The proposed project consists of two parts. The objective of the first part is to calibrate the planktic foraminiferal Mg/Ca-paleothermometer of a shallow dwelling and a deep dwelling species using existing sediment trap and plankton net samples from the North Atlantic Ocean and Arabia Sea.The extent to which Mg/Ca data vary between foraminiferal genotypes and morphotypes will be assessed for the first time. The sample set used for these calibrations is unique in its absolute amount and temporal and spatial resolution and thus yields a great opportunity to improve our existing understanding of Mg/Ca-paleothermometry.Such an extensive calibration is needed for unequivocal application of the Mg/Ca-thermometer in paleoceanographic studies. The objective of the second part is to apply the new calibrations together with paired 18O measurements in a paleoceanographic study of the 3.6 to 2.6 Ma (Late Pliocene) time slice during the onset of the Northern Hemisphere Glaciation.New paleoceanographic records will be reconstructed to quantify the variability of the Arctic ice shield and its relationship of the final closure of the Panama Gateway to surface-water temperature and hydrographic stratification.The work will employ an integrated approach, generating geochemical data (oxygen isotopes and Mg/Ca ratios) using well-preserved planktic foraminifera from Site 1308, recently (2004) drilled during IODP Leg 303 in the central North Atlantic. Site 1308 provides the opportunity to work at unprecedented stratigraphic resolution and age control.The proposed work will be integrated into the scientific activities of other Leg 303 members. The proposed analyses will provide critical information for the understanding of the processes forcing the transition from a world without large-scale Arctic ice sheets to one dominated by significant Northern Hemisphere Glaciation.