Geochemie van nummulieten als proxy voor Eocene klimaatsveranderingen in het Zuidelijke Noordzeebekken

Understanding past greenhouse climates is essential for assessing the potential impacts of current anthropogenic climate change. The Eocene (56.0-33.9 Ma) was the last greenhouse period, characterized by elevated atmospheric CO₂ levels and ice-free polar regions. Global temperatures rose throughout the early Eocene, reaching a maximum during the Early Eocene Climatic Optimum (EECO; ca. 53.0-50.0 Ma), followed by long-term cooling toward the Oligocene icehouse. This thesis reconstructs an Eocene palaeotemperature record and environmental changes in the southern North Sea Basin (NSB). This mid-latitudinal region provides an extensive Eocene stratigraphy. This research focuses on Nummulites, a genus of larger benthic foraminifera, using the Mg/Ca ratio of the calcite of their tests as a palaeotemperature proxy. The first step was to propose a standardized methodological approach. Reliable Mg/Ca-derived temperatures can be obtained from well-preserved, small specimens using a simplified cleaning protocol. The specimens are then analysed using ICP-OES. The results indicate a ca. 7 °C increase in average seawater temperature from the pre-EECO (ca. 20-21 °C; NP11, 54-53 Ma) to the peak EECO (ca. 28-30 °C; NP12, 51-50 Ma). Additional stable isotope data from Cibicidoides spp. and grainsize analyses indicate a positive shift in the reconstructed δ¹⁸Osw record, representing salinity increase, at the onset of the EECO. These data suggest changes in regional water mass characteristics. During the early Eocene, Nummulites distribution was primarily influenced by water depth and -clarity rather than temperature or salinity. During the middle to late Eocene, seawater temperatures cooled from ca. 26 °C to ca. 20 °C. Simultaneously, the Nummulites abundance and size was reduced. The environmental setting also shifted toward higher-energy deposition. These changes indicate that aside from decreasing temperatures, habitat degradation and depositional dynamics played an important role in the regional decline of Nummulites. Comparison of the generated data with global temperature records shows a reduced latitudinal temperature gradient during the EECO, supporting climate model predictions of poleward expansion of tropical conditions during greenhouse climates.