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

Nannoconus, an extinct group of calcareous nannoplankton, characterized by a skeleton formed of interlocking calcite lamellae, spanned around a central canal, was a major carbonate producer in the Early Cretaceous seas (~150-120 Ma). With the deposition of the largest (size ~5-20 μm) and heaviest (mass ~200-1400 picogram) skeletons among the calcareous nannofossils of that time period, it contributed to massive marine carbonate accumulations for ~30 million years. However, the calcification site (intra- versus extracellular) of Nannoconus, remains unknown till date. Notably, the extracellularly produced calcite is often Mg-enriched than the intracellularly produced one. Braarudosphaera bigelowii, an extant extraceullarly calcifying nannoplankton sharing similar taxonomy as Nannoconus, shows similar Mg-enrichment in its biocalcite. Hence, to understand the Mg content along with other trace (e.g., Sr, Mn) elements of Nannoconus’s skeleton, it is chemically characterized through micro X-ray fluorescence (μ-XRF) at the ID21 beamline of European Synchrotron Radiation Facility (ESRF). The results indicate extensive modification of the elemental signals of the skeleton through post-depositional recrystallization and clay contamination. Synchrotron-based μ-XRF, for the first time, provides a Mg/Ca value (in mmol/mol) of a single Nannoconus, i.e., a ~150 Myr old calcareous nannofossil. Nannoconus’s Mg/Ca, calculated as lower than 3.27 mmol/mol, is very similar to that of intracellular calcite. Yet, structural and taxonomic similarity between Nannoconus and B. bigelowii suggests an extracellular calcification for both of them. Thus, chemical data alone remain inconclusive to infer the calcification site of Nannoconus.