Onset of persistent surface ocean oxygenation during the Great Oxidation Event

Domain: earth_science
Published: 2025-12-09
Editorial outcome: accept

Abstract
Free oxygen (O2) first began accumulating in Earth’s atmosphere shortly after the Archean-Proterozoic transition during the ‘Great Oxidation Event’ (GOE). The nature of surface ocean oxygenation at this time is, however, poorly quantified, limiting our understanding of planetary oxygenation thresholds. Geochemical records of global ocean redox may potentially shed light on this critical interval. Here, we show that vanadium (V) isotope ratios in 2.32-2.26-billion-year-old (Ga) shales from the Transvaal Supergroup, South Africa, capture a unidirectional transition in global ocean redox conditions shortly above the stratigraphic level marking the canonical rise of atmospheric O2. Around 2.32 Ga, sedimentary sinks were dominated by anoxic environments that drove extensive seawater V drawdown. A positive shift in seawater V isotopic composition in the overlying stratigraphy indicates global expansion of marine settings with ≥10 μM dissolved O2 in bottom water, likely restricted to shallow-water environments and attributable to widespread equilibration with an oxygenated atmosphere.

Introduction
The rise of atmospheric O2 partial pressure (pO2) during the GOE, ca. 2.43-2.22 billion years ago1–3, marks the permanent oxygenation of Earth’s surface. Despite intense study of this time interval, there is little consensus on the tempo and amplitude of O2 accumulation in the atmosphere4, and even less consensus on its accumulation in the ocean5. Marine oxygenation in response to the GOE fundamentally changed the trajectory of biological innovation on Earth, ultimately laying the groundwork for complex multicellular life, and constituted a critical step in defining the ultimate nature of Earth’s habitability6,7.

The GOE is marked most clearly by loss of sulfur isotope mass-independent fractionation (S-MIF) signatures, generated by photochemical reactions in an oxygen-free atmosphere, from the sedimentary record8. After the GOE, there was a sufficient pO2, above a threshold of >10–6 of the present atmospheric level (PAL), to prevent preservation of S-MIF signatures9. The disappearance of S-MIF was initially thought to occur as a unidirectional transition2,3, approximately corresponding to the boundary separating the Rooihoogte and Timeball Hill formations of the Transvaal Supergroup, South Africa, dated to 2.316 ± 0.007 Ga10 (Fig. 1A). Subsequent work has provided possible evidence for older and younger Paleoproterozoic S-MIF disappearances1,11,12. The significance of younger S-MIF is still debated (see Supplementary Information for points of relevance to this study), but multiple possible fluctuations across the GOE interval suggest that atmospheric pO2 may have oscillated across the 10–6 PAL threshold during a transition lasting from ca. 2.43 to 2.22 Ga1,12–14.

The ca. 2.43–2.22 Ga time interval encapsulates a knowledge gap in the marine response to the GOE. A widely recognized signal for marine biogeochemical overhaul is not seen until the ca. 2.22–2.06 Ga Lomagundi carbon isotope excursion (LCIE), the largest positive carbonate carbon isotope excursion in Earth history, which has been linked to unprecedented organic carbon burial that drove ocean oxidation15. Evidence of coupled atmospheric and marine oxygenation was reported at the ca. 2.32 Ga Rooihoogte-Timeball Hill formation boundary (Fig. 1), with thallium (Tl) isotopic data indicating widespread burial of Mn oxides, requiring oxygenated bottom waters on shallow marine shelves at almost the same stratigraphic level where persistent, large S-MIF signals disappear16.

However, evidence for Mn oxide burial provides only one, qualitative index for rising O2, so to provide further texture to our understanding of ocean oxygenation across the GOE, we measured sedimentary V isotope ratios (reported as δ51V=(51/50V sample / 51/50V sw – 1) × 1000) in the same Rooihoogte and Timeball Hill shale samples previously analyzed for Tl isotope values16. Vanadium isotopes track the global marine redox state as Tl isotope values do, but the oxidized sink for V in the oceans records a threshold dissolved O2 level17 (>10 μM; outlined in detail below), rather than the specific burial of Mn oxides18. As such, combined V and Tl isotopic data can provide more nuance to reconstructions of global ocean oxygenation events and their impacts on multiple redox-sensitive element cycles19. In this study, we targeted organic-rich shales and analyzed δ51V in the authigenic V fraction (δ51Vauth). This fraction represents the V scavenged from Paleoproterozoic seawater by sinking organic matter, the isotopic composition of which allows reconstruction of relative changes in the global ocean redox state20.

Results
The δ51Vauth values range from –1.50 to –0.68‰ in the Rooihoogte and Timeball Hill formations (Fig. 1G, Supplementary Data S1). Much of this variability comes from a few outliers low in the stratigraphy; the drivers of which are discussed below and in Fig. S1. The samples define two populations based on their stratigraphic position. In the samples deposited deeper than 1,300 m drill core depth (hereafter referred to as the ‘lower section’), the average value of δ51Vauth is −1.07 ± 0.07‰ (2SE). In the samples from above 1100 m drill core depth (hereafter referred to as the ‘upper section’), the average value of δ51Vauth is −0.80 ± 0.05‰ (2SE). There is therefore an increase in the average δ51Vauth value of 0.27 ± 0.12‰ between the lower and upper sections.

Geochemical indicators from the EBA-2 drill core show that local redox conditions fluctuated between oxic and anoxic throughout deposition of the Rooihoogte and Timeball Hill formations12,16 (Fig. 1).

Almost all samples we targeted have highly reactive Fe to total Fe ratios higher than those expected for oxic conditions (FeHR*/FeT > 0.22, where FeHR* includes a correction for highly reactive Fe incorporated into clay minerals during diagenesis; Fig. 1B), and most have FeHR*/FeT > 0.38, which is above the calibrated threshold for anoxic deposition12,44. A handful of samples in the Rooihoogte Formation have pyrite to total reactive iron (Fepy/FeHR*) values that place them in the possibly euxinic (Fepy/FeHR* = 0.6‒0.8) or euxinic (Fepy/FeHR* > 0.8) fields45. Additionally, bulk samples generally feature elevated enrichment factors (XEF) relative to the UCC (XEF=(X/Al)sample / (X/Al)UCC) for the redox-sensitive elements V, Mo and U16, which requires reducing (and in the case of Mo, euxinic) conditions46–49. Two samples in the lower part of the section with FeHR*/FeT < 0.22, which potentially indicates deposition under oxic conditions, have V, Mo and U EFs much higher than 1, suggesting deposition under reducing conditions, where Fe2+ was remobilized back to the water column from anoxic non-sulfidic pore waters at the sediment-water interface45.

Despite capturing an important interval of Earth history in the immediate aftermath of the first S-MIF disappearance, we did not analyze authigenic V isotopes in the stratigraphic interval between ~1100 and 1300 m depth. All available samples from this interval were likely deposited under oxic conditions based on FeHR*/FeT ratios that are lower than 0.22 (Fig. 1B). This sedimentary redox condition requires different leaching procedures due to a different dominant host phase for authigenic V (vanadate adsorbed to Fe oxyhydroxides), and this leaching procedure has yet to be applied to or calibrated for ancient sedimentary rocks, having only been tested in modern marine sediments17.

Methods
Geological setting of samples
Samples were obtained from the EBA-2 diamond drill core (26.4700° S, 27.5883° E) drilled near Carltonville, South Africa (Kloof Goldfields Property, Eastern Boundary Area). This drill core intersected a well-preserved interval of the Paleoproterozoic Transvaal Supergroup, and the specific depth intervals analyzed in this study have been sampled and analyzed in several previous studies2,3,12,16. The interval of the EBA-2 drill core we studied comprises the Rooihoogte and Timeball Hill formations. The succession has undergone regional metamorphism to only lower greenschist facies grade54.

The Rooihoogte Formation has upper and lower members in the drill core EBA-2, and only the upper Rooihoogte Formation was analyzed and is thus described in this study. It dominantly consists of mudstones and black shales that coarsen upwards into siltstone and are capped by a thin chert breccia42. The Timeball Hill Formation contains two upward-coarsening sequences. In each sequence, the lower part is highly carbonaceous black mudstone, and these mudstones become less carbonaceous up-section, where they are interbedded with dark-gray to gray siltstones42. The mudstones and black shales of the upper Rooihoogte and Timeball Hill formations are interpreted to have been deposited in a pro-delta setting within the basin, with a connection to the open ocean towards the southwest55,56. Diamictite and conglomerate of the Rietfontein Member form a cap for the upper Timeball Hill Formation42. The Rietfontein diamictite has been interpreted to be a glacial deposit based on the presence of faceted and striated pebbles57, and was not sampled for authigenic V isotopic analysis in our study.

Vanadium isotope analysis
Samples were prepared for V isotope analysis in a Class 100 clean laboratory at Woods Hole Oceanographic Institution (WHOI). All reagents used were double-distilled in the laboratory or purchased at trace-metal grade or higher. Approximately 200 mg of powdered sample material was weighed into acid-cleaned Savillex Teflon vials and leached overnight in 2 M HNO3 at 130 °C, following a protocol that is shown in numerous previous studies to isolate authigenic V17,40. The leached fraction was then separated by centrifugation and pipetting of the supernatant into cleaned Teflon vials for multiple further acid digestion steps. Acid digestion steps included the use of concentrated HCl+HNO3 (aqua regia) and concentrated HNO3+H2O2, which destroy organics, but avoid digesting any detrital silicate material. Aliquots of these exact same leach solutions were utilized for previous Tl isotope analyses16. Aliquots of the samples were then brought into solution in 2 mL of 0.8 M HNO3 and processed through a four-step ion-exchange column chromatography procedure to purify V from matrix elements, particularly Ti and Cr, which have isobaric interferences with V58,59. Load and matrix fractions collected for each column were analyzed using a Thermo Fisher iCAP quadrupole inductively coupled plasma mass spectrometer (Q-ICPMS) at WHOI to determine that no V was lost during column purification.

Vanadium isotope ratios were measured on a Thermo Neptune multi-collector ICP-MS (MC-ICP-MS) at WHOI following established methods60. Both samples and standards were prepared to matched concentrations of 800 ng/ml in 0.1 M HNO3. Sample introduction was conducted using a Cetac Aridus II desolvating nebulizer, and in the Neptune, nickel ‘Jet’ type sampler and ‘X’ type skimmer cones. Analyses were conducted in medium resolution mode to resolve flat-topped V (and Cr, Ti) peak shoulders away from various ArC+ and ArN+ (plus hydride) interferences. In this configuration, we typically achieved 150–250 V/800 ppb V sensitivities on 51V. To correct for isobaric interference from minor contaminant 50Cr and 51Ti on the V isotope signals, Cr and Ti isotopes were analyzed simultaneously with 50V and 51V and isobar contributions were corrected for using a mass bias coefficient that was determined at the end of the analytical session by analysis of a pure Cr plasma tuning solution. All isotopes of interest were measured on Faraday cups fitted with 1011 Ω resistors, except 51V, which was measured using a 1010 Ω resistor. Analysis used standard-sample bracketing with the Alfa Aesar (AA) V specpure standard solution to correct for instrumental mass fractionation, with V isotopic ratios reported in delta notation: (δ51V (‰)=1000 × {[(51V/50V)sample / (51V/50V)AA] – 1}). Each sample was bracketed with analysis of the BDH Chemicals V solution internal standard with known δ51V17,35,58 that was analyzed identically to samples to monitor instrument performance, giving values in agreement with the published δ51V value of −1.19‰ over each full analytical session (δ51V=−1.18 ± 0.08‰ (2SD) in session 1; δ51V=−1.21 ± 0.15‰ (2SD) in session 2). Samples were analyzed in duplicate or triplicate, according to the amount of available material, and uncertainties are presented as the larger of either the 2SD uncertainty of BDH analyses run in the same sequences or the 2SD of replicate analyses of the samples. Bulk digests of the BHVO-2 and AGV-2 USGS igneous rock geostandards, and an authigenic leach of the SCo-1 USGS shale geostandard, were processed through the column chromatography and mass spectrometry protocols alongside drill-core EBA-2 sedimentary rock samples and yielded values in agreement with those previously published17,19,59,61,62.

Our V isotope dataset was generated by analysis of two separate batches of samples over separate analytical sessions (Supplementary Data File). We saw no evidence for systematic differences in δ51V values measured in each session. Of the geostandards measured in both sessions (SCo-1 leach, BHVO-2), one was lighter (but within error) and another was equal to or heavier (but within error) than the recommended value for each session, and the BDH values were almost identical (second session slightly lighter), and always within error of the recommended value. For the EBA-2 drill-core shale samples, no upper section samples (<1300 m depth) were analyzed in the first session. However, the average value of the lower section (>1300 m) samples was identical in each session (first session: δ51Vauth=−1.07 ± 0.08 2SE, second session: δ51Vauth=−1.07 ± 0.11 2SE). Therefore, the positive δ51V shift (averaging 0.27‰) observed from the lower to upper section appears not to be an artifact of different analytical sessions.

Trace element concentrations
Bulk and authigenic V and a suite of other trace-element concentrations were determined from a separate bulk rock digest, and an aliquot of the authigenic leachate produced for V isotope analysis, respectively. These concentration measurements were made during the previous Tl isotopic study, and the methods used for these analyses are briefly outlined below for ease of reference. Bulk digestion of ~20 mg of rock powder was performed using concentrated acid steps, including HF+HNO3, HCL+HNO3, and HNO3+H2O2. For both bulk and leachate material, elemental concentrations were generated using a Q-ICP-MS analyses at WHOI with reference to a five-point calibration curve based on dilutions of a gravimetrically prepared, multi-element standard. Prior to analysis, samples were diluted in 2% HNO3 and doped with indium (In) to act as an internal standard for monitoring matrix effects and instrumental drift. Trace-element concentration measurements performed this way at WHOI have been shown to be accurate and have a precision of 5–10% depending on individual elements, based on comparison to replicate analyses of USGS reference materials AGV-1, AGV-2, BHVO-1, BHVO-2, BIR-1, and BCR-263,64.

Vanadium isotope fractionation calculations
We do not attempt to pinpoint the exact values of δ51Vsw, or the globally averaged effective Δ51V that prevailed during deposition of the Rooihoogte and Timeball Hill formations, due to limited constraints in the local fractionation of V into sediments. Two end-member scenarios are suggested to explain the relative change in δ51Vsw of a +0.27‰ between the average values inferred for the lower and upper sections. We considered two scenarios: one where there was no oxic V sink, and a positive shift in δ51Vsw resulted from a shift to non-quantitative, fractionated vanadyl drawdown in reducing environments that became slightly more oxidized after 2.32 Ga and one where anoxic (O2 > 10 μM) V sink appeared or expanded.

In the scenario that the increase in δ51Vsw solely reflects a decrease in the extent of vanadyl drawdown within a single global reducing sink: δ51Vrivers = δ51Vsediments,out (1) and δ51Vsediments,out = δ51Vsw + Δ51Veff (2) where Δ51Veff is the effective isotopic difference between seawater and sediment. In the extreme case that initially in the lower section, Δ51Veff = 0‰ (full drawdown of vanadyl)20, an increase of δ51Vsw by +0.27‰ would need to be offset by a decrease of Δ51Veff to −0.27‰ during deposition of the upper section. If we treat this sink as the cumulative product of Rayleigh distillation with an instantaneous isotopic difference Δ51Vinst = −0.7‰ (as seen during V drawdown to the Cariaco Trench sediments)17, then: Δ51Veff = δ51Vcumulative – δ51Vinitial,dissolved = (f – 1) × Δ51Vinst × ln[(f – 1) / f] (3) The smallest change in f associated with a 0.27‰ decrease in Δ51Veff is found when f goes from 1 to 0.81, giving a minimum 19% decrease in organic particle-associated V burial (Fig. S2). We note that this end member scenario poses an unrealistic initial (lower section) condition of total, quantitative drawdown of vanadyl from seawater, but we use this end member as it sets the minimum relative change in V drawdown burial that could have occurred going from the lower to upper section.

For the oxic sink scenario, we can again determine the minimum relative change in the size of the oxic sink using the end member scenario that the whole shift in δ51Vsw reflects a global change in Δ51Veff from 0‰ in the lower section, and −0.27‰ in the upper section. If we assume, in this case, that the entire 0.27‰ increase in δ51Vsw solely reflects new V removal to an oxidized vanadate sink with Δ51V O2>10μM = −1.1‰17, while the remainder of V is still removed to highly reducing sink with Δ51Vred = 0‰, then the increase in fraction of oxidized V burial can be found by solving: Δ51Veff = fox × Δ51V O2>10μM + (1 – fox) × Δ51Vred = –0.27 (4) setting Δ51Vred = 0, and rearranging to find that fox = 0.27/1.1 = 25%, which is approximately 1/3 of the modern ocean oxidized V sink fraction20.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This is my second time to review this paper. Considering this is a new submission to Nature Communications, I go through the whole MS as a new one. Of course, this is still a very important study, and the new V isotope dataset is with high quality and scientific significance. In addition, authors have addressed most of the points raised by the previous reviewers, and the MS has been certainly improved. I appreciate their effort to take the reviewers' comments into account, and suggest publication after considering my new comments listed below.\nLine 37. Could you please introduce more details on the trajectory of biological innovation during the GOE?\nLine 60. I encourage authors to consider a more important scientific question, rather than complementing the Tl isotope perspective on ocean oxygenation across the GOE using V isotopes.\nLine 74–77. But the residence time of oceanic V may be shorter than 90 kyr in ancient oceans.\nLine 77–78. Deposition of dissolved V into sediments with different isotope fractionation should be involved in the reasons why V isotope geochemistry provides information on the global ocean redox state, which I think is the most important.\nLine 86. I don’t think this isotope fractionation is only controlled by Fe-oxyhydroxide.\nLine 97–99. This should be indicated in Fig. 2.\nLine 118. You should at least provide the location of this drill core.\nLine 156. Referred to as the “upper section”?\nLine 161. Again, you should make sure if the paleoseawater δ51V value was globally homogeneous!\nLine 165–166. I can see a robust positive correlation between the TOC and V EF, which could support the removal of oceanic V by organic matter.\nLine 201–207. Maybe, some paleo-salinity proxies can help to eliminate this effect.\nLine 221–226. Figures are necessary to visually illustrate the Rayleigh distillation model (Line 223) and two-component carbon isotope mass balance model, at least in Supplementary materials (Line 225–226).\nLine 233–236. See the comment above.\nLine 260. “These data provide”.\nLine 472–488. Figs. 1H-N are not described.\nFigure 1. The relationship of (A-G) with (H-N) is not shown clearly. Authors can revise as Ostrander et al. (2024).\nFigure 2. The V isotope fractionation between euxinic sediments and open-ocean seawater is from –0.7‰ to 0.0‰, controlled by local V drawdown efficiency. Please revise it.\nWei Wei\nUniversity of Science and Technology of China", "reviewer_2": "I was reviewer #3 in the previous round of submission of this manuscript. In re-reviewing the manuscript here, I found that the responses to my queries were answered satisfactorily and that the overall clarity of the manuscript was substantially improved by the revisions suggested by all reviewers.", "reviewer_3": "This study builds on work done on the EBA-2 drill core from South Africa that goes from the Rooihoogte to Rietfontein formations, spanning age constraints of 2.32 to 2.26 Ga, with samples previously measured for S-MIF (Poulton et al., 2021) and Tl isotopes (Ostrander et al., 2024). In this new study, V isotopes are measured that could offer additional insight on ocean anoxia and oxygenation during the time of the Great Oxygenation Event (GOE) and its aftermath. Although the use of d51V is interesting and novel, there are important limitations in this study."}, "author_response": "We thank the reviewer for two rounds of useful comments and appreciate their recommendation that our manuscript be published pending these last recommended changes.\nLine 37. Could you please introduce more details on the trajectory of biological innovation during the GOE?\nAdded as requested:\n“ultimately laying the groundwork for complex multicellular life”\nLine 60. I encourage authors to consider a more important scientific question, rather than complementing the Tl isotope perspective on ocean oxygenation across the GOE using V isotopes.\nWhile focusing solely on the title sentence of this paragraph arguably misses the important science question we formulate within this paragraph, we take the reviewer’s point and revised this first clause to read:\n“However, evidence for Mn oxide burial provide only one, qualitative index for rising O , so to provide 2 further texture to our understanding of ocean oxygenation across the GOE”.\nLine 74–77. But the residence time of oceanic V may be shorter than 90 kyr in ancient oceans.\nAddressed with the added text:\n“This global homogeneity is also expected to hold in the Paleoproterozoic, because V concentrations in black shales from this time period suggest a comparable order-of-magnitude size of the marine dissolved V pool to the modern22,23, and thus there is no obvious reason why the oceanic V residence time would be orders of magnitude lower19. ”\nLine 77–78. Deposition of dissolved V into sediments with different isotope fractionation should be involved in the reasons why V isotope geochemistry provides information on the global ocean redox state, which I think is the most important.\nWe had added this point about isotope fractionation to point 1) of this paragraph, as suggested:\n“Vanadium isotope geochemistry provides information on the global ocean redox state because 1) V is redox sensitive, behaving differently and taking on different isotopic compositions in different redox environments (Fig. 2); and 2) it has a long (ca. 90 kyr) residence time relative to modern and ancient ocean mixing timescales on the order of 1 kyr20, such that the seawater dissolved V reservoir and its isotopic signature should be globally well-mixed in the open ocean and unrestricted basins19.”\nLine 86. I don’t think this isotope fractionation is only controlled by Fe-oxyhydroxide.\nWe indicated the fractionation is explainable by this mineral control, rather than that this is the sole control. However we have edited the text to be more inclusive:\n“and adsorbed onto a range of Fe-Mn oxyhydroxide surfaces19,21”\nLine 97–99. This should be indicated in Fig. 2.\nThis has been fixed as requested.\nLine 118. You should at least provide the location of this drill core.\nThis information was already in the Methods section on Geological setting of samples but sure, we have added it here too.\nLine 156. Referred to as the “upper section”?\nThanks for catching this, we added the parenthetical “(hereafter referred to as the ‘upper section’)” as suggested.\nLine 161. Again, you should make sure if the paleoseawater δ51V value was globally homogeneous!\nThis is now addressed in our response to requested addition above, pertaining to the residence time of V in the oceans.\nLine 165–166. I can see a robust positive correlation between the TOC and V EF, which could support the removal of oceanic V by organic matter.\nWe agree, there is some supporting evidence for that point. We edited those sentences to read:\n“A possible negative co-variation with of δ51V V EF, and apparent positive correlation of TOC with V auth EF, occur in the lower section, which may indicate water column V depletion that plausibly could be driven by organic matter (Fig. S1)”\nLine 201–207. Maybe, some paleo-salinity proxies can help to eliminate this effect.\nThis is a nice suggestion for future studies to consider, but given the limited track record of paleo- salinity studies in such ancient shales, where seawater trace element ratios may have been quite different to those assumed where these techniques are applied in Phanerozoic shales, it is not something we are readily able to employ in our study at this time.\nLine 221–226. Figures are necessary to visually illustrate the Rayleigh distillation model (Line 223) and two-component carbon isotope mass balance model, at least in Supplementary materials (Line 225– 226).\nLine 233–236. See the comment above.\nWe have provided supplementary figure panels (Figure S2) to illustrate these simple fractionation calculations, as requested.\nLine 260. “These data provide”.\nChanged as requested\nLine 472–488. Figs. 1H-N are not described.\nFigure 1. The relationship of (A-G) with (H-N) is not shown clearly. Authors can revise as Ostrander et al. (2024).\nAddressing both above comments at the same time: We added text to the caption:\n“(H-N) show the same data as for (A-G), with the narrow stratigraphic range at the base of the section from 1,315 to 1,350 m drill core depth expanded for clarity.”\nWe also added a small bracket annotation to the figure indicating that the lower panels are a zoomed in inset of the upper panels, in the manner of the Ostrander et al. paper.\nFigure 2. The V isotope fractionation between euxinic sediments and open-ocean seawater is from – 0.7‰ to 0.0‰, controlled by local V drawdown efficiency. Please revise it.\nDone"}]

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