Nanostructural and Geochemical Features of the Jurassic Isocrinid Columnal Ossicles

Abstract

Calcite isocrinid ossicles from the Middle Jurassic (Bathonian) clays in Gnaszyn (central Poland) show perfectly pre− served micro− and nanostructural details typical of diagenetically unaltered echinoderm skeleton. Stereom pores are filled with ferroan calcite cements that sealed off the skeleton from diagenetic fluids and prevented structural and geochemical alteration. In contrast with high−Mg calcite skeleton of modern, tropical echinoderms, the fossil crinoid ossicles from Gnaszyn contain only 5.0–5.3 mole% of MgCO3. This low Mg content can be a result of either a low temperature environ− ment (ca. 10C) and/or low Mg/Ca seawater ratio. Both conditions have been proposed for the Middle Jurassic marine en− vironment. Occurrence of Mg−enriched central region of stereom bars of Jurassic columnal ossicle of Chariocrinus an− dreae is consistent with the concept of magnesium ions involvement in earliest growth phases of calcium carbonate biominerals. Key wor ds: Echinodermata, Crinoidea, calcite, nanostructure, geochemistry, AFM, NanoSIMS, Jurassic.