Evidence of manganese incorporation and distribution at the nanoscale in coccolithophore calcite

The coccolithophore algae (Calcihaptophycidae) produce an extracellular covering of calcium carbonate plates (coccoliths) and are responsible for up to 10% of global carbon fixation through photosynthesis and biomineralization. Therefore, any alteration in cell functioning and calcification may strongly affect the ocean-atmosphere system. In this regard, the identification of any variation of the element pathways to the cell and their incorporation in the coccolith is important to understand the response of these organisms to changes in (past) environmental conditions and to identify (paleo)ecological proxies. Here, we focus on manganese which is an essential element for coccolithophore functioning: understanding whether manganese is also incorporated in coccolith calcite and if its distribution is species-specific is of seminal importance. In order to obtain ultra-high resolution (50 x 50 nm) spatial elemental distribution maps of Early to mid-Cretaceous (ca. 122 to 110 Ma) coccoliths we used synchrotron-based XRF analyses at the nano scale. The studied nannofossils belong to different taxa preferring low to higher surface-water fertility and were extracted from samples of different ages (from early Aptian to early Albian) collected in distant oceanic settings including the western Tethys, the Pacific and proto-North Atlantic Ocean. The analyses showed a high correlation in the spatial distribution of calcium and manganese in the coccoliths, suggesting that manganese is present in the coccolith calcite, and it was likely incorporated during coccolith formation. Moreover, the studied species show different Mn/Ca ratios suggesting that there is a species-specific control on the manganese uptake. Secondary calcite was also identified but it displays higher Mn/Ca ratios compared to biogenic calcite. Only the specimens collected from a black shale displayed secondary calcite crusts probably formed at the sediment-water interface after coccolith precipitation to the seafloor. Suboxic/anoxic conditions promoted manganese remobilization from Mn-oxides and Mn-oxyhydroxides which was incorporated in the secondary calcite. The Mn/Ca ratio in coccolith calcite crusts may therefore be indicator of (past) oxygen variations in bottom waters.