Multiscale Microbial Preservation and Biogeochemical Signals in a Modern Hot-Spring Siliceous Sinter Rich in CO2 Emissions, Krýsuvík Geothermal Field, Iceland
Round 1
Reviewer 1 Report
This study presents very detailed textural and geochemical observations of silica-sinter hosted microbial fossils at scales from field/outcrop to TEM. The analyses are well-documented (with a few missing details discussed below).
Where this paper could be improved is in the discussion of the broader implications. The field and analytical work here have implications beyond this one particular setting, and I am sure the authors know it. But, as it stands, there is very limited discussion of this kind of broader meaning and context for this study.
What do the results here tell us about what to expect from other similar modern-day sinter deposits - are there any important generalizations that can be made to other similar systems?
Can the authors elaborate on what their work has revealed about the applicability of these modern analogues to ancient Si-rich systems (such as the very ancient examples they cite in the introduction)?
Early in the paper, the presence of silica-rich (including opaline silica) deposits on Mars is mentioned. Are any of the findings and observations here applicable to the exploration of Mars?
For both Mars and early Earth, what aspects of this kind of system are closely analogous and what aspects are going to be wildly different, and how can that help us investigate such ancient/extraterrestrial deposits?
Here are some minor items
The study seems to presume that silica is opal-A everywhere (and that's reasonable) but did the authors observe any evidence for the presence of other silica phases?
Fig. 2, there is a thin horizontal white line across panels C and D
Fig. 4 panel b, the "mm" is quite hard to read.
Fig. 5, again horizontal white lines, possible pdf or image file production problem?
Fig. 7, the lower left panel, are the numbers on the vertical axes counts? Weight percentages are mentioned in the caption (also lines 407-408) - but something like C/Si isn't usually expressed in wt.%, it ought to be dimensionless. If what is meant is wt.% C/wt.% Si, doesn't that bias things by the difference in atomic weight, and wouldn't it be better to express everything in atomic/molar percentages (which is after all what is being measured here). A few other things: How are those percentages estimated (and, what are the approximate detection limits)? Back at line 191, it is stated that "analytical results display an error between ± 5% to 7%." and here its mentioned that the maps for elements with <1 wt% concentration are blurry.
What is the section coated with? Is there any chance that epoxy or other lab materials have gotten into pore space artificially increasing the C concentration?
Fig. 8 and 9 again white horizontal lines
Author Response
REVIEWER 1
- This study presents very detailed textural and geochemical observations of silica-sinter hosted microbial fossils at scales from field/outcrop to TEM. The analyses are well-documented (with a few missing details discussed below). Where this paper could be improved is in the discussion of the broader implications. The field and analytical work here have implications beyond this one particular setting, and I am sure the authors know it. But, as it stands, there is very limited discussion of this kind of broader meaning and context for this study. What do the results here tell us about what to expect from other similar modern-day sinter deposits - are there any important generalizations that can be made to other similar systems? Can the authors elaborate on what their work has revealed about the applicability of these modern analogues to ancient Si-rich systems (such as the very ancient examples they cite in the introduction)? Early in the paper, the presence of silica-rich (including opaline silica) deposits on Mars is mentioned. Are any of the findings and observations here applicable to the exploration of Mars? For both Mars and early Earth, what aspects of this kind of system are closely analogous and what aspects are going to be wildly different, and how can that help us investigate such ancient/extraterrestrial deposits?
We have added the following section:
- Astrobiological implications
On Earth, aging of silica sinter deposits is accompanied by dehydration, increase in density and loss of porosity [49]. Preservation of microbial fossils entombed within silica precipitates is favoured by host rocks resistant to chemical weathering, such as cherts. But, during increasing burial depth, cherts are subjected to progressively higher temperatures and pressures, which destroy primary biogenic signatures. Above 80 °C, organic matter begins to undergo thermal cracking (catagenesis), liberating the most volatile constituents and leaving behind only refractory components (kerogen) with modified molecular and isotopic composition. Under low-grade metamorphic conditions (ca. 150‒200 °C), microbial evidence becomes more difficult to interpret or is lost completely.
However, hydrated silica occurrences associated with sulfide precipitates have been identified in situ by rovers and orbiting spacecrafts on Mars, which have been interpreted as a result of sinter deposition from hot springs [6, 127-128]. Unlike on Earth, the amorphous hydrated opal form (opal-A) is not transformed to paracrystalline forms (Opal-C/CT) and fully crystalline anhydrous quartz, but Opal-A seemingly persists on Mars due to extreme aridity conditions [129].
As documented in this paper, sinter fabrics and textures are mainly controlled by microbial communities, influencing the preservation of microbially induced facies at macro-, meso- and microscales. Recognition of these biomediated sinter multiscales is key to understand the significance of hot spring deposits on Mars. The microbial diversity of sinter deposits varies greatly as a function of physical, chemical and biological factors that relate to the functional requirements of life in that environment and/or the specific organisms that first successfully colonize these substrates. As documented in this paper, gradients in both physical parameters, such as temperature, pH and sulfur content, control multi-scale variations in microbial diversity. Therefore, a strategy for sample collection on Mars seems necessary based on the heterogeneity of microbial fabrics and textures in their analogous silica sinters on Earth.
Farmer, J. D.; Des Marais, D.J. Exploring for a record of ancient Martian life. Journal of Geophysical Research 1999, 104, 26977–26995, doi: 10.1029/1998je000540.
Ruff, S.W.; Campbell, K.A.; Van Kranendonk, M.J.; Rice, M.S.; Farmer, J.D. The case for ancient hot springs in Gusev crater, Mars. Astrobiology 2020, 20, 475–499, doi: 10.1089/ast.2019.2044.
Frydenvang, J.; Gasda, P.J.; Hurowitz, J.A.; Grotzinger, J.P.; Wiens, R.C.; Newsom, H.E.; Edgett, K.S.; Watkins, J.; Bridges, J.C.; Maurice, S.; Fisk, M.R.; Johnson, J.R.; Rapin, W.; Stein, N.T.; Clegg, S.M.; Schwenzer, S.P.; Bedford, C.C.; Edwards, P.; Mangold, N.; Cousin, A.; Anderson, R.B.; Payré, V.; Vaniman, D.; Blake, D.F.; Lanza, N.L.; Gupta, S.; Van Beek, J.; Sautter, V.; Meslin, P.Y.; Rice, M.; Milliken, R.; Gellert, R.; Thompson, L.; Clark, B.C.; Sumner, D.Y.; Fraeman, A.A.; Kinch, K.M.; Madsen, M.B.; Mitrofanov, I.G.; Jun, I.; Calef, F.; Vasavada, A.R. Diagenetic silica enrichment and late-stage groundwater activity in Gale crater, Mars. Geophysical Research Letters 2017, 44, 4716–4724, doi: 10.1002/2017GL073323.
- The study seems to presume that silica is opal-A everywhere (and that's reasonable) but did the authors observe any evidence for the presence of other silica phases?
Not in the study material. This is explained in line 162: “Amorphous opal-A (SiO2·nH2O) is the dominant component of the sinter deposits in the Krýsuvík geothermal field”.
- Figs. 2, 5, 8 and 9. There is a thin horizontal white line across panels C and D.
We do not know the origin of these lines or the changes in letter font and size. Probably it is a problem related to file production. The final version does not show these problems.
- Fig. 4 panel b, the "mm" is quite hard to read.
Here and in other figures with colour pictures, the scale is labelled in the figure caption.
- Fig. 7, the lower left panel, are the numbers on the vertical axes counts? Weight percentages are mentioned in the caption (also lines 407-408) - but something like C/Si isn't usually expressed in wt.%, it ought to be dimensionless. If what is meant is wt.% C/wt.% Si, doesn't that bias things by the difference in atomic weight, and wouldn't it be better to express everything in atomic/molar percentages (which is after all what is being measured here). A few other things: How are those percentages estimated (and, what are the approximate detection limits)? Back at line 191, it is stated that "analytical results display an error between ± 5% to 7%." and here its mentioned that the maps for elements with <1 wt% concentration are blurry.
Yes, the vertical axes are counts, a term added now in the figure. The C/Si ratio is obviously dimensionless, as modified now in the text. The error of the analysis is stated in section Methodology so the blurry aspect refers to concentrations below this error (5%), as modified in the text.
- What is the section coated with? Is there any chance that epoxy or other lab materials have gotten into pore space artificially increasing the C concentration?
As explained in Section Material and methods, ESEM analyses were made on uncoated samples at room temperature. To avoid any misunderstanding, the term “uncoated” is repeated in line 414.
Thanks again for your revision.
Reviewer 2 Report
The authors have written a very compelling manuscript that I think would be a wonderful contribution to this journal.
Below I have noted slight formatting revisions and the inclusion of some necessary references that are missing from the text that acknowledge relevant prior work. After these changes are made, I believe this manuscript is ready for publication.
I congratulate the authors on their work and look forward to seeing more from them in the future.
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General Comments:
1. The References are not properly numbered, which made following them difficult. It starts from 1-4 then goes back to 1.
2. The font randomly changes throughout the manuscript (e.g., line 276).
3. For temperatures, sometimes you put a space between the number and “ºC”, other times not (both in text and in figures/legends). Make whichever you choose consistent.
Line Comments:
L40-43: “The discovery of surface hydrothermal silica sinters on Mars has increased interest in sinter-related biosignatures on Earth as a proxy to understand the origin of life itself.”
Not only the origin of life, also the general search for evidence of life on ancient Earth and Mars. Please acknowledge and reference these works here:
Gangidine, A.; Havig, J.R.; Fike, D.A.; Jones, C.; Hamilton, T.L.; Czaja, A.D. Trace Element Concentrations in Hydrothermal Silica Deposits as a Potential Biosignature. Astrobiology 2020, 20, 525–536. DOI: 10.1089/ast.2018.1994
Gangidine A, Walter MR, Havig JR, Jones C, Sturmer DM, Czaja AD. Trace Element Concentrations Associated with Mid-Paleozoic Microfossils as Biosignatures to Aid in the Search for Life. Life. 2021; 11(2):142. https://doi.org/10.3390/life11020142
Konhauser, K.O., Jones, B., Reysenbach, A.L., and Renaut, R.W. (2003) Hot spring sinters: keys to understanding Earth’s earliest life forms. Can J Earth Sci 40:1713–1724. https://doi.org/10.1139/e03-059
L85: “SEM” – define at first usage.
L154-160: For Figure 2, please put in-figure scale bars, even if approximate. It is difficult to determine what the scale is, especially when some of the panels are at very different scales. The text descriptions such as “left scarp is about 1.6 m high” and “shelf is about 3 m wide” are not sufficient for those who are not familiar with the terminology/geologic setting, and even if so it is not exactly clear what you are referring to by “left scarp” and “shelf”. If in-figure scale bars are not possible, at least put an arrow or other indicator to the structure you are referring to when giving scale references.
L164: Please reference:
Lynne, B.Y.; Campbell, K.A.; James, B.J.; Browne, P.R.; Moore, J. Tracking Crystallinity in Siliceous Hot-Spring Deposits. Am. J. Sci. 2007, 307, 612–641.
L258: Please define/cite what palisade texture is - a good reference for this is:
Jian Gong, Kimberly D. Myers, Carolina Munoz-Saez, Martin Homann, Joti Rouillard, Richard Wirth, Anja Schreiber, and Mark A. van Zuilen. Astrobiology. Apr 2020.500-524. http://doi.org/10.1089/ast.2019.2025
L262: “They and composed of” presumably you meant “They are composed of”?
L307: For Fig 3., it is difficult to see both “c” and “d” as well as the numbers over the scale bars. I would suggest placing c and d over a white circle like you did for other parts of the figure (like C1) and for the scale bars, either place a white box behind and make the text/bar black or leave as is but outline the text.
L333: For Fig 4., the scale bar numbers are difficult to read again, and are quite small. Please make larger and change color, outline, or add a background behind the text.
L394: For Fig. 6, difficult to read scale bar numbers again.
L411: For Fig 7, difficult to read scale bar numbers again.
L411: For Fig 7, please make panel indicators consistent with other figures. This figure uses capital letters to indicate panels – all other figures use lowercase letters in parentheses such as “(a)”.
L515-562: Somewhere in this section (section 6) or earlier in the manuscript please reference this citation for mechanisms responsible for fossilization processes in hot springs:
Cady, S.L.; Farmer, J.D. Fossilization Processes in Siliceous Thermal Springs: Trends in Preservation along Thermal Gradients. Ciba Found Symp. 1996, 202, 150–170.
L577: For Section 7, acknowledge and cite how your conclusions compare/differs from prior biomarker analyses from hot spring sinters:
Bronwyn L. Teece, Simon C. George, Tara Djokic, Kathleen A. Campbell, Steven W. Ruff, and Martin J. Van Kranendonk. Astrobiology. Apr 2020.537551. http://doi.org/10.1089/ast.2018.2018
Author Response
REVIEWER 2
- The References are not properly numbered, which made following them difficult. It starts from 1-4 then goes back to 1.
Yes, this is another problem related to files conversion. It has been checked and updated.
- The font randomly changes throughout the manuscript (e.g., line 276).
As explained above, this change did not occur in the original files. All this has been checked in the complete file received from the editorial.
- For temperatures, sometimes you put a space between the number and “ºC”, other times not (both in text and in figures/legends). Make whichever you choose consistent.
Thanks, this has been homogenised.
L40-43: “The discovery of surface hydrothermal silica sinters on Mars has increased interest in sinter-related biosignatures on Earth as a proxy to understand the origin of life itself.”
Not only the origin of life, also the general search for evidence of life on ancient Earth and Mars. Please acknowledge and reference these works here:
Gangidine, A.; Havig, J.R.; Fike, D.A.; Jones, C.; Hamilton, T.L.; Czaja, A.D. Trace Element Concentrations in Hydrothermal Silica Deposits as a Potential Biosignature. Astrobiology 2020, 20, 525–536. DOI: 10.1089/ast.2018.1994
Gangidine A, Walter MR, Havig JR, Jones C, Sturmer DM, Czaja AD. Trace Element Concentrations Associated with Mid-Paleozoic Microfossils as Biosignatures to Aid in the Search for Life. Life. 2021; 11(2):142. https://doi.org/10.3390/life11020142
Konhauser, K.O., Jones, B., Reysenbach, A.L., and Renaut, R.W. (2003) Hot spring sinters: keys to understanding Earth’s earliest life forms. Can J Earth Sci 40:1713–1724. https://doi.org/10.1139/e03-059
Thanks, both the new idea and the references are added in the same sentence.
L85: “SEM” – define at first usage.
Done.
L154-160: For Figure 2, please put in-figure scale bars, even if approximate. It is difficult to determine what the scale is, especially when some of the panels are at very different scales. The text descriptions such as “left scarp is about 1.6 m high” and “shelf is about 3 m wide” are not sufficient for those who are not familiar with the terminology/geologic setting, and even if so it is not exactly clear what you are referring to by “left scarp” and “shelf”. If in-figure scale bars are not possible, at least put an arrow or other indicator to the structure you are referring to when giving scale references.
All the scales are referred to in the caption.
L164: Please reference:
Lynne, B.Y.; Campbell, K.A.; James, B.J.; Browne, P.R.; Moore, J. Tracking Crystallinity in Siliceous Hot-Spring Deposits. Am. J. Sci. 2007, 307, 612–641.
Added.
L258: Please define/cite what palisade texture is - a good reference for this is:
Jian Gong, Kimberly D. Myers, Carolina Munoz-Saez, Martin Homann, Joti Rouillard, Richard Wirth, Anja Schreiber, and Mark A. van Zuilen. Astrobiology. Apr 2020.500-524.
Reference added.
L262: “They and composed of” presumably you meant “They are composed of”?
Yes, there was a typing mistake. Modified.
L307, 333, 394, 411: The scales of all the figures with colour pictures are now labelled in the caption.
L515-562: Somewhere in this section (section 6) or earlier in the manuscript please reference this citation for mechanisms responsible for fossilization processes in hot springs:
Cady, S.L.; Farmer, J.D. Fossilization Processes in Siliceous Thermal Springs: Trends in Preservation along Thermal Gradients. Ciba Found Symp. 1996, 202, 150–170.
Done.
L577: For Section 7, acknowledge and cite how your conclusions compare/differs from prior biomarker analyses from hot spring sinters:
Bronwyn L. Teece, Simon C. George, Tara Djokic, Kathleen A. Campbell, Steven W. Ruff, and Martin J. Van Kranendonk. Astrobiology. Apr 2020.537551. http://doi.org/10.1089/ast.2018.2018
We have added a short discussion because the biomarkers analysed in the paper mentioned by the reviewer are restricted to the apolar fraction, while we analysed the whole total lipid extracts. The new text reads “Their scarcity contrasts with their relative abundance yielded by biomarker analysis in El Tatio (Chile) and the Taupo Volcanic Zone (New Zealand) sinters [119], which are focused on the apolar fraction (comprising about 1‒2% of all biomarkers) with high C17 alkane peaks, whereas in our analysis the alkanes were dominated by plant-derived (C27‒C33) ‘contaminants’”.
Thanks for your revision.
