The late Miocene to early Pliocene climate transition in the Southern Ocean

We have constructed a relatively high resolution (5–10 kyr) Globigerina bulloides stable isotope record from Ocean Drilling Program Site 1088 (South Atlantic sector of the Southern Ocean). Together with the two published records from the high latitudes of the Southern Hemisphere that each span at least a portion of this interval of time, we begin to assemble a spatial picture of sea surface hydrography during the late Miocene early Pliocene climate transition (~ 8–3 Ma). We hypothesize that meridional movements in the frontal zones (Polar Front and Subtropical Convergence) with their impact on hydrographic gradients would be evident in temporal changes in the planktonic foraminiferal stable isotopic gradients among the sites. Our results indicate that there were no large changes in δ18O gradients during the interval of study. Relatively small changes such as a gradual warming of about 3–4 °C at Site 1088 during the late Miocene fall well within the realm of modern ocean seasonality in this region. The early Pliocene δ18O records largely parallel the global decrease defined by benthic foraminifera in the deep Pacific suggesting no amplified warming of surface waters in the Southern Ocean. The planktic δ13C records vary positively with the δ18O records both in the long term trend as well as on the scale of individual data points. We believe that this association reflects primarily reduced nutrient utilization during warmer intervals although a small component of the signal would also be consistent with reduced carbon isotope fractionation during air–sea gas exchange during relatively warmer conditions. If correct, this argument would have implications for carbon cycling during the early Pliocene and would support scenarios of elevated CO2 levels.