Dear dr. Vuillemin,

Thank you for your replies to the comments of the reviewers. All three reviewers provided thorough reviews with constructive comments. Based on these comments, my assessment is that a major revision is required. I see from your replies that you have already modified the text and figures and I hereby kindly ask you to upload your revised manuscript and your detailed reply letter with all changes on the (internal) BG website. I will then be able to move forward to the next phase.

best regards,
Caroline Slomp

Dear dr. Vuillemin,

Thank you for submitting your revised manuscript. Based on the revisions and your replies to the comments of the reviewers, I recommend minor revisions. These are detailed below. Please note that these changes are primarily meant to improve the clarity of your manuscript. If you disagree with the suggestions, please provide a motivated reply.

best regards,
Caroline Slomp

Detailed comments:

Reviewer #1 - Comment 2: The method used to determine the pH is not commonly used for sediments, and is unlikely to provide accurate insight in the in-situ pH, as indicated by the reviewer. Typically, pH in sediment porewaters is either determined with micro-electrodes (e.g. Reimers et al., 1996; GCA) or by inserting a pH probe in the porewater sample. To address the reviewer’s comment fully a comment providing this context could be added e.g.:

“We note, however, that direct measurements of pH in the sediment porewater with, for example, microsensors would provide a better indication of the in-situ pH (e.g. Reimers et al. 1996). “

Reviewer #2
Comment 1: I’m not convinced the change in title is supported by your manuscript. Of course practically all redox reactions in sediments are microbially-driven. However, no specific evidence for a key role of a specific type of microbes in inducing vivianite formation is presented but this current title could be interpreted as such. This title also could be misunderstood to indicate that you find evidence that the vivianite is formed in microbial cells (which we know is possible). I would suggest to keep the original title.

Other comments on the revised text:
P1. Line 23: “represent specific cases” could be replaced by “are characterized by a specific type of”
P1. Line 35-36. Text could be changed to “Together with the porewater profiles, these data suggest….”
P2. Line 1, P16, line 23 and P17, line 3. Can you clarify what “pore water electron acceptors” refers to, i.e. can you be more specific with respect to the electron acceptors you are referring to? How do you know the electron acceptors are solutes and not solids? Alternatively, you could keep the text more general and refer to the progression of the typical sequence of electron acceptors during diagenesis.
P2. Line 2-3. It is not clear how the decrease in microbial activity and P recycling are linked here. Could you be more specific? Alternatively, this text (starting at “as the likely…”) could be removed.
P12. Why is it relevant to mention “because Ni is also abundant”? What data does that refer to? I would suggest to omit this from the sentence.
P13. It is not clear what is meant by “drawdown PO43- cycling by microbes”
P13. Line 23-24. What about parallel precipitation of siderite and vivianite?
P13, starting with line 14: why are you referring to Fe3+ in the porewater whereas your measurements show that Fe3+ is absent from in the porewater (see section 2.3).
P13. Line 30. “Such a relationship…”
P15. Line 13. What is meant exactly by “Fe pathways”? Can you be more specific?
P16. Line 4: “are highly sorbent for” could be replaced by “that can react with porewater”
P16. Line 12. It is not clear what is meant by “in terms of ancient rock record”. I would suggest to rephrase this.
P16. Line 13. “Concentrations estimates” could be changed to “Estimates of concentrations of P in deep anoxic waters, as deduced from…”

Dear dr. Vuillemin,

Thank you for your response to the comments (which were all from the editor, not from the reviewers).

I kindly ask you to consider making three more changes:

(1) In addressing the comment regarding the pH, you added the following text: “We acknowledge that measuring the pH directly in the pore water using microsensors would provide more accurate in situ results (Reimers et al., 1996).” This suggests that the method that you used to measure pH is an accurate method for in situ pH values in sediments and that the microsensors are simply more accurate. You provide no citation for this and I doubt that this is correct (in line with the reviewer’s comment). Hence, I would propose to use the formulation that I suggested previously: “We note, however, that direct measurements of pH in the sediment pore water with, for example, microsensors would provide a better indication of the in-situ pH (e.g. Reimers et al. 1996). “

(2) Line 23, page 13: porewater Fe3+ should be porewater Fe2+

(3) Line 26, page 13: In addressing the comment regarding “drawdown PO4
3- cycling by microbes” the following text was added: “decrease the rates of PO4
- uptake by microbes”. It is still not clear what this refers to exactly. I would suggest to either clarify or remove.

best regards,
Caroline Slomp

Dear Dr. Vuillemin,

Thank you for making these final changes. I look forward to seeing your manuscript published.

best regards,
Caroline Slomp