High-amplitude variations in North Atlantic sea surface temperature during the early Pliocene warm period

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 similar to 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 delta O-18 record obtained from the same section. We find that while ocean surface temperatures were significantly warmer (similar to 6 degrees 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 (similar to 4.5 degrees C) occurred from 3.5 to 2.5 Ma and was followed by an interval of more modest cooling (an additional 1.5 degrees C) from 2.5 Ma to the present. Evolutionary orbital-scale phase relationships between North Atlantic SST and benthic delta O-18 show that SST began to lead delta O-18 significantly coincident with the onset of strong cooling at Site 982 (similar to 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.