NORTH ATLANTIC SUBTROPICAL SEA SURFACE TEMPERATURES DURING THE MIOCENE

Earth’s climate transitioned to a colder state during the Miocene. This cooling was punctuated by the Miocene Climatic Optimum (MCO; 17-14.0 Ma), when sea surface temperatures (SSTs) were approximately 3-4°C warmer than today. The planet then experienced a period of global cooling, termed the Middle Miocene Climate Transition (MMCT; 14-12.8 Ma), during which permanent ice sheets formed in East Antarctica. Despite the transition’s climatic importance, there are few North Atlantic reconstructions of SST changes during this period. Here we present new data from trace metals (Mg/Ca) and stable isotope (δ¹⁸O, δ¹³C) analyses of surface-dwelling foraminifera (Globigerinoides quadrilobatus) from subtropical Deep Sea Drilling Project Site 563 (33.64°N). A decrease in Mg/Ca by approximately 1.0 mmol/mol associated with the end of the MCO (14.8-12.8 Ma) indicates a SST cooling of approximately 3°C in the surface ocean. Oxygen isotope data from G. quadrilobatus show a 0.75‰ decrease during the same time interval. Of this, approximately 0.35‰ can be attributed to changes in δ¹⁸O of seawater due to permanent ice sheet formation in East Antarctica (Cramer et al., 2010), suggesting a surface water cooling of ~2˚C. The apparent lower cooling at Site 563 measured by stable isotope analysis could be accounted for by higher evaporation in the subtropical gyre. Further stable isotope and trace element analyses of surface-dwelling foraminifera are needed to increase the resolution of and extend the records of the North Atlantic SSTs at Site 563 during the Miocene. This study is part of a collaborate effort to obtain SST variations at three other sites from intermediate and high latitude sites in the North Atlantic, thus essential in reconstructing the meridional thermal gradient in the Atlantic Ocean during the Miocene.