the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Mar 2025
Quantitative reconstruction of deglacial bottom-water nitrate in marginal Pacific seas using the pore density of denitrifying benthic foraminifera
Abstract. Quantifying past ocean nitrate concentrations is crucial for understanding the global nitrogen cycle. Here, we reconstruct deglacial bottom-water nitrate concentrations ([NO3-]BW) reconstruction in the oxygen-deficient zones of the Sea of Okhotsk, the Gulf of California, the Mexican Margin, and the Gulf of Guayaquil. Using the pore density of denitrifying benthic foraminifera as a nitrate proxy, differences in [NO3-]BW are observed at the study sites spanning the Last Glacial Maximum to the Holocene. Changes in water-column denitrification, water-mass ventilation, primary productivity, and sea surface temperatures may account for nitrate differences at the study sites. The [NO3-]BW in the Sea of Okhotsk, the Gulf of California, and the Gulf of Guayaquil are influenced by the intermediate water masses while, the [NO3-]BW at the Mexican Margin is likely influenced by deglacial changes in the Pacific Deep Water. The comparison of past and present [NO3-] shows that the modern Gulf of Guayaquil and the Gulf of California currently have stronger oxygen-deficient zones with higher denitrification than during the Last Glacial Maximum. In contrast, the modern Mexican Margin and the Sea of Okhotsk may have higher oxygen, indicated by low modern denitrification than during the Last Glacial Maximum.
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Status: open (until 29 May 2025)
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RC1: 'Comment on egusphere-2025-1182', Anonymous Referee #1, 22 Apr 2025
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Thank you for giving me the opportunity to review this manuscript, entitled "Quantitative reconstruction of deglacial bottom-water nitrate in marginal Pacific seas using the pore density of denitrifying benthic foraminifera". The paper is well written and very pleasant to read, but I would suggest shortening it as it is very long and we tend to get a bit lost in the results and discussion sections. Overall, the manuscript is of significant interest in this field of research, even If I usually question the quality of the reconstruction, either O2 or nitrate, based on benthic foraminifera porosity. I have developed all my concerns below, overall I encourage publication after minor revisions.
-L88: So what you consider suboxic here is everything below 5 µmol/kg? Including microxic, anoxic, dysoxic? (Following Kranner et al., 2022; Hoogakker et al., 2025)
-L93: Do not start a sentence with an acronym
-L113: Maybe avoid repetition here: "The D15N records from the bulk sediment..." Plus, how about "interlinked" or something similar suggesting dependance, rather than "various"?
-L123: "either planktic or benthic" sounds a bit strange
-L124: less than what?
-L138: "Some species, for example Bolivina spissa, which are..."
-L149-152: Be cautious here, Bolivinidae contains a lot of bolivina species, on how many species has this hypothesis been tested?
-L152-154: Reference for this?
-L156: Bolivina in italics
-Figure 1: not sure I fully understand panel B: if the red cross is for B. subadvena, does it mean all the other symbol are for B. spissa? What location is B. subadvena then? Not sure about mixing species and location here.
-Figure 2: not very colorblind friendly, please use a gradient with only one color or two colorblind friendly colors;
-Also, why is the chosen depth 700m? why not an average of your core depth for example?
-Figure 3: I recommend using this color scale for Fig. 2.
-Also, we are still in the introduction on page 8, and there are already 3 figures. It might be nice to try to shorten the manuscript a bit.
-L201 to 214: please check between RV and R/V
L215-220: to bit hard to read and to follow. I would just write "A total of 1541 specimens were investigated (1268 B. spissa and 273 B. subadvena) across X samples from location A, B, C..." without detailing too much. You can also put these information in a supplementary table for example., but I'm not sure all the information given in this paragraph are very important in the end.
-L229: Do not detail everything you did not do.
-L230: I am quite surprised that you worked with the oldest chambers, as porosity investigated is usually performed on the latest chambers (penultimate or antepenultimate) to avoid too much pore calcification, infilling, pore plate, which are quite common in Bolivina species, can you justify this choice?
-L233: can you develop what are pore density, porosity, and so on?
-L234: previously trained deep learning algorithm: reference for that?
-L235: this sentence is not necessary
-L288: Overall in paragraph 2.4, why are you sometimes using Intcal20, sometimes Marine20?
-Table 1: MD01-2415: It's a zero, not an "O"
-L344: We are already page 14, and only at the results section, please try to be more concise and shorten the manuscript, even if I have to admit it is very pleasant to read.
-Figure 4: I suggest to remove the big arrow on the left. By looking at this figure, I would say that the link between NO3- and D15N is not clear, or that the link between porosity and Nitrate is not clear. I tried to develop method for reconstructing O2 based on porosity and my experience led me to think that this is very tricky indeed as we try to retrieve information from the response of a single species, or here 2, that usually live in a restricted range of the parameters we are trying to investigate. Have you tried to compare the signal you extract from the porosity to existing O2 reconstruction for these core site?
In my experience, porosity reconstruction or using it as a transfer function also show a large margin of error, so I'm always very dubitative when I see new transfer function based on foraminiferal porosity.
I think this is important as you suggest later on that O2 can also play a role, and I think it actually is. Finally, maybe put the letters a, b, c, and d on the lower part of each graph?
-Legend of Fig. 4: maybe put the dashed line in black? As red is the color you used for D15N?
-L432 to 436: maybe you don't need to explain all of that?
-L438: Yes, but at the same time, all your records don't have modern samples, so don't shoot you in the foot like this, unless you can argue that the Holocene shows not variability.
-Figure 5: Here would be a good place to add a comparison with O2, to try to investigate which part of your signal is actually dependent on oxygenation. Now, several reconstructions exist for these areas; If not I think this is redundant with Fig. 4 as you have no new information here, so I would suggest removing it to try to shorten the manuscript
-L450: what is a SEM?
-L453 to L456: is it necessary to develop that?
-L493: again here, try to add a comparison with O2 reconstructions?
-L522-524: Not sure I get it there: If the argument is using B. species for nitrate reconstruction because they rely on denitrification and don't need O2, why would they be dependent on O2 and redox conditions here? If they still do depend on O2, then this relationship should be discussed here if O2 can account for part of the porosity investigated here.
Overall in the rest of the manuscript, I feel like we have these porosity reconstructions, that are supposedly related to nitrate concentration here, but I won't question that further, and then lots of supposition around these reconstructions during the whole discussion. I feel like you are not bringing argument to discuss or defend a hypothesis, but rather throwing a lots of questions around that.
-L667-668: again, at least a few O2 reconstructions exists for these periods of time and location, so you can use them instead of wondering if the O2 level was different.
Citation: https://doi.org/10.5194/egusphere-2025-1182-RC1
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