Editor: L. Reisberg Keywords: Rhenium Osmium Molybdenum Archean Oxidative weathering Seawater Aut... more Editor: L. Reisberg Keywords: Rhenium Osmium Molybdenum Archean Oxidative weathering Seawater Authigenic enrichments of Re and Mo in 2.7-2.5 Ga organic-rich mudrocks (ORM) indicate that dissolved Re and Mo were present in late Archean seawater. However, it can be difficult to determine if the dissolved Re and Mo were sourced from oxidative crustal weathering or magmatic/hydrothermal/extraterrestrial (MHE) inputs. Comparison of Re, Mo, and 192 Os (non-radiogenic Os isotope) concentrations, and initial 187 Os/ 188 Os isotope compositions from Re-Os isochron regressions of ORM can shed insight on the source of Re and Mo to Archean seawater. Seawater 187 Os/ 188 Os reflects the relative contributions of radiogenic crustal versus unradiogenic MHE Os inputs to seawater. During the Phanerozoic, transient increases in magmatic/hydrothermal activity, marked by unradiogenic 187 Os/ 188 Os excursions and higher 192 Os concentrations in ORM, are accompanied in some cases by higher Re concentrations. These unradiogenic Os sources could thus have played a greater role in the Archean marine Re (and possibly Mo) budget given that the riverine Re (and Mo) flux at that time was likely smaller than the Phanerozoic flux. Hence, a positive correlation between Re, Mo, and 192 Os concentrations in Archean ORM that yields unradiogenic initial 187 Os/ 188 Os may reflect a MHE origin for dissolved Re and Mo in the overlying paleo-seawater. Local oxidative weathering in the presence of photosynthetic O 2 of maficultramafic rocks and crustal sulfide minerals with low Re/Os ratios is an alternative explanation, but the two scenarios cannot be readily distinguished on the basis of initial 187 Os/ 188 Os. By contrast, an initial 187 Os/ 188 Os from Archean ORM that is statistically higher than the MHE baseline of 0.10-0.11 points to oxidative mobilization of Re, Os, and Mo from crustal sulfide minerals. Rhenium and Mo concentrations that are negatively correlated with 192 Os concentrations in Archean ORM yielding unradiogenic initial 187 Os/ 188 Os suggests that some Re and Mo were derived from oxidative weathering. Such negative correlations may reflect an increased marine Os sink (adsorption to oxide minerals) during a mild increase in local environmental O 2 levels. These concepts may help trace the onset of photosynthetic O 2 production and accumulation along Archean ocean margins.
A new Re-Os radiometric age date for an organic-rich shale horizon from the Vazante Group in Braz... more A new Re-Os radiometric age date for an organic-rich shale horizon from the Vazante Group in Brazil, coupled with geological observations, provide evidence for late Mesoproterozoic glacial episodes, conflicting with the general view of greenhouse conditions marked by a eustatic high stand at this time. Field observations of a reverse fault juxtaposing older Mesoproterozoic sedimentary rocks above younger Neoproterozoic strata provide a new stratigraphic framework and reconcile the apparent inversion of U-Pb detrital zircon ages through the succession. Combined, the geochronological, geochemical and stratigraphic evidence suggest that the Vazante Group sediments accumulated along a passive margin of the São Francisco craton and are correlative with the neighboring Paranoá Group. Biomarker, sulfur isotope and iron speciation analyses support the interpretation of a strongly stratified water column during post-glacial transgression and deposition of one of the bituminous shale horizons. The relationship of the glaciogenic Vazante Group to other late Mesoproterozoic successions, such as the non-glacial Atar Group in West Africa and Bylot Supergroup in arctic Canada, however, remains enigmatic.
Metazoans are likely to have their roots in the Cryogenian period 1-3 , but there is a marked inc... more Metazoans are likely to have their roots in the Cryogenian period 1-3 , but there is a marked increase in the appearance of novel animal and algae fossils shortly after the termination of the late Cryogenian (Marinoan) glaciation about 635 million years ago 4-6 . It has been suggested that an oxygenation event in the wake of the severe Marinoan glaciation was the driving factor behind this early diversification of metazoans and the shift in ecosystem complexity . But there is little evidence for an increase in oceanic or atmospheric oxygen following the Marinoan glaciation, or for a direct link between early animal evolution and redox conditions in general 9 . Models linking trends in early biological evolution to shifts in Earth system processes thus remain controversial 10 . Here we report geochemical data from early Ediacaran organic-rich black shales ( 635-630 million years old) of the basal Doushantuo Formation in South China. High enrichments of molybdenum and vanadium and low pyrite sulphur isotope values (D 34 S values $65 per mil) in these shales record expansion of the oceanic inventory of redox-sensitive metals and the growth of the marine sulphate reservoir in response to a widely oxygenated ocean. The data provide evidence for an early Ediacaran oxygenation event, which pre-dates the previous estimates for post-Marinoan oxygenation 11-13 by more than 50 million years. Our findings seem to support a link between the most severe glaciations in Earth's history, the oxygenation of the Earth's surface environments, and the earliest diversification of animals.
Recent geochronological studies have raised concerns regarding the accuracy of regional and globa... more Recent geochronological studies have raised concerns regarding the accuracy of regional and global correlations of Neoproterozoic sedimentary successions using relative dating methods such as carbon isotope chemostratigraphy and glaciogenic diamictite-cap carbonate lithological and geochemical characteristics. Precise and accurate radio-isotopic age constraints for Neoproterozoic sedimentary rocks thus remain a high priority for establishing a reliable Neoproterozoic chronostratigraphy. Here, we present a new Re-Os age of 640.7 ± 4.7 Ma from black shales of the upper Black River Dolomite (Togari Group, northwestern Tasmania) that represents a minimum age for deposition of the immediately underlying, diamictite-bearing Julius River Member. The upper Black River Dolomite Re-Os age is statistically identical to a Re-Os age of 643.0 ± 2.4 Ma previously determined for the Sturtian post-glacial Tindelpina Shale Member (Umberatana Group, Adelaide Rift Complex, southern Australia). Consistent with previous lithostratigraphic and stromatolite biostratigraphic evidence, the Re-Os ages suggest broadly coeval deposition of the Julius River Member and Sturtian diamictites and overlying organic-rich marine shales. Consideration of the Neoproterozoic geochronological database suggests that "Sturtian"-type diamictite-cap carbonate pairs cannot be used as chronostratigraphic marker horizons for global correlation of the Neoproterozoic rock record.
The Vazante Group consists of Precambrian carbonate-dominated platform deposits that extend along... more The Vazante Group consists of Precambrian carbonate-dominated platform deposits that extend along more than 300 km in the external zone of the Brasilia Fold Belt of the São Francisco Basin in east central Brazil. The sequence is about 4.8 km thick and contains a preserved glaciomarine diamictite unit (containing dropstone) at the top and a lower diamictite unit at the bottom. Previous C-and Sr-isotope profiles suggested the correlation of the upper diamictite unit with the "Sturtian" glacial event (ca. 750-643 Ma). However, new Re-Os isotope data from the shales associated with the upper diamictites yield radiometric age estimates between 993 ± 46 and 1100 ± 77 Ma. U-Pb measurements on a suite of clear euhedral zircon crystals that were separated from the same shales associated with the upper diamictite and from the arkosic sandstone above the lower diamictite yield ages as young as 988 ± 15 and 1000 ± 25 Ma, respectively. Based on the Re-Os and U-Pb ages, the best age estimate of the Vazante Group is constrained to be ∼1000-1100 Ma and thus the two diamictite units are not correlative with the Sturtian glaciation(s) but most likely are records of glacial events that occurred during the late Mesoproterozoic.
We present new Re-Os isotope data from the early Paleoproterozoic Klipputs Member (Nelani Formati... more We present new Re-Os isotope data from the early Paleoproterozoic Klipputs Member (Nelani Formation, Ghaap Group, Transvaal Supergroup, South Africa) to further evaluate low-level Re-Os geochronology of organic-rich sedimentary rocks and improve global correlations of Paleoproterozoic sedimentary rocks. The Klipputs Member contains Re (0.3-0.8 ppb), total Os (34-177 ppt) and common Os ( 192 Os; 10-66 ppt) abundances that overlap upper crustal abundances of 0.2-2 ppb, 30-50 ppt, and 12-21 ppt, respectively. Five samples yield a Model 1 Re-Os date of 2479 ± 20 Ma (±22 Ma if the 187 Re decay constant uncertainty is included, 2 , Mean Square of Weighted Deviates [MSWD] = 0.01) using the Cr VI -H 2 SO 4 digestion protocol. Dissolution of the same sample powders using inverse aqua regia yields a Model 3 Re-Os date of 2862 ± 190 Ma (2 , MSWD = 148). Comparison of Re-Os isotope data between the two methods reveals that inverse aqua regia liberated a component of radiogenic detrital Os whereas the Cr VI -H 2 SO 4 medium did not. The Re-Os date of 2479 ± 22 Ma for the Klipputs Member is consistent with published geochronological ages from underlying and overlying stratigraphic units, suggesting the Cr VI -H 2 SO 4 protocol yields accurate Re-Os depositional ages from ORS containing very low levels of hydrogenous Re and hydrogenous + radiogenic Os. Hence, there is potential for widespread application of low-level Re-Os ORS geochronology, including sedimentary successions deposited before the Great Oxidation Event. The initial 187 Os/ 188 Os (0.12 ± 0.01) from the Klipputs Member Cr VI -H 2 SO 4 regression indicates that dissolved Os in the contemporaneous local seawater originated from hydrothermal alteration of unradiogenic oceanic crust and/or cosmic dust rather than oxidative weathering of the radiogenic upper crust. In light of the Re-Os depositional age for the Klipputs Member, we propose that the stratigraphic units in the upper Hamersley Group above the Brockman Iron Formation (Western Australia) are younger than the Ghaap Group. Diamictite from the Doradale Formation in the Ghaap Group is constrained to be older than 2479 ± 22 Ma and thus may not be correlative with glacial deposits assigned to the oldest Paleoproterozoic glaciation.
To improve estimates of the extent of ocean oxygenation during the late Ediacaran Period, we meas... more To improve estimates of the extent of ocean oxygenation during the late Ediacaran Period, we measured the U and Mo isotope compositions of euxinic (anoxic and sulfidic) organic-rich mudrocks (ORM) of Member IV, upper Doushantuo Formation, South China. The average d 238 U of most samples is 0.24 ± 0.16& (2SD; relative to standard CRM145), which is slightly higher than the average d 238 U of 0.02 ± 0.12& for restricted Black Sea (deep-water Unit I) euxinic sediments and is similar to a modeled d 238 U value of 0.2& for open ocean euxinic sediments in the modern well-oxygenated oceans. Because 238 U is preferentially removed to euxinic sediments compared to 235 U, expanded ocean anoxia will deplete seawater of 238 U relative to 235 U, ultimately leading to deposition of ORM with low d 238 U. Hence, the high d 238 U of Member IV ORM points to a common occurrence of extensive ocean oxygenation ca. 560 to 551 Myr ago.
We use the molybdenum isotope paleoredox proxy to look for evidence of small amounts of O 2 in th... more We use the molybdenum isotope paleoredox proxy to look for evidence of small amounts of O 2 in the environment ˜50 Ma before the Great Oxidation Event (GOE) in a high resolution profile from the ˜2.5 Ga Mt. McRae Shale. The molybdenum isotope compositions (δ 98/95Mo) from samples throughout the sequence span a range from 0.99‰ to 1.86‰. All samples have heavier δ 98/95Mo 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 distinct patterns of δ 98/95Mo variation. Heavier δ 98/95Mo values occur in the S1 unit, where δ 98/95Mo correlates with Mo enrichments. In the S2 unit, δ 98/95Mo 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 removal from bottom waters was likely quantitative. Thus, δ 98/95Mo in this interval likely records coeval seawater. The lighter δ 98/95Mo values in the S2 unit may indicate a less fractionated ocean Mo inventory relative to the S1 unit. However, sedimentary Fe proxies suggest that S2 sediments accumulated under a water column that was ferruginous rather than euxinic, raising the possibility of non-quantitative Mo scavenging and hence an expressed δ 98/95Mo fractionation relative to coeval seawater. Because any associated fractionations during this process would have favored the light isotope in sediments, the lighter δ 98/95Mo 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 δ 98/95Mo 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 in surface oceans. The extent of environmental oxygenation in either unit is difficult to assess due to uncertainties in the global Mo isotope budget. Because of the small ocean Mo inventory in the Late Archean, documented by low concentrations of Mo and low Mo/TOC, the extent of oxygenation required to account for the observed fractionations is much smaller than in modern oceans. However, when juxtaposed against the record of δ 98/95Mo through time, our findings provide further evidence of the onset of environmental oxygenation before the GOE.
High-resolution chemostratigraphy reveals an episode of enrichment of the redox-sensitive transit... more High-resolution chemostratigraphy reveals an episode of enrichment of the redox-sensitive transition metals molybdenum and rhenium in the late Archean Mount McRae Shale in Western Australia. Correlations with organic carbon indicate that these metals were derived from contemporaneous seawater. Rhenium/osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 +/- 8 million years ago (Ma). Molybdenum and rhenium were probably supplied to Archean oceans by oxidative weathering of crustal sulfide minerals. These findings point to the presence of small amounts of O2 in the environment more than 50 million years before the start of the Great Oxidation Event.
The redox state of the oceans strongly influences the concentration of dissolved trace metals in ... more The redox state of the oceans strongly influences the concentration of dissolved trace metals in sea water. Changes in the redox state of the oceans are thought to have limited the availability of some trace metals in the past, particularly during the Proterozoic eon, 2,500 to 542 million years ago 1-4 . Of these trace metals, zinc (Zn) is of particular importance to eukaryotic organisms, because it is essential for a wide range of basic cellular functions. It has been suggested 5 that during the Proterozoic, marine environments were broadly euxinic-that is, anoxic and sulphidic-which would have resulted in low Zn availability. Low Zn bioavailability could therefore be responsible for an observed delay in eukaryote diversification 2 . Here we present a compilation of Zn abundance data from black shales deposited under euxinic conditions from the Precambrian time to the present. We show that these values track first-order trends in seawater Zn availability. Contrary to previous estimates 6 , we find that Zn concentrations during the Proterozoic were similar to modern concentrations, supporting recent studies 7,8 that call for limited euxinia at this time. Instead, we propose that predominantly anoxic and iron-rich deep oceans, combined with large hydrothermal fluxes of Zn, maintained high levels of dissolved Zn throughout the oceans. We thus suggest that the protracted diversification of eukaryotic Zn-binding proteins was not a result of Zn biolimitation.
New Re-Os dates obtained from black shales overlying the Sturtian and Areyonga glacial deposits i... more New Re-Os dates obtained from black shales overlying the Sturtian and Areyonga glacial deposits in southern and central Australia, respectively, challenge the prevailing consensus of three Neoproterozoic glaciations. The end of Sturtian glaciation in the Adelaide Rift Complex is constrained by a Re-Os date of 643.0 ؎ 2.4 Ma from the overlying Tindelpina Shale Member (basal Tapley Hill Formation). A Re-Os date of 657.2 ؎ 5.4 Ma for the basal Aralka Formation constrains the age of the underlying Areyonga glacial deposits in the Amadeus Basin. The Re-Os ages show that the Sturtian and Areyonga glacial deposits are younger than other radiometrically dated (ca. Ma) Neoproterozoic glacial intervals previously regarded as possible correlatives. Thus, the ''Sturtian'' ice age was markedly diachronous, and/or there was more than one ''Sturtian''-type glaciation. Some Neoproterozoic glacial deposits may represent the products of regional and diachronous glaciation associated with protracted breakup of the supercontinent Rodinia rather than ''snowball'' or ''slushball'' Earth ice ages.
We use the molybdenum isotope paleoredox proxy to look for evidence of small amounts of O 2 in th... more We use the molybdenum isotope paleoredox proxy to look for evidence of small amounts of O 2 in the environment $50 Ma before the Great Oxidation Event (GOE) in a high resolution profile from the $2.5 Ga Mt. McRae Shale. The molybdenum isotope compositions (d 98/95 Mo) from samples throughout the sequence span a range from 0.99& to 1.86&. All samples have heavier d 98/95 Mo 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 distinct patterns of d 98/95 Mo variation. Heavier d 98/95 Mo values occur in the S1 unit, where d 98/95 Mo correlates with Mo enrichments. In the S2 unit, d 98/95 Mo 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 removal from bottom waters was likely quantitative. Thus, d 98/95 Mo in this interval likely records coeval seawater. The lighter d 98/ 95 Mo values in the S2 unit may indicate a less fractionated ocean Mo inventory relative to the S1 unit. However, sedimentary Fe proxies suggest that S2 sediments accumulated under a water column that was ferruginous rather than euxinic, raising the possibility of non-quantitative Mo scavenging and hence an expressed d 98/95 Mo fractionation relative to coeval seawater. Because any associated fractionations during this process would have favored the light isotope in sediments, the lighter d 98/95 Mo 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 d 98/95 Mo 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 in surface oceans. The extent of environmental oxygenation in either unit is difficult to assess due to uncertainties in the global Mo isotope budget. Because of the small ocean Mo inventory in the Late Archean, documented by low concentrations of Mo and low Mo/TOC, the extent of oxygenation required to account for the observed fractionations is much smaller than in modern oceans. However, when juxtaposed against the record of d 98/95 Mo through time, our findings provide further evidence of the onset of environmental oxygenation before the GOE.
Joint application of the Mo isotope paleoredox proxy and Re-Os deposition-age geochronometer to e... more Joint application of the Mo isotope paleoredox proxy and Re-Os deposition-age geochronometer to euxinic black shales has potential for tracing the evolution of ocean redox chemistry over geological time. Here, we report new Re-Os and Mo isotope data for the Mesoproterozoic Velkerri Formation (Roper Group) and Paleoproterozoic Wollogorang Formation , McArthur Basin, northern Australia. New Re-Os ages of 1361 ± 21 Ma (2r, n = 14, mean square of weighted deviates [MSWD] = 1.3, Model 1) and 1417 ± 29 Ma (2r, n = 12, MSWD = 1.3, Model 1) constrain the depositional age of the Velkerri Formation and its contained biomarkers, as well as acritarchs and microfossils from the Roper Group. Black shales from the upper Velkerri Formation have high Mo abundances (105-119 ppm) and degree of pyritization [DOP] values (0.90-0.92) implying quantitative conversion of molybdate (MoO 4 2À ) to thiomolybdate (MoS 4 2À
New Re -Os isotopic data were obtained from chlorite-grade black shales from the upper Old Fort P... more New Re -Os isotopic data were obtained from chlorite-grade black shales from the upper Old Fort Point Formation (Windermere Supergroup), a post-glacial Neoproterozoic marker horizon in western Canada. A Re -Os isochron date of 634 F 57 Ma (MSWD = 65, n = 5) was determined using the conventional inverse aqua regia digestion medium. However, dissolution of the same samples with a new CrO 3 -H 2 SO 4 dissolution technique [Chem. Geol. 200 (2003) 225] yielded a much more precise date of 607.8 F 4.7 Ma (MSWD = 1.2). Both dates are in agreement with existing U -Pb age constraints that bracket the Old Fort Point Formation between f 685 and f 570 Ma. The distinctive Re -Os systematics recorded by the two analytical protocols is explained by dissolution of a variably radiogenic, detrital Os component by the aqua regia method. In contrast, the CrO 3 -H 2 SO 4 technique minimizes this detrital component by selectively dissolving organic matter that is dominated by hydrogenous (seawater) Re and Os. The date of 607.8 F 4.7 Ma is thus interpreted as the depositional age for the upper Old Fort Point Formation providing a minimum age constraint for the timing of the second Windermere glaciation in western Canada. This ice age is correlative with the Marinoan ( f 620 -600 Ma) ice age and older than the f 580-Ma Gaskiers glaciation of northeastern North America. The new Re -Os age determined from the CrO 3 -H 2 SO 4 digestion technique thus provides further support to a growing body of evidence for a global Marinoan glacial episode. Such an interpretation would not be discernable from the imprecise Re -Os date obtained with the aqua regia protocol. These results also indicate the potential for Re -Os radiometric dating of black shales that was not previously recognized. Importantly, neither chlorite-grade metamorphism nor the low organic content (TOC < 1%) of the Old Fort Point Formation precluded the determination of a precise Re -Os depositional age using the CrO 3 -H 2 SO 4 analytical protocol. D
The use of molybdenum isotope data (δ 98 Mo) from organic-rich shales to draw inferences concerni... more The use of molybdenum isotope data (δ 98 Mo) from organic-rich shales to draw inferences concerning marine paleoredox conditions at a global scale is predicated upon the assumptions of (1) a residence time of Mo in seawater much greater than the ocean mixing time, and (2) quantitative removal of Mo from a strongly euxinic ([H 2 S] aq > 11 μM) water column to the sediment, thus preserving the seawater δ 98 Mo signature. In this study we analyze Mo isotopic variation in the Hushpuckney Shale, a 73-cm-thick unit representing the late transgressive to early regressive stages of a glacio-eustatic cyclothem (Swope Formation) deposited in the Late Pennsylvanian Midcontinent Sea (LPMS) of North America. The Hushpuckney can be subdivided into four stratigraphic zones of distinctive geochemical character. Zones I and III, which accumulated under weakly euxinic conditions, acquired relatively high δ 98 Mo values (+0.9 to +1.1‰), whereas Zone II, which accumulated under intensely euxinic conditions, acquired lower δ 98 Mo values (~+0.6‰). Zone IV, which accumulated under suboxic conditions in the water column, acquired the heaviest δ 98 Mo values (+1.1 to + 1.8‰). These results contrast with the pattern of redox -δ 98 Mo covariation in modern marine environments, in which the heaviest δ 98 Mo values are found in the most intensely euxinic facies. We evaluate three different hypotheses to account for the Mo isotopic patterns of the Hushpuckney Shale. One hypothesis, seawater-freshwater mixing, is rejected owing to isotopic mass balance considerations. A second hypothesis is a local control on δ 98 Mo by water-column redox cycling of Mn, with particulate Mnoxyhydroxides adsorbing isotopically light Mo and transferring it to the sediment, a process that was most active during deposition of Zone II. The significance of this scenario is that euxinic black shales may not reliably record global seawater δ 98 Mo in areas where a Mn-particulate shuttle is operative. A third hypothesis is based on rapid secular variation of the Mo isotope composition of Late Pennsylvanian global seawater. In order to account for δ 98 Mo trends within the Hushpuckney Shale, seawater δ 98 Mo must have varied by~1.2‰ at a~100-kyr timescale, which would have been possible only if the residence time of Mo in Late Pennsylvanian seawater was b 100 kyr. Although both the second and third hypotheses are viable based on the present limited δ 98 Mo dataset, we discuss how each model might be tested through additional Mo isotope data.
The photosynthetic production of oxygen in the oceans is thought to have begun by 2.7 billion yea... more The photosynthetic production of oxygen in the oceans is thought to have begun by 2.7 billion years ago, several hundred million years before appreciable accumulation of oxygen in the atmosphere. However, the abundance and distribution of dissolved oxygen in the late Archaean oceans is poorly constrained. Here we present geochemical profiles from 2.6-to 2.5-billion-year-old black shales from the Campbellrand-Malmani carbonate platform in South Africa. We find a high abundance of rhenium and a low abundance of molybdenum, which, together with the speciation of sedimentary iron, points to the presence of dissolved oxygen in the bottom waters on the platform slope. The water depth on the slope probably reached several hundred metres, implying the export of O 2 below the photic zone. Our data also indicate that the mildly oxygenated surface ocean gave way to an anoxic deep ocean. We therefore suggest that the production of oxygen in the surface ocean was vigorous at this time, but was not sufficient to fully consume the deep-sea reductants. On the basis of our results and observations from the Hamersley basin in Western Australia, we conclude that the productive regions along ocean margins during the late Archaean eon were sites of substantial O 2 accumulation, at least 100 million years before the first significant increase in atmospheric O 2 concentration.
Editor: L. Reisberg Keywords: Rhenium Osmium Molybdenum Archean Oxidative weathering Seawater Aut... more Editor: L. Reisberg Keywords: Rhenium Osmium Molybdenum Archean Oxidative weathering Seawater Authigenic enrichments of Re and Mo in 2.7-2.5 Ga organic-rich mudrocks (ORM) indicate that dissolved Re and Mo were present in late Archean seawater. However, it can be difficult to determine if the dissolved Re and Mo were sourced from oxidative crustal weathering or magmatic/hydrothermal/extraterrestrial (MHE) inputs. Comparison of Re, Mo, and 192 Os (non-radiogenic Os isotope) concentrations, and initial 187 Os/ 188 Os isotope compositions from Re-Os isochron regressions of ORM can shed insight on the source of Re and Mo to Archean seawater. Seawater 187 Os/ 188 Os reflects the relative contributions of radiogenic crustal versus unradiogenic MHE Os inputs to seawater. During the Phanerozoic, transient increases in magmatic/hydrothermal activity, marked by unradiogenic 187 Os/ 188 Os excursions and higher 192 Os concentrations in ORM, are accompanied in some cases by higher Re concentrations. These unradiogenic Os sources could thus have played a greater role in the Archean marine Re (and possibly Mo) budget given that the riverine Re (and Mo) flux at that time was likely smaller than the Phanerozoic flux. Hence, a positive correlation between Re, Mo, and 192 Os concentrations in Archean ORM that yields unradiogenic initial 187 Os/ 188 Os may reflect a MHE origin for dissolved Re and Mo in the overlying paleo-seawater. Local oxidative weathering in the presence of photosynthetic O 2 of maficultramafic rocks and crustal sulfide minerals with low Re/Os ratios is an alternative explanation, but the two scenarios cannot be readily distinguished on the basis of initial 187 Os/ 188 Os. By contrast, an initial 187 Os/ 188 Os from Archean ORM that is statistically higher than the MHE baseline of 0.10-0.11 points to oxidative mobilization of Re, Os, and Mo from crustal sulfide minerals. Rhenium and Mo concentrations that are negatively correlated with 192 Os concentrations in Archean ORM yielding unradiogenic initial 187 Os/ 188 Os suggests that some Re and Mo were derived from oxidative weathering. Such negative correlations may reflect an increased marine Os sink (adsorption to oxide minerals) during a mild increase in local environmental O 2 levels. These concepts may help trace the onset of photosynthetic O 2 production and accumulation along Archean ocean margins.
A new Re-Os radiometric age date for an organic-rich shale horizon from the Vazante Group in Braz... more A new Re-Os radiometric age date for an organic-rich shale horizon from the Vazante Group in Brazil, coupled with geological observations, provide evidence for late Mesoproterozoic glacial episodes, conflicting with the general view of greenhouse conditions marked by a eustatic high stand at this time. Field observations of a reverse fault juxtaposing older Mesoproterozoic sedimentary rocks above younger Neoproterozoic strata provide a new stratigraphic framework and reconcile the apparent inversion of U-Pb detrital zircon ages through the succession. Combined, the geochronological, geochemical and stratigraphic evidence suggest that the Vazante Group sediments accumulated along a passive margin of the São Francisco craton and are correlative with the neighboring Paranoá Group. Biomarker, sulfur isotope and iron speciation analyses support the interpretation of a strongly stratified water column during post-glacial transgression and deposition of one of the bituminous shale horizons. The relationship of the glaciogenic Vazante Group to other late Mesoproterozoic successions, such as the non-glacial Atar Group in West Africa and Bylot Supergroup in arctic Canada, however, remains enigmatic.
Metazoans are likely to have their roots in the Cryogenian period 1-3 , but there is a marked inc... more Metazoans are likely to have their roots in the Cryogenian period 1-3 , but there is a marked increase in the appearance of novel animal and algae fossils shortly after the termination of the late Cryogenian (Marinoan) glaciation about 635 million years ago 4-6 . It has been suggested that an oxygenation event in the wake of the severe Marinoan glaciation was the driving factor behind this early diversification of metazoans and the shift in ecosystem complexity . But there is little evidence for an increase in oceanic or atmospheric oxygen following the Marinoan glaciation, or for a direct link between early animal evolution and redox conditions in general 9 . Models linking trends in early biological evolution to shifts in Earth system processes thus remain controversial 10 . Here we report geochemical data from early Ediacaran organic-rich black shales ( 635-630 million years old) of the basal Doushantuo Formation in South China. High enrichments of molybdenum and vanadium and low pyrite sulphur isotope values (D 34 S values $65 per mil) in these shales record expansion of the oceanic inventory of redox-sensitive metals and the growth of the marine sulphate reservoir in response to a widely oxygenated ocean. The data provide evidence for an early Ediacaran oxygenation event, which pre-dates the previous estimates for post-Marinoan oxygenation 11-13 by more than 50 million years. Our findings seem to support a link between the most severe glaciations in Earth's history, the oxygenation of the Earth's surface environments, and the earliest diversification of animals.
Recent geochronological studies have raised concerns regarding the accuracy of regional and globa... more Recent geochronological studies have raised concerns regarding the accuracy of regional and global correlations of Neoproterozoic sedimentary successions using relative dating methods such as carbon isotope chemostratigraphy and glaciogenic diamictite-cap carbonate lithological and geochemical characteristics. Precise and accurate radio-isotopic age constraints for Neoproterozoic sedimentary rocks thus remain a high priority for establishing a reliable Neoproterozoic chronostratigraphy. Here, we present a new Re-Os age of 640.7 ± 4.7 Ma from black shales of the upper Black River Dolomite (Togari Group, northwestern Tasmania) that represents a minimum age for deposition of the immediately underlying, diamictite-bearing Julius River Member. The upper Black River Dolomite Re-Os age is statistically identical to a Re-Os age of 643.0 ± 2.4 Ma previously determined for the Sturtian post-glacial Tindelpina Shale Member (Umberatana Group, Adelaide Rift Complex, southern Australia). Consistent with previous lithostratigraphic and stromatolite biostratigraphic evidence, the Re-Os ages suggest broadly coeval deposition of the Julius River Member and Sturtian diamictites and overlying organic-rich marine shales. Consideration of the Neoproterozoic geochronological database suggests that "Sturtian"-type diamictite-cap carbonate pairs cannot be used as chronostratigraphic marker horizons for global correlation of the Neoproterozoic rock record.
The Vazante Group consists of Precambrian carbonate-dominated platform deposits that extend along... more The Vazante Group consists of Precambrian carbonate-dominated platform deposits that extend along more than 300 km in the external zone of the Brasilia Fold Belt of the São Francisco Basin in east central Brazil. The sequence is about 4.8 km thick and contains a preserved glaciomarine diamictite unit (containing dropstone) at the top and a lower diamictite unit at the bottom. Previous C-and Sr-isotope profiles suggested the correlation of the upper diamictite unit with the "Sturtian" glacial event (ca. 750-643 Ma). However, new Re-Os isotope data from the shales associated with the upper diamictites yield radiometric age estimates between 993 ± 46 and 1100 ± 77 Ma. U-Pb measurements on a suite of clear euhedral zircon crystals that were separated from the same shales associated with the upper diamictite and from the arkosic sandstone above the lower diamictite yield ages as young as 988 ± 15 and 1000 ± 25 Ma, respectively. Based on the Re-Os and U-Pb ages, the best age estimate of the Vazante Group is constrained to be ∼1000-1100 Ma and thus the two diamictite units are not correlative with the Sturtian glaciation(s) but most likely are records of glacial events that occurred during the late Mesoproterozoic.
We present new Re-Os isotope data from the early Paleoproterozoic Klipputs Member (Nelani Formati... more We present new Re-Os isotope data from the early Paleoproterozoic Klipputs Member (Nelani Formation, Ghaap Group, Transvaal Supergroup, South Africa) to further evaluate low-level Re-Os geochronology of organic-rich sedimentary rocks and improve global correlations of Paleoproterozoic sedimentary rocks. The Klipputs Member contains Re (0.3-0.8 ppb), total Os (34-177 ppt) and common Os ( 192 Os; 10-66 ppt) abundances that overlap upper crustal abundances of 0.2-2 ppb, 30-50 ppt, and 12-21 ppt, respectively. Five samples yield a Model 1 Re-Os date of 2479 ± 20 Ma (±22 Ma if the 187 Re decay constant uncertainty is included, 2 , Mean Square of Weighted Deviates [MSWD] = 0.01) using the Cr VI -H 2 SO 4 digestion protocol. Dissolution of the same sample powders using inverse aqua regia yields a Model 3 Re-Os date of 2862 ± 190 Ma (2 , MSWD = 148). Comparison of Re-Os isotope data between the two methods reveals that inverse aqua regia liberated a component of radiogenic detrital Os whereas the Cr VI -H 2 SO 4 medium did not. The Re-Os date of 2479 ± 22 Ma for the Klipputs Member is consistent with published geochronological ages from underlying and overlying stratigraphic units, suggesting the Cr VI -H 2 SO 4 protocol yields accurate Re-Os depositional ages from ORS containing very low levels of hydrogenous Re and hydrogenous + radiogenic Os. Hence, there is potential for widespread application of low-level Re-Os ORS geochronology, including sedimentary successions deposited before the Great Oxidation Event. The initial 187 Os/ 188 Os (0.12 ± 0.01) from the Klipputs Member Cr VI -H 2 SO 4 regression indicates that dissolved Os in the contemporaneous local seawater originated from hydrothermal alteration of unradiogenic oceanic crust and/or cosmic dust rather than oxidative weathering of the radiogenic upper crust. In light of the Re-Os depositional age for the Klipputs Member, we propose that the stratigraphic units in the upper Hamersley Group above the Brockman Iron Formation (Western Australia) are younger than the Ghaap Group. Diamictite from the Doradale Formation in the Ghaap Group is constrained to be older than 2479 ± 22 Ma and thus may not be correlative with glacial deposits assigned to the oldest Paleoproterozoic glaciation.
To improve estimates of the extent of ocean oxygenation during the late Ediacaran Period, we meas... more To improve estimates of the extent of ocean oxygenation during the late Ediacaran Period, we measured the U and Mo isotope compositions of euxinic (anoxic and sulfidic) organic-rich mudrocks (ORM) of Member IV, upper Doushantuo Formation, South China. The average d 238 U of most samples is 0.24 ± 0.16& (2SD; relative to standard CRM145), which is slightly higher than the average d 238 U of 0.02 ± 0.12& for restricted Black Sea (deep-water Unit I) euxinic sediments and is similar to a modeled d 238 U value of 0.2& for open ocean euxinic sediments in the modern well-oxygenated oceans. Because 238 U is preferentially removed to euxinic sediments compared to 235 U, expanded ocean anoxia will deplete seawater of 238 U relative to 235 U, ultimately leading to deposition of ORM with low d 238 U. Hence, the high d 238 U of Member IV ORM points to a common occurrence of extensive ocean oxygenation ca. 560 to 551 Myr ago.
We use the molybdenum isotope paleoredox proxy to look for evidence of small amounts of O 2 in th... more We use the molybdenum isotope paleoredox proxy to look for evidence of small amounts of O 2 in the environment ˜50 Ma before the Great Oxidation Event (GOE) in a high resolution profile from the ˜2.5 Ga Mt. McRae Shale. The molybdenum isotope compositions (δ 98/95Mo) from samples throughout the sequence span a range from 0.99‰ to 1.86‰. All samples have heavier δ 98/95Mo 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 distinct patterns of δ 98/95Mo variation. Heavier δ 98/95Mo values occur in the S1 unit, where δ 98/95Mo correlates with Mo enrichments. In the S2 unit, δ 98/95Mo 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 removal from bottom waters was likely quantitative. Thus, δ 98/95Mo in this interval likely records coeval seawater. The lighter δ 98/95Mo values in the S2 unit may indicate a less fractionated ocean Mo inventory relative to the S1 unit. However, sedimentary Fe proxies suggest that S2 sediments accumulated under a water column that was ferruginous rather than euxinic, raising the possibility of non-quantitative Mo scavenging and hence an expressed δ 98/95Mo fractionation relative to coeval seawater. Because any associated fractionations during this process would have favored the light isotope in sediments, the lighter δ 98/95Mo 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 δ 98/95Mo 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 in surface oceans. The extent of environmental oxygenation in either unit is difficult to assess due to uncertainties in the global Mo isotope budget. Because of the small ocean Mo inventory in the Late Archean, documented by low concentrations of Mo and low Mo/TOC, the extent of oxygenation required to account for the observed fractionations is much smaller than in modern oceans. However, when juxtaposed against the record of δ 98/95Mo through time, our findings provide further evidence of the onset of environmental oxygenation before the GOE.
High-resolution chemostratigraphy reveals an episode of enrichment of the redox-sensitive transit... more High-resolution chemostratigraphy reveals an episode of enrichment of the redox-sensitive transition metals molybdenum and rhenium in the late Archean Mount McRae Shale in Western Australia. Correlations with organic carbon indicate that these metals were derived from contemporaneous seawater. Rhenium/osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 +/- 8 million years ago (Ma). Molybdenum and rhenium were probably supplied to Archean oceans by oxidative weathering of crustal sulfide minerals. These findings point to the presence of small amounts of O2 in the environment more than 50 million years before the start of the Great Oxidation Event.
The redox state of the oceans strongly influences the concentration of dissolved trace metals in ... more The redox state of the oceans strongly influences the concentration of dissolved trace metals in sea water. Changes in the redox state of the oceans are thought to have limited the availability of some trace metals in the past, particularly during the Proterozoic eon, 2,500 to 542 million years ago 1-4 . Of these trace metals, zinc (Zn) is of particular importance to eukaryotic organisms, because it is essential for a wide range of basic cellular functions. It has been suggested 5 that during the Proterozoic, marine environments were broadly euxinic-that is, anoxic and sulphidic-which would have resulted in low Zn availability. Low Zn bioavailability could therefore be responsible for an observed delay in eukaryote diversification 2 . Here we present a compilation of Zn abundance data from black shales deposited under euxinic conditions from the Precambrian time to the present. We show that these values track first-order trends in seawater Zn availability. Contrary to previous estimates 6 , we find that Zn concentrations during the Proterozoic were similar to modern concentrations, supporting recent studies 7,8 that call for limited euxinia at this time. Instead, we propose that predominantly anoxic and iron-rich deep oceans, combined with large hydrothermal fluxes of Zn, maintained high levels of dissolved Zn throughout the oceans. We thus suggest that the protracted diversification of eukaryotic Zn-binding proteins was not a result of Zn biolimitation.
New Re-Os dates obtained from black shales overlying the Sturtian and Areyonga glacial deposits i... more New Re-Os dates obtained from black shales overlying the Sturtian and Areyonga glacial deposits in southern and central Australia, respectively, challenge the prevailing consensus of three Neoproterozoic glaciations. The end of Sturtian glaciation in the Adelaide Rift Complex is constrained by a Re-Os date of 643.0 ؎ 2.4 Ma from the overlying Tindelpina Shale Member (basal Tapley Hill Formation). A Re-Os date of 657.2 ؎ 5.4 Ma for the basal Aralka Formation constrains the age of the underlying Areyonga glacial deposits in the Amadeus Basin. The Re-Os ages show that the Sturtian and Areyonga glacial deposits are younger than other radiometrically dated (ca. Ma) Neoproterozoic glacial intervals previously regarded as possible correlatives. Thus, the ''Sturtian'' ice age was markedly diachronous, and/or there was more than one ''Sturtian''-type glaciation. Some Neoproterozoic glacial deposits may represent the products of regional and diachronous glaciation associated with protracted breakup of the supercontinent Rodinia rather than ''snowball'' or ''slushball'' Earth ice ages.
We use the molybdenum isotope paleoredox proxy to look for evidence of small amounts of O 2 in th... more We use the molybdenum isotope paleoredox proxy to look for evidence of small amounts of O 2 in the environment $50 Ma before the Great Oxidation Event (GOE) in a high resolution profile from the $2.5 Ga Mt. McRae Shale. The molybdenum isotope compositions (d 98/95 Mo) from samples throughout the sequence span a range from 0.99& to 1.86&. All samples have heavier d 98/95 Mo 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 distinct patterns of d 98/95 Mo variation. Heavier d 98/95 Mo values occur in the S1 unit, where d 98/95 Mo correlates with Mo enrichments. In the S2 unit, d 98/95 Mo 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 removal from bottom waters was likely quantitative. Thus, d 98/95 Mo in this interval likely records coeval seawater. The lighter d 98/ 95 Mo values in the S2 unit may indicate a less fractionated ocean Mo inventory relative to the S1 unit. However, sedimentary Fe proxies suggest that S2 sediments accumulated under a water column that was ferruginous rather than euxinic, raising the possibility of non-quantitative Mo scavenging and hence an expressed d 98/95 Mo fractionation relative to coeval seawater. Because any associated fractionations during this process would have favored the light isotope in sediments, the lighter d 98/95 Mo 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 d 98/95 Mo 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 in surface oceans. The extent of environmental oxygenation in either unit is difficult to assess due to uncertainties in the global Mo isotope budget. Because of the small ocean Mo inventory in the Late Archean, documented by low concentrations of Mo and low Mo/TOC, the extent of oxygenation required to account for the observed fractionations is much smaller than in modern oceans. However, when juxtaposed against the record of d 98/95 Mo through time, our findings provide further evidence of the onset of environmental oxygenation before the GOE.
Joint application of the Mo isotope paleoredox proxy and Re-Os deposition-age geochronometer to e... more Joint application of the Mo isotope paleoredox proxy and Re-Os deposition-age geochronometer to euxinic black shales has potential for tracing the evolution of ocean redox chemistry over geological time. Here, we report new Re-Os and Mo isotope data for the Mesoproterozoic Velkerri Formation (Roper Group) and Paleoproterozoic Wollogorang Formation , McArthur Basin, northern Australia. New Re-Os ages of 1361 ± 21 Ma (2r, n = 14, mean square of weighted deviates [MSWD] = 1.3, Model 1) and 1417 ± 29 Ma (2r, n = 12, MSWD = 1.3, Model 1) constrain the depositional age of the Velkerri Formation and its contained biomarkers, as well as acritarchs and microfossils from the Roper Group. Black shales from the upper Velkerri Formation have high Mo abundances (105-119 ppm) and degree of pyritization [DOP] values (0.90-0.92) implying quantitative conversion of molybdate (MoO 4 2À ) to thiomolybdate (MoS 4 2À
New Re -Os isotopic data were obtained from chlorite-grade black shales from the upper Old Fort P... more New Re -Os isotopic data were obtained from chlorite-grade black shales from the upper Old Fort Point Formation (Windermere Supergroup), a post-glacial Neoproterozoic marker horizon in western Canada. A Re -Os isochron date of 634 F 57 Ma (MSWD = 65, n = 5) was determined using the conventional inverse aqua regia digestion medium. However, dissolution of the same samples with a new CrO 3 -H 2 SO 4 dissolution technique [Chem. Geol. 200 (2003) 225] yielded a much more precise date of 607.8 F 4.7 Ma (MSWD = 1.2). Both dates are in agreement with existing U -Pb age constraints that bracket the Old Fort Point Formation between f 685 and f 570 Ma. The distinctive Re -Os systematics recorded by the two analytical protocols is explained by dissolution of a variably radiogenic, detrital Os component by the aqua regia method. In contrast, the CrO 3 -H 2 SO 4 technique minimizes this detrital component by selectively dissolving organic matter that is dominated by hydrogenous (seawater) Re and Os. The date of 607.8 F 4.7 Ma is thus interpreted as the depositional age for the upper Old Fort Point Formation providing a minimum age constraint for the timing of the second Windermere glaciation in western Canada. This ice age is correlative with the Marinoan ( f 620 -600 Ma) ice age and older than the f 580-Ma Gaskiers glaciation of northeastern North America. The new Re -Os age determined from the CrO 3 -H 2 SO 4 digestion technique thus provides further support to a growing body of evidence for a global Marinoan glacial episode. Such an interpretation would not be discernable from the imprecise Re -Os date obtained with the aqua regia protocol. These results also indicate the potential for Re -Os radiometric dating of black shales that was not previously recognized. Importantly, neither chlorite-grade metamorphism nor the low organic content (TOC < 1%) of the Old Fort Point Formation precluded the determination of a precise Re -Os depositional age using the CrO 3 -H 2 SO 4 analytical protocol. D
The use of molybdenum isotope data (δ 98 Mo) from organic-rich shales to draw inferences concerni... more The use of molybdenum isotope data (δ 98 Mo) from organic-rich shales to draw inferences concerning marine paleoredox conditions at a global scale is predicated upon the assumptions of (1) a residence time of Mo in seawater much greater than the ocean mixing time, and (2) quantitative removal of Mo from a strongly euxinic ([H 2 S] aq > 11 μM) water column to the sediment, thus preserving the seawater δ 98 Mo signature. In this study we analyze Mo isotopic variation in the Hushpuckney Shale, a 73-cm-thick unit representing the late transgressive to early regressive stages of a glacio-eustatic cyclothem (Swope Formation) deposited in the Late Pennsylvanian Midcontinent Sea (LPMS) of North America. The Hushpuckney can be subdivided into four stratigraphic zones of distinctive geochemical character. Zones I and III, which accumulated under weakly euxinic conditions, acquired relatively high δ 98 Mo values (+0.9 to +1.1‰), whereas Zone II, which accumulated under intensely euxinic conditions, acquired lower δ 98 Mo values (~+0.6‰). Zone IV, which accumulated under suboxic conditions in the water column, acquired the heaviest δ 98 Mo values (+1.1 to + 1.8‰). These results contrast with the pattern of redox -δ 98 Mo covariation in modern marine environments, in which the heaviest δ 98 Mo values are found in the most intensely euxinic facies. We evaluate three different hypotheses to account for the Mo isotopic patterns of the Hushpuckney Shale. One hypothesis, seawater-freshwater mixing, is rejected owing to isotopic mass balance considerations. A second hypothesis is a local control on δ 98 Mo by water-column redox cycling of Mn, with particulate Mnoxyhydroxides adsorbing isotopically light Mo and transferring it to the sediment, a process that was most active during deposition of Zone II. The significance of this scenario is that euxinic black shales may not reliably record global seawater δ 98 Mo in areas where a Mn-particulate shuttle is operative. A third hypothesis is based on rapid secular variation of the Mo isotope composition of Late Pennsylvanian global seawater. In order to account for δ 98 Mo trends within the Hushpuckney Shale, seawater δ 98 Mo must have varied by~1.2‰ at a~100-kyr timescale, which would have been possible only if the residence time of Mo in Late Pennsylvanian seawater was b 100 kyr. Although both the second and third hypotheses are viable based on the present limited δ 98 Mo dataset, we discuss how each model might be tested through additional Mo isotope data.
The photosynthetic production of oxygen in the oceans is thought to have begun by 2.7 billion yea... more The photosynthetic production of oxygen in the oceans is thought to have begun by 2.7 billion years ago, several hundred million years before appreciable accumulation of oxygen in the atmosphere. However, the abundance and distribution of dissolved oxygen in the late Archaean oceans is poorly constrained. Here we present geochemical profiles from 2.6-to 2.5-billion-year-old black shales from the Campbellrand-Malmani carbonate platform in South Africa. We find a high abundance of rhenium and a low abundance of molybdenum, which, together with the speciation of sedimentary iron, points to the presence of dissolved oxygen in the bottom waters on the platform slope. The water depth on the slope probably reached several hundred metres, implying the export of O 2 below the photic zone. Our data also indicate that the mildly oxygenated surface ocean gave way to an anoxic deep ocean. We therefore suggest that the production of oxygen in the surface ocean was vigorous at this time, but was not sufficient to fully consume the deep-sea reductants. On the basis of our results and observations from the Hamersley basin in Western Australia, we conclude that the productive regions along ocean margins during the late Archaean eon were sites of substantial O 2 accumulation, at least 100 million years before the first significant increase in atmospheric O 2 concentration.
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