EVOLUTION OF EASTERN EQUATORIAL PACIFIC SEA SURFACE TEMPERATURES DURING THE PLIOCENE CLIMATE TRANSITION

The Pliocene spans the most recent major climate transition in earth's history. The early Pliocene (5.3-3 Ma), the most recent period of sustained warmth, was followed by the more highly variable late Pliocene (3-1.8 Ma), which witnessed the cooling of high latitude climate and the onset of Northern Hemisphere Glaciation. Two recent hypotheses suggest that high latitude cooling may have been triggered by tectonically induced changes in the paleoceanography of the tropics (Driscoll and Haug, 1998; Cane and Molnar, 2001). Using the alkenone organic proxy, we document orbital-scale sea surface temperature (SST) and productivity variations through the Pliocene climate transition (5-1.8 Ma) at Ocean Drilling Program Site 846 (4º S, 91º W) in the Eastern Equatorial Pacific (EEP). SSTs ranged from 21-26 ºC and coccolithophorid productivity varied substantially, with higher values occurring during glacial intervals. Pliocene SSTs decreased by ~1ºC / Myr during the interval of significant Northern Hemisphere ice sheet growth. Superimposed on this temperature trend is a ~400 kyr "sawtooth" pattern, characterized by a ~240 kyr interval of low amplitude (1ºC) obliquity-dominated SST fluctuations followed by a ~160 kyr interval of higher amplitude obliquity-dominated SST variations. These high amplitude intervals correspond to orbital eccentricity minima and are marked by strong cooling events (3-4ºC) which are associated with pronounced (3-6 times greater than average) 41-kyr productivity blooms. Spectral analysis indicates strong 41-kyr and limited 23-kyr periodicity in both SST and productivity records throughout the interval. Our results imply a shoaling of the EEP thermocline from early to late Pliocene, consistent with recent propositions that the tropics transitioned from a perennial El Niño state to a more La Niña-like state during this time period. The strong response of tropical SST and productivity to eccentricity forcing suggests the existence of significant non-linear feedbacks in the Pliocene climate system. The presence of strong obliquity, a typically 'polar' signal, and negligible precession, a typically 'tropical' signal, in climatic records at this tropical locality indicates that a remote connection from high latitudes may have play an important role in driving low latitude climate.