Environmental change in the subtropics during the late middle Eocene greenhouse and global implications

Oxygen isotope records from ODP Site 1052 (Blake Nose, subtropical North Atlantic Ocean) indicate significant short‐term, high‐amplitude variability (up to 1.4‰ in about 2500 to 4000 years) during the late middle Eocene (37.9–37.5 Ma). These variations reflect a combination of changes in sea surface temperature and the oxygen isotope composition of the regional seawater. In order to independently evaluate the magnitude of SST changes at Blake Nose and better understand the nature of environmental change during the late middle Eocene, we present planktonic foraminiferal Mg/Ca data combined with δ 18 O data from the same samples. The calculated Mg/Ca paleotemperatures indicate a decrease in SSTs over the study interval from ∼33 to 28°C at the same time that previously published foraminiferal δ 18 O calcite values from the same samples also decrease. Thus, the δ 18 O calcite values must reflect a significant component of seawater δ 18 O change in order to reconcile the opposite paleotemperature trends reflected by both proxies. Calculated δ 18 O seawater values decreased from ∼3‰ at 37.83 Ma to ∼2‰ at 37.6 Ma. The combined trends of the SST and δ 18 O seawater cannot be explained by an increase of continental ice over this time interval. Instead, the data favor an overall weakening of the hydrological cycle as global climate transitioned from the greenhouse to icehouse.