| 03 Feb 2026
Biogeochemical Dichotomy and Intra-Order Variability in Miliolid and Rotaliid Foraminifera
Abstract. Foraminiferal geochemical records reflect both environmental and biological influences. Disentangling these factors is essential for improving their application in marine monitoring and contributing valuable insights into the evolution across major foraminiferal lineages. Calcifying foraminifera evolved independently, with miliolids and rotaliids represent the most widespread and ecologically dominant calcifying foraminiferal groups. Most geochemical studies to date have focused on rotaliids, despite the importance of miliolids in ecological and environmental roles as prolific calcifiers. This study leverages the unique southeastern Mediterranean Israeli coastal waters, where dominant representatives of both groups co-occur in the same habitats, allowing for a direct comparison of bioincorporation differences, known as the vital effect. This setting also allowed for within-group variability and the identification of biological and environmental elemental signatures characteristic of specific taxa. Elemental incorporations in tests of six co-occurring taxa were analyzed: three rotaliids and three miliolids, from an oligotrophic Mediterranean marine reserve using whole-test ICP-MS analyses. Results reveal a clear geochemical dichotomy, with miliolids exhibiting consistently higher element/Ca ratios than rotaliids for nearly all measured elements, except Li, which shows the opposite trend. The contrast is strongest for rare earth elements (REEs) with order of magnitude differences (up to 45 times), and moderate but systematic for other elements (e.g., Zn, Cd, Fe). This dichotomy likely reflects fundamental differences in biomineralization pathways between the two orders. Within each order, element/Ca ratios show distinct patterns: in some taxa, variability appears to be biologically controlled through biomineralization processes, while in others it seems environmentally driven, reflecting the chemical composition of the surrounding water.
Dear editor,
I have carefully read the manuscript by Hoober and co-workers on the El/Ca of several rotaliid and miliolid foraminifera (egusphere-2026-38). Overall, I am very enthusiastic about the presented dataset! Adding ‘unusual’ elements and overlooked species to the global dataset is indeed the only way to come to an integrated understanding of foraminiferal biomineralization. It is also necessary to improve the applicability (e.g. development of new proxies) of foraminiferal El/Ca downcore. I do recommend publication of this manuscript in EGUsphere, but only after major revisions. The important issues I have with this work are listed below and concern 1) a lack of ICP-MS- and statistics-related details in the methods, 2) representation of the main results and 3) a more elaborate discussion of the main findings. In addition, minor corrections are provided through the annotated pdf.
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Sincerely,
Lennart de Nooijer
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Methods
I see that cleaning and dissolution of the foraminiferal shells did not lead to samples with a similar Ca concentration. This is, however, important to minimize matrix-effects. Also, the names of the standards need to be included; I suspect the second ‘standard’, the homogenized shells of A. lobifera, does not count as a standard (see e.g. Boer et al., 2022). Geostandards and Geoanalytical Research 46(3): 411-432. DOI: 10.1111/ggr.12425. Finally, basic metrics of the measurements should be provided: e.g. accuracies or LODs, as well as the masses scanned (e.g. which isotope(s) of Mg were measured?) and any procedures to account for interferences.
Vital statistical information is missing. A PCA is presented, but not mentioned in the method section. What software was used to do the PCA and how was the data treated (normalized, transformed, etc.)? Was all data included? I guess the larger dots are the averages for that species, but the caption doesn’t say so. What happened to Rosalina (not included in the PCA)?
Results
Figures 2, 3 and 4 overlap with each other and therefore the results may be better presented differently. Btw, the box-whisker plots of figure 2 are unclear: often the whiskers extend towards the minimum and maximum values, but here there seem to be outliers identified. If/ how this is done, however, is not mentioned in the Methods. And what is the difference between two (Sr) and three asterisks (e.g. Mg)? Horizontal axes do not have to have ‘Rotaliids’ and ‘Miliolids’; those are already in the legend.
More importantly, figure 3 is confusing and I am not sure what it says relative to figure 2. For example, the Nd/Ca vary greatly within the Rotaliida (more than an order of magnitude), while those of the miliolids are much more similar. Figure 2, however, suggests the opposite. Figure 3 is maybe meant to show the differences between species, but that can better be included in figure 2. But then it becomes just like figure 4. The overall differences between elements (i.e. in being enriched in the miliolids compared to the rotaliids) is also summarized in figure 5. So, I think figures 2 and 3 can be removed: figure 4 essentially has all the data and figure 5 summarizes all that data. Table 1 can also be removed: the actual data is in table S2 and the numbers that are now in table 1 are already in figure 2/ 4. The order in which the figures are referred to in the text is incorrect (line 157), which to me also indicates that the order of the figures/ their contents should be changed. Section 3.2 is called ‘intra-order variability’, but this is already shown in the preceding section (figure 3).
Discussion
The authors are keen on stating that there is a systematic difference between rotaliids and miliolids, but that is not apparent from this dataset. The difference in biomineralization mechanisms is well documented and it makes perfect sense that this translates to an overall difference in elemental (and isotopic) composition. However, the results presented here paint a more nuanced picture: Mg/Ca varies within rotaliids, with one species having a similar Mg/Ca as the three miliolids (figure 4). For As, La and Nd, the averages may be different, but ratios within the rotaliids and miliolids for these elements are large compared to the between-group variability. This doesn’t convince me of a clear dichotomy (e.g. line 272).
In short, these data require some more in-depth thinking. When not considering rotaliids versus miliolids, there is a pattern that seems very consistent, but not thematized by the authors: high and low El/Ca are correlated within species. This is likely the ground for ordering the species as was done in figure 4: A. lobifera has lowest El/Ca for almost all elements, P. calcariformata second lowest, etc. This inter-element correlation may indicate the influence of certain (physiological) processes. See Branson and De Nooijer, 2025. Elements 21: 105-111. DOI: 10.2138/gselements.21.2.105 for some ideas on this. Marchitto et al. (2018), EPSL 481: 20-29 is another great article that aims at mechanistically explaining observed correlations between elements. I encourage the authors to explore similar ideas.
In addition, there is much more literature on miliolid (and large benthic rotaliid) El/Ca. It may be that after combining these new data with existing data provides indeed a (more) robust picture of rotaliid versus miliolid shell chemistry, but that should than be included in this discussion. It may may also lead to the acknowledgement that in rotaliids in particular, the El/Ca can vary greatly, which begs the question what ‘the rotaliid calcification mechanism’ really is and how it relates to El/Ca…
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