Assessing Nummulites geochemistry as a proxy for Early Eocene palaeotemperature evolution in the North Sea Basin

Abstract: Early Eocene climate is characterized by gradual warming toward the Early Eocene Climatic Optimum (EECO). Environmental changes related to this global long-term warming trend are recorded in Ypresian marine sediments in Belgium. Test geochemistry of Nummulites enables the reconstruction of palaeotemperature changes, based on the Mg-incorporation in the calcite test without complications connected to other proxies. Early Eocene Nummulites occur in high abundances in the inner shelf deposits of the southern edge of the North Sea Basin, providing multiple sample locations. Since this palaeotemperature proxy has only recently been developed, we investigated a methodological approach. We assessed the relationship between test geochemistry and (1) preservation state, (2) test size, (3) cleaning procedure in order to evaluate the use of this new proxy. We propose a simplified cleaning procedure (ethanol-methanol-peroxide) of well-preserved specimens within the smaller size range (radius < 3 mm). Using this methodology, a long-term palaeotemperature increase of  ± 7 °C is reconstructed from the pre-EECO ( ± 54 Ma) to the EECO ( ± 51 Ma), with a mean EECO palaeotemperature of  ± 28 °C. This corresponds with previous studies, strengthening the use of Nummulites geochemistry as a proxy for reconstructing long-term climate change in the southern North Sea Basin.