Multispecies planktonic and benthic foraminiferal stable isotopes from north atlantic subtropical site 558: thermocline intensification during the mid-miocene climate transition

We reconstruct the subtropical North Atlantic water column structure during the Miocene Climate Optimum warming (MCO; 17-14.8 Ma) and the Middle Miocene Climate Transition cooling (MMCT; 14.8-12.8 Ma) by analyzing delta O-18 and delta C-13 in four species of foraminifera (surface dwellers Dentoglobigerina altispira and Trilobatus quadrilobatus, thermo-cline dweller Dentoglobigerina venezuelana, and benthic Plan-ulina wuellerstorfi) from Site 558 (37.8 degrees N). At the end of the MCO, delta O-18 of surface and thermocline dwellers increased by >1%0, suggesting at least 2 degrees C cooling in the upper ocean as ice growth increased global delta O-18(sw) by similar to 0.5%0. The difference in delta O-18 values between thermocline and surface-dwelling species increased during the MMCT, coinciding with the development of a largely permanent East Antarctic Ice Sheet, and persisted into the Late Miocene. We interpret this increase in vertical delta O-18 gradient as a strengthening of the thermocline due to in-tensification of subtropical gyre circulation in response to the MMCT cooling.