Ocean deoxygenation after the Sturtian Snowball

Domain: earth_science
Published: 2025-07-01
Editorial outcome: accept

Abstract
The abrupt ending of the Sturtian ‘Snowball’ glaciation was characterised by enhanced chemical weathering and carbon cycle perturbations, but there is less certainty over how oxygen levels responded to those changes. Here we reconcile conflicting views using a carbonate-based multiproxy dataset from the Taishir Formation in Mongolia. The geochemical data reveal an episode of ocean deoxygenation, followed by a shift toward less reducing, but still largely anoxic conditions in a post-glacial ocean characterised by nutrient and sulfate limitation. Ocean redox dynamics and biogeochemical cycling following the Sturtian deglaciation were likely dictated by unique tectonic and climatic regimes that facilitated the buildup of a recalcitrant dissolved organic carbon pool in the deep ocean. Post-glacial eutrophication may help to explain the delayed diversification of algal clades, but the persistence of ocean anoxia, excepting transient oxidation pulses, likely hindered the emergence of obligate aerobes, such as animals, until the Ediacaran Period.

Introduction
The Cryogenian Period (c. 717 to 635 million years ago, or Ma) encompasses the Sturtian and Marinoan ‘Snowball Earth’ glaciations1, together with an intervening non-glacial interlude during which a seemingly ‘normal’ hydrological cycle returned. The period represents a fundamental turning point in Earth-Life evolution, linking the ‘Tonian Transformation’2 with the ‘Ediacaran-Cambrian Radiation’3. The non-glacial interlude (c. 661–650 Ma) is of particular interest due to it being a key stage in the rise toward ecosystem dominance by multicellular eukaryotes in the form of both algae and animals (putative sponges), as evidenced by organic biomarkers4,5. This ecosystem reshaping was accompanied by significant perturbations to the global carbon cycle as indicated by a high carbonate carbon isotope (δ13C carb) baseline punctuated by transient negative δ13C carb excursions. Examination of the Cryogenian carbon cycle has raised the hypothesis that decreased organic carbon remineralisation associated with sulfate-poor deep ocean conditions led to the growth of a large dissolved organic carbon (DOC) pool6, but the evidence remains controversial7. High δ13C carb values have also been related to ocean-atmosphere oxygenation, possibly caused by increased organic carbon burial and corresponding oxygen release due to a surplus of glacially induced nutrient supply, thus providing a potential link to early animal evolution8. Recent studies highlight the role of uninhabitable environments for limiting animal evolution, specifically pinpointing the immediate aftermath of the Sturtian deglaciation as one such key interval9,10. Proxy evidence and modelling studies point to enhanced volcanic activity10, a brief burst of chemical weathering11,12 and global cooling following a super-greenhouse climate9,13 in the aftermath of the Sturtian deglaciation. A short-lived euxinic interval has also been inferred from iron speciation data9, but the underpinning mechanisms are not fully understood. Moreover, while Mo isotopes from siliciclastic deposits support extensive marine euxinia14,15, this would appear to contradict U isotope records from carbonate rocks that are interpreted to indicate transient ocean oxygenation16. These conflicting views hamper our understanding of mechanistic links between climate, ocean dynamics and biological evolution during such a critical interval.

In order to resolve these uncertainties, we revisit the Taishir Formation in Mongolia, which represents one of the few continuous successions deposited on an open carbonate ramp during the non-glacial Cryogenian interval12 (Fig. 1). Details of the geological setting are provided in the Supplementary Information. We focus on well-preserved carbonate samples from the lowermost interval of the Taishir Formation, which was deposited over several million years following the Sturtian Snowball deglaciation11,17 (Supplementary Information). To attain insight into carbon and sulfur cycling, nutrients, weathering and marine redox conditions, we investigate a wide range of carbonate-based geochemical proxies that include carbon, sulfur, zinc, strontium and uranium isotopes, and rare earth elements (REE), phosphorus and iodine concentrations. These proxies are extracted from samples with high carbonate contents following established leaching protocols, and no significant contamination from non-carbonate phases is identified (Methods and Supplementary Information).

One major assumption in such chemostratigraphic studies is that geochemical proxies have regional or even global significance due to the chemical and isotopic homogeneity of the world’s oceans. However, transient heterogeneity may have occurred in the immediate aftermath of Cryogenian glaciations due to the abrupt release of a meltwater plume, as evidenced perhaps by the relatively radiogenic 87Sr/86Sr ratios of carbonates overlying Sturtian diamictites in NW Canada18. Sr isotope values in the studied interval are not anomalous but are instead consistent with the least radiogenic values from Twitya18 and Rasthof19 carbonates. Together with the lack of typical cap carbonate sedimentary features, the studied interval seems unlikely to have been affected by a meltwater plume during deposition (Supplementary Information). Additionally, petrographic observations and critical scrutiny of diagenetic effects (Supplementary Information) show that the first-order stratigraphic trends (Fig. 2) are indicative of changes in seawater composition, and have not been significantly overprinted by diagenetic processes. Therefore, these records enable us to depict a more holistic picture of post-Sturtian environments and interrogate potential links between environmental change and biological evolution during the rise to organic complexity.

Results
Local redox conditions
Cerium anomalies archived in carbonate rocks can provide insights into local water column redox conditions, whereby negative Ce anomalies (values below 0.9–1), accompanied by seawater-like REE patterns, are commonly considered to indicate oxic conditions20,21. Samples with seawater-like REE patterns in this study show average Ce anomalies of 1.11 and 0.78 for the lower (0–32m) and upper (50–120m) parts of the succession, respectively. While we observed non-seawater-like REE patterns from the lower part (Supplementary Fig. 6; Supplementary Information), the exclusion of these values does not affect the overall Ce anomaly trend, which is consistent with previous studies11,16 (Fig. 2). This indicates that the depositional settings transitioned from largely suboxic/anoxic seafloor conditions20 during deposition of the lower part, to oxic conditions for the upper part. The very low sulfur contents in decarbonated residues of the samples11 further indicate that the local seawater and/or pore water was mostly non-sulfidic during deposition of the lower part.

Carbonate-associated iodine, expressed as I/(Ca+Mg), is emerging as another useful proxy for tracking shallow seawater redox conditions22. This is mainly because only iodate is incorporated into carbonates23, is prevalent in oxic seawater and reduces rapidly to iodide in low-oxygen environments24. Owing to the relatively slow oxidation kinetics of iodide25,26, carbonate I/(Ca+Mg) ratios are also sensitive to oxycline proximity27,28. The low I/(Ca+Mg) values (<0.5μmol/mol) in our samples are consistent with most Proterozoic carbonate records (Fig. 2), which have been interpreted to indicate a redox-stratified water column with a shallow oxycline29. Nevertheless, I/(Ca+Mg) is highly susceptible to diagenesis30 and generally decreases during diageneticalteration29. Considering that the samples may have experienced neomorphism, diagenetic iodine loss cannot be ruled out entirely (Supplementary Information).

Transient ocean deoxygenation
The sulfur isotope composition of carbonate-associated sulfate (δ34S CAS) is a useful tool for reconstructing the dynamic sulfur cycle, which is linked to Earth’s surface oxidation state through pyrite burial31. Recent advances in geochemical redox proxies have also allowed the extent of global seafloor anoxia to be estimated by uranium isotopes in carbonate rocks16,32,33. In short, during the reduction of soluble U(VI) to particulate-reactive U(IV), heavy 238U is favoured due to the nuclear field shift effect34. Intensified U(IV) removal to sediments during periods of expanded marine anoxia thus drives seawater δ238U toward lower values. Although there appears to be some effect from organic matter loading on uranium isotope fractionation35, large isotopic fractionation is generally associated with uranium reduction. Therefore, marine anoxia is considered to exert a dominant control on the seawater δ238U.

Our carbonate δ238U values (Fig. 2), combined with published δ238U records16 from different stratigraphic sections, reveal a decreasing trend following the Sturtian deglaciation (Fig. 3). The decreasing trend cannot readily be explained by sea-level changes or chemical weathering (Supplementary Information) and instead suggests significant expansion of anoxic waters. This evidence for expanded anoxia is accompanied by extremely high δ34S CAS values (mean 63‰; Supplementary Information), which require enhanced microbial sulfate reduction and removal of 32S-enriched pyrite from the ocean36. Therefore, the δ238U and δ34S CAS data together suggest greatly expanded marine anoxia and pyrite burial following the Sturtian deglaciation. Indeed, coupled high δ34S CAS (> +40‰) and low δ238U (<−1‰) values are also evident during the late Ediacaran32,33,36,37 and similarly attributed to the spread of euxinic seawater and enhanced pyrite burial, along with an elevated isotopic ratio of sulfur input (δ34S in)33,36–38. This coupling may also explain the observed trend following the Sturtian deglaciation, as glacial erosion could have exposed Tonian evaporites leading to elevated δ34S in, while the expansion of euxinic seawater is further supported by iron speciation records9 (Fig. 3).

The integrated carbonate δ238U records then show a shift towards relatively higher values with a mean of −0.49‰ (Fig. 3). This δ238U value has been associated with the post-Sturtian ocean oxygenation, with ~0.5% of the seafloor being anoxic compared to ~0.2% in the modern ocean16,39. Alternatively, this δ238U value is similar to that of most Proterozoic carbonates, which has been interpreted to indicate limited euxinia in an ocean that was still largely anoxic40. Such a discrepancy is related to limited U isotope fractionation under suboxic and ferruginous conditions41,42. Recent studies suggest that U isotope fractionation associated with ferruginous conditions may vary significantly, the magnitude of which is possibly linked to primary productivity with limited δ238U fractionation under the low-productivity, ferruginous environment43–45. Given the potentially decreased primary productivity46 (see also the following discussion), we consider that the positive δ238U shift, instead of indicating widespread oxygenation, is more likely to reflect the return to less reducing conditions with a concomitant reduction in marine euxinia. The inference of a reduction in marine euxinia is in accordance with low δ98Mo values in coeval shales14,15 and decreasing δ34S CAS and pyrite δ34S (δ34S py) values (Fig. 3), which in turn implies an increase in sulfate levels10. Nevertheless, seawater sulfate concentrations overall remained reasonably low as evidenced by low Δ34S (δ34S CAS – δ34S py) values47,48 and low [CAS] in these samples (see Methods and Supplementary Data) and coeval carbonates48. Taken together, integrating δ238U and δ34S evidence suggests that the post-Sturtian ocean experienced a transient ocean deoxygenation marked by the expansion of low-sulfate euxinic seawater, followed by a return to less reducing conditions.

Nutrient dynamics
Zinc is a bioessential micronutrient that functions as a cofactor in important enzymes such as carbonic anhydrase and alkaline phosphatase. The use of Zn isotopes to trace perturbations to micronutrient cycling is relatively new49–52, but builds on extensive investigation of Zn isotope systematics. The modern deep ocean is mostly homogeneous with respect to δ66Zn at around +0.45‰53, which is isotopically heavier than riverine input (~0.33‰)54,55. Euxinic sediments could potentially record seawater δ66Zn due to near quantitative removal of Zn56,57. Carbonate rocks are estimated to represent a minor sink for Zn54,58. Biogenic carbonates such as corals appear to be able to record the seawater δ66Zn value59, yet inorganic carbonates are isotopically heavier than the deep ocean with an offset of around 0.4‰ potentially due to Zn2+ incorporation through tetrahedral coordination58,60,61. While Fe-Mn crusts and nodules are 0.4–0.5‰ heavier than seawater54,55, recent studies suggest that the quantitatively most important oxic sink is dominated by pelagic oxic sediments62, which is 0.1–0.2‰ lighter than seawater58,63. Importantly, the only significant sink of isotopically light Zn in largely anoxic oceans seems to be organic-rich continental margin sediments64. In light of this global Zn isotope mass balance framework, past variations in seawater Zn isotope compositions as recorded in carbonate rocks and/or shales have been used to infer changes in Zn input to, and output from the ocean49–52.

Carbonate δ66Zn values exhibit a shift from +0.74‰ to +0.45‰, and remain around +0.45‰ in the lower part of the succession, followed by a gradual increase upsection to +0.68‰ (Fig. 2). The relatively low background δ66Zn values compared to modern carbonates61 might relate to the largely diminished pelagic oxic sink in deep ferruginous oceans. The δ66Zn stratigraphic variation is unlikely to be accounted for by changes in relative sea-level and/or local redox conditions (Supplementary Information). Instead, considering the isotopic offset between carbonates and seawater, δ66Zn variation in the lower part suggests a shift towards low regional seawater δ66Zn values. This negative shift broadly coincides with the prominent rise of 87Sr/86Sr indicating significantly elevated weathering rates11 (Fig. 2). Hence, one possible explanation is that strong postglacial weathering delivered abundant light Zn from continents either due to enhanced deglacial volcanism10 and/or exposed large igneous provinces9. Shifts to lower seawater δ66Zn values have also been attributed to a diminished light Zn sink (i.e., removal into organic-rich sediments) compensated for by expanded euxinic50 or oxic (Fe-Mn crusts/nodules)52 sinks, or to enhanced isotopically light Zn input from benthic52,53 or hydrothermal53 sources. A strong direct influence of hydrothermal Zn seems unlikely given the insignificant carbonate Eu anomalies in samples (~1.0), while increasing 87Sr/86Sr values are also inconsistent with a more globally significant hydrothermal source. Considering the U and S isotope evidence for expanding anoxia/euxinia during deposition of the lower part, any increased oxic sink (Fe-Mn crusts/nodules) or benthic input due to oxygenation can also be largely excluded. An increased euxinic sink alone would lead to a decrease in the marine Zn reservoir and hence carbonate Zn concentrations50, but Zn/(Ca+Mg) shows a consistent increasing trend in the lower part (Fig. 2). The relatively stable δ66Zn during the expansion of anoxia/euxinia as indicated by δ238U and δ34S records (Fig. 2) also suggests that an expanded euxinic sink is unlikely to give rise to δ66Zn variability in the lower part. On the other hand, the δ66Zn values then gradually increase upsection, and its onset occurs prior to the shift to less reducing conditions as indicated by Ce/Ce* and U isotopes (Fig. 2), implying a limited role for an expanding oxic sink. Furthermore, the transition to less reducing conditions in the upper part may have decreased seawater δ66Zn values via remobilization of light Zn from continental margin sediments52. Considering the coincidence with the plateauing of 87Sr/86Sr ratios, the increase in δ66Zn suggests that weathering-derived light Zn input was progressively outweighed by a relative increase in the burial of isotopically light organic-rich sediments. Overall, δ66Zn evidence suggests an enhanced nutrient supply into the ocean following the Sturtian deglaciation likely leading to eutrophication, followed by a period of diminished influx.

Phosphorus is considered to be the ultimate limiting nutrient for primary productivity in oceans over geological timescales65. Carbonate P/(Ca+Mg) can record relative changes to oceanic phosphate levels in deep time as long as other potential influences can be deconvolved66,67. The lower part of the succession has a relatively higher mean P/(Ca+Mg) (0.11 mmol/mol) ratio with a prominent peak to 0.2 mmol/mol near its top, compared to the upper part (mean 0.08 mmol/mol) (Fig. 2). The partitioning of phosphate into carbonate minerals is affected by the fluid phosphate concentration, pH, temperature, alkalinity, mineralogy and precipitation rates68. Following the post-Sturtian super-greenhouse climate, the oceans could have received enhanced alkalinity and calcium input from weathering13, which might have increased carbonate precipitation rates68. Meanwhile, there seem to be no significant variations in ocean pH69 and mineralogy (Supplementary Information). The combined effects would likely decrease carbonate P/(Ca+Mg) values, and hence, the prominent peak in the lower part implies relatively elevated seawater phosphate levels. Because of the nutrient-type distribution of dissolved phosphate in the ocean, shallow seawater would be expected to exhibit phosphate depletion, making the prominent increase with basin shallowing even more remarkable. The peak of P/(Ca+Mg) broadly coincides with high δ34S CAS and low δ238U values, suggesting that the increase in dissolved phosphate concentrations resulted from low P burial efficiency due to intensified recycling under euxinic/sulfidic conditions70. This positive nutrient recycling feedback is also supported by phosphorus phase association and iron speciation records from coeval strata9.

Weathering induced biogeochemical cascades
Redox dynamics and the global carbon cycle following the Sturtian deglaciation were dictated by an extreme climatic, but also a highly unusual tectonic regime (Fig. 3). Specifically, the peneplanation of Rodinia after its amalgamation71 likely led to a relatively low-relief supercontinent, on which chemical weathering was more transport- than rate-limited. During the long-lasting Sturtian glaciation, ice sheets scoured these relatively flat-lying continental interiors, removing regolith and surface rocks, and producing highly reactive rock flour6,72. As a consequence, the high pCO2 levels accumulated during glaciation decreased rapidly after deglaciation due to enhanced silicate weathering13,72 but were not readily replenished due to unusually low arc volcanism73,74 and mid-ocean ridge carbon outflux75 (Fig. 3). The brief episode of intense chemical weathering that followed deglaciation led therefore into a more prolonged interval of global cooling, in accordance with the temporal evolution of seawater 87Sr/86Sr (Fig. 3), and markedly lower nutrient and sulfate delivery to the ocean46,72.

The ocean at the end of the Sturtian deglaciation (~661 Ma) was probably dominated by anoxic ferruginous conditions (Fig. 3a) with euxinia delayed by limited sulfate availability in the oceans and a high proportional influx of iron relative to sulfate6. With increasing nutrient and sulfate input, a tipping point must have been crossed whereby a relative excess of H2S over iron promoted the expansion of euxinia. Enhanced P recycling under euxinic/sulfidic conditions in turn exerted a positive productivity feedback9,70 (Fig. 3b). Consequently, elevated organic carbon and pyrite burial, as evidenced by a coupled increase in δ13C carb and δ34S, would have resulted in a pulse of oxygen production. However, stimulated primary production would also have increased organic export to the deep ocean, increasing the oxygen demand for organic matter remineralisation76. The overall low seawater sulfate levels may also have increased the seabed methane flux, helping to keep seafloors anoxic77. Hence, increased oxygen consumption in the marine realm associated with organic matter (and possibly methane) oxidation resulted in ocean deoxygenation.

Following the eutrophication episode, the gradual decrease in weathering-derived nutrient and sulfate influx led to a lessening of marine euxinia9, in part due to a negative productivity feedback caused by less efficient P recycling (Fig. 3c). Together with increased oxygen solubility driven by a cooling climate78, this change would have been conducive to the expansion of less reducing environments in the marine realm. The cooling climate could also have decreased microbial respiration and hence induced a more efficient biological carbon pump79. In the absence of abundant oxidants, this process may have favoured organic matter preservation as supported by increasing δ13C carb (Fig. 3). Nonetheless, δ34S records broadly show an inverse relationship with δ13C carb during this stage (Fig. 3). Such a decoupling appears to suggest that the oxidant (oxygen) generated by organic carbon burial was partly transferred into sulfate through a surplus of pyrite weathering over burial, thereby limiting the extent of atmospheric oxygenation. Collectively, the above scenarios illustrate that oxygenation of the atmosphere and oceans (especially the deep marine realm) was likely asynchronous during the Cryogenian non-glacial interlude.

Implications for the carbon cycle and biological evolution
Sustained high δ13C carb values during the Cryogenian non-glacial interlude are conventionally interpreted to infer elevated organic carbon burial rates8. We suggest that the redox and nutrient dynamics, as discussed earlier, should also have enlarged the marine DOC inventory and hence impacted the carbon cycle. During post-glacial eutrophication, abundant organic substrates were available but would not have been efficiently degraded under expanded anoxia conditions, thus promoting DOC accumulation80. The later nutrient-limited condition, in conjunction with decreased microbial respiration driven by a cooling climate, would also have limited microbial DOC consumption81–83. Therefore, post-Sturtian environmental change led to the net accumulation of a recalcitrant DOC pool in the deep ocean, which is supported by a relatively muted δ13C org shift compared with the prominent rise of δ13C carb (Fig. 3). The DOC pool could have switched from net accumulation to net oxidation once certain oxidant thresholds were crossed, buffering against extensive oceanic oxygenation and atmospheric CO2 drawdown78,84. Such a prediction is consistent with coincident negative shifts of both δ238U and δ13C carb through the Taishir anomaly16. Taken together, we suggest that the sustained high δ13C carb following the Sturtian Snowball deglaciation is at least in part due to the buildup of a deep ocean DOC pool.

Marine anoxia and high sea surface temperatures have been invoked to explain the delayed rise of green algae and putative demosponges after the Sturtian deglaciation5,9,10. There is a broad coincidence between the development of less reducing environments, cooling climate and the rise of biotic complexity (Fig. 3). A synergistic effect can be envisioned whereby a lessening of marine anoxia alongside limited nutrient availability and moderate climate would have extended the habitable space, perhaps setting the stage for the expansion of opportunistic species. Therefore, our dataset appears to support the idea that severe environmental stress after the Sturtian deglaciation delayed further biological diversification. Nonetheless, conditions would have been favourable only to aerobic forms with low oxygen demands because the oceans were still not significantly oxygenated. Later oceans remained largely anoxic and the episodic transitions towards a more oxygenated system throughout the late Neoproterozoic appear to have relied on external stimuli such as tectonic drivers. This reliance may explain the prolonged rise of oxygen and delayed occurrence of energetic lifestyles over the late Ediacaran to Cambrian.

Methods
Bulk limestone samples were crushed into small rock chips. The clean chips without visible veins and weathered surfaces were selected and ground into fine powders using a TEMA agate disc mill or agate mortar and pestle for further processing.

Rare earth elements and phosphorus
Rare earth elements (REE) and phosphorus were extracted using sequential leaching85. For each sample, ~50 mg powder was prewashed with 5 ml neutral 1 M ammonium acetate. The mixture was sonicated in the ultrasonic bath for 30 min and then centrifuged for 5 min, from which the supernatants were discarded. The residues were rinsed with de-ionized (DI) water (18.2 MΩ·cm) three times, followed by partial leaching with dilute acetic acid (0.3 M) for 30 min in the ultrasonic bath. The resultant supernatants obtained by centrifugation were filtered into acid cleaned Teflon beakers using 0.22 μm syringe filters, dried on a hot plate and redissolved in 2% HNO3. The solutions were further diluted in 2% HNO3 for major and minor element analysis (Ca, Mg, Fe, Mn, Al, P, Sr, Ba) on a Varian 720 inductively coupled plasma optical emission spectrometer (ICP-OES) at University College London. Phosphorus was analysed at a wavelength of 213.618 nm using the polyboost function. The leachates were then diluted to ~100 ppm Ca with 2% HNO3 for rare earth elements analysis via an Agilent 7900 inductively coupled plasma mass spectrometer (ICP-MS) at University College London. The internal standard of 2 ppb indium was added to all analysed solutions to monitor the instrumental drift and matrix effects. The formation of 2+ ions and oxide interference were monitored using Ba2+ and the formation rate of Ce oxide before analysis. Correction of BaO interference on Eu was achieved by monitoring single Ba elemental solutions throughout the analytical session. The accuracy was monitored by two in-house bulk digested solution standards of SRM-88a and SGR-1, which showed agreeable values with certified values within uncertainty. Repeated analysis of the drift monitor and standards gave a precision better than 5%. Blanks showed negligible concentrations below the detection limit. REE concentrations are normalised against post-Archean Australian Shale86 (denoted by the subscript N). The bell-shaped index (BSI) is calculated after ref. 87 to evaluate the enrichment of middle REE. The Ce anomaly is calculated following ref. 88 Ce/Ce*=CeN / (PrN × NdN) 0.5.

Carbonate-associated iodine
The carbonate-associated iodine (CAI) was extracted according to an updated method89. Around 10 mg powder was sonicated in 1 ml DI water that was then centrifuged and discarded. The residue was leached with 1 ml 3% HNO3 in the ultrasonic bath for 10 min. After centrifugation, an aliquot of the supernatant was pipetted into the stabiliser (3% ethylenediaminetetraacetic acid in 3% ammonium hydroxide). Such stock solution was then diluted in the stabiliser for Ca and Mg analysis using a Varian 720 ICP-OES at University College London. The accuracy was checked with two in-house solution standards of SRM-88a and SGR-1, and blanks were negligible. Based on the results, the stock solution was further diluted to ~100 ppm Ca with the stabiliser for iodine analysis on an Agilent 7900 ICP-MS at University College London. Calibration standards were made freshly from potassium iodate powder, dissolved in the stabiliser with 100 ppm Ca matched to samples. The detection limit of I/Ca is ~0.1 µmol/mol. Repeated analysis of the drift monitor gave the relative standard deviation better than 3%. Analyses of JCp-1 and CRM-393 duplicates yielded I/Ca values of 3.90 ± 0.04 µmol/mol (1 SD, n=3) and 0.39 µmol/mol ± 0.001 (1 SD, n=3), which were comparable to published literature values22,89. The stabilisers and procedure blanks showed negligible iodine concentrations.

Carbonate-associated sulfate
Approximately 15–30 g of sample powder was first immersed in excess 12% NaClO followed by ultrasonication in a water bath for 1 h and shaken overnight. The residues after centrifugation were rinsed with DI water three times. The bleaching process was repeated five times, after which the residues were saturated in excess 1% H2O2 overnight under constant agitation. The residues were rinsed with DI water three times and dried out in the oven. The dried residues were cleaned with excess 10% NaCl for 24 h on an orbital shaker and then rinsed with DI water three times. The cleaning process was repeated five times, after which the residues were dissolved in 6 M HCl within 45 min. The leachates were filtered through 0.22 μm syringe filters immediately, to which a 200 g/L BaCl2 solution was added to precipitate BaSO4. The resultant precipitates were washed with 6 M HCl once and DI water three times before being dried and weighed for sulfur isotope analysis. This multistep process for carbonate-associated sulfate (CAS) extraction follows the method of ref. 90 to minimise any potential contamination from non-carbonate phase sulfur. No precipitate was observed from the blank. Sulfur isotope analysis of dried precipitates (δ34S CAS) was carried out at Iso-Analytical Laboratory (UK) using the EA-IRMS. The sample and vanadium pentoxide catalyst contained in a tin capsule were combusted at 1080°C in the presence of oxygen, which was further combusted by passing through tungstic oxide/zirconium oxide in a helium stream, followed by reduction by high purity copper wires. Water is removed using a Nafion™ membrane, while SO2 is resolved from N2 and CO2 on a packed gas chromatograph column at a temperature of 45°C before it was introduced into the IRMS. The analysis was based on monitoring of m/z 48, 49 and 50 of SO+ produced from SO2, and δ34S values are reported relative to the Vienna Canon Diablo Troilite (VCDT) standard. Three lab working standards (IA-R025, IA-R026, IA-R061) were used for calibration, which have been analysed against the international standards NBS-127 (+20.3‰), IAEA-SO-5 (+0.5‰) and IAEA-S-1 (−0.3‰). The accuracy was monitored by analysing IA-R061 (+20.51‰ ± 0.11, 1 s.d., n=3) and IAEA-SO-5 (+0.71‰ ± 0.03, 1 s.d., n=2), which are consistent with reported values. The CAS concentrations were calculated based on the weights of dried precipitates and the sample purity determined during sulfur isotope analysis. It should be noted that while we processed all the samples, only five samples yield sufficient sulfur for isotopic analyses.

Uranium isotopes
Approximately 500 mg sample powder was partially leached with 20 ml 0.2 M acetic acid for 24 h at room temperature in order to limit the contamination of uranium associated with non-carbonate phases91. Following centrifugation, the supernatants were filtered through 0.22 μm syringe filters, aliquoted and diluted for measurement of elemental concentrations (Ca, Mg, Fe, Mn, Al, Sr, U) using ICP-OES and ICP-MS at University College London. The accuracy was assessed by the analysis of in-house solution standards of TMDA-70, SGR-1, and SRM-88a, which were in agreement with certified values. Repeated analysis of the drift monitor gave the relative standard deviation better than 5%. Based on measured U concentrations, around 40 ng U was pipetted into acid-cleaned Teflon beakers and mixed with IRMM 3636 uranium double spike to achieve a spike/sample ratio of approximately 0.1. The mixtures were dried down and redissolved in 1 ml 3 M HNO3. Uranium was then purified using Eichrom UTEVA resin following the protocol of ref. 92. Briefly, columns and resin were cleaned with 0.05 M HCl and conditioned with 3 M HNO3 before loading samples. The matrix elements and Th were eluted using 3 M HNO3 and 6 M HCl, respectively, and U was collected with 0.05 M HCl. Purified samples were oxidised with concentrated HNO3 and redissolved in 3% HNO3 before analysis at Royal Holloway University of London. Uranium isotope ratios were measured on a Thermo-Finnigan Neptune Plus multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) equipped with an Aridus III desolvating nebulizer system for sample introduction. Acid blank was analysed before every solution analysis to correct measured intensities. Uranium isotope ratios are reported relative to CRM-112A standard:
δ238U = [(238U/235U)sample / (238U/235U)CRM-112A − 1] × 1000

Propagated analytical uncertainties for individual δ238U measurements were typically 0.03–0.08‰ (2 SE) for ~15 ppb solutions. A limestone standard CRM-393 was processed in the same manner as samples and gave a δ238U of -0.35 ± 0.08‰ (2 SE), which agrees with known values. Two duplicates were processed through the same full procedure and showed excellent agreement. Total procedural blank was ~5 pg of U and negligible. The error is reported as propagated uncertainty on individual isotope ratios.

Zinc isotopes
Carbonate-bound zinc was extracted following the protocol of ref. 61. Around 200 mg of sample powder was rinsed with DI water three times, then leached with 20 ml buffered 1 M ammonium acetate solution (pH 5) for 24 h at room temperature under constant agitation. The supernatants after centrifugation were pipetted into new acid-cleaned tubes. The aliquots of leachates were diluted in 2% HNO3 for elemental analysis (Ca, Mg, Fe, Mn, Al, Sr, Zn) using ICP-OES and ICP-MS at University College London. The in-house solution standards of TMDA-70, SGR-1, and SRM-88a were analysed to check the accuracy, which exhibited values consistent with certified values. Repeated analysis of the drift monitor gave the relative standard deviation better than 5%. Based on the Zn concentrations, 100–200 ng Zn was pipetted into acid-cleaned Teflon beakers, which were dried down on the hot plate, oxidised with excess concentrated HNO3 and refluxed with 1 ml 6 M HCl. The certain amount of a 64Zn-67Zn double spike was added to all samples to achieve a spike/sample ratio of approximately ~1.1–1.293, followed by evaporation to dryness and refluxed with 1 ml 6 M HCl. Solutions were further dried down and redissolved in 1 ml 1 M HCl to ensure spike-sample equilibration. Zinc was then purified through column chromatography using AG MP-1M resin (BioRad) following refs. 54,94. In brief, columns and resin were cleaned with 2% HNO3 and conditioned with 1 M HCl before loading samples. The matrix elements were eluted with 1 M HCl, and Zn was collected with 0.01 M HCl. Prior to analysis, purified samples were oxidised with concentrated HNO3 and redissolved in 2% HNO3. Zinc isotopes were measured on a Nu Instruments Plasma 3 MC-ICP-MS at University College London following the protocol of refs. 59,93,95. Solutions of ~100 ppb Zn were introduced via an Aridus III desolvating nebulizer system and a PFA nebulizer (50 μL/min). A standard-sample bracketing approach was adopted during analysis, and the raw data collection was preceded by an analysis of 2% HNO3 for signal correction. Interference corrections for 64Ni and Ba2+ were negligible. Instrumental mass bias was corrected through the double-spike technique93 using the off-line data reduction procedure of ref. 96. Zinc isotope ratios were determined relative to a new standard AA-ETH Zn:
δ66Zn = [(66Zn/64Zn)sample / (66Zn/64Zn)AA-ETH − 1] × 1000

Final δ66Zn values are reported with respect to JMC-Lyon by applying a correction of +0.28‰97. Total procedural blanks were ~1–2 ng. Accuracy and external reproducibility of δ66Zn values were assessed by two secondary solution standards (London Zn and NCS DC70303) and a limestone standard (CRM-393) that was processed through the same full procedure as the samples. Solution standards gave a δ66Zn of 0.11 ± 0.04‰ (2 SD, n=14) for London Zn and a δ66Zn of 0.73 ± 0.06‰ (2 SD, n=6) for NCS DC70303, which are consistent with previously reported values59. CRM-393 gave a δ66Zn of 0.75 ± 0.05‰ (2 SD, n=4). Three duplicates were processed along with samples to assess internal reproducibility and yielded agreeable values within the error of uncertainty. The reported error on figures is the external 2 SD reproducibility of NCS DC70303, which was the largest external uncertainty during the measurement period.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Zhang and co-authors present a suite of novel paleoenvironmental and paleonutrient proxies (CAS S isotopes, CAU isotopes, iodine ratios, REE patterns and Ce anomaly, Zn isotopes, and CAP) in a carbonate succession from Mongolia deposited after the Sturtian glaciation. They compare their data to published records and present a model for variable redox conditions, nutrient delivery, and climate events to explain the geochemical records. They also speculate that this combination of biogeochemical conditions could explain the anomalous C isotope behavior observed during this time. This manuscript presents a large amount of novel data that are rarely discussed in tandem and therefore this multi-proxy dataset is notable. It is not a simple task to discuss a diversity of proxy data in a digestible way and the authors have done a nice job in this regard. The time interval is also of broad interest for its geobiological importance. Especially given the wealth of data, the paper is logically structured and the authors consider many different angles (contamination, diagenesis, sea level etc.) in the supplement. The interpretation is mostly grounded in the data in this paper and presented elsewhere.\n1. My main comment is that there is a lot of attention and interpretation on the basal 10’s of meters of the section (the lowermost ~30 m in Figure 2 and stages I and II in Figure 3) that are interpreted to represent a period of transient deoxygenation/euxinia and intense eutrophication. This would be novel because it resolves some of the differences from previous studies. However, the geochemistry of these strata should be interpreted with caution and I encourage the authors to consider alternative processes driving this portion of the data. The immediate deglacial cap carbonate and postglacial sequence have been proposed to represent a poorly mixed ocean resulting from a fresh meltwater lens that was strongly stratified from saline ocean water that evolved under snowball conditions (e.g., Yang et al., 2017 Geology; Hoffmann et al. 2017 Science Advances p.26-31). The mixing time scale was modeled to be 10^4 to 10^5 yr (Yang et al., 2017), and the unusual ocean dynamics of the deglacial and postglacial period present a challenge in interpreting the basal samples as representative of a global seawater signal. The duration of a stable, density-stratified ocean is also difficult to determine and likely also is variable depending on the specific oceanography of a given site, but caution in interpreting geochemical data in the “T1” member of the Taishir Formation is warranted (Lau et al., 2017).\nThe potential for a stably stratified ocean to influence carbonates in the Taishir Formation is supported by data presented in this study. Specifically, the REE patterns in Figure S4, which are discussed in the supplement, lack features diagnostic of seawater (heavy REE enrichment compared to light REE enrichment, inconsistently or lack of a developed Y anomaly, relatively low Pr and Nd and relatively high middle REEs). Lines 132-134 of the supplement attribute the “non-seawater-like REE patterns” to early diagenesis but it is also reasonable to consider that these are non-seawater-like because they are in fact recording a syndepositional signal of poorly mixed fluids. Attributing these patterns to early diagenesis is also contradictory to the supplemental discussion that these samples do not otherwise correlate with other geochemical tests or markers for diagenesis. Therefore, this alterative explanation for the REE patterns (as well as the fluid influencing these carbonates) is feasible and should be considered. Given this prior work, the authors should consider the possibility that the basal ~32 m of section should not be interpreted in the context of global secular seawater evolution. If this were to be the case, what are the impacts on the overall interpretation of environmental change after the Sturtian snowball?\n2. Carbonate U isotope records. The interpretation of the U isotopes is primarily driven by one quite negative d238 value (of ~-1.2‰ at ~32 m). This value is low for carbonates, for this section, and for this time. In addition to the lower samples not containing a seawater-like REE sample and for the potential for a poorly mixed, stratified ocean to be driving the geochemistry of the basal “Sequence 3”, there is also potential for this sample to be an outlier. The supplement discusses that this sample has anomalously high Al and dismisses siliciclastic contamination; I think this reasoning is solid, but it still remains that there is something unusual about this sample. It is possible that this sample is representative of a larger trend, but without further samples from this section it is not possible to interpret this confidently. Moreover, Figure 3 compares the Tsagaan Gorge U isotope record with that from other carbonate sections from this basin in Lau et al. (2017). When combined, the significance of this one sample as well as the proposed negative trend in d238U is less obvious. Why not combine the entire record if the U isotope record in the interpretation if a compilation of global S isotope data are also interpreted in reconstructing redox conditions? How would a broader compilation of data from the Taishir Formation modify the interpretations of anoxia --> to a more oxidizing ocean?\n3.Iodine to calcium+magnesium ratios. Iodine ratios are particularly susceptible to diagenesis and will be readily altered before other carbonate geochemical proxies (see Hardisty et al., 2017, cited in the main text; Lau and Hardisty, 2022 GCA). Importantly, iodine ratios that are below detection are also the result of diagenetic alteration. Therefore, the interpretations of iodine ratios below detection as anoxic (or positioned near the oxycline) are possible but cannot be done with confidence when the alternative of diagenetic alteration is also possible. Using the positive Ce anomaly of the lowermost strata to support the interpretations of anoxia isn’t convincing because both myself and the authors state that the REE patterns are not representative of seawater. In addition, low I/(Ca+Mg) may also represent a redox stratified ocean and not just local anoxia. The reason for this is because the kinetics of iodide to iodate oxidation are slow, so an ocean poorly oxygenated bottom waters will not be able to produce a large iodate reservoir (Hardisty et al., 2021 EPSL).\n3. Age model and correlation of the Taishir Formation to the global compilation. The overall interpretation of the new geochemical data presented here (Figure 3) relies on comparison to the global S isotope compilation, the Fe speciation compilation, the biomarker compilation, and modeled climate data--none of which are from Mongolia. It should be noted that the age model discussion in the supplement proposes that the time interval covered by the studied section is 2-4 Myr (line 83), which is well shorter than the six million years shown in Figure 3.\nComparing records from different sites (e.g., Mongolia to other sites) should be done cautiously as it relies on a highly speculative correlation and age model. The authors acknowledge the challenge here in the supplement, and for correlating Mongolia specifically (lines 68-85). The timing of deglaciation is potentially diachronous (Rooney et al., 2020 Geology) and the duration of the carbonate units within the interglacial period are very difficult to constrain. The age model proposed by Bowyer et al. and presented in Figure 3 proposes that the duration of the T1 member is ~two million years. Based on C isotope chemostratigraphy of a unit that may not be representative of seawater, the age here is not well constrained given the thickness of the unit relative to T2 and T3 and the potential for the cap carbonate at the base of T1 to represent an unsually high sedimentation rate. The challenge of correlating Cryogenian units is certaintly not unique to this study. However, because the interpretations combine S isotopes from elsewhere and the implications of this study are interpreted with other geochemical, biomarker, climate, and tectonic data collected elsewhere, additional nuance should be added to the conceptual model to account for this uncertainty.\nMinor comments\nLine 61: “high purity samples” – what does this mean?\nAs a note, the authors discuss that previous interpretation of the U isotope record grouped together the ferruginous and euxinic sinks (Lau et al., 2017). Others have proposed that if suboxic or ferruginous sediments have a more muted isotope fractionation compared to sulfidic sediments, the relatively high d238U in the lower part of the Taishir Formation (compared to the T3 member which is not discussed here) could represent not oxic but suboxic or ferruginous conditions, at the expense of sulfidic sediments. It should be noted that evidence for ferruginous and euxinic conditions having distinct U isotope fractionation is equivocal (e.g., Lau et al., 2022 GCA, Rutledge et al., 2024 GCA). Nonetheless, I do want to emphasize that I am excited that, while this study interprets the extent of oxygenation to be less widespread than in the 2017 paper, the overall trend in less reducing conditions in T2 compared to T3 is also shown here, and the data in T2 are generally well aligned. (Note that T1 was interpreted with caution due to the potential for deglacial and postglacial stratification leading to non-steady-state behavior and large scatter).\nFigure 3\n- Panel J: it is hard to distinguish between literature data and data presented in this study. The colors should be changed.\n- Yellow bar is also difficult to see (on both printed PDF and on my screen)\n- Kimberly Lau, Penn State", "reviewer_2": "In this manuscript, Zhang et al. present a new multi-proxy carbonate dataset from Mongolia that spans the interval immediately following the Sturtian glaciation. They argue for an initial pulse of eutrophication and widespread anoxia, followed by a return to less reducing but not fully oxygenated ocean conditions. Overall, I find the manuscript to be comprehensive and well-written, and the interpretations are broadly supported by the data. I recommend publication with minor to moderate revisions. I have given detailed line-by-line comments in both the main text and the supplement (attached files), so please follow those closely. Overall, I feel that some more explanations are needed about some specific diagenetic issues such as fractionation of Zn isotopes during incorporation into carbonate and the correlations between d34SCAS and variables such as [Al] and Sr/Ca. Including a plate of photomicrographs in the supplement would be helpful to the reader. I point out numerous issues such as this in my comments, but overall I am positive about this manuscript and don't have any overarching criticisms that should prevent publication."}, "author_response": "Response to Reviewer Comments\nReviewer #1\nZhang and co-authors present a suite of novel paleoenvironmental and paleonutrient proxies\n(CAS S isotopes, CAU isotopes, iodine ratios, REE patterns and Ce anomaly, Zn isotopes,\nand CAP) in a carbonate succession from Mongolia deposited after the Sturtian glaciation.\nThey compare their data to published records and present a model for variable redox\nconditions, nutrient delivery, and climate events to explain the geochemical records.\nThey also speculate that this combination of biogeochemical conditions could explain the\nanomalous C isotope behavior observed during this time. This manuscript presents a large\namount of novel data that are rarely discussed in tandem and therefore this multi-proxy\ndataset is notable. It is not a simple task to discuss a diversity of proxy data in a digestible\nway and the authors have done a nice job in this regard. The time interval is also of broad\ninterest for its geobiological importance. Especially given the wealth of data, the paper is\nlogically structured and the authors consider many different angles (contamination,\ndiagenesis, sea level etc.) in the supplement. The interpretation is mostly grounded in the\ndata in this paper and presented elsewhere.\nWe thank Kimberly Lau for the constructive comments on our manuscript.\n1. My main comment is that there is a lot of attention and interpretation on the basal 10’s of\nmeters of the section (the lowermost ~30 m in Figure 2 and stages I and II in Figure 3) that\nare interpreted to represent a period of transient deoxygenation/euxinia and intense\neutrophication. This would be novel because it resolves some of the differences from\nprevious studies. However, the geochemistry of these strata should be interpreted with\ncaution and I encourage the authors to consider alternative processes driving this portion of\nthe data. The immediate deglacial cap carbonate and postglacial sequence have been\nproposed to represent a poorly mixed ocean resulting from a fresh meltwater lens that was\nstrongly stratified from saline ocean water that evolved under snowball conditions (e.g., Yang\net al., 2017 Geology; Hoffmann et al. 2017 Science Advances p.26-31). The mixing time scale\nwas modeled to be 10^4 to 10^5 yr (Yang et al., 2017), and the unusual ocean dynamics of\nthe deglacial and postglacial period present a challenge in interpreting the basal samples as\nrepresentative of a global seawater signal. The duration of a stable, density-stratified ocean\nis also difficult to determine and likely also is variable depending on the specific\noceanography of a given site, but caution in interpreting geochemical data in the “T1” member\nof the Taishir Formation is warranted (Lau et al., 2017).\nThe potential for a stably stratified ocean to influence carbonates in the Taishir Formation is\nsupported by data presented in this study. Specifically, the REE patterns in Figure S4, which\nare discussed in the supplement, lack features diagnostic of seawater (heavy REE\nenrichment compared to light REE enrichment, inconsistently or lack of a developed Y\nanomaly, relatively low Pr and Nd and relatively high middle REEs). Lines 132-134 of the\nsupplement attribute the “non-seawater-like REE patterns” to early diagenesis but it is also\nreasonable to consider that these are non-seawater-like because they are in fact recording a\nsyndepositional signal of poorly mixed fluids. Attributing these patterns to early diagenesis is\nalso contradictory to the supplemental discussion that these samples do not otherwise\ncorrelate with other geochemical tests or markers for diagenesis. Therefore, this alterative\nexplanation for the REE patterns (as well as the fluid influencing these carbonates) is feasible\nand should be considered. Given this prior work, the authors should consider the possibility\nthat the basal ~32 m of section should not be interpreted in the context of global secular\nseawater evolution. If this were to be the case, what are the impacts on the overall\ninterpretation of environmental change after the Sturtian snowball?\nThis is a good point. To our knowledge, the above-mentioned hypothesis of ‘meltwater plume’\nis first proposed by Shields (2005) to explain the global occurrence of unique cap dolostone\nafter the Marinoan glaciation, which was deposited during the transgressive system tract\n(TST) and is characterised by tube-like structures and giant wave ripples (Hoffman et al.,\n2017). However, the TST is absent or highly condensed in non-glacial Cryogenian\ncarbonates directly overlying the Sturtian diamictites (Hoffman et al., 2017). The sedimentary\nfeatures similar to Marinoan cap carbonates have also not been observed in the studied\ninterval of Taishir Formation. From the perspective of geochemical signals, the meltwater-\naffected Marinoan cap carbonates are characterised by unusually high 87Sr/86Sr values due\nto the highly radiogenic feature of meltwater (Liu et al., 2014). Similar phenomenon has been\nobserved from the very basal part (~3m) of the Twitya Formation overlying Rapitan\ndiamictites in northwest Canada (Rooney et al., 2014). This observation indicates the\npossible existence of locally preserved Sturtian cap carbonates. However, we do not observe\nsuch high 87Sr/86Sr values from the basal part of the studied interval in Mongolia (Fig. 2),\nwhich are instead consistent with the values from Twitya carbonates (above the highly\nradiogenic interval; Rooney et al., 2014) and Rasthof carbonates (Yoshioka et al., 2003).\nThese values are now widely regarded as representing the seawater composition (Bold et al.,\n2016; Cox et al., 2016; Zhou et al., 2020; Bowyer et al., 2022). Together, we suggest that the\nstudied interval has not been affected by the meltwater during deposition and so our\ninterpretation would not be influenced.\nOn the other hand, we would like to clarify that not all samples from the lower part are\ncharacterised by non-seawater-like REE patterns (Fig. S6), which is contrary to what would\nbe expected if a stable meltwater lid impacted the carbonate deposition. Attributing the non-\nseawater-like REE patterns to meltwater is also inconsistent with the Sr isotope data, as\ndiscussed above. Indeed, we have discussed the origin of these non-seawater like REE\npatterns in detail in our prior study (Zhang and Shields, 2023), and we reached a conclusion\nsimilar to that of Lau and Hardisty (2022): proxies' sensitivities to diagenetic processes are\nvariable for different systems and under different local conditions. We showed that REE may\nbe particularly prone to early diagenetic alteration because of the high partition coefficients\nof REE in carbonate minerals and contrasting REE concentrations in REE carrier phases. It\nthus may be not surprising that there are no obvious correlations with diagenetic indicators.\nThis is also why we discussed whether other geochemical proxies are altered in conjunction\nwith the distortion of REE in the supplement. It may be interesting to note here that non-\nseawater-like REE patterns are indeed not uncommon in Neoproterozoic carbonate rocks\npreserving δ13C excursions (e.g., Shuram anomaly), while these rocks are shown to preserve\nother geochemical proxies that can be globally comparable (e.g., Dodd et al., 2023).\nWe thank Kimberly for highlighting the potential influence of meltwater plume and for making\nus aware that readers may have the same question. We have now mentioned this point in\nthe main text (Line 64-67) and added a new section in the Supplementary Information for\nfurther clarification.\n2. Carbonate U isotope records. The interpretation of the U isotopes is primarily driven by\none quite negative d238 value (of ~-1.2‰ at ~32 m). This value is low for carbonates, for this\nsection, and for this time. In addition to the lower samples not containing a seawater-like REE\nsample and for the potential for a poorly mixed, stratified ocean to be driving the geochemistry\nof the basal “Sequence 3”, there is also potential for this sample to be an outlier. The\nsupplement discusses that this sample has anomalously high Al and dismisses siliciclastic\ncontamination; I think this reasoning is solid, but it still remains that there is something\nunusual about this sample. It is possible that this sample is representative of a larger trend,\nbut without further samples from this section it is not possible to interpret this confidently.\nMoreover, Figure 3 compares the Tsagaan Gorge U isotope record with that from other\ncarbonate sections from this basin in Lau et al. (2017). When combined, the significance of\nthis one sample as well as the proposed negative trend in d238U is less obvious. Why not\ncombine the entire record if the U isotope record in the interpretation if a compilation of global\nS isotope data are also interpreted in reconstructing redox conditions? How would a broader\ncompilation of data from the Taishir Formation modify the interpretations of anoxia --> to a\nmore oxidizing ocean?\nThank you for the comment. As mentioned above, the studied interval was not influenced by\nthe meltwater during deposition, and our independent evaluation of U isotope records\nindicates that the stratigraphic δ238U trends are not governed by diagenetic processes. We\nhave reprocessed and remeasured the sample with the lowest δ238U value and obtained the\nsame value within uncertainty. The consistency rules out the possibility of experimental error.\nAs suggested, we then combined the entire carbonate δ238U records and used LOESS fitting\nto interpret the trends (Fig. 3), as with recent published U isotope studies (e.g., Zhang et al.,\n2022; Remírez et al., 2024). It turns out that despite the apparent gap in the dataset caused\nby the lithology change (Fig. 2), the overall decreasing trend in the lower part followed by\nrelatively higher δ238U values remains quite robust. Therefore, a broader compilation of data\nfrom the Taishir Formation further strengthens our argument for the expansion of seafloor\nanoxia following Sturtian deglaciation. We have now modified the Section ‘Transient ocean\ndeoxygenation’ based on the trends derived from integrated U isotope dataset. Detailed\nchanges can be found in Line 148-183. We have also updated the Figure 3, as shown below.\nFig. 3. Conceptual model and biogeochemical indicators following the Sturtian\ndeglaciation. a, Vigorous weathering induced eutrophication after the Sturtian deglaciation.\nb, Expansion of euxinia on productive margins owing to excess of H S over iron. c,\n2\nseawater oxygenation. In (a-c), the thicknesses of arrows indicate the relative magnitude of\nfluxes, and the sizes of ellipses represent the relative sizes of reservoirs. Abbreviations: OM,\norganic matter burial; Fe , pyrite burial; DOC, dissolved organic carbon pool. d, Global length\npy\nof continental arcs (Cao et al., 2017) and mid-ocean ridges (Dutkiewicz et al., 2024; Müller\net al., 2022). The blue band indicates the studied interval. e, Occurrences of biomarkers\n(modified after Bowyer et al., 2023). f, Possible evolution of atmospheric CO and global\n2\nmean surface temperature (modified after Bowyer et al., 2023; Le Hir et al., 2009). PAL,\npresent atmospheric level. g-l, Compilation of carbon, strontium, sulfur, uranium isotopes and\niron speciation records (modified after Bowyer et al., 2023). Solid lines in g,h,j,k represent\nthe LOESS (locally estimated scatterplot smoothing) fitting curves, and the shaded area\nrepresents 1σ (68%) confidence interval. The dashed lines in j indicate the major range of\nδ34S . In l, each boxplot represents the distribution of global Fe /Fe data in each one-\npy py HR\nmillion-year time bin. The light shade of pale red band indicates the second stage. The age\nmodel of e-l is based on model A of Bowyer et al. (2023) (also see Supplementary Fig. 2 for\nthe alternative age model D).\n3.Iodine to calcium+magnesium ratios. Iodine ratios are particularly susceptible to diagenesis\nand will be readily altered before other carbonate geochemical proxies (see Hardisty et al.,\n2017, cited in the main text; Lau and Hardisty, 2022 GCA). Importantly, iodine ratios that are\nbelow detection are also the result of diagenetic alteration. Therefore, the interpretations of\niodine ratios below detection as anoxic (or positioned near the oxycline) are possible but\ncannot be done with confidence when the alternative of diagenetic alteration is also possible.\nUsing the positive Ce anomaly of the lowermost strata to support the interpretations of anoxia\nisn’t convincing because both myself and the authors state that the REE patterns are not\nrepresentative of seawater. In addition, low I/(Ca+Mg) may also represent a redox stratified\nocean and not just local anoxia. The reason for this is because the kinetics of iodide to iodate\noxidation are slow, so an ocean poorly oxygenated bottom waters will not be able to produce\na large iodate reservoir (Hardisty et al., 2021 EPSL).\nThank you for the comment. We agree that the carbonate-associated iodine is especially\nprone to diagenesis. The interpretation of low I/(Ca+Mg) is thus always plagued by the\npossibility of iodine loss during diagenetic processes. Our samples are characterised by very\nhigh Sr concentrations and so they were likely originally aragonites and could have\nexperienced neomorphism during early diagenesis. In this context, we acknowledge that\ndiagenetic iodine loss cannot be ruled out entirely in our case. Therefore, we have largely re-\nwritten the relevant sections in both the main text and the supplement, discussing the\npossibility of iodine loss during diagenetic processes. Nevertheless, as pointed out by\nKimberly, the alteration of I/(Ca+Mg) does not necessarily indicate alteration of other proxies\nand so this would not affect our conclusions. Again, we would like to clarify that not all\nsamples from the lower part is characterised by non-seawater-like REE patterns (Fig. S6).\nThe potential influence of meltwater has also been excluded as discussed above. Therefore,\nthe interpretation of locally reducing environments is in agreement with positive Ce anomalies,\nwhich is also consistent with the previous interpretation of Ce anomalies on the same\ndepositional unit from different sections (Lau et al., 2017). Detailed changes can be found in\nLine 87-106.\n3. Age model and correlation of the Taishir Formation to the global compilation. The overall\ninterpretation of the new geochemical data presented here (Figure 3) relies on comparison\nto the global S isotope compilation, the Fe speciation compilation, the biomarker compilation,\nand modeled climate data--none of which are from Mongolia. It should be noted that the age\nmodel discussion in the supplement proposes that the time interval covered by the studied\nsection is 2-4 Myr (line 83), which is well shorter than the six million years shown in Figure 3.\nComparing records from different sites (e.g., Mongolia to other sites) should be done\ncautiously as it relies on a highly speculative correlation and age model. The authors\nacknowledge the challenge here in the supplement, and for correlating Mongolia specifically\n(lines 68-85). The timing of deglaciation is potentially diachronous (Rooney et al., 2020\nGeology) and the duration of the carbonate units within the interglacial period are very difficult\nto constrain. The age model proposed by Bowyer et al. and presented in Figure 3 proposes\nthat the duration of the T1 member is ~two million years. Based on C isotope\nchemostratigraphy of a unit that may not be representative of seawater, the age here is not\nwell constrained given the thickness of the unit relative to T2 and T3 and the potential for the\ncap carbonate at the base of T1 to represent an unsually high sedimentation rate. The\nchallenge of correlating Cryogenian units is certaintly not unique to this study. However,\nbecause the interpretations combine S isotopes from elsewhere and the implications of this\nstudy are interpreted with other geochemical, biomarker, climate, and tectonic data collected\nelsewhere, additional nuance should be added to the conceptual model to account for this\nuncertainty.\nThank you for the comment and suggestion. The age model would only be speculative if\ndeglaciation was considered diachronous, but there is little evidence for this. The above-\nmentioned reference Rooney et al. (2020) indeed summarised available radiometric age\nconstraints and suggested that ‘deglaciation was globally synchronous’. As discussed above,\nthere is also no evidence to suggest that the studied interval represents cap carbonates. The\nunit thickness could be helpful on estimating duration, but this should also be built on the\nconsideration of lithology. Specifically, T1 is dominated by fine-grain laminated limestone,\nwhereas T2 & T3 are mainly composed of coarse-grain limestone with the presence of ooids.\nIt is reasonable to suggest that sedimentation rates for T2 & T3 are different from and\nprobably higher than that of T1. Hence, the relative thin thickness of T1 could indicate a\nrelatively condensed deposition rather than a short duration. Nevertheless, our estimation\nassuming constant sediment accumulation rates could provide constraints on the magnitude\nof duration, which is our main purpose. The estimation suggests that the deposition of studied\ninterval spanned several million years. This estimation agrees with the age model of Bowyer\net al. (2023), and hence, we have invoked their age model A in Figure 3 to facilitate the\ncomparison with global data.\nThe feasibility of the age model can also be tested independently with Sr isotopes (please\nsee Zhou et al. (2020) for an example of its application). Basically, one can use the well\ncharacterised seawater 87S/86Sr change rate in the Phanerozoic (McArthur et al., 2012) to\nestimate the duration. The underlying assumption is that the oceanic Sr residence time has\nnot been radically different through the Neoproterozoic to the Phanerozoic, which is likely\nvalid. The magnitude of 87S/86Sr change in the T1 is ~ 0.0004. If we take the maximum change\nrate of 87S/86Sr during the Phanerozoic, i.e., 0.000192, then the estimated duration should be\n~2 Myr. Certainly, this estimation carries some uncertainties, but consistent results across\ndifferent approaches support the conclusion that the deposition of studied interval spanned\nseveral million years. We have now added detailed description in the supplement section\n‘Geological, stratigraphic context and age model’.\nWe agree that any age model, particularly in deep time, comes with its own uncertainties.\nIn this study, we used the chronostratigraphic framework of Bowyer et al. (2023), which is the\nbest age model we can currently obtain. The way they constructed the age model can be\nfound in their Method section. We note that the Mongolian data have been central to the\nconstruction of the model from the beginning. They have also thoroughly discussed the\nalternative possibilities and limitations of the age model in their supplement. Importantly, they\nshowed that the overall geochemical trends and the sequence of the trends relative to fossil\noccurrences and climatic steady state would remain consistent in spite of the uncertainties.\nTo illustrate this, we have now tested the alternative age model in the supplement (Fig. S2),\nwhich represents an extreme case of a non-glacial duration of c. 21 Myr. As shown below,\nthe overall trends maintain, and this alternative model does not significantly impact our\ninterpretation.\nFig. S2. Conceptual model and biogeochemical indicators following the Sturtian deglaciation.\na, Vigorous weathering induced eutrophication after the Sturtian deglaciation. b, Expansion\nof euxinia on productive margins owing to excess of H S over iron. c, Continuous atmospheric\n2\nc), the thicknesses of arrows indicate the relative magnitude of fluxes, and the sizes of\nellipses represent the relative sizes of reservoirs. Abbreviations: OM, organic matter burial;\nFe , pyrite burial; DOC, dissolved organic carbon pool. d, Global length of continental arcs\npy\n(Cao et al., 2017) and mid-ocean ridges (Dutkiewicz et al., 2024; Müller et al., 2022). The\nblue band indicates the studied interval. e, Occurrences of biomarkers (modified after Bowyer\net al., 2023). f, Possible evolution of relative O saturation of surface seawater and global\n2\nmean surface temperature (modified after Bowyer et al., 2023; Le Hir et al., 2009). g-l,\nCompilation of carbon, strontium, sulfur, uranium isotopes and iron speciation records\n(modified after Bowyer et al., 2023). Solid lines in g,h,j,k represent the LOESS (locally\nestimated scatterplot smoothing) fitting curves, and the shaded area represents 1σ (68%)\nconfidence interval. The dashed line in j indicates the major range of δ34S . In l, each boxplot\npy\nrepresents the distribution of global Fe /Fe data in each one-million-year time bin. The\npy HR\nlight shade of pale red band indicates the second stage. The age model of e-l is based on\nmodel D of Bowyer et al. (2023).\nMinor comments\nLine 61: “high purity samples” – what does this mean?\nIt refers to samples with high carbonate contents. We have modified the sentence for clarity.\nAs a note, the authors discuss that previous interpretation of the U isotope record grouped\ntogether the ferruginous and euxinic sinks (Lau et al., 2017). Others have proposed that if\nsuboxic or ferruginous sediments have a more muted isotope fractionation compared to\nsulfidic sediments, the relatively high d238U in the lower part of the Taishir Formation\n(compared to the T3 member which is not discussed here) could represent not oxic but\nsuboxic or ferruginous conditions, at the expense of sulfidic sediments. It should be noted\nthat evidence for ferruginous and euxinic conditions having distinct U isotope fractionation is\nequivocal (e.g., Lau et al., 2022 GCA, Rutledge et al., 2024 GCA). Nonetheless, I do want to\nemphasize that I am excited that, while this study interprets the extent of oxygenation to be\nless widespread than in the 2017 paper, the overall trend in less reducing conditions in T2\ncompared to T3 is also shown here, and the data in T2 are generally well aligned. (Note that\nT1 was interpreted with caution due to the potential for deglacial and postglacial stratification\nleading to non-steady-state behavior and large scatter).\nWe are also reassured to see that our data align well with those of Lau et al. (2017) for T2.\nIndeed, δ238U data from T1 are also generally consistent, except for the very positive δ238U\nvalues (Fig. 3). It is true that the uranium isotope fractionation under ferruginous conditions\ncould have a large range, the magnitude of which appears to relate to primary productivity\n(Lau et al., 2022; Rutledge et al., 2024; Gilleaudeau et al., 2025). The work of Kimberly and\nher colleagues has greatly advanced our understanding of this aspect and has actually\ninspired our interpretation. We have now re-written the Section ‘Transient ocean\ndeoxygenation’, where we expanded the discussion on this point and added the above\nmentioned references.\nFigure 3\n- Panel J: it is hard to distinguish between literature data and data presented in this study.\nThe colors should be changed.\n- Yellow bar is also difficult to see (on both printed PDF and on my screen)\nWe have now modified Figure 3, as shown above. Thank you for the suggestion.\n- Kimberly Lau, Penn State\nReviewer #2\nIn this manuscript, Zhang et al. present a new multi-proxy carbonate dataset from Mongolia\nthat spans the interval immediately following the Sturtian glaciation. They argue for an initial\npulse of eutrophication and widespread anoxia, followed by a return to less reducing but not\nfully oxygenated ocean conditions. Overall, I find the manuscript to be comprehensive and\nwell-written, and the interpretations are broadly supported by the data. I recommend\npublication with minor to moderate revisions. I have given detailed line-by-line comments in\nboth the main text and the supplement (attached files), so please follow those closely. Overall,\nI feel that some more explanations are needed about some specific diagenetic issues such\nas fractionation of Zn isotopes during incorporation into carbonate and the correlations\nbetween d34SCAS and variables such as [Al] and Sr/Ca. Including a plate of\nphotomicrographs in the supplement would be helpful to the reader. I point out numerous\nissues such as this in my comments, but overall I am positive about this manuscript and don't\nhave any overarching criticisms that should prevent publication.\nWe thank the reviewer for the positive and thoughtful comments.\nLine 45: \"of the\"\nRevised.\nLine 82: How was this determined? Please point the reader to the appropriate supplementary\nfigure and text that shows how this was determined. How was detrital influence on REE\npatterns assessed? Did you screen samples based on Y/Ho?\nThank you for the comment. We have now modified the sentence for clarity and added\nrelevant discussion in the supplement. The influence of detritus was first minimised by\nconducting sequential leaching with dilute acetic acid (Methods) and further assessed by the\nAl concentration and its correlation with the REE content (Fig. S3). Y/Ho is certainly an\nimportant parameter, but since nearly all samples show moderate positive La and Y\nanomalies, the samples were screened here mainly based on REE pattern parameters\nincluding Pr /Yb , Sm /Yb , and BSI (bell shaped index; Tostevin et al., 2016).\nN N N N\nLine 99: This needs further explanation. Ce anomalies show oxic conditions, but I/(Ca+Mg)\ndoes not because the site was too close to the oxycline to have appreciable iodate in the\nwater column? Please explain.\nThe reviewer is correct. The reason relates to the oxidation-reduction kinetics of iodine: while\niodate reduction is considered to be rapid, iodide oxidation is relatively slow even with high\n[O ] in ambient waters (Chance et al., 2014). Therefore, it has been proposed that low\npoint also raised in Kimberly’s comment. An example of the decoupling between Ce/Ce* and\nI/(Ca+Mg) is the late Paleozoic, during which consistently low I/(Ca+Mg) similar to\nProterozoic values has been observed (Lu et al., 2018), but the Ce anomaly during this period\nshows significantly negative values (Zhang and Shields, 2022). We omitted these\nexplanations in the original manuscript to keep the word count concise and apologize for any\nconfusion this may have caused. However, as mentioned above, we agree with Kimberly that\ndiagenetic iodine loss cannot be ruled out entirely in our case. Hence, we have revised the\nrelevant discussion about I/(Ca+Mg) in both the main text and supplement, but we would like\nto emphasize that our interpretation of other proxies and our overall conclusion remain\nunchanged.\nLine 104: Fig.2 I think this figure needs a proper stratigraphic log showing the sedimentology\nand facies of the succession to compare with the geochemical data. Also, why are there only\n4 points for d34SCAS? And maybe some more explanation of the dashed lines is needed in\nthe caption. For example, why are you presenting the median Archean value for P/(Ca+Mg)\nwhen these rocks are Cryogenian? Lastly, it may be best to plot a dashed line in the d238\npanel showing the average d238U value of modern carbonates as opposed to modern\nseawater because of the potential offset in d238U between carbonates and seawater. best\nto compare carbonates to carbonates.\nThank you for the suggestion. We have now added the stratigraphic column, showed the\ndash line for the mean δ238U value of Bahamian carbonates (Chen et al., 2018), and changed\nthe caption. There are only 4 δ34S datapoints because only five samples yield sufficient\nCAS\nsulfur for isotopic analyses (Methods), with one of them removed from discussion due to the\npotential of being influenced by pyrite oxidation (Supplement Information). We presented the\nmedian P/(Ca+Mg) value of Archean carbonates because it represents the first reported\nstatistical result from Precambrian carbonates (Ingalls et al., 2022) and is comparable to our\ndataset. Nevertheless, our main purpose is to visually separate different data groups, and we\nhave updated the dashed lines accordingly. The revised Figure 2 is shown below.\nFig. 2. Geochemical data profiles for the studied succession from the lower part of\nTaishir Formation at Tsagaan Gorge. Sequence stratigraphy is from Lindsay et al. (1996).\nLithostratigraphy, δ13C and 87Sr/86Sr data (circles) are from Shields et al. (2002). 87Sr/86Sr\ncarb\ndata (squares) of Bold et al. (2016) are correlated based on the age model of Bowyer et al.\n(2023). The mean δ66Zn value of the modern deep ocean is from Lemaitre et al. (2020) and\nthe mean δ238U value of Bahamian carbonates is from Chen et al. (2018). The shaded band\nhighlights the lower part of the succession. Note that I/(Ca+Mg) values below the detection\nlimit are regarded as zero, and errors for some measurements of δ238U are smaller than the\ndata symbols.\nLine 147: Important to also note that the measured d238U value of -0.43 per mil is still much\nlower than modern carbonates, which average -0.15 per mil. The carbonate values of -0.43\nare similar to most values seen in the mid-Proterozoic as well (Gilleaudeau et al., 2019,\nEPSL), which may be worth mentioning because it could be a similar interpretation.\nThank you for the comment. We have now added the reference and relevant discussion to\nthe revised manuscript. Detailed changes can be found in Line 164-183.\nLine 150: Why would it be weakly euxinic? You have d238U values less than -1 per mil and\nyou have very heavy d34SCAS values, so euxinia must have been expansive. Why are you\ncalling it weakly euxinic?\nThe inference of a weakly euxinic condition is primarily based on the study of Clarkson et al.\n(2023). Here weakly euxinic refers to water column [H S ] < 100 μmol/mol, rather than the\n2 total\nspatial extent of euxinia. They found that U isotope fractionation could be very high (∆238U >\n+0.6‰) under such a condition and so they suggested that this condition could potentially\nexplain the very low seawater δ238U values observed during the early Tonian and the late\nEdiacaran. The weakly euxinic condition also appears to be consistent with the background\nof low-sulfate seawater in our case. However, we find the term somewhat misleading and\nhave therefore removed it.\nLine 159-160: What about carbonates? What is the offset between seawater and carbonate\nd66Zn? This is critical to discuss as you are presenting carbonate d66Zn data.\nThank you for the comment. Carbonate rocks are estimated to represent a minor sink for Zn\n(Little et al., 2014; Dickson, 2022). Biogenic carbonates such as corals and foraminifera\nappear to be able to record the seawater δ66Zn values (Little et al., 2021; Druce et al., 2022),\ninorganic carbonates generally prefer the heavy Zn isotope with an offset of around 0.4‰\npotentially due to Zn2+ incorporation through tetrahedral coordination (Mavromatis et al., 2019;\nMüsing et al., 2022; Dickson, 2022). We have now added the relevant content in Line 209-\n212.\nLine 180-181: But looking at Fig. 2, Zn concentrations are lower in the lower part of the\nsuccession compared to the upper part, so couldn't this be because of widespread euxinic\nZn drawdown?\nThis is a good point. It is true that [Zn] is relatively lower in the lower part of the succession,\nbut it does not decrease upsection as would be expected with expanding anoxia/euxinia.\nInstead, [Zn] shows a consistent increasing trend in the lower part (Fig. 2). Additionally, δ66Zn\nremains relatively stable and low after the initial decrease, despite the expansion of\nanoxia/euxinia (Fig. 2). These observations lead us to consider it less likely that the expanded\neuxinic sink is responsible for the δ66Zn variation in the lower part. We have expanded the\ndiscussion on this point (Line 241-246).\nLine 187: But why would more organic-rich sediments be deposited during the less reducing\nupper interval compared to the strongly anoxic lower interval? For the upper part, you are\narguing for less reducing global seafloor conditions but expanded deposition of organic-rich\nsediments. This seems contradictory. Please explain.\nThank you for the comment. We would like to clarify that we were not comparing the burial\nflux of organic-rich sediments between the lower and upper parts here. Instead, we focused\non the relative importance of the processes that could have caused the δ66Zn variation. It is\npossible that, as the light Zn input from continents gradually decreased as indicated by the\nflattening of seawater 87Sr/86Sr curve, the organic matter burial required to counteract the\nnegative δ66Zn shift also decreased accordingly. Additionally, the expansion of less reducing\nconditions could have led to the remobilization of light Zn from continental margin sediments\ndue to the increased dissolved oxygen on the shelf (Sweere et al., 2020). The process, if\nunaccompanied by increased organic matter burial, would likely lead to the decrease of\nseawater δ66Zn values; but this conflicts with what we observed (Fig. 2). Hence, we think that\nthe inference of relatively increased burial of organic-rich sediments is reasonable. On the\nother hand, the change in oceanic redox conditions may be largely related to the shallow\nocean (or margin seafloor) as there is no evidence for full ocean oxygenation. Meanwhile,\norganic matter burial may persist in deeper oceans or upwelling zones, as supported by the\nconsistently high TOC in the Xiangmeng Formation deposited in the basinal environment\n(Bowyer et al., 2023). Therefore, it appears that relatively increased organic matter burial\ndoes not necessarily conflict with the less reducing seafloor conditions. We have modified\nthe content in Line 246-253.\nLine 252: Fig.3 How do you explain the different d238U values at very similar times between\nthis study and Lau et al. (2017)? And how was the age model constructed to put these points\nhere on the same plot?\nThank you for the comment. As mentioned in our response to Kimberly, we adopted the age\nmodel of Bowyer et al. (2023), which is the best age model we can currently obtain. We have\nalso tested the alternative age model and updated figures (Figs. 3 and S2) as shown above.\nThis information is now clearly mentioned in the updated caption of Figure 3 (Line 339) and\nin the Supplement Information. Our δ238U data are generally consistent with those in Lau et\nal. (2017) for the upper part (Fig. 3). When combined together, their data also largely follow\nthe decreasing trend in the lower part, except for the very positive δ238U values (Fig. 3). It is\npossible that the uncertainties related to the age model may have caused the different δ238U\nvalues at very similar times. Alternatively, some noise in the carbonate δ238U dataset is quite\ncommon in published literature, which has been associated with analytical uncertainties and\ndiagenesis (Kipp and Tissot, 2022), and so the trend is the most important for the\ninterpretation. Particularly, it has been recently suggested that carbonate δ238U values >> -\n0.3‰ have a strong likelihood of being altered during diagenesis (Kipp and Tissot, 2022;\nWang et al., 2022). In this regard, and considering Kimberly’s comment about our very\nnegative value being an outlier, one may argue that the two contrasting values at very similar\ntimes should both be filtered out. However, this would still not significantly change the trend.\nOverall, we think that despite some variability in the integrated δ238U records, the general\ntrend and, therefore, our interpretation will likely remain valid.\nLine 378: Could this be further evidence for sulfate limitation in the oceans at this time?\nYes. We have now mentioned this in Line 179. Thank you for pointing this out.\nSupplement Comments\nAn R-squared of 0.92 between Al and d34SCAS is a strong correlation that is hard to explain\naway as spurious due to insufficient data. I think you need to discuss what other factors could\ncause this correlation? I would expect d34SCAS to be most prone to diagenesis of all the\ncarbonate-based proxies you’ve employed, so I think you can entertain the idea of alteration\nof d34SCAS signals while still interpreting the other signals are well-preserved.\nThank you for the comment. We also found the positive correlation between δ34S and Al\nCAS\nto be perplexing. This is becauseδ34S values of terrestrial reservoirs, except for some\ncarbonates and pyrites, typically fall below +20–30‰ (Hammerli et al., 2021). As such, any\nincrease in clastic-derived sulfur as implied by increasing Al contents would likely decrease\nrather than increase δ34S values in our case. The initial Al data were from leachates prepared\nfor Zn isotopes, which were obtained by dissolving samples in buffered ammonium acetate\n(pH 5.0). Considering that Al concentrations are highly sensitive to the acid strength and that\nδ34S data were obtained using 6M HCl leaching, we suspect that the spurious correlation\nCAS\nmay have been influenced by the Al data. To address this, we redissolved the samples in 6M\nHCl, measured the major elements and re-plotted them again (Fig. S3). The updated analysis\nreveals that [Al] no longer shows a significant covariation with δ34S (R2=0.35), as shown\nCAS\nbelow. We have now modified the Supplement Information, updated the Supplementary Figs.\n3 and 5, and included the new data in the Supplementary Data.\nFig. S3. Cross plots of geochemical proxies against Al, Fe, Mn and TOC contents. TOC\ndata are from Shields et al. (2002).\nFig. S5. Cross plots of geochemical proxies against commonly used diagenetic indicators.\nδ18O data are from Shields et al. (2002).\nIt would be best to present a plate of photomicrographs as a supplementary figure so the\nreader can see the petrographic textures for himself/herself.\nThank you for the suggestion. The petrographic observations were conducted by the author\nGraham Shields over 20 years ago, with the findings published in Shields et al. (2002). We\nutilized the same sample set in this study and have now added the reference to the\nsupplement. Although we made every effort to locate the thin sections, it appears that they\nhave been lost over time. We also checked our samples and unfortunately found no\nremaining rock pieces suitable for preparing new thin sections, as all of them had been\ncrushed into powder for sulfur isotope analysis.\nRepeated word.\nModified.\nAgain, it is d34SCAS that is showing potential problems, so I think you need to be open to\npotential alteration of this signal, which could have occurred while many of the other signals\nremained well-preserved. d34SCAS has been shown to be the most easily altered of all the\ncarbonate-based proxies you’ve analyzed.\nWe agree that the possibility of a genuine covariation should be considered here, in addition\nto the potential for a spurious covariation as discussed in the original supplement. Sr/Ca is\ngenerally lower in altered carbonates due to the loss of Sr during diagenesis. Therefore, the\npositive covariation between Sr/Ca and δ34S implies that the relatively lower δ34S\nCAS CAS\nvalues may be the result of diagenetic alteration. It should be noted that we do not observe\nsignificant covariations between Mn/Sr, δ18O and Sr/Ca, which are expected to covary during\ndiagenesis (Brand and Veizer, 1980). In addition to diagenesis, the variation of carbonate\nSr/Ca can also be affected by other factors such as seawater Sr/Ca. Hence, the observed\ncovariation may also represent closely linked environmental factors that caused the\nsimultaneous variation of δ34S and Sr/Ca. Regardless of the reason behind this covariation,\nCAS\nit appears that the extremely high δ34S are unlikely to be significantly altered.\nCAS\nConsequently, this would not influence our interpretation of euxinia in the lower part. We have\nnow added the above discussion to the Supplement Information.\nI would like to see a stratigraphic plot similar to main text Fig. 2 that shows d18O, Mg/Ca,\nand Mn/Sr, as well as any other detrital/diagenetic indicators that may be useful. Also, looking\nat the cross-plots, you have some samples that have very low d18O values less than -10 per\nmil. These are typically taken to indicate diagenesis. How do you explain those when arguing\nthat the other signals are well-preserved?\nThank you for the suggestion. We have added the figure, as shown below, to the Supplement\nInformation. Carbonate δ18O is very sensitive to diagenesis and can be altered at very low\nfluid/rock ratios. The low δ18O values may suggest the alteration of oxygen isotopic system.\nHowever, the resistance of different proxies to diagenesis is variable, and the same proxy\ncould exhibit different behaviour under different ambient fluid conditions (Lau and Hardisty,\n2022; Zhang and Shields, 2023). Therefore, the alteration of sensitive proxies such as δ18O\ndoes not necessarily indicate distortion of other signals.\nFig. S4. Chemostratigraphic profiles of carbonate δ18O, Mg/Ca, Mn/Sr ratios and Sr\nconcentrations for the studied succession from the lower part of Taishir Formation at Tsagaan\nGorge. δ18O data are from Shields et al. (2002).\nMore discussion is needed on the criteria you used for identifying seawater-like REE signals.\nThank you for the comment. We have now modified this part accordingly. Detailed changes\ncan be found in the Supplement Information.\nOK, thanks for discussing this here. But what about the correlation between d34SCAS and\nAl and Sr/Ca? I still don’t think this have been adequately explained.\nPlease see our responses above.\nDiscussion of this is need in the text.\nThank you for the suggestion. We have now incorporated this into the text in the supplement.\nResponse to Reviewer Comments\nReviewer #1\nI reviewed the initial submitted manuscript. I appreciate the consideration of my previous\ncomments and concerns, which the authors have done satisfactorily. Specifically, the new\ndiscussion in the supplement and in the rebuttal addresses my main comment about the\nmeltwater pulse and I appreciate that the revised manuscript has integrated the data more\ncompletely than in the initial version.\nI have some minor comments/suggestions prior to acceptance.\nWe are glad to know that Dr Kimberly Lau is satisfied with our previous revision. We also\nthank her for taking the time to give constructive comments on our manuscript, which have\nled to improvement of the work.\nLines 172-175: This is an interesting hypothesis which is made compelling by the tie-in to the\nproductivity trends.\nWe thank Kimberly for the comment and appreciate the recognition of our interpretation.\nLine 179: “in the study samples” or “in these samples”\nThanks, modified.\nLine 216: Given that burial of organic matter can result from multiple factors, I suggest\nconnecting organic-rich continental margin sediments to organic carbon flux to productivity\nmore explicitly here.\nThank you for the suggestion. We agree that organic matter burial could relate to other factors\nin addition to productivity. Nevertheless, our current understanding of Zn isotope systematics\nin modern oceans indicates that the direct relationship between δ66Zn and productivity\nremains uncertain (Little et al., 2016; Horner et al., 2021). Therefore, we wrote the sentence\nthis way intentionally to strike a cautious note.\nLines 244-246: can the link between expanded euxinia and d66Zn be made more clear? I\ndidn’t quite follow this.\nThank you for the comment. This interpretation or link was first proposed by John et al. (2017).\nThey argued that at steady state, a smaller organic matter burial sink could be compensated\nfor by a greater euxinic sink, which would lead to lower δ66Zn values as there would be less\nisotopically light Zn being removed from the ocean. In our case, however, δ66Zn values\nremain relatively stable despite the further expansion of euxinic conditions, which implies only\na limited role for euxinia in controlling δ66Zn values through the lower part of the section. We\nhave now rephrased the sentence for clarity (Line 226-228).\nLines 247-249: Perhaps, but what if it’s local redox change and not global redox change that\nis driving the Zn isotope record? It seems like (also?) pulling in the Ce/Ce* would be useful\nin making this argument.\nThank you for the comment. We have now modified the sentence to incorporate Ce/Ce*\nevidence (Line 233).\nLine 253: This paragraph discusses several mechanisms and trends in Zn isotopes. It is\nuseful but I wasn’t clear what the sum of the arguments boiled down to with regard to what\nthe authors are interpreting from the Zn record. It would be useful to add a summary sentence\nthat links the record back to what can be confidently stated about nutrient cycling/availability.\ni.e. extend the statement about burial flux of isotopically light organic rich sediments\noutpacing weathering to when and how productivity increased.\nThis is a good point. We have now added the summary sentence at the end of this paragraph\n(Line 238-240). Thank you for the suggestion.\nFig 2. Now some vertical lines are not labeled/explained in caption. I think this was useful in\nthe previous version, even if there was some debate over which threshold was meaningful.\nThat could be explained further in the caption.\nThank you for the suggestion. We have now modified the figure and its caption, as shown\nbelow.\nFig. 2. Geochemical data profiles for the studied succession from the lower part of Taishir\nFormation at Tsagaan Gorge. Sequence stratigraphy is from Lindsay et al. (1996). Lithostratigraphy,\nδ13C and 87Sr/86Sr data (circles) are from Shields et al. (2002). 87Sr/86Sr data (squares) of Bold et\ncarb\nal. (2016) are correlated based on the age model of Bowyer et al. (2023). The Precambrian I/(Ca+Mg)\nbaseline is from Lu et al. (2017). Estimation of seawater sulfate δ34S is from Wang et al. (2019). The\nmean δ66Zn value of the modern deep ocean is from Lemaitre et al. (2020) and the mean δ238U value\nof Bahamian carbonates is from Chen et al. (2018). The shaded band highlights the lower part of the\nsuccession. Note that I/(Ca+Mg) values below the detection limit are regarded as zero, and errors for\nsome measurements of δ238U are smaller than the data symbols.\nFig S2 replicates Fig 3?\nThank you for the comment. Fig. S2 represents the output of the alternative age model D.\nIt looks very similar to Fig. 3 because the overall trends remain consistent.\nReviewer #2\nThe authors have done a nice job revising the manuscript. I do have additional comments\nthat should be addressed in the attached annotated PDF, but they are now relatively minor.\nNote the comments are in the merged PDF in the portion with the accepted (not tracked) new\nmanuscript. I recommend publication once these few remaining issues are addressed.\nWe thank the reviewer for the continued positive feedback and constructive comments, which\nhave helped improve the manuscript.\nLines 65-67: This statement lacks context. The reader does not know what you are talking\nabout without looking at the SI. Some more information is needed here to make this stand on\nits own here in the main text.\nThank you for the suggestion. We have now expanded the relevant content to provide more\ninformation (Line 65-74).\nLine 146: I think you should add \"indicate limited euxnia in an ocean that was still largely\nanoxic\". This will help clarify this alternative interpretation.\nThanks, added.\nLine 147: I would change this to \"limited\".\nThanks, modified.\nLine 153: I agree but it is also important to note that this does not require widespread\noxygenation. It could have been a reduction in euxinia in a still largely anoxic ocean.\nThank you for the comment. We have now made the point more clearly that the positive δ238U\nshift does not necessarily indicate widespread oxygenation (Line 176).\nLine 210: I think you need a few concluding sentences about what your interpretation of the\nZn isotope data is. You exclude a lot of potential causes of the Zn isotope variability, but it\nseems very uncertain and I'm not sure what the final interpretation is. Please summarize in a\nfew sentences.\nThank you for the suggestion. This point was also raised by the other reviewer, and we have\nnow added the summary sentence at the end of the paragraph (Line 238-240).\nLine 230: What effect would P availability and primary productivity have on the Zn isotopes?\nThank you for the comment. Increased P availability and primary productivity could enhance\norganic matter burial, which may in turn drive seawater δ66Zn toward higher values if there\nare no significant changes in other fluxes.\nLine: 293-294: This is not clear from the Zn isotope discussion above.\nThank you for the comment. We have now removed this sentence for clarity.\nLine 323: I'm still having trouble reconciling this with the Lau et al. (2017) paper that argues\nfor ocean oxygenation in the immediate aftermath of the Sturtian followed by a return to\nanoxia afterwards. You seem to be arguing the opposite. I still don't think this is adequately\naddressed in the revised paper. Does Lau's oxgyenation interval coincide with your Stage 1?\nIf so, this needs to be explicitly stated to allow readers to make the connection between these\n2 papers on the same rocks.\nThank you for the comment. The return to anoxia described in Lau et al. (2017) corresponds\nto the T3 member of the Taishir Formation, which lies well above the interval investigated in\nour study. This stratigraphic relationship is illustrated in Fig. S1, which shows the regional\ncorrelation of the Taishir Formation. Conversely, the oxygenation interval proposed by Lau\net al. (2017) corresponds to Stage 3 in our study. As detailed in the Section ‘Transient ocean\ndeoxygenation’, our δ238U data align well with those of Lau et al. (2017), but our interpretation\ndiffers slightly, reflecting advances in our understanding of U isotope systematics since that\npublication. Importantly, the episode of deoxygenation identified in the basal part of the\nTaishir Formation in our study is a novel finding. Professor Lau has confirmed in their review\nthat the δ238U records from both studies are broadly consistent and that our new data are not\nin conflict."}]

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