NOAA/WDS Paleoclimatology - North Atlantic ODP982 Pliocene Alkenone SST Reconstruction

We provide the first continuous, orbital-resolution sea surface temperature (SST) record from the high-latitude North Atlantic, a region critical to understanding the origin of the Plio-Pleistocene ice ages and proximal to regions that became frequently glaciated after ~2.7 Ma. We analyzed sediments from Ocean Drilling Program Site 982 over the last 4 Ma for their alkenone unsaturation index and compared this surface water signal to a benthic d18O record obtained from the same section. We find that while ocean surface temperatures were significantly warmer (~6°C) than modern temperatures during the early Pliocene, they were also as variable as those during the late Pleistocene, a surprising result in light of the subdued variance of oxygen isotopic time series during the interval of 3-5 Ma. We propose two possible explanations for the high orbital-scale SST variability observed: either that a strong, high-latitude feedback mechanism not involving large continental ice sheets alternately cooled and warmed a broad region of the northern high latitudes or that by virtue of its location near the northern margin of the North Atlantic Drift, the site was unusually sensitive to obliquity-driven climate shifts. On supraorbital time scales, a strong, sustained cooling of North Atlantic SSTs (~4.5°C) occurred from 3.5 to 2.5 Ma and was followed by an interval of more modest cooling (an additional 1.5°C) from 2.5 Ma to the present. Evolutionary orbital-scale phase relationships between North Atlantic SST and benthic d18O show that SST began to lead d18O significantly coincident with the onset of strong cooling at Site 982 (~3.5 Ma). We speculate that these changes were related to the growth and subsequent persistence of a Greenland ice sheet of approximately modern size through interglacial states.