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

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.