NanoXRF cartography of Middle Jurassic (~-170 Ma) coccoliths
Résumé
Coccolithophores, golden-brown algae, produce micrometric calcite platelets called coccoliths. Coccoliths geological record is continuous from their appearance 220 Ma ago until today. Hence, coccoliths have huge potential for geochemical reconstruction of paleoclimate and paleoceanography through time. We have performed sub-micrometric XRF analysis on 3 species of Early Bajocian (Middle Jurassic) coccoliths at European Synchrotron Radiation Facility (ESRF) on the beamline ID22NI, Grenoble, France. Based on this analysis, which had never been applied on coccoliths before, we endeavor to have a better understanding of the chemical composition of coccoliths and thus of marine waters where they were precipitated. Samples have been collected at Cabo Mondego, Portugal, the international reference section (GSSP) for the Early Bajocian interval and they were selected based on their preservation. Beforehand, coccoliths were separated from the matrix using a picking technique and deposited on 500 nm-thick silicon nitride membranes. With an excitation beam at 17 KeV and a resolution of 100 m, we have mapped 14 elements in coccoliths, i.e., S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Cu, Zn, Br, Rb and Sr. The mapping allows the recognition of biological incorporation of elements from the diagenetic overprint. Calcium, Strontium and Manganese are biologically organized in coccoliths. Sulfur, Chlorine, Chromium, and Bromine are also incorporated in coccoliths without suffering any diagenetic process. Conversely, Potassium, Iron, Copper, Zinc, Titanium and Rubidium are related to diagenetic processes and clays contamination.
These results, namely the first record of sub-micrometric chemical organization documented for coccoliths, will (1) help in a better understanding of the chemical incorporation of elements, and (2) in the recognition of diagenetic effects on coccoliths chemistry, and finally (3) develop new paleoceanographic proxies, e.g. the Chlorine and Bromine incorporation in coccoliths for salinity reconstructions. Further analysis especially on living species will greatly help in the calibration of such a proxy.
