Successive Modes of Carbonate Precipitation in Microbialites along the Hydrothermal Spring of La Salsa in Laguna Pastos Grandes (Bolivian Altiplano)

Round 1

Reviewer 1 Report

In the article presented by Muller et al. the authors use in-situ observations, chemical determinations and an empirical model to provide insight into spatial and chemical successive modes of carbonate precipitation in organo-sedimentary structures that change along a hydrothermal system.

The study site is very interesting and has been explored in previous reports, which include some of the coauthors. Here, the authors use some of this published information (e.g., mineral composition) to develop hypotheses about carbonates formation. But often, particularly in the discussion section it is hard to separate the new from previous information because references to previous data are not included. 

Although the article well written in general, it is way too long and it is plagued of partly-sustained hypotheses and conclusions. In multiple occasions the authors describe cause-effect assumptions instead of spatial or chemical relationships. As a consequence, the reader gets confused in multiple ways, for example about the data acquired for this study and the data used from other studies, or the assumption of the results of the model as estimations of biological processes (see comments about Results).

Also, the research gap is not clearly described in the introduction section, instead the long introduction (more than a 1000 words) is a review of literature carbonates formation in of organo-sedimentary systems. The authors should consider to replace for example paragraphs form line 48 to 79 and paragraph from line 98 to 114  with information that allown the reader to better understand the relevance of pCO2, DIC, DIC isotopes and geochemical parameters to understand carbonate formation, and overall to frame the geochemical development they propose.

I strongly recommend to re-write the introduction towards guiding the reader to understand the particularities and relevance of the model and the research gap. I suggest to shorten the discussion section as well. For this, I suggest the authors limit the discussion to the main findings of the study and their implications, also to constrain their hypotheses, some of them, related to biological interactions that were not determined, but mainly inferred (see comments on Discussion).

An important concern is the calculation of evaporation; this is a process deeply involved in different parts of the model results and discussion. I recommend to include parameters such as temperature and Cl interactions as additional factors to calculate evaporation. In the present model Cl concentration is the only parameter used to calculate evaporation, as a result evaporation is reported as zero in proximity to the thermal spring. This is hard to believe in a high-altitude system where water temperature is around 47 celsius and air temperature is also high.

Methods

The lack of clarification of some steps of the methods section makes the reading of results and conclusion difficult. Particularly because some of the key aspects of the model are not comprehensibly developed/explained.

Please add, probably in Supplementary information, the parameters and assumptions used in the PHREEQC 3 software so your analysis can be somewhat reproducible.  

Can you briefly elaborate on the system-specific processes involved in CO2 degassing?

Please clarify if the wet season has influence in your model.

Evaporation is a major participant of your model and results, however important concerns emerge on its calculation, where Cl concentration seems the only parameter used for this calculation. It is hard to understand that evaporation equals zero in this high-altitude  hot spring, where both water temperature and air temperature are high.

Do you think Cl pattern is affected bu Cl.Na or Cl-K interactions? 

Please incorporate this interactions in the model and/or discuss Cl interactions in your system and brines.

Do you think integration of temperature affects the Relative proportions model?

What is the depth for physicochemical measurements from water samples?

Results

Please remove Photosynthesis from Fig. 3 or make clear this is hypothetical. Is carbonate dissolution marked as a hypothesis or as a result?

Please consider to revise, explain, justify or rephrase that you detected the following with CSLM, SEM microscopy:

relics of pigments.

silicified cyanobacteria,

Calcite and Fe-As oxides

Discussion

Discussion is way too long and somewhat repetitive. Please shorten substantially.

I understand the vision of the model but please consider to revise/rephrase that you "found three modes of CaCo3 precipitation"...this is contrasting with the large explanation of the organo-sedimentary diversity you presented in the Geological setting section.The authors should reconsider if the field work and microscopy efforts are enough to conclude this. 

The authors should consider to remove/revise/rephrase baseless statements, for example those in:

line 919 "...cyanobacterial calcification starts to contribute to DIC balance...

line 809-811.

line 880 "photosynthesis is high in these ponds"..."control at least partly the DIC balance"

line 822-824

line 824-825

Paragraph 827-833

line 857-861

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Execellent paper! Diverse methods clearly described and absolutely necessary for this type of investigation. This investigation is a milestone in the understanding of hydrothermal and hypersaline environments. The combination between microscopic analyses, water and isotope chemistry is exemplary. The only weakness of the paper, that the cited papers are not really state of the art. I would recommend to discuss and incorporate the papers published by the Goettingen group about the hypersaline lakes of Kritimati (Arp, Ionescu, Schneider, etc.) and also the newly published paper of Della Porta et al about the hydrothermal travertins in Italy.

I´ll recommed to publish this paper after minor revision

Author Response

Please see the attachment.

Author Response File: Author Response.docx