| 12 Feb 2026
Planktonic foraminifera Iodine/Calcium ratio: is it a proxy for dissolved oxygen in the ocean?
Abstract. Direct observations indicate a declining trend in ocean oxygen concentrations, which is not quantitatively captured by models. The complexity of oxygenation variability, linked to both physical and biochemical parameters, must be investigated across different climate contexts. Foraminiferal iodine-to-calcium (I/Ca) ratios have emerged as a proxy for subsurface oxygen concentrations although its capacity for quantitative reconstructions remains to be elucidated. We provide a new database, including the first results from the Mediterranean Sea and new samples in the Arabian Sea together with the parameters of biochemistry (nutrient concentration, pH, chlorophyll, oxygen, net primary productivity), physical and geographical (temperature, salinity, latitude, distance from the coast, water depths) and diagenesis potential (depth in core, sediment age) to better understand the proxy behaviour. Considering the strong spatiotemporal variability in dissolved oxygen in subsurface ocean, we propose to use statistically robust 25th percentile of oxygen concentration (p25 [O2]) instead of minimum concentration in the upper 500 m in the water column. Our results affirm that oxygen concentration is the primary driver of foraminiferal I/Ca and we propose a new calibration equation of foraminiferal I/Ca against p25 [O2]. Based on a new database, we identify a complex relationship between p25 [O2] and iodine speciation, which is one of the main sources of scatter. A comparison with synthetic calcite reveals that planktonic foraminiferal tests can incorporate either more or less iodate at high p25 [O2] than abiotic calcite, probably due to “vital effects”. The Mediterranean Sea samples present a wide range from low to high I/Ca (0.6 to 6.9 µmol/mol) in well-oxygenated water, which cannot be explained solely by authigenic calcite precipitation. Our results highlight the complex behaviour of the I/Ca proxy, while reinforcing its semi-quantitative reconstruction in palaeoceanography.
This is a much-needed examination of the controls on the foraminiferal I/Ca proxy. The new data are of high quality and so is the global I/Ca compilation and associated statistical analyses.
A few important considerations before publication:
1. Since 2026, 2 papers were published (Wang et al., 2026 and He et al., 2026) that are highly relevant to this topic. These papers should be integrated into the discussion and analyses of the paper at hand. In particular, this includes the impacts of oxycline depth and latitude on I/Ca.
2. The authors need to clarify under what circumstances they suggest the proxy is quantitative, if at all. My general reading of the authors text as well as my own impression from this paper and others in the field is that there are large limitations on the degree to which I/Ca can be used predict O2, but I don't think those limitations are clearly outlined. For example, several other papers identified I/Ca ranges linked to O2 ranges, but not explicit O2 values. It should be explained how this approach does or does not differ from these past more conservative approaches and provide a more prescriptive breakdown of when, where, and why I/Ca can or can’t be used quantitatively for O2, if at all. For example, Line 242: “The objective of this representation is not to find the best fit, but rather to determine whether the relationship between iodate proportion and dissolved oxygen content could explain the variation in foraminiferal I/Ca-based oxygen reconstruction.” Here the authors express that the I/Ca vs O2 equations are intended solely to explore potential relationships. However, I feel that this apparent intention is lost in the Discussion since it is not mentioned again although multiple equations are presented. I find this confusing and fear that readers will over extrapolate such equations to define O2 from I/Ca. Regardless of whether I correctly understand the authors intentions, clarifications should be presented appropriately in the discussion as a caution to readers for when and when not to apply I/Ca as quantitative for O2 reconstructions.
3. How would mixing impact the I/Ca proxy relationship with local O2? The IO3 vs O2 relationship has a major mixing component and so I/Ca is expected to as well in addition to vital effects, diagenesis, etc. See Cheng et al., 2024, which demonstrates the challenges of modeling iodate in the ocean with reduction and oxidation dependent on solely O2. See also the Wang et al., 2026 paper referenced above which shows mixing signals and oxycline depth. Mixing is not discussed as an impact on iodate in ODZs. See Moriyasu et al., 2020; Hardisty et al., 2021; Evans et al., 2020, which together make a case for mixing impacts on iodate in the Tropical North Pacific ODZ. This is also summarized in Cheng et al., 2024. This is because iodide oxidizes slowly, so low iodate signals can be mixed and integrated over large areas of ODZs without a clear local O2 dependency.
https://doi.org/10.5194/bg-21-4927-2024
https://doi.org/10.1016/j.epsl.2020.116676
https://doi.org/10.1029/2019GB006302
https://doi.org/10.1002/lno.11412
Detailed comments:
Figure S2: Wang et al., 2026 was published recently. It demonstrates a relationship between iodate and oxycline depth. Is the same relationship observed in the data here? I recognize that the Wang 2026 paper was published after the manuscript in hand was submitted, but that paper is highly relevant to this one, so should be incorporated into the discussion. https://doi.org/10.1016/j.gloplacha.2026.105412
Figure S2: He et al., 2026 also demonstrate a relationship between I/Ca and latitude that mimics the same relationship seen for IO3 and latitude. Does the dataset assembled here also reveal that pattern? https://doi.org/10.1038/s41561-025-01896-w
Line 351: “This mismatch between oxygen and iodate concentration may at least partly explain the apparent presence of iodate in waters here defined as oxygen-depleted.” Is it not the other way around: i.e., that the presence of iodate in low O2 waters explains the mismatch between O2 and IO3?
Figure 4: Why are data from samples younger than 8kyr used here? Elsewhere 2kyr is used. It seems difficult to expect a comparison between I/Ca from samples from last 8kyr and contemporary water column O2.
Figure 1: Do the color of the symbols on the map in part A marking locations correspond to I/Ca legend? Their colors overlap but the values all seem very low and thus inconsistent with the subsequent graphs of I/Ca. I’m wondering if the maximum values on the color scale are too large thus giving the impression of uniformly low I/Ca? If this is the case, adjust the color scale range so the spread in I/Ca is more clearly visualized.
Line 395 and Figure 6: Zhou et al., 2014 also demonstrated a temperature dependency between I/Ca and iodate in medium (see their supplement). It looks like this was utilized but a uniform temp of 19 deg was used, though the color bar shows a large range of temperatures. It would be helpful to use the iodate concentrations and temperature from each site at the depth of p25(O2) to plot the synthetic calcite predicted I/Ca value using the temperature dependent I/Ca equations from Zhou et al., 2014 and plot these against the actual measured I/Ca values. This could be done for each site and demonstrate the relationships, if any. This could aide in interpreting deviations from “ideal”.
Line 408: The highly elevated values of I/Ca >6 µmol/mol are not necessarily specific to foraminiferal calcite. It is noteworthy that similarly elevated values are observed for core-top aragonite (see Hardisty et al., 2017 data from Great Bahamas Bank), which may reflect partition coefficient differences for aragonite or other environmental influences. Also see Lu et al., 2022 for a similar discussion from benthic forams.
https://doi.org/10.1016/j.gca.2022.06.001 and https://doi.org/10.1016/j.epsl.2017.01.032
Figures: If the authors intend to present I/Ca as quantitative for O2 under any circumstance, it would be useful to make another map like Figure 1A showing marine O2 (showing p25O2 in this case) but with the fill of the circles marking sites shaded with the predicted O2 based on the I/Ca measured at this site. This would provide a global visualization not apparent in the other figures for which regions follow the predicted trend line and which do not.