Variations in the western Pacific warm pool across the mid-Pleistocene: Evidence from oxygen isotopes and coccoliths in the West Philippine Sea

Abstract

We present planktonic foraminifera oxygen isotope and Florisphaera profunda abundance data from the International Marine Global Change Study Program (IMAGES) Core MD06-3050, which was collected in the West Philippine Sea on the margin of the Western Pacific Warm Pool (WPWP). Our records reveal marked changes in the upper water structure in the Kuroshio source region as early as 1.5Ma, continuing through the mid-Pleistocene climate transition (MPT). These changes are evidenced by reconstructions of the thermocline (based on planktonic foraminiferal oxygen isotope differences) and the nutricline (based on the relative abundance of the coccolithophore species Florisphaera profunda) in the Kuroshio source region. The evolution of the thermocline/nutricline featured three prominent phases: 1) phase I (1.5–1.1Ma), which was characterized by low-amplitude variations in a shallow thermocline; 2) phase II (1.1–0.8Ma), which was characterized by high-amplitude variations in a relatively shallow thermocline/nutricline; and 3) phase III (0.8–0.5Ma), which was characterized by low-amplitude variations in a deep thermocline/nutricline. Combined with the zonal sea surface temperature gradient in the tropical Pacific, the evolution of the upper water structure can be linked to tropical climatic changes across the MPT, including the intensification of atmospheric circulation and a gradual cessation of the long-term El Niño-like state. The upper water structure of the WPWP was asymmetric before 0.8Ma, with a shallow thermocline/nutricline along the northern margin and a deep thermocline/nutricline in the center of the WPWP and South China Sea (SCS). The modern WPWP was established by approximately 0.8Ma, and the upper water structure has changed synchronously as a whole since that time. A comparison of the δ18O values derived from Globigerinoides ruber tests from the MD06–3050 core with those from other cores in the western Pacific suggests that the lower glacial/interglacial G. ruber δ18O values in MD06-3050 before 0.8Ma could be related to the enhancement of the WPWP and to changes in atmospheric circulation patterns.