NOAA/WDS Paleoclimatology - Planktonic and Benthic Foraminiferal Stable Isotope Data from North Atlantic Subtropical Site 558 from 7.8 to 20.09 Ma

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 d18O and d13C in four species of foraminifera (surface dwellers Dentoglobigerina altispira and Trilobatus quadrilobatus, thermocline dweller Dentoglobigerina venezuelana, and benthic Planulina wuellerstorfi) from Site 558 (37.8°N). At the end of the MCO, d18O of surface and thermocline dwellers increased by >1‰, suggesting at least 2°C cooling in the upper ocean as ice growth increased global d18Osw by ~0.5‰. The difference in d18O 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 d18O gradient as a strengthening of the thermocline due to intensification of subtropical gyre circulation in response to the MMCT cooling.