the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 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.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-38', Lennart de Nooijer, 04 Feb 2026
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RC2: 'Comment on egusphere-2026-38', Ellen Thomas, 01 Mar 2026
Comments on Biogeochemical Dichotomy and Intra-Order Variability in Miliolid and Rotaliid Foraminifera
Ellen Thomas
Overall, I liked this dataset on El/Ca in the two main clades of calcifying benthic foraminifera, including some commonly analyzed El/Ca data (e.g. Mg/Ca) and some El/Ca on which not many data are available (e.g., REE/Ca), and think it is suitable for publication. However, I see problems in the discussion/interpretation of the data, and the authors do not clearly explain what is novel in their story, since dichotomy between miliolid/rotaliid test formation and El/Ca has been long and extensively documented. I therefore recommend potential acceptance after major revision.
As I understand the main goal of this work, the authors aim to evaluate the (relative or absolute) importance of 'biological' and 'environmental' effects on incorporation of trace elements in foraminiferal calcite, and do this by analyzing representatives of different, widely divergent foraminiferal clades from the same samples (i.e., same environment). My problems are the following:
- The authors have not collected/presented environmental data on factors that could influence element incorporation at the sample locality, e.g., temperature (clearly of importance of Mg/Ca), salinity, carbonate saturation, Ca-concentration, pH. Without such data, we do not know what El/Ca would be at equilibrium precipitation at the location, and we cannot compare data correctly to data from other locations. In my opinion, one should compare El/Ca in non-biologically precipitated calcite with that in foraminifera in order to fully evaluate the relative importance of 'environmental' and 'biological'; effects. I would expect that for most elements discussed here, distribution coefficients are available, so one could calculate at least the theoretical El/Ca (and in quite a few cases, e.g., see below, cultivation data are available).
- Linked to 1: this may well be minor at the studied locality, but I wonder about the comparison of data on larger and smaller foraminiferal taxa and a potential source of differences by taxon. In general, larger foraminifera have longer life times than smaller ones (possibly years vs. weeks), thus whole-test analyses of larger foraminifera would represent longer time periods. Is there short-term variability in the environment which might show up differently in analysis of shorter-lived vs. longer-lived taxa?
- Importantly, I think the authors do not sufficiently compare their data with the large amount of published material - I provide just a few instances below, where I list e.g. several publications on El/Ca in Amphistegina, a genus studied here. Yes, many fewer data are available on miliolids than on rotaliids, but the authors do not refer to e.g., the classical data in Blackmon and Todd 1959 (J of Paleontology 33, 1-15) including data points on Mg/Ca in miliolids in similarly shallow environments as the present study. Nardelli et al., 2016, comment on Zn in a miliolid (Mar Micropal 126, 42-49). Planktic forams are all rotaliids, so how do their values in e.g., Nd/Ca in planktic forams sampled in the water column (in contrast to sediment) compare with the ones shown here? (e.g. Pomies et al., EPSL 2002, 1031-1045)? How about Palmer 1985, EPSL 73, 285-298 on REE, specifically 'lattice REE', or Haley et al., 2005, EPSL 239, 79-97? How about Li/Ca (LeHouedec et al., 2021, G3, 22, e2020GC009496)? or Langer et al., 2015, and Charrieau et al., 2023, on Li incorporation, in Amphistegina (Charriaeu et al., 2023; Minerals 2023, 13, 127; including cultivation data)? Levi et al., 2019, on various trace elements and intrashell variability within Amphistegina (Front Earth Sci 7, 247)? Raitsch et al., 2010, Biogeosciences 7, 869-881,: Ca-saturation state effects on Mg and Sr incorporation in Heterostegina? I thus think that the authors do not look into possible reasons for the extent and /or lack thereof of the dichotomy in sufficient detail, by comparison with relevant literature.
I would have greatly appreciated a table in which the authors, for every El/Ca analyzed, compare their values with a range of published ones (as far as possible), to show the reader which of their El/Ca data are novel (or novel for miliolids), to what extent El/Ca at the studied location compare to data in other environments, e.g. in other species of rotaliids, in order to better evaluate the importance of environmental effects. In my opinion, the authors' conclusions on the predominance of the biotic effects may not be as convincing if more literature is considered. Specifically, the authors pay little attention to the long-known great variability within Mg/Ca in rotaliids (where environmental effects, of temperature, are obviously present), with some rotaliids having Mg/Ca values as high as miliolids (as they show in their figs. 2, 4). In Mg/Ca data discussion if variability there is no mention of the potential effects on the dichotomy of of the commonly observed banding in rotaliids, which obviously does not occur in miliolids (e.g., see van Dijk et al., 2019, for review and reference to plankton and benthos incl. Amphistegina; Front Earth Sci 7, 281; Geerken et al., 2019, Sci Reports, 9, 3598; Amphistegina and Ammonia). In conclusion, I think that there is insufficient evaluation of the abundant literature, to justify the statement that the authors established 'baseline element ranges for both orders' (282-283).
Lesser importance:
- I do not think Figure 3 particularly informative; what is its goal?
- Typos in name R. globularis/glabolaris (fig 4).
- 233: the authors state that 'Mn/Ca in Lachlanella provides one of the clearest examples of an environmentally influenced signal .This is of course a subjective statement, possibly based mainly on data in this paper, but I would say that the widespread use of Mg/Ca as temperature proxy shows that Mg/Ca is possibly the clearest example of an environmentally based signal.
- 261: give reference for statement that Mg/Ca rose in Eocene - in many publications time resolution is too low to state so with confidence, and it may have risen much later, in the Neogene (e.g. see Evans et al., 2026, PNAS 123, e251178112)
- 278: any evidence for this non-linear uptake?
Citation: https://doi.org/10.5194/egusphere-2026-38-RC2
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- 1
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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