A SPATIAL PERSPECTIVE ON PLIO-PLEISTOCENE OCEAN COOLING

The last major climate transition in Earth's history occurred between the Pliocene and Pleistocene epochs. The warm stable conditions of the early Pliocene (5-3 Ma) were followed by significant global cooling, which eventually led to the development of extensive Northern Hemisphere icesheets during the late Pliocene and Pleistocene. The cause of this climatic transition remains unclear. Existing data suggest that the most significant cooling occurred at high latitudes while negligible climatic change took place in the tropics. However, most existing data sets are temporally discontinuous, providing only snapshots of Plio-Pleistocene climatic conditions. We use the alkenone organic proxy to document the continuous evolution of Plio-Pleistocene sea surface conditions at a high latitude site: ODP Site 982 (58ºN, 16ºW) in the North Atlantic and at two low latitude sites: ODP Site 846 (3ºS, 91ºW) in the Eastern Equatorial Pacific (EEP) and ODP Site 662 (1ºS, 12ºW) in the Eastern Equatorial Atlantic (EEATL). These records show that surface ocean cooling during the Plio-Pleistocene transition was heterogeneous, with greatest cooling occurring at our high latitude site (~2ºC /Myrs) and appreciable (~1ºC /Myrs in the EEP) to slight (~0.3ºC/Myrs in the EEATL) cooling occurring at our tropical sites. Our results further indicate that significant changes in sea surface conditions occurred at both low and high latitudes just prior to the onset of Northern Hemisphere Glaciation at ~3 Ma. In the North Atlantic, we find a dramatic change in the amplitude and structure of sea surface temperature (SST) variations, while our low latitude data imply that a major change in ocean productivity occurred ~3 Ma. Spectral analysis indicates that obliquity cycles dominate records at all sites through this climatic transition. At our low latitude sites, obliquity band coherency and phase relationships between sea surface indices and benthic oxygen isotopes, a proxy for high latitude climate, imply a linkage between low latitude and high latitude climates, which remained essentially constant throughout the Plio-Pleistocene Transition. The presence of strong obliquity power, typically a 'polar' signal, in climatic records from the tropics indicates that a remote connection from high latitudes may have played an important role in driving low latitude climate for the past 5 Myrs.