Morphotypes of the coccolithophore Gephyrocapsa as a proxy for sea surface temperature

Abstract

Previous works have established that six morphotypes of the genus Gephyrocapsa are recognized and related to the sea surface temperature. However, applying Gephyrocapsa morphotypes to reconstruct this parameter is challenging because morphotype descriptions and photographs are done in an Electron Scanning Microscope (SEM), while the majority of quantitative analysis research of fossil assemblages is done with an Optical Microscope (OM). Therefore, we aim to evaluate coccolith image correspondence in these two kind of microscopes. Four sediment core samples were used from the offshore section of the Pelotas Basin. For each sample and each analysis type, 60 coccoliths were measured, photographed, and classified by morphotype in the SEM and the OM. The morphotype proportions found in the SEM and the OM were correlated using a sum of squares statistical analysis. Weak correlations were found. It does not seem possible to securely identify Gephyrocapsa morphotypes in the OM, as the image differs considerably from that found in the SEM. Paleotemperatures were calculated from morphotype proportions in the SEM and the OM and discrepancies were very large, demonstrating that this proxy is not viable when studies are completed in the OM.