Texas A&M University, United States

To reconstruct changes in tropical Pacific surface hydrography, we used samples from Site 1241 (5 degrees 50'N, 86 degrees 26; 2027-m water depth) to establish high-resolution records of Mg/Ca and delta (super 18) O for the mixed-layer dwelling planktonic foraminifer Globigerinoides sacculifer for the Pliocene time interval from 4.8 to 2.4 Ma. An increase in average sea-surface temperatures (SSTs) (24.5 degrees -25.5 degrees C) between 4.8 and 3.7 Ma can probably be explained by a southward shift of the Intertropical Convergence Zone, thereby increasing the influence of the warmer North Equatorial Countercurrent. The general global cooling trend, a response to intensification of Northern Hemisphere glaciation (NHG), started at approximately 3.2 Ma (shown by the delta (super 18) O (sub benthic) record) and is paralleled by tropical east Pacific cooling (indicated by SST (sub Mg/Ca) ). Tropical east Pacific cooling, however, had already commenced by approximately 3.7 Ma, suggesting that global cooling, probably related to decreasing atmospheric CO (sub 2) concentrations, might have started well before intensification of NHG. Relative changes in local sea-surface salinity (indicated by delta (super 18) O (sub salinity) ) show a decoupling from global high-latitude processes (shown by the delta (super 18) O (sub benthic) record). Long-term regional freshening started with decreasing SST (sub Mg/Ca) at approximately 3.7 Ma, suggesting that changes in the tropical wind field combined with latitudinal shifts of the tropical rainbelt were related to general decreases in tropical east Pacific SST-controlled delta (super 18) O (sub salinity) . The similarity of Pliocene SST (sub Mg/Ca) for G. sacculifer with modern SSTs in the east Pacific, in combination with the early development of a shallow thermocline at Site 1241, gives no direct support to the idea that a permanent El Nino-like Pliocene climate might have existed during the early Pliocene.