the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Active microbial sulfur cycling in 13,500-year-old lake sediments
Paula C. Rodriguez
Cara Magnabosco
Longhui Deng
Stefano M. Bernasconi
Hendrik Vogel
Marina Morlock
Mark A. Lever
Abstract. The addition of sulfur (S) to organic matter to form organosulfur compounds is generally thought to protect organic matter from microbial degradation and promote its preservation. While most microbial sulfur cycling occurs in sulfate-rich sediments above the sulfate-methane transition zone, recently discovered active sulfur cycling in deeper sulfate-poor environments may have a yet-unquantified impact on the mineralization of organic matter. Here we investigated the fate of buried S-compounds down to 10-m sediment depth representing the entire ~13.5 kya history of the sulfate-rich alpine Lake Cadagno. Chemical profiles of sulfate and sulfur reveal that these oxidized species are depleted at the sediment surface with the concomitant formation of iron sulfide minerals. An underlying aquifer provides a second source of sulfate and other oxidants to the deepest and oldest sediment layers generating an inverse redox gradient. At both sulfate depletion zones, isotopes of chromium-reducible sulfur (CRS) and humic-bound sulfur are highly negative (−30 to −65 per mil) compared to background sulfate suggesting ongoing microbial sulfur cycling. Interestingly, humic-bound S from intermittent sediment layers within the sulfate-depleted methanogenesis zone consistently exhibits a lower δ34S than CRS in lacustrine deposits but a higher δ34S than CRS in terrestrial deposits, which could possibly be due to different reactivities of organic matter types (lacustrine versus terrestrial origin) to sulfide or the ability of microorganisms to form/degrade organic S. Although sulfate concentrations are extremely low between the sulfate depletion zones, dsrB gene libraries reveal a huge potential for microbial sulfur reduction throughout the sediment column.
- Preprint
(1129 KB) - Metadata XML
-
Supplement
(362 KB) - BibTeX
- EndNote
Jasmine S. Berg et al.
Status: open (extended)
-
RC1: 'Comment on egusphere-2023-2102', Anonymous Referee #1, 06 Nov 2023
reply
Sulfur species in sediments have been used extensively to reconstruct ancient environments, but most of these analyses rely on the canonical assumption that the d34S values preserved in pyrite reflect “sulfide.” Here, the authors present pyrite d34S values from well-constrained, anoxic Lake Cadagno in the context of concentrations and d34S values for other sulfur phases. The observed patterns in near-surface sediments broadly conform to expectations about early microbial sulfate reduction in this environment, but they also include intriguing signals in organic sulfur and in the contrast between sedimentary layer types (lacustrine vs terrestrial). These data are valuable contributions to growing compilations and provide an interesting case study. Still, I was left wanting more in terms of analysis and potential hypotheses for organic S sources, beyond the established (but valid) observation that we lack a real understanding of the mechanisms behind sedimentary sulfur isotopes. I would encourage the authors to consider expanding their analysis to enhance the novelty of the study, prior to eventual publication. The authors should also better address potential sources for high C:S, very low d34S humic acid S values, and the relative abundances of HAS vs. TOS vs. CRS (see below).
Specific comments:
The abundances of organic and inorganic sulfur forms are difficult to compare in their differing units. In Line 220, the text states that humic acid sulfur is the dominant phase present in the deep euxinic sediments, but this species is presented in Fig. 2Ba different unit (mass/mass). The methods indicate that HAS was measured as a sulfur concentration, so why is its molecular weight / mass shown here? Please present the HAS concentrations in a molar S unit (directly measured or calculated from C:S) so it can be more directly compared with the other phases. This becomes especially relevant in the context of Line 237, which appears to make the opposite observation, that most sulfur is inorganic. This balance is a critical aspect of the discussion, so please define these relationships as clearly as possible.
Please also explictly define the relationship between “TOS” used in ratios, kerogen, and humic acid S.
It would increase the future utility of this data to briefly discuss how using a humic acid extraction (versus looking at residual kerogen) might impact comparisons with kerogen data. (Which components of sedimentary organics are included/excluded? Is the HA extraction chemically likely to extract sulfurization products vs reduced vs oxidized organic S species?)
Line 384 – Please lay out your argument more explicitly as to why the observation that specific intervals with very high pyrite concentrations are lacustrine means that most pyrite across the section is authigenic. Not all lacustrine layers show this pattern, what might explain these very high pyrite zones?
Section 4.2 – This discussion should address the abundance of organic S (esp. quantitatively relative to pyrite) alongside its isotope data. If the C:S ratio of HA is representative, it is very high (5,800-13,000), making its very low 34S value especially notable. These very high ratios do also increase the probability that this is an analytical issue – are there sufficient counts on this sulfur peak to be confident in these values? If so, this is a remarkable observation and should be discussed in greater detail.
(How do these C:S ratios compare with other humic acid fractions from terrestrial environments? What does this imply for sulfurization relative to other sources?)
Please note the data types used in Hebting et al. 2006 and Urban 1999 that were used to define early organic matter sulfurization and compare them explicitly with your new results.
Section 4.3 – This section would benefit from an explicit re-statement of how organic S concentration varies across the section in the context of pyrite concentration. There are also some key observations from the data that could be better addressed here.
What sequence of processes do the authors propose to explain the relatively light d34S values in HA lacustrine layers? Why might lacustrine layers differ from each other? Are there alternative explanations for organic S being so extremely light relative to pyrite?
How is sediment remobilization expected to affect pyrite vs organic matter d34S values? How specifically might a shift to more labile organic matter (associated with more lacustrine-type sources) affect these values? How often is that what you observe in the lacustrine-type layers?
The manuscript is motivated in large part by the potential contribution of sulfurization to organic matter preservation, but this is not returned to in the Discussion. How does the data overall provide insights into organic matter preservation? If this is not a primary conclusion, consider de-emphasizing it in the abstract and introduction.
Citation: https://doi.org/10.5194/egusphere-2023-2102-RC1
Jasmine S. Berg et al.
Jasmine S. Berg et al.
Viewed
HTML | XML | Total | Supplement | BibTeX | EndNote | |
---|---|---|---|---|---|---|
127 | 55 | 10 | 192 | 18 | 6 | 6 |
- HTML: 127
- PDF: 55
- XML: 10
- Total: 192
- Supplement: 18
- BibTeX: 6
- EndNote: 6
Viewed (geographical distribution)
Country | # | Views | % |
---|
Total: | 0 |
HTML: | 0 |
PDF: | 0 |
XML: | 0 |
- 1