The formation of a permanent temperature SST gradient at the Plio-Pleistocene boundary: Highlights from the Western Pacific Warm Pool periphery area

We present a continuous record of sea surface temperatures (SSTs) derived from Mg/Ca ratio of plantonic foraminifera (G. sacculifer , 300-355 μm) over the last 2.2 Ma at ODP Site 1115B, Woodlark Basin (9o S, 151oE, water depth 1149m), western equatorial Pacific. The site is located at about the southern periphery of modern Western Pacific Warm Pool (WPWP) and offers an valuable record showing the expansion and contraction history of the warm pool through time. The reconstructed SSTs increase from 25 to 28oC with glacial-interglacial fluctuations ranging between 3.5 and 4.5 oC during 1.8-1.5 Ma, and then show a long-term decreasing trend. Comparing various SST records in the western and eastern Pacific, we found that the a permanent tropical SST gradient was formed at about 1.73 Ma. No direct link can be found among the formation of the permanent SST gradient, the Northern Hemisphere glaciation, and the reconstructed contents of greenhouse gases. However, the planktonic foraminiferal oxygen isotope records from the subtropical-subantarctic transitional area in southern Atlantic do show similar, step-wise decreasing trends. It has been interpreted that the southern hemispheric westerlies were enhanced during 2.0-1.5 Ma in several steps and resulting in stronger upwellings in the eastern tropical Pacific. Cosequently, the cooling in the eastern tropical Pacific induced stronger trade winds and had warm waters to pile up in the west, creating a permanent tropical SST gradient between the eastern and western equatorial Pacific. The exact timing of formation of the permanent SST gradient, its relationship to the evolution of the southern hemisphere westerlies, as well as its global climaticimpact, however, still need further examination.