Sulfate triple-oxygen-isotope evidence confirming oceanic oxygenation 570 million years ago

The largest negative inorganic carbon isotope excursion in Earth’s history, namely the Ediacaran Shuram Excursion (SE), closely followed by early animal radiation, has been widely interpreted as a consequence of oceanic oxidation. However, the primary nature of the signature, source of oxidants, and tempo of the event remain contested. Here, we show that carbonate-associated sulfate (CAS) from three different paleocontinents all have conspicuous negative 17O anomalies (Δ′17OCAS values down to −0.53‰) during the SE. Furthermore, the Δ′17OCAS varies in correlation with its corresponding δ34SCAS and δ18OCAS as well as the carbonate δ13Ccarb, decreasing initially followed by a recovery over the ~7-Myr SE duration. In a box-model examination, we argue for a period of sustained water-column ventilation and consequently enhanced sulfur oxidation in the SE ocean. Our findings reveal a direct involvement of mass-anomalously 17O-depleted atmospheric O2 in marine sulfate formation and thus a primary global oceanic oxygenation event during the SE.

and a rise of the sulfate concentration coincided with an episode of large sulfate 17O depletion at the aftermath of Marinoan glaciation". This sentence should be moved to the Discussion section, after the authors show their new data.
Here the authors only need to present the hypothesis. It is redundant and distractive to discuss any other intervals here.
Line 88: This paragraph is too long. I suggest the authors split it into two shorter ones.
Line 113: "The NaCl-leached sulfate potentially represents a mixture of primary and secondary sulfate…" Theoretically, primary sulfate cannot be released before the host carbonates are acidified. Why here secondary sulfate could be mixed with primary sulfate in NaCl-leached sulfate? Why NaCl could release primary sulfate here? <MQI ))/2 b?JWIQ% VXOJEWI JURP R[MHEWMRQ RJ VXOJMHI PMQIUEOV LEV ORZIU f+,A EQH f)0? YEOXIV WLEQ the primary seawater sulfate." If this sentence is not talking about the authors' own results, please add citations here. Line 157: "will" The word "will" in the discussion section should all be replaced with "would". I suggest the authors double-check this issue throughout the entire manuscript.
Line 213: "which provides a new dimensional information" The word "information" is an uncountable noun.
Line 247: "Let us then explore another possibility, i.e., an increased"… The writing looks informal. The modeling section is a bit lengthy, with lots of numbers that are not previously introduced. It is not easy to follow.
Line 276: "linked to global cooling" Global cooling? This "global cooling" comes out of nowhere. Please either delete it or discuss it in detail.
Line 280 "On the other hand, it begs the question if the atmosphere O2 had been distinctly depleted in 17O throughout the Ediacaran Period and the high sulfate concentrations at the aftermath of Marinoan Snowball Earth and during the SE merely facilitated the preservation of the sulfate 17O record, or atmospheric O2 was distinctly depleted in 17O only during the two episodes". This manuscript shows lots of novel 17O data. But I feel that the 17O data have not been deeply discussed. Could the authors offer an in-depth discussion on the origin, pathway, and preservation of the 17O anomaly.
Line 290: "28 samples from one of the Canyon-Shoulder sections (31°8'46"S, 138°32'3"E) in South Australia" There are many sections in Jon Husson's paper. Please tell the readers which exact section in Husson's paper the authors analyzed in this study.
Line 293: citation 59 It is inappropriate for the authors to incorrectly cite the 's paper for the Mochia-Khntuk section. The  paper does not contain any original data of the Mochia-Khntuk section.
Lines 294 to 300: Method Please delete the "(a) (b) (c) (d)" in this paragraph. This paragraph should be rewritten. This method section looks like a lab manual.
Lines 305-311: Method Please delete the "(1) (2) (3)" in this paragraph. This method section looks like a lab manual. Figure 1: I suggest the authors plot data of different sections along the same X-axis. I noticed that the authors stretched or squeezed the X-axis when plotting the O and S isotope data. It is difficult for me to compare. Line 490: "filled with a graduated color denote" Gradual, not graduated.

Reviewer #1 (Remarks to the Author):
Wang et al., have made a remarkable discovery in strata bearing the Shuram excursion.  (Chen and Morris, 1972;Oba and Poulson, 2009;Siang et al., 2017), with the chemical reaction being exothermic. While the exact pathways of this reaction are not fully understood and there are some unresolved steps, it is widely believed to occur in various natural environments where both compounds (H 2 S and O 2 ) are present, such as sulfur springs and geysers (Xu et al., 1998), anoxic lakes (Gingras et al., 2011), swamps and wetlands (Wu et al., 2011), or marine sulfide-rich environments (e.g., the Black Sea; Neretin et al., 2001). We have included additional discussion about modern H2S oxidation by O2 in the revised manuscript, see lines 236-238. The authors should discuss these possibilities.
Response: Thanks for these comments. From your comments, we can tell that the statement of 'After all, the SE is only ~60 Myr ...' mentioned above has achieved its goal: getting reader to ponder deeply the origin of the 17 O-depletion in air O2 in geological history. Meanwhile, we also felt the statement can generate misunderstanding. We have, therefore, discarded the statement in the revised Response: We understand the reviewer's concern regarding the potential role of old evaporites as the sulfate source for the SE oceans. However, it is unlikely that old evaporites serve as a significant source of the SE sulfate due to several reasons.
(1) Our CAS data from three examined paleocontinents all show distinctly (3) The pre-Ediacaran evaporites are all small in scale. The current pre-Ediacaran evaporite volume is ~4.7 × 10 5 km 3 in total (Evans, 2006). If being all gypsum and all dissolved, they could only supply ~0.6 Myr of the sulfate flux (i.e., ~1 × 10 13 mol/yr) needed in Shields et al. ( 2019)'s model estimate. Although the current pre-Ediacaran evaporite volume is the residue after experiencing dissolution, these numbers require an unreasonably large volume of evaporites exposed in the pre-Ediacaran time, and evidence for massive pre-Ediacaran evaporite dissolution is lacking. We have added additional relevant discussion regarding these possibilities in the revised manuscript (see lines 220-232 and also Supplementary Fig. 4).
The reviewer entertained the idea that the extreme weathering brought on by Cryogenian glaciations may have unearthed some of the older deposits. This is likely, but we do not think it played a role for the SE event, given the quite long-time lag of >60 Ma between SE and the end of Cryogenian glaciations.
We agree with the reviewer that other possibilities, such as subterranean groundwater discharge and ocean destratification, that have been put forward for Marinoan barites may have played a role during the SE as well. We note that both these events are transient or short-termed by nature and are therefore cannot be entirely we discussed a scenario in which later O2-rich water (e.g., groundwater) circulated through pyrite-rich carbonates and oxidized the pyrite to sulfate. This is a distinct possibility if the "late" water carried 17 O-depleted air O2. However, we have presented arguments that this scenario was less likely (see discussion at lines 176-189).
I hope these suggestions help. Again, this is an extremely important discovery that the authors have found and I am very supportive of this work for publication.
Response: We show our appreciation again for the reviewer's positive evaluation and valuable inputs.  (that has the same triple oxygen isotope composition as atmospheric O2) caused the SE.
The manuscript is extremely well-written and tells a compelling story. There are a few major comments that should be addressed before publication. today. This is due to the fact that only a small fraction (<25%) of the oxygen in product sulfate is derived from O2 even under aerobic conditions. Another factor is the kinetic isotope effect (KIED+nFD-) of ~0.985 (Cao and Bao, 2021) supported not only by our data (see discussion in lines 205-217), but also independently by previous studies (e.g., Fike et al., 2006;Kaufman et al., 2007;McFadden et al., 2008;Shi et al., 2018).  open question (e.g., Derry, 2010;Grotzinger et al., 2011;Knauth and Kennedy, 2009;Tahata et al., 2013). Our study found that sulfur and triple oxygen isotope composition in sulfate co-vary with the 13 Ccarb excursion, supporting the SE as a primary event. This 10 finding seemingly underscores the necessity to explore primary processes that can N`XTJRV \QN LW&^JZRJ\RWV WO m 13 Ccarb JVM m 18 Ocarb, but at this time our report cannot delve deeper into this topic as it requires more data and goes beyond the scope of the research.

+# GQN qo
Please see the added discussion at lines 190-204 in the revised manuscript.
2) Our modeling, based on sulfur cycling, demonstrates that an enhanced oxidation of the pre-existing reduced sulfur pool lasting for only ~1.5 Myrs can explain the sulfate sulfur and oxygen isotopes. However, the sulfate isotopes are closely associated with \QN LJZKWVJ\N m 13 C, which exhibits a negative excursion lasting ~7 Myrs (i.e., 574-567 Ma). This suggests that it may not be necessary to propose a sustained, slow oxidation of organic matter over 7-8 Ma to account for the SE event. Nevertheless, additional models, such as those focused on surface carbon cycling, would be required to Response: We may misunderstand the reviewer's meaning. In general, O2 generated by photosynthesis, terrestrial or aquatic, sources its oxygen entirely from the ambient water. Response: Thanks. We could have expressed it better. A minor revision has been made to enhance the clarity of this statement (see lines [205][206][207]. Spatial heterogeneity refers to geographic or depth heterogeneity of sulfate's sulfur and triple oxygen isotope composition. Such heterogeneity is expected when seawater sulfate concentration is low, such as during the pre-or post-SE interval.

Reviewer #3 (Remarks to the Author):
This manuscript presents sulfur and oxygen isotope profiles of the Shuram excursion at Additionally, Vendotaenid fossils have been reported from the Shuiquan Formation, 13 which are commonly found in post-Marinoan rocks (Xiao et al., 2004). We argue that

\QN VNPJ\R^N LJZKWV R[W\WXN N`L]Z[RWV _R\Q J VJMRZ WO l**'2k ZNLWZMNM RV \QN FQ]RY]JV
Formation at Mochia-Khutuk section should be correlated to the Shuram excursion for the following reasons. based on multiple lines of evidence and precise dating constraints . We have extensively discussed this issue in the revised Supplementary Information and have cited the relevant references (Xiao et al., 2004;Wang et al., 2022 PR;Wang et al., 2023 NSR;Dodd et al., 2023). However, we have decided not to include much of the above discussions in the main text of our manuscript as they are not directly related to the topic of this study. Response: The reviewer is correct. The reference has been replaced by 'Wang et al., 2023 NSR', and also see our response above.
I also noticed that the carbon isotope data of two sections (i.e., the Wonoka and Shuiquan formations) are missing in the online supplement. This is frustrating. The readers cannot compare these sections on their own. These carbon isotope data service as the essential foundation of this study. Without these carbon isotope data being available, I do not think this manuscript should be published.
Response: Thanks for pointing out our omission. We now have included the carbon isotope data of the Wonoka and Shuiquan formations in Supplementary Data 1.
2. The language should be further improved. The current version is still not satisfactory.
There are grammatical errors from time to time. Perhaps the senior author of this manuscript (e.g., Huiming Bao) could make more efforts in revising the language.
Response: Thanks for the comment. The full text was carefully revised.
3. The organization should be further improved. The model part is quite lengthy and not easy to follow. I suggest the authors shorten the model section.
Response: Thanks for the suggestion. We wanted to ensure that readers or other researchers can reproduce our model and results, hence the much details in the original manuscript. We have done some revisions and taken steps to shorten it by moving the following information to Supplementary  Response: Thanks. This sentence has been removed in the revised manuscript.
Line 88: This paragraph is too long. I suggest the authors split it into two shorter ones.
Response: Revised, see lines 89-95. 16 Line 113: "The NaCl-leached sulfate potentially represents a mixture of primary and secondary sulfate…" Line 157: "will" The word "will" in the discussion section should all be replaced with "would". I suggest the authors double-check this issue throughout the entire manuscript.
Response: Thanks for pointing it, and all were revised.
Line 175: "circum-SE" This is not a commonly used word.
Response: Modified as "pre-or post-SE" in the revised manuscript, see line 206. Line 213: "which provides a new dimensional information" The word "information" is an uncountable noun.
Response: The sentence has been revised, see lines 253-255.
Line 247: "Let us then explore another possibility, i.e., an increased"… 17 The writing looks informal. The modeling section is a bit lengthy, with lots of numbers that are not previously introduced. It is not easy to follow.
Response: Thanks for bringing this to our attention. The sentence has been revised (see lines 281-283). We have done some revisions on the modeling section (explanations for parameters were added, see lines 281-308) and shortened it by moving the following information to Supplementary  There are many sections in Jon Husson's paper. Please tell the readers which exact section in Husson's paper the authors analyzed in this study.