Dear Michael and co-authors,

please be invited to reply to the second anonymous reviewer and upload an updated version of your manuscript.

Sincerely,

Lennart de Nooijer

Additional comments Henehan et al., bg-2016-459

Dear Michael and co-authors,

Thanks for uploading an updated version of your manuscript. I think you handled the comments of the two reviewers adequately. In addition, I have some minor, and one more substantial issue that may help to improve your manuscript. I am looking forward to see the final version of your paper.

All the best,

Lennart

Introduction
page 2, lines 24-27: I don’t understand what ‘conditions during life’ means here. Don’t these conditions include inorganic carbon chemistry at the sea surface (lines 24-25)? Or do you mean that ‘conditions during life’ within a foraminifer’s lifetime are assumed to remain constant?
page 3, line 11: ‘Be’ should be ‘Bé’. Please also check other references to Bé in the rest of the manuscript.
page 3, first paragraph: perhaps it is useful to include somewhere in the introduction the notion that SNW is often thought of/ assumed to reflect the ‘average thickness of the chamber walls’. I would say that this may indeed be largely true, but in-/ decreased porosity (Bé et al., 1976. Progress in Micropaleontology 1-9) and differences in densities (i.e. shell walls may not be completely ‘filled’ with CaCO3) may also determine the SNW.
Methods
page 3, line30: shouldn’t this be ‘µmol m-2 s-1’?
page 3, line 32: ‘petri’ should be capitalized.
page 4, line 17: include ‘mol/mol’ after ‘6.25’.
page 5, line 10: the ‘area’ reflects the size of the newly formed chambers. It should be noted here that the CI is therefore a species-specific measure (as is done in the appendix). When comparing CI’s between species, morphology of the chambers should be taken into account (particularly when comparing spherical to more flattened chambers).
page 5, line 24: Reiss (1957. Contribution from the Cushman Foundation for Foraminiferal Research 8, 127-145) is probably the first to have described the added layer of calcite on top of previously formed chambers. Moreover, this only applies to rotallid foraminifera (line 22), rather than foraminifera in general.
page 5, line 25: ‘thickness’ applies to the chamber wall, not to the chamber as such.
page 5, lines 27-28: does the thickening of the chamber walls in this case result in the formation of a crust?
page 7, line 7: ‘more times’ should be ‘with one more layer’ for this particular example.
Results
page 8, lines 32-33: sounds like a contradictory to me.
page 8, line 34: effect of temperature on CI?
page 9, line 6: this is an important point, that may be stressed more in the discussion. A recently proposed model for (rotalid) foraminiferal calcification suggests that foraminifera are able to convert any DIC into [CO32-] at the site of calcification (Toyofuku et al., 2017. Nature Communications 8: 14145). This would hint to a primary control of [DIC] rather than ambient [CO32-], saturation state or seawater pH.
Discussion
page 9, lines 25-26. This is not true. There are a number of papers providing an explanation for the contrasting responses of marine calcifyers to ocean acidification by Justin Ries. Foremost, these include Ries et al., 2009 (Geology 37: 1131-1134) and Ries, 2011 (GCA 75: 4053-4064). Please include this in the discussion.
page 10, lines 17-32. I am not sure that I follow this completely. The distinction between smaller specimens that do not have an internal pool and larger foraminifera that require storage of DIC prior to chamber formation does make little sense to me. The references for small and larger specimens (Nehrke et al., 2013 versus Erez/ Ter Kuile) coincides with large inter-species differences (in morphology as well as carbonate chemistry). Without implying to impose my own favoured model for calcification (see e.g. Toyofuku et al., 2017), the basic mechanism in which protons are exchanged for calcium during chamber formation is suggested for (benthic) species across a large range of sizes (Glas et al., 2012. Journal of Experimental Marine Biology and Ecology 424-425: 53-58). It may be, however speculative, that the steepness of the resulting external pH gradient (Glas et al., 2012; Toyofuku et al., 2017) determines the uptake rate of DIC. This steepness may well be a function of size and hence relates CI to volume/surface ratios.
Depending on the authors’ preference, I suggest to re-word this paragraph somewhat. Without making a distinction between ‘pools’ versus ‘no-pools’, the mechanism leading to the uptake of DIC from the surrounding seawater likely depends on the amount of ions necessary for the production of one new chamber (which is a function of the specimen’s volume) and the surface area across which these ions have to be transported. These two units change with foraminiferal life-time and may hence result in a more or less cost-effective uptake of DIC (and Ca2+).
page 11, line 10: it may be informative to include Flako-Zaritsky et al. (2011. Marine Micropaleontology 80: 74-88), who show that a number of benthic species are capable of producing their calcite despite very low pH and consequently, undersaturated conditions.
page 13, line 11: there is a comma too much.
Figures and figure captions
figure 1: include panels (d) and (g) to the description for panel (a). Similar for (e)/(h) and (f)/(i)
figure 3: what happened to the uncertainties in the seawater Mg/Ca (panel a)?
figure 5: related to re-phrasing of the third paragraph of section 4.1, the authors may have to change the text within this figure.
Tables
table 3: CI’s should have a unit (µg/mm2).
Appendix
page 30, line 6: ‘our’
Figure A2: I don’t understand how in model 2, b can vary by 10%, but never exceeds beyond ~0.1075 or drops below ~0.0975 instead of 0.110 and 0.900.

Dear Michael and co-authors,

I have once more read your manuscript and the response to my suggestions. I am looking forward to see your paper published as is in Biogeosciences.

Sincerely,

Lennart de Nooijer