the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Evidence of deep subsurface sulfur cycle in a sediment core from eastern Arabian Sea
Aninda Mazumdar
Aditya Peketi
Namrata Khadke
Subhashree Mishra
Ankita Ghosh
Sai Pavan Pillutla
Mohd Sadique
Kalyani Sivan
Anjali Zatale
Abstract. Anaerobic microbial sulfate reduction and oxidative sulfur cycling have been studied in long sediment cores mainly acquired as part of IODP explorations. The most remarkable observation in many of these studies is the existence of an active sulfur cycle in the deep subsurface sediments that have very low organic carbon content and are presumably refractory. Here we investigate the interstitial sulfate concentrations and sulfur isotope ratios in a 290 m long core collected from the eastern Arabian Sea at a water depth of 2663 m. Continuous decrease in pore water-sulfate concentrations with depth (up to 75 mbsf) coupled with enrichment in δ34SSO4 values suggests organoclastic sulfate reduction (OSR) processes attributed to the activity of sulfate-reducing bacteria (SRB) and retention of labile organic substrates amenable to the SRBs. Below a depth of 75 mbsf, the absence of a further reduction in sulfate concentrations indicates insufficient labile substrate to drive SRB. An increase in sulfate concentrations at the deeper subsurface (below 128.5 mbsf) coupled with decreasing δ34SSO4 values may be attributed to a ferric-oxyhydroxide driven oxidation of Fe-sulfide. This study reveals that even under deep aerobic water columns, organic matter may continue to be a source of labile organic substrates at significantly deeper subsurface. Enhanced sulfate concentrations in the deeper depths may be attributed to the oxidation of sulfides via ferric-oxyhydroxides buried deep within the sediment. A microbiological investigation may reveal further details of the sulfur cycle at the deep surface.
Aninda Mazumdar et al.
Status: final response (author comments only)
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RC1: 'Comment on bg-2023-3', Anonymous Referee #1, 02 Mar 2023
Dear authors, dear editor,
Please find below my review for the submitted manuscript entitled “Evidence of deep subsurface sulfur cycle in a sediment core from eastern Arabian Sea” by Dr. Aninda Mazumdar and collaborators.
I hope they will be useful to the authors.
Best regards------------------------------------------------------------------------------------------------------------------
In the submitted manuscript, the author investigated the sulfur cycling in a 290-meter-long core collected from the eastern Arabian Sea. They investigated the porewater sulfate concentration, sulfate sulfur isotopic compositions, total organic matter, alkalinity etc to evaluate the (bio)geochemical processes during the early diagenesis. Based on the decrease in porewater sulfate and increase in sulfur isotopic composition, they suggested that organoclastic sulfate reduction can exist in the top 75 meters sediments. Further down, patterns of increased sulfate concentration and decreased sulfur isotope compositions were observed, which were interpreted as the results of iron sulfide oxidation in iron oxide rich environments. Overall, the research topic, exploring the sulfur cycle in deep subsurface marine sediments, is of high interest and suits the research topic of this journal. The explanation of OSR in the top sediment is convincing, but the interpretation on the iron oxide-driven Fe sulfide oxidation is speculative. Some points in the discussion need to be strengthened to make a more compelling story.
Mainpoints:
In fact, as mentioned in the introduction, other possibilities (e.g., seawater intrusion through deep cracks, deeper brine flux) can cause the anomalous sulfate concentrations and isotope pattern in porewater at deeper sediments. Consider to evaluate any possibility for your case.
I think a key evidence might come from the iron sulfide (e.g., pyrite) in the sediments.
Can you observe depletion of iron sulfide contents and enrichment of iron oxides if the oxidation of iron sulfide was the main processes in deep sediments?
What is the sulfur isotope composition of iron sulfide? Can they be low enough to lower the d34S of porewater sulfate?
Please note that sulfate sulfur isotope composition is still higher than the seawater sulfate value (~21 permill). An seawater-like fluid intrusion might lead to similar porewater patterns.
The interpretation of high alkalinity in most deep sediment need to be considered further.
Abstract:
Lines 30-31: “Enhanced sulfate concentrations …… via ferric-oxyhydroxides buried deep within the sediment” – repetition with the sentence abovd starting from Line 25 “An increase in sulfate concentrations at deeper … a ferric-oxyhydroxide driven oxidation of Fe-sulfide”.
Introduction:
Line 40: Mn reduction should come prior to Fe reduction. Change the positions of Mn4+ and Fe3+.
Line 46: Correct order.
Line 72: The reason for the depletion of sulfate within SMTZ could also be result from sulfate-driven anaerobic oxidation of methane.
Line 182: Regarding to Fe-sulfide formation, the absence of H2S suggest a hydrogen sulfide-limited and iron-rich environment.
Line 186: It is useful that the author can provide more details about the pyrite formation in the sediments. For example, the distributional pattern and sulfur isotopic composition (if they existed).
Line 197-199: Or there is additional sulfate input.
Line 203: Can you add some pictures of iron oxide aggregates or gypsum into the manuscript? This might help to strength your point.
Line 210-212: If the elevated alkalinity was due to the carbonate dissolution, one would expect a decreased, rather than an increased calcium carbonate content. You might better describe the background of sediment at the beginning. Can you observe some changes of the sedimentary phase in different zones (I - IV)?
Line 244-245: How to explain the variation of organic matter in the sediment? In addition to OSR, can it affect by the dynamic of productivity in the water column?
Line 246: I agree the OSR in the top 75 m sediment. However, the interpretation on the iron oxide-driven Fe sulfide oxidation is speculative. For example, a deep crack lead to instruction of seawater in deeper sediment, or deeper brine fluid might also lead to the similar patterns. These hypotheses might not be answered in the end, but at least they should be evaluated.
Citation: https://doi.org/10.5194/bg-2023-3-RC1 -
RC2: 'Comment on bg-2023-3', Anonymous Referee #2, 17 Mar 2023
Thank you for submitting your manuscript to Biogeosciences. This study explains the anomalous SO42- concentrations and δ34SSO4 profiles in the interstitial waters in a long sediment core to discuss sulfur cycle. But, after careful evaluation, I regret to inform you that there are some problems with your manuscript, and it needs to be rejected.
- The abstract is a brief introduction to the full text. First, you should briefly introduce the research background and limitations of your current study, and then introduce your work. In addition, the sulfate concentrations and sulfur isotope ratios were introduced in the abstract, and the explanation were showed. But I think these reasons are over-stated because there is lack of evidence.
- The introduction should try to quote the domestic and foreign scientific and technological papers published in the last 5 years. In addition, explain their importance, mention the past work that has tried to solve or answer the research question, and finally conclude with a clear research objective. Besides, Geology section is redundant, and should be simplified.
- In general, the description of material methods is not clear and rigorous enough, there is a lack of explanation of scientific and replicates methods used in the study: (a) Study site and sample collection: latitude and longitude information, sampling depth, sample number, biological repetition, etc., should be accurately described. I have another concern, what is the significance of analyzing sediment core? In my opinion, the authors need to clarify the vertical interactions of these parameters (SO42-, ions, TOC, etc.). (b) Biogeochemical analysis methods are best described together. Why is the δ34SSO4 values measured?
- In Results and Discussion, I think the main problem is that some explanation is lack of evidence. For example, OSR processes attributed to the activity of sulfate-reducing bacteria, but SRB was not analyzed in this manuscript; decreasing d34SSO4 values may be attributed to a ferric-oxyhydroxide driven oxidation of Fe-sulfide, which is also the speculation.
Citation: https://doi.org/10.5194/bg-2023-3-RC2
Aninda Mazumdar et al.
Aninda Mazumdar et al.
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