SHELL CARBONATE CHEMISTRY OF TURONIAN PRISTINE FORAMINIFERA FROM THE TANZANIA DRILLING PROJECT

Upper Cretaceous sediments recovered during the Tanzania Drilling Project in 2007 to 2009 reveal long intervals with exceptionally well preserved planktic and benthic foraminifera, allowing for single species carbon and oxygen isotope and geochemical analyses. We present results of an ongoing study in which we focus on the paleoceanographic and paleoclimatic history at low latitudes during the Turonian period of global warmth. The studied material contains abundant and well diversified foraminifera faunas indicating an outer shelf to upper slope position. Consistently present calcareous species are used for carbon and oxygen isotope measurements. To enhance our understanding of the stable isotope data we study intra-species offsets and their relation to carbonate geochemistry of the shells, by applying a combination of light-microscope and SEM imaging with microprobe and cathodoluminescence-based geochemical analyses. For example, various aragonitic-shelled species of the genus Epistomina show a striking offset in carbon isotopes while displaying similar values in oxygen isotopes, as compared to other benthic species. Cross plots of carbon and oxygen isotopes show distinct clustering of data points for individual species, pointing to species-specific fractionation during the foraminifer’s life time. Cross plots of the elemental shell compositions also show species-specific clusters of data points, possibly indicating primary and/or early-diagenetic mineralogical differences that potentially affect the isotopic shell chemistry apart from or along with the vital effects. The influence of diagenetic alteration is studied by comparing isotope data and geochemical composition of the shells from samples with different degrees of preservation. Our study aims at providing important information for selecting species that are suitable for studies of stable isotopes and temperature reconstructions in the Cretaceous. Furthermore, we compare single species measurements of pristine shells with isotope data from bulk sediments to assess the reliability of bulk sediment data, as they are increasingly used for chemostratigraphic correlation and reconstruction of relative temperature trends in the Cretaceous.