Ideas and perspectives: Mineralizing Fluid Control on Minor Elements in Biogenic CaCO<sub>3</sub>: Insights from Otoliths

Kekelou, Athina; Langer, Gerald; Ziveri, Patrizia

doi:10.5194/egusphere-2025-5251

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https://doi.org/10.5194/egusphere-2025-5251

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07 Nov 2025

| 07 Nov 2025

Status: this preprint is open for discussion and under review for Biogeosciences (BG).

Ideas and perspectives: Mineralizing Fluid Control on Minor Elements in Biogenic CaCO₃: Insights from Otoliths

Athina Kekelou, Gerald Langer, and Patrizia Ziveri

Abstract. The minor element composition of calcium carbonate (CaCO₃) biominerals from marine calcifying organisms leaving a sedimentary record has been used for decades to reconstruct various biogeochemical parameters. Advancing geochemical proxies and understanding their underlying mechanisms is essential for climate reconstructions, environmental research, and investigations of biomineralization processes. Despite considerable success of proxy applications, limited mechanistic understanding still restricts their full potential. The problem is often summarized by the term "vital effect", i.e. minor element partitioning due to biological activity. The element partitioning from the calcifying fluid into the biomineral, however, is usually described in terms of inorganic precipitation of a mineral from an aqueous solution of inorganic ions. Although this assumption is central to many partitioning models it has not been tested because the calcifying fluid of classic proxy archives such as foraminifera, molluscs, and coccolithophores has not been successfully sampled for element analysis. The calcifying fluid of fish otolith formation (endolymph), by contrast, was sampled and chemically analysed accompanied by corresponding otolith data. However, previous datasets have not been compared to inorganic partitioning coefficients to test this assumption. In this study, we address this gap using published data from four fish species and six elements. Our results indicate that the final stage of otolith minor element incorporation is influenced by organic matter in the endolymph fluid and therefore cannot be considered purely inorganic. Our conclusion questions a central assumption of many minor element partitioning models. This does not imply that existing models are questionable, but that they share a common oversimplification. By removing this oversimplification all kinds of different models can be improved. Our study contributes broadly to the understanding of biogenic CaCO₃ geochemistry, and it is relevant to the majority of existing models.

Received: 27 Oct 2025 – Discussion started: 07 Nov 2025

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Athina Kekelou, Gerald Langer, and Patrizia Ziveri

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RC1:
'Comment on egusphere-2025-5251', Nina Keul, 09 Dec 2025 reply
The manuscript “Ideas and perspectives: Mineralizing Fluid Control on Minor Elements in Biogenic CaCO3: Insights from Otoliths” (https://doi.org/10.5194/egusphere-2025-5251) by Athina Kekelou, Gerald Langer and Patrizia Ziveri investigates how the minor element composition of biogenic calcium carbonate (CaCO₃) is influenced by the chemical environment of the mineralizing fluid, using published datasets from fish otoliths. The authors synthesize data from four fish species and six elements, and compare otolith partitioning to inorganic partitioning coefficients to assess the role of organic matter in the endolymph fluid. The research is original and addresses an important mechanistic question in the field of biogeochemistry and proxy development. The study is well-positioned to advance understanding of “vital effects” in biomineralization, and the methods and datasets used are appropriate for the research questions. However, certain sections, particularly the Results, require more precise data presentation (e.g. comprehensive data tables!) and clarification to fully support the conclusions. Furthermore, this section needs to be carefully re-worked, as currently it reads like a draft version. Despite these issues, the manuscript is clearly written, conceptually strong, and provides valuable insights for the community. I would recommend publication after the authors address the points raised in the review. Also, while I am from a biomineralization background and feel confident assessing the present manuscript, I am not a fish biology expert and therefore cannot fully assess the fish-specific literature. I trust the authors have ensured appropriate citation and coverage of these aspects.
I wish the authors success with the revisions and remain available for further feedback.

Best wishes,
Nina Keul

Recommendation: Major revision.
Specific Comments
14: is a graphical abstract actually required in BG? But it’s a nice idea and summarizes the study well

Introduction
Line 44: Cite additional earlier publications alongside Pallacks et al., 2023.

L. 65 “but a well-known riddle is the Mg-problem” at least allude to the problem in a few words, as the readership of BG is too large to all be familiar with this.

L. 66 end sentence with full stop.

L. 68/ 69: Rephrase the Figure caption into proper English (add depicts, and literature should be replaced by references.

L. 85: different formatting (size, font, colour of Tab and Fig, should be checked for consistencey)

L. 99: pleas rephrase, since foraminifera exist in all these environments (see. E.g. Siemensma et al., for taxonomy of freshwater forams), however, they do not form CaCO3 shells.

;. 102 – 104:paragraph is a bit short, either expand or merge

Materials and Methods
2: linewidth between rows 1 and 2 is different to rest of table, please adjust, why are there brackets around the references? (same for Tab. 1), add spaces after commas in elements list

-add sub-chapters to M&M, e.g. data collection (literature review), calculations

148/ 149:”Tthe first study byThomas et al., 2017 used a marine species”-> what makes this the first study? I d suggest rephrasing all allusions to some artificial order which I assume has not meaning throughout the text, e.g. also l. 156/ 157: the second and third paper”)

Line 150: Use spectrometer instead of -metry.

Line 158: Sp. For S1, S2, S3, and S4 ( as in Sp1, Sp2, Sp3., Sp4) would make it easier for the reader

Line 155: Explain what the indices in N represent (e.g. N_b)

Line 150: Spell out the full word inductively coupled plasma mass spectrometry (ICP-MS) before using the abbreviation, plasmamass spectrometry is incorrect

equation 3: consistent formatting (square roots have different sizes)

several formatting inconsistencies throughout manuscript: it should always be referred to Fig. x not Figure x. Please check throughout manuscript
Results
Provide comprehensive data tables including all raw data and partition coefficients, and reference these in the Results section.

The manuscript needs to be carefully language edited, I am sure the co-authors are capable of that. E.g. l. 191: instead of writing “Figure 2 illustrates that cows are green” it should be formulated in a way that the statement is made and the reference is added in a bracket, e.g. Cows are green (Fig. 2). This is just one example of many and would transfer this piece from a student draft to a scientific manuscript.

Currently there are so many imprecise statements that all need clarification, I am just listing a few to demonstrate my point.

Line 188: Specify the differences at the species level.

Line 190: Clarify what is similar and which comparisons are made.

Line 193-195 Rephrase for proper English, I do not understand what you want to day..

Overall, the Results section reads like a draft and requires substantial revision, I am confident that the co-authors can guide this process as I feel like this is beyond my role as a reviewer. The same applies to Figure and Table captions, they need to be carefully checked, e.g. if they are all in italics, species names should not be in italics.

Discussion
While this section is very relevant and in broad of interest to the broader community it is a bit hard to follow. The discussion jumps back and forth between several elements, e.g. in section 4.2., for example: For Zn, consider addressing seawater organic complexing (line 278) and endolymph organics/differential partitioning (lines 296) in the same paragraph.

Line 246: Rename species as Sp. 1–Sp. 4 to avoid confusion with Fig. S2.

Please incorporate in the discussion the endolymph-otolith system is aragonite based, while foram proxies are calcite based. An extrapolation is further complicated by the fact that there are low (e.g. Globigerinoides) and high Mg (e.g. Amphistegina) foraminifer species.

Line 306: Mg, Na and Zn: given the previous sections, where it was argued in detail what processes different from inorganic precipitation are involved for these elements, this header is misleading and maybe can be renamed? While “invisible vital effects” might be an obvious choice I do not think it makes this any clearer, maybe something along the lines “The special case when apparently the biogenic and inorganic partitioning coeff. are indistinguishable?

Section 4.4.: great section, I esp. like Figs 3/ S2

Figures
Supplementary Information
Consistencies: in actual document it is refered to e.g. Figure S2, however, in the supplementary material it is the labeled as Figure Supp. 2

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Citation: https://doi.org/10.5194/egusphere-2025-5251-RC1

Athina Kekelou, Gerald Langer, and Patrizia Ziveri

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Short summary

Fish otoliths formation is key for understanding the incorporation of elements into biominerals. It is often assumed that the final step of biomineralization consists of inorganic precipitation as the fluid where biominerals form can hardly be sampled. Thanks to fish ear anatomy this can be overcome with otoliths. By comparing otolith formation and inorganic precipitation, we proved that this assumption is not always true. Our findings could refined models and shed light on biomineralization.


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