DIAGENESIS IN ECHINODERM SKELETONS AND THEIR APPLICATION TO THE RECONSTRUCTION OF PALEOCEAN MG/CA

Well preserved fossil echinoderms have been considered reliable Mg/Ca paleoseawater proxies. However, accurate characterization of the diagenetic alternation of their skeletons is a prerequisite for evaluation of their original, major-element geochemical composition. Here, I expanded the existing model of diagenesis of echinoderm skeleton by integration of various analytical methods that up to now only rarely have been used to assess the diagenetic changes of fossil echinoderms. Diagenetic study of ~100 differently preserved echinoderms (ranging in age from the Cambrian through Miocene) revealed that the most common transformation to low magnesium calcite leads to obliteration of the primary geochemical (i.e., specific distribution of sulphates) and micro/nanostructural (stereom, layered and nanocomposite structures) features of echinoderm skeleton. By contrast, echinoderm ossicles with preserved porous microstructure (stereom) without any major signs of its internal change commonly display geochemical (i.e., specific distribution of sulphates) and micro/nanostructural details (stereom structure, relicts of growth layering, nanograins), which are comparable to those observed in Recent echinoderm skeletons, and therefore they likely retain their original major-element skeletal composition. Intriguingly, my results do not support the earlier suggestions that switchovers from the skeletal high-Mg to low-Mg calcite consistently occurred in echinoderms according to the seawater type. For instance, well preserved fossil echinoderms from the Devonian (calcitic sea with low Mg/Ca ratio) are preserved as high-Mg calcite. These data are consistent with recent studies emphasizing a key-role of physiological factors (i.e. vital effects) in biomineralization in extant echinoderms, in which significant variations in the skeletal Mg content at different length scales are observed. [This work was funded by the National Science Centre (NCN) grant no DEC-2011/03/N/ST10/04798 and was performed in the NanoFun laboratory co-financed by the European Regional Development Fund within the Innovation Economy Operational Programme POIG.02.02.00-00-025/09].