Multiple Sulfur Isotope Evidence for Bacterial Sulfate Reduction and Sulfate Disproportionation Operated in Mesoarchaean Rocks of the Karelian Craton

Round 1

Reviewer 1 Report

review of Vysotskiy et al. (Multiple sulfur isotope evidence for microbially driven processes (bacterial sulfate reduction and sulfur disproportionation) operated in Mesoarchean rocks of the Karelian Craton 

General Comments

This paper reports samples from an Archean province in Fennoscandia.  The samples indeed look really interesting with textural, and S isotopic data used to argue for a microbially mediated process in their formation. 

The structure of the paper is quite often based around assertions with the assumption of microbial fractionation.  For example in the abstract: “This set of multiple sulfur isotope fractionation data from the Karelian Craton can be considered as traces of microbial life”.  There is a fair bit of circumstantial evidence involved here, but nothing definitive.  The textures (to me anyway) look as though they are most likely microbially mediated.  The S isotopes may be as well, but I get the feeling this paper hasn’t really put it all together and leaves significant gaps in the story (such as the explanation of high d34S), and how the different S isotope generations stayed distinct.

While the microstructures look as though they might be microbial, textural information is subject to a lot of interpretations.  As the authors note (line 597), textures are subject of fierce debate, so tread carefully.  Apparently the samples were etched in HNO3 but this is only mentioned in the appendix.  I would have like to have seen greater use of this in the description of the data.

There is no mention of Archean photolytic fractionation in the Introduction.  I see this as a significant omission but the photolytic story is used in the discussion so the authors are certainly aware of it.  

But also missing from anywhere in the text is the use of 36S to distinguish various sources.  This is an important issue because the photolytic fractionation lies on a slope -1 line (in D33S-D36S space) but microbially mediated fractionations lie on a slope of -7.  Therefore the departure of data from the -1 line (to lower D36S than expected) has been used by Ono et al. to argue for microbial fractionation in the early Archean (e.g. Dresser Formation).  Can 36S measurements be accomplished with this analytical system?  

The samples show a reasonably clear distinction in the different populations, save for a few aberrant points (to higher D33S).  Given that the spot is quite large, what are the chances of mixed analyses?  Were the spots revisited after analysis to see if mixing of domains could have occurred?

Marcasite was evidently identified in bulk XRD analyses.  I would suspect that it would have a quite different etching response.  Was this noted at all?

Specific comments:

L26 Main point of paper is S isotope systematics, but data is not really put in to context in abstract.  No mention of how the reservoirs formed in the abstract in terms of D33S.  No mention of photolysis.  

L35 D36S should be mentioned somewhere.

L45 Very general statement:  “δ34S data published in these studies range from values of approximately −30‰ to +20‰), that supports the existence of microbial life during the Neoarchean.”  whereas above (L37-39) it is stated “The strongly negative δ34S values recorded in sulfides from ancient sediments have been considered as one of the principal arguments for a biological genesis of sulfides”.  No question that S isotopes are complicated.  Make sure it is laid out in a clearly developing framework.

L70 No mention of Photolytic fractionation in Introduction.

L75 “Therefore, sulfide samples were taken from sedimentary rocks, not from the ore zone of the Leksa deposit where sulfides were formed at higher temperatures (>150C), eliminating the possibility of bacterial activity”  Yes and no.  The ores may represent migration of the original sedimentary pyrite, which was result of “bacterial” activity. What evidence for hydrothermal activity?  How widespread is this thermal event?

L78 Should probably mention the appendix up front.  There is a lot of useful information in the appendix.

L102 So if you have an 80µm spot, you cant necessarily get clean analyses of the domains as shown.  How was the targeting of these assemblages made?

L119 If fluorination is used why cant 36S be measured?

L121 What is the standard?  If it is tank SO2, how was it calibrated?

L125  What does “typical” reproducibility actually mean?  

L181 I’d lead with the high precision zircon age rather than the Pb-Pb and Sm-Nd of lower precision.

L194 presume carbon = carbonate and is dolomite according to appendix.  

L227 The data presented appears to show two distinct parageneses.  Be worth looking at the high D33S from yellow and red points to make sure they have sampled only these associations (and you haven’t gone through at depth to another domain).

L227 To me this seems to show an abrupt change in solution chemistry bringing in a high D33S source.  High D33S is somewhat unusual.  

L255 Yes it ranges up to +2.7, but it ranges from +1.5 to +2.7.  It is totally separate from the colloform and matrix disseminated pyrite.  

L256 Is there anything to associate the colloform and matrix disseminated pyrite? Looks like they are forming from near identical S isotope fractionation processes.

L263 I agree that taking into account modern settings is important;  There’s very little discussion about this.  But Archean is complicated because of photolytic S.

L279 “biological sulfate reduction, as most reliable mechanism to produce very large fractionation of sulfur isotopes”  Yes, but you note below that you need the Rayleigh fractionation to produce strong positive d34S.

L282 First mention of photolytic.

L285 “produce sulfate aerosols with a negative ∆33S signature and elemental sulfur aerosols with a positive ∆33S signature.”  This should be explained in a bit more detail because it provides important constraints on the reservoirs.

L288 “would take place via microbially mediated pathways followed by reactions (R1-R3)”

Not clear that these are successive reactions.  Both are likely happening at same time.  But issue is getting sulfur and/or sulfate  to H2S.    

L293 Lots of discussions in literature regarding seawater D33S value.  Probably around -1 ‰.  

L296 Confusing use of symbols: three used.  Don’t use filled grey circles for literature = previous studies.  Who knows what’s been analysed.  I think this diagram messes up the interpretation and doesn’t add much beyond Figure 4.  Could combine is a judicious use of symbols is used.

L346 “Thus, the current model does not 346 provide an explanation for the high δ34S values measured in concentrically laminated pyrite grains.” and also the matrix disseminated grains. And why the pyrite surrounding colloform is totally distinct.  Did the system open up again?

L339 The mixing relations need development.  Seawater with d34S of 0 ‰ has D33S of -1 ‰.  If fractionation of -10 ‰ occurring then the d34S = -10 ‰ should show the seawater D33S.  But it doesn’t.  It looks like it is zero, or thereabouts.

L400 I’m not so sure that the zero permil is just a mixture.  

L425 Yes, this process can produce high d34S.  And you need to produce sulfide.  But these compositions scatter along the d34S space suggesting a variety of samples of the Rayleigh fractionation.  It’s not just a mixing end member.  Plus there can be multistage Raleigh.  But I’m still perturbed about the subsequent overprint of isotopically distinct pyrite.  How did this solution get in?

L441 “We associate these structures with the fossilization of microorganisms.”  Might be a step too far. But at least refer to some Phanerozoic examples that are clear.

L452 Lay out the detail of this association.

L461 The arguments need to be carefully laid out.  Too many conclusions and assertions presented.

L476 “Although the fact of the existence of microorganisms in the Archaean …”  Yes, but relating this to the signatures in the rocks is key.

L511 “The present results can enhance understanding of sulfur cycling and microbial life in the Mesoarchean”.  Without the d36S data it only adds to the complexity.

L528  I liked the Appendix and think some of it could be used in the sample descriptions.

L537 “This resulted in the formation of a fine-grained texture”  May need more explanation.

L539 No images with etching.  No images of analytical spots.

L549 “Pyrite crystallized from shallow or interstitial waters of sedimentary host-rocks while they were being locally enriched with ore elements.”  This is an example of a declaration/assertion that is literally incorrect.   Yes, the general context is correct, but this paper is about the specific relationships.  In detail, what is shallow water in this context?  Why do they need to be undergoing local enrichment?  Pyrite is meant to be crystallised during microbial reduction. Phrasing needs to be exact.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript entitled “Multiple sulfur isotope evidence for microbially driven processes (bacterial sulfate reduction and sulfur disproportionation) operated in Mesoarchaean rocks of the Karelian Craton” examines sulfides from the ca. 2.8-billion-year-old (Mesoarchaean) volcanic-sedimentary Leksa massive sulfide deposit in the Fennoscandian Shield in NW Russia. Wider range of sulfur isotopes (δ³⁴S) is reported compared to previous studies of Mesoarchaean sulfides – a feature that the authors link to microbial processes. The studied euhedral and colloidal pyrites also differ in their sulfur isotopic composition (Δ³³S), which is linked to disproportionation of photochemically derived sulfur during the formation of the colloidal pyrites. The study is an important addition to Archaean sulfur isotope record with implications to early life and elemental cycles, and I recommend the manuscript for publication in Minerals after minor revision, as indicated line by line below:

Lines 2-4: The title can be shortened to “Multiple sulfur isotope evidence for bacterial sulfate reduction and sulfate disproportionation operated in Mesoarchaean rocks of the Karelian Craton”, thereby also avoiding the use of parenthesis.

Line 13: Sulfur isotope effects can be shortened to sulfur isotopes or alternatively “effects” should be replaced by “patterns”, “features” or characteristics”.

Line 14: "…this metabolism.." could be changed to "…sulfur metabolism…" for clarity.

Line 20: When rounded, the upper end of the δ³⁴S range should be 33‰ rather than 32‰ (in Table A1 the sample H2/24.0 has δ³⁴S of 32.7‰).

Line 71à The section “Material and Methods” should be moved after the section “Geological setting”, as it would be logical to introduce the geological background and site before giving details on the samples.

Line 72: It is not yet introduced where the site is, and the reader is thus puzzled where are these drill holes located. This can be fixed by changing the order of sections 2. and 3.

Line 107: You may refer here that more details on sulfide petrography are provided in the Appendix.

Line 145: 2+ should be in superscript.

Line 146: …determined by X-ray…

Line 150à The section “Geological setting” should be moved before the section “Material and Methods”

Line 152: Replace “They…” with “It is…”

Figure 2: In Figure 2A NK should be NC (Norrbotten Craton) as indicated in the figure text. In Figure 2B numbers adjacent to the legend should be moved in front of the boxes as in Figure 2A for clarity.

Line 174: Replace “them” with “it”.

Line 181: Replace “volcanic” with “volcanism”.

Line 186: … this study

Line 194: In which form does the carbon occur in these rocks? Carbonates? Graphite? Other? Please clarify.

Line 202: …sphalerite, and galena.

Figure 3: The symbol shown in the legend (box number 6) is not visible in the figures and can be removed. Explanation for the box number 7 is missing from the figure text.

Line 261: check the font size and style for “microbial life”

Lines 261-262: The sentence “Interpretation is determined…” is unclear, seems unnecessary, and can be deleted.

Line 274: add t in “first”

Line 279: Replace “reliable” with “likely”

Line 285: Add commas: …should, according to photochemical experiments [52-59],…

Line 298: representing

Line 305: …progressively increase…

Line 306: …sulfate is consumed…

Line 310: Replace “where a” with “with"

Line 311: Add comma: …water, and…

Line 232: Add comma: …authors, more…

Line 329-332: This sentence can be shortened to: “Although one feasible explanation we propose for the strongly ³⁴S-enriched sulfides is microbial processes, it remains unclear whether such isotopic enrichments can be achieved only through microbial sulfate reduction, even in restricted pore waters.”

Line 333-347: It seems in Figure A4 that the δ³⁴S values are decreasing towards the rim. This is another proof that microbial sulfate reduction according to Rayleigh fractionation in a setting where sulfate source is diminishing cannot explain the results – the pattern is actually opposite.

Line 354: “… do not contradict the conclusion…” can be shortened to “…are in line with…”

Line 391: remove “[e.g.,”

Line 399-400: The sentence can be shortened: “…inputs, but may result from mixing of two…”

Lines 411-425: A picture/sketch illustrating this evolution would be informative addition

Lines 415-419 (point 2): Could this be also related to increased temperature that prohibits microbial activity?

Line 450: “Once…”

Line 484: correct typo in “disproportionating”

Line 558: From where are these forms found? In the host-rock? In carbonaceous sediments? Please clarify.

Figure A3: Box J should probably be G instead. Are F and J part of the area E? If yes, from which part? Please indicate in the figure, if possible

Line 572: Figure A3 E-G instead of E-J (see previous comment)

Line 576-577: Where do the letters a, B and “c” refer to? They dot not seem to fit with figure A3.

Figure A4: It could be mentioned that this is the same section as in Figure A3 (A) (at least it seems so to me).

Line 605: This should not be a new paragraph but continuation from the previous page.

Line 606: Sugitani 2019 is not found in the reference list. Consider adding a separate reference list for the appendix.

Table A1: Font size can be decreased such that the table will fit in two pages.

References: The authors may consider referring to some additional sources such as Strauss H., 2003, Sulphur isotopes and the early Archaean sulphur cycle, Precambrian Research 126, 349-361 (for Archaean sulfur cycles and isotopes)

and

Drake H., et al. 2015, Extreme fractionation and micro-scale variation of sulphur isotopes during bacterial sulphate reduction in deep groundwater systems, Geochimica et Cosmochimica Acta 161, 1-18

and

Drake H. et al. 2018, Unprecedented ³⁴S-enrichment of pyrite formed following microbial sulfate reduction in fractured crystalline rocks, Geobiology 16, 556-574 (for extreme sulfur isotope fractionation).

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Review of Vysotskiy et al. Multiple sulfur isotope evidence for bacterial sulfate reduction and sulfate disproportionation operated in Mesoarchaean rocks of the Karelian Craton

Revision

I think this version is much improved and has addressed any concerns I had.

But I would suggest that the d36S results are incorporated into the main manuscript.  Even if they are presented as a standalone aside regarding the nature of the D33S-D36S relationship in a limited subset of the samples.  It is a very useful constraint on the processes active in the formation of this deposit.

I support publication of this manuscript.

Specific Comments (all suggestions)

L120-121 This greenstone belt is a system of tectonic sheets and consists of … 

L122 Paleoproterozoic basaltic desire overlies the Archean sequence.

L146 represented mainly as pyrite

L147 rarer as pyrrhotite 

L166 Definition of VSMS.  Include “deposits” after VSMS in this instance.

L171 I would note that calling this island arc volcanics could cause a reaction in some Archean geologists because of the implications for plate tectonics.  I’m happy for it to stand here.

L192 In further discussion…

L200 …less than 20 µm

L239 Estimate uncertainty on d36S, even if just internal measurement errors.

L289 Report that you have measured d36S in a limited subset of the samples to understand the relationship between D33S and D36S.

L319 that operated

L343 Since the samples of coliform pyrite studied here archived opposite signs …

L429 can be explained by 

L444 This interpretation suggests…

L459 would be apparent

L469 We consider such an environment unlikely to exist …

L474 highly inhomogeneous

L480 …, which was observed

L494 (just an observation, but Rayleigh fractionation can subtly change D33S through application of a slightly different mass fractionation law compared to the instrumental mass fractionation law (e.g. Rayleigh vs Power laws).  But the action of microbes can also change D33S (and D36S) as noted below in the manuscript).

L585 Somewhat inappropriate to bring the d36S results in here without noting them in Results.

L789 Fig. A6 - also consider putting this in the Results.  But make sure scales are the same for D33S and D36S.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf