Early Pleistocene formation of the asymmetric east-west pattern of upper water structure in the equatorial Pacific Ocean

Surface- and subsurface-dwelling planktonic foraminifera from the upper 43 m of Hole A at the Ocean Drilling Program (ODP) Site 807, which was recovered from the western Pacific warm pool during ODP Leg 130, were analyzed for stable oxygen and carbon isotopes. By comparing these results with data from ODP Site 851 in the eastern equatorial Pacific, this study has reconstructed the paleoceanographic changes in upper ocean waters in the equatorial Pacific since 2.5 Ma. During the period from 1.6–1.4 Ma, the oxygen isotopes of surface and subsurface waters were found to markedly change in the western and eastern equatorial Pacific, further confirming the final formation of the well-defined asymmetric east-west (E-W) pattern at that time. This feature was similar to the zonal temperature gradient (sea surface temperature is higher in the west and lower in the east) and the asymmetric upper water structure (thermocline depth is deeper in the west and shallower in the east) in the modern equatorial Pacific. The zonal gradient change of subsurface water δ ¹⁸O was greater than that of surface water δ ¹⁸O, indicating that the formation of the asymmetric E-W pattern in the equatorial Pacific should be much more related to the shoaled thermocline and markedly decreased subsurface water temperature in the eastern equatorial Pacific. Moreover, since ∼1.6 Ma, the carbon isotopic differences between surface and subsurface waters clearly decreased in the equatorial Pacific, and their long-term eccentricity periods changed from 400 ka to ∼500 ka, reflecting the reorganization of the ocean carbon reservoir. This probably resulted from the deep water reorganization in the Southern Ocean at that time and its enhanced influence on the tropical Pacific (especially subsurface water). Our study demonstrates that the tropical ocean plays an important role in global climate change.