 is low. The weakly euxinic sink is associated withsediments deposited under water columns in which[H2S]aq <11 lM, which results in non-quantitative Mo con-version to tetrathiomolybdate. The BIF sink is included inFM instead of FO because the formation of BIF does notnecessarily require the presence of free O2.

The estimated average Mo isotopic value for modernrivers which are equilibrated with the fully oxygenatedmodern atmosphere is �0.7& (Archer and Vance, 2008).Although a lack of a thick oxidized soil horizon for absorb-ing and retaining light Mo isotopes under Late Archeanweathering conditions might have resulted in less fraction-ation, the presumably smaller quantities of dissolved Moin surface waters would have tended to create more oppor-tunity for Rayleigh-type fractionation of dissolved Mo. Inthe absence of information about which of these factorswas more important, our baseline assumption for this con-ceptual model is that the isotopic composition of riverineMo was the same then as now.

We estimate d98/95Mo of the low temperature hydrother-mal source mainly based on an endmember calculation fora single modern low temperature ridge-flank hydrothermalsystem, which yields a value of �0.8& (McManus et al.,2002). In modern oceans, high-temperature hydrothermalsystems most likely act as sinks instead of sources for Modue to the low solubility of Mo sulfides (Anbar et al.,2007). The modern Mo contribution from low temperaturehydrothermal sources is estimated to be about 1/9 of theriverine input (Morford and Emerson, 1999; McManuset al., 2002). This value may be higher in Late Archeanoceans, but because its magnitude and isotopic compositionare poorly constrained, we ignore it for now and thus sim-plify Eq. (1) to get:

dRFR ¼ dOFO þ dEFE þ dMFM: ð2Þ

Dividing both sides of Eq. (3) by FR we obtain:

dR ¼ dOfO þ dEfE þ dMfM; ð3Þ

where fO = FO/FR, fM = FM/FR and fE = FE/FR. If weassume there are no other sinks, i.e.,

fO þ fE þ fM ¼ 1; ð4Þ

then we can combine Eqs. (3) and (4) to obtain:

fO ¼ dR þ dMfE � dEfE � dMð Þ= dO � dMð Þ: ð5Þ

Following on these considerations, we can then obtainan expression for the Mo removal flux associated with theoxic sink (FO):

FO ¼ FR � fO

¼ FR � dR þ dMfE � dEfEð Þ � dMÞ= dO � dMð Þ: ð6Þ

We can solve this equation for particular cases by adoptingrepresentative values for dE and dM as discussed below.

For euxinic sinks, we use 1.39& for dE, the averaged98/95Mo value measured for S1. If we assume that dE

records the contemporaneous seawater value, usingD98/95Mosw-oxic = 3.0& (Barling and Anbar, 2004; Was-ylenki et al., 2008a,b), then dO = dE � 3.0&.

The combined intermediate sink FM is a mixture of mul-tiple components. Molybdenum isotope studies in modernanoxic sediments suggest an anoxic fractionation of�0.7& from the seawater value (Poulson et al., 2006).The fractionation factor associated with Mo removal tosuboxic sediments is loosely constrained, ranging from1.0& to 2.8& in the modern ocean system (Siebert et al.,2006). There is no documented definite fractionation factorfor the weakly euxinic sink, but we can consider it to bebetween euxinic and anoxic sediments. BIF are assumedto have a fractionation factor of about 1.11&, based onexperimental determination of D98/95Moaq-ferrihydrite (Gold-berg et al., 2009). To simplify, we use an estimated overallfractionation factor (D98/95MoSW-M) of 1.0& for this com-bined sink. Therefore, dM = dE � 1.0&.

In Fig. A1, FO is expressed as a fraction relative to theMo riverine input in modern oceans, plotted as a functionof fE. Each curve represents the calculated results obtainedby substituting different FR as a certain fraction of the mod-ern day Mo riverine flux. Based on scaling between Mo/TOC and Mo seawater content (Algeo and Lyons, 2006),low Mo/TOC ratios imply that the Late Archean seawaterMo content probably only approached 1% of that in mod-ern oceans (Anbar et al., 2007; Scott et al., 2008). This hasimportant implications for the Mo ocean isotope budget, asseen in Fig. A1. For example, if we assume the riverine Moinput was 1% of the present-day flux, the oxic flux need nothave been more than �0.35% of the Mo riverine flux today(Fig. A1). In other words, because of the much smaller sizeof the Mo oceanic inventory at that time, a much smallerextent of oxygenation than today could have generatedthe heavy seawater isotope signatures seen in the Mt.McRae Shale. This general conclusion is insensitive to theassumptions made in deriving the equations above.

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Associate editor: Derek Vance