Chemical characterization of Nannoconus based on synchrotron micro X-ray fluorescence

  1. Rajkumar CHOWDHURY
  2. Baptiste Suchéras-Marx
  3. Luis Carlos Colocho Hurtarte
  4. Hiram Castillo-Michel
  5. Alejandro Fernandez-Martinez
  6. Fabienne Giraud

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Abstract

Nannoconus, an extinct calcareous nannoplankton genus, characterized by a heavy calcite skeleton (micaliths; ~200-1400 picogram), was a major planktonic producer in the Early Cretaceous seas (~150-120 Ma) contributed to massive marine carbonate accumulations for over ~30 million years. However, the calcification site (intra- versus extracellular) of its skeleton remains unknown till date. Notably, the extracellularly produced biocalcite is often Mg-enriched compared with the intracellularly produced one. Braarudosphaera bigelowii, an extant extracellularly calcifying nannoplankton closely related to Nannoconus, shows such Mg-enrichment in its biocalcite. To assess the Mg content along with other trace (e.g., Sr, Mn) elements in the micaliths of different Nannoconus species, their chemical composition has been analysed using synchrotron micro X-ray fluorescence (μ-XRF). The results show that the elemental signals of the micaliths are affected by post-depositional recrystallization and clay contamination. However, for the first time, a Mg/Ca value (in mmol/mol) of a single micalith of Nannoconus, i.e., a ~150 Myr old calcareous nannofossil is given. Mg/Ca of the micalith, calculated as lower than 3.27 mmol/mol, is very similar to that of intracellular calcite. Thus, chemical data alone remain inconclusive to infer the calcification site of the Nannoconus skeleton.

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  1. Peer Community in Paleontology

    Nannoconus was, for some thirty million years, the dominant pelagic carbonate producer of the Early Cretaceous oceans (Mutterlose and Bottini, 2013). While intracellular biomineralization in calcareous nannoplankton is well documented and even visualised (e.g., Triccas et al., 2025), some extant representatives are reported to calcify extracellularly (e.g., Hagino et al., 2016). The distinction matters well beyond taxonomy, because the calcification site leaves a chemical fingerprint and may influence downstream proxy reconstructions. The study uses an approach akin to the nearest living relative: extant, extracellularly calcifying organisms typically produce Mg-enriched biocalcite, since the trans-membrane ion transport that fractionates Mg in intracellular calcification is bypassed (e.g., Couradeau et al., 2012). Braarudosphaera bigelowii, an extant relative within the same order as Nannoconus (Braarudosphaerales), calcifies extracellularly and shows exactly this Mg enrichment. The study exploits this extant model following the logic: if Nannoconus calcified extracellularly, its micaliths should be Mg-enriched too.

    What Chowdhury et al. (2026) present is, to my knowledge, the first Mg/Ca value reported for an individual Mesozoic calcareous nannofossil. That is achieved thanks to a substantial technical expertise. Earlier work on Mg and Sr in Nannoconus (Renard et al., 2007) necessarily worked with bulk carbonate fractions that mixed the genus with other nannofossils and abiotic calcite; here the signal is resolved down to a single micalith. The headline result, an upper limit of Mg/Ca below 3.27 mmol/mol, places Nannoconus in the range of intracellularly calcified living nannoplankton and Cenozoic coccolith, with no Mg enrichment despite the genus's proposed extracellular calcification. This proposition has been intensively questioned by one of the reviewers and the discussion offered by the authors has been most instructive in considering the complexity of the problem, including the size of the micaliths. The authors refrain from overclaiming and conclude that chemistry alone cannot settle the question of the calcification site. Instead, they offer a reconciliation (a possible membrane-mediated regulation of Mg²⁺ transport from an extracellular organic template) that is presented as a hypothesis.

    I evaluated this article because of its advanced mapping methodology and data analysis, which may help resolve the debates over the sites of calcification beyond nannoplankton. It exploits synchrotron spectromicroscopy of carbonate biominerals, which is extremely powerful, but also demanding in terms of sample cleanness and measurement calibration (e.g., Cosmidis et al., 2015). Synchrotron-based μ-XRF at submicron resolution, a cutting-edge method used e.g. by Bottini et al. (2020) or Chevrier et al. (2025), is applied to map elements across single micaliths of three Nannoconus species, plus a Tubodiscus verenae coccolith and a Micrantholithus obtusus micalith for comparison. The dual-energy mapping strategy (below the Ca K-edge to improve sensitivity for low-concentration Mg and Sr, and above it for Ca) is thoughtful and well chosen for the elements of interest. I would like to emphasize the rigorous treatment of contamination. The authors do not just report ratios, as is common in the field; they classify all fifteen elements into four groups by how their 2D distribution tracks micalith morphology, then use bivariate count plots (e.g., Ca/Al, Ca/Si, Ca/Mg, Ca/Sr) to separate a high-Ca, micalith-bound cluster from a low-Ca cluster originating in surrounding clay. A pixel-masking approach removes the surrounding-clay signal, and a region-of-interest analysis isolates the least-contaminated zone to derive defensible upper limits. The data processing and presentation has been improved thanks to the comments of one of the expert referees.

    I would like to highlight several further strengths of this study:

    Diagenetic alteration may be difficult to distinguish from primary composition and from contamination. Bottini et al. (2022) proposed that this particularly applies to manganese, which they interpreted as being incorporated during the biomineralization at concentrations that appear to be species-specific and further enriched in secondary calcite. Mn and Fe in vivo presence in intracellular bodies involved in calcification had been also observed by Chevrier et al. (2025), whereas Suchéras-Marx et al. (2021) emphasized the diagenetic pathways of Mn incorporation. This study contributes to this discussion, as it interprets the contrast in Mn contents between Aptian and Barremian micaliths through the redox behaviour of Mn and tied to the oxic-anoxic interface position. This explanation has been thoroughly questioned by the reviewers, but the authors delivered a detailed defence of this interpretation, based on their identification of Fe-Mn-Cr-V micronodules in a N. globulus central canal.

    The data and reproducibility practices are exemplary: the raw maps and mass-fraction tables are archived and the masking code is deposited in Zenodo, with samples curated and identifiable in a public collection.

    The study’s limitations are honestly acknowledged by the authors and I repeat them here only because they point to constructive next steps. The central Mg/Ca conclusion rests on a single micalith of N. steinmannii subsp. minor, so the inference about the genus is provisional; clay contamination that is intimately intergrown with the micalith cannot be removed by masking and so persists in the reported upper limit; and the absence of any published Mg/Ca for B. bigelowii itself means the key comparison is made against a clade-level expectation rather than a direct measured value. The authors propose that techniques capable of physically cleaning surfaces or resolving finer structure, such as FEG-SEM, TEM, or SIMS with sputter cleaning, are the natural steps forward.

    To summarize, this manuscript pairs a clean, biologically motivated question with a state-of-the-art analytical approach, well documented data analysis, and a disciplined contamination control. The claim of being “first” is genuinely justified in this case: it is the first Mg/Ca value for an individual Mesozoic nannofossil. The interpretive restraint, i.e., declining to force a resolution to the calcification-site debate that the data cannot support, is a model for how single-fossil geochemistry should be reported.

    References

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