Dear Dr DeCarlo and co-authors,

Please be invited to respond to the two reviews that have been uploaded. I am looking forward to see an updated version of your manuscript and your reply to the reviewers.

Sincerely,

Lennart de Nooijer

Dear Dr. DeCarlo and co-authors,

Thank you for uploading the revised version of your manuscript. In general, you have uploaded a considerably revised manuscript in which most issues raised by the reviewers have been addressed. Some minor issues remain that need to be corrected before this manuscript can be accepted for publication in Biogeosciences. In addition, I would like to ask you to have a critical look at your title: perhaps is does not (100%) reflect the content anymore.

Sincerely,

Lennart de Nooijer

Page 18, line 16-18: “Finally, our analysis indicates that the Raman ν1 FWHM is an accurate proxy of fluid ΩAr, making it a complementary approach to B/Ca and δ11B because combining information from the two approaches allows calculating the full carbonate system via [CO32−], pH and now importantly [Ca2+] .”. The authors should be careful when making these kind of statements. The results from the inorganic precipitation experiments do not justify this interpretation promoted by the authors. In my opinion, it is currently a potential proxy for fluid ΩAr in inorganic aragonite, which can be used to develop a way to indirectly measure ΩAr of the calcification fluid of aragonite producing organisms, like corals.
Fig. 4. The authors seem to neglect the distinct offset between their observations and the ones made by Alkhatib and Eisenhauer (2017, GCA). How can you explain this offset?

Minor comments:
Please make change solid Mg/Ca to Mg/Casolid or Mg/Caaragonite to avoid confusion with media/seawater Mg/Ca.
Throughout the manuscript: pCO2 instead of pCO2. (i.e. please italicize the “p”). Also in the references please check pCO2 and CO2 (example page 25, lines 16 and 24).

Dear Thomas and others,

I think your manuscript is ready for publication. You have dealt adequately with the reviews and I would like to thank you for submitting your work to Biogeosciences.

All the best,

Lennart