Abstract:
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 Pliocene climatic conditions. We use the alkenone organic proxy to document the continuous evolution of Plio-Pleistocene sea surface conditions at both a high latitude site: Ocean Drilling Program Site 982 (58 degrees N, 16 degrees W) in the North Atlantic and a low latitude site: ODP Site 846 (3 degrees S, 91 degrees W) in the Eastern Equatorial Pacific (EEP). Both records indicate a substantial cooling of the sea surface, over the past 5 Myrs, with greater cooling at high latitudes - the North Atlantic cooled by approximately 2 degrees C /Myrs and the EEP cooled by approximately 1 degrees C/Myrs. Our results show that significant changes in sea surface conditions occurred at both low and high latitudes just prior to the onset of Northern Hemisphere Glaciation at approximately 3 Ma. In the North Atlantic, we find a dramatic change in the amplitude and structure of sea surface temperature (SST) variations, while in the EEP our data imply that a major change in ocean productivity occurred approximately 3 Ma. Spectra analysis indicates that obliquity cycles dominate records at both sites through this climatic transition, with SST and productivity variations approximately in phase and both of these indices leading benthic oxygen isotope variations by 2-5 kyrs. At our low latitude site, 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 EEP indicates that a remote connection from high latitudes may have played an important role in driving low latitude climate for the past 5 Myrs.