Quaternary Planktonic Foraminiferal Fauna And Variations Of The Warm Pool In The Western Equatorial Pacific

As the main source of global heat and vapor, the Western Pacific Warm Pool (WPWP) acts as an engine of the global climatic system and, together with the high-latitude North Atlantic, constitutes the switch of the global environmental changes. Located in the joint of the three major atmospheric circulations, the WPWP is inseparable from the Southern Oscillation, El Nino and East Asian Monsoon, so any changes of the upper sea structure and sea-surface temperature (SST) in this area often influence the global climatic system. Therefore, the variation of the WPWP during glacial cycles has become the primary issue in Quaternary paleoclimatologic studies.The analytic and calculation results from a study of planktonic foraminifera in the upper 25 meters of ODP 130 Site 807 reveal, for the first time, the periodical SST and thermocline changes in the tropical western Pacific during the last 1.5 Ma glacial cycles, and contribute the scientific evidence to research of the WPWP stability.To study distinct paleoceanographic indices, this paper has established an age model based on the identification of planktonic foraminifera marker species at Site 807 combined with earlier paleogeomagnetic and calcareous nannofossil data, and identified glacial/interglacial marine isotopic stages based on oxygen isotopic data and carbonate dissolution events, providing a framework for discussing various paleoceanographic indices in the WPWP.All the dissolution index from ODP Site 807 (for example, the planktonic foraminiferal fragmentation) indicates that carbonate dissolution was stronger during interglacial than during glacial stages, representing the typical "Pacific model of carbonate dissolution cycles". But in general, the preservation of fossil fauna is good enough to be used for this quantitative study. Also several faunal transfer functions were used to calculate sea surface temperature and the depth of thermocline. The results indicate that both the SST and upper water structure had significantly changed since 1.5 Ma. The glacial-interglacial winter SST difference was about 5.0 7.5; the fluctuation range of the depth of thermocline (DOT) was beyond 100 meters. All the evidence mentioned above proved the variability of the WPWP, testifying the SST changes in tropical oceans.The variation of the WPWP is stepwise in the Quaternary, with the mid-Pleistocene Revolution at ~0.9 Ma as the most significant transition boundary. Prior to this boundary, the fluctuation ranges of the SST and DOT were very small; after that, the temperature increased in steps and the DOT deepened gradually which indicates the thickening of the upper warm water. Furthermore, the percentage abundance of the cold and warm planktonic foraminifera and the DOT changed in an opposite way during glacial/deglacial cycles across 0.9 Ma. According to spectrum analyses, the WPWP deep water was influenced more by the high-latitude forcing, with the coarse fraction and test fragmentation showing obvious -100 ka eccentricity periodicity. At the same tune, however, the surface and subsurface waters were affected more directly by the low-latitude forcing thereby exhibiting the tropical half precession periodicity of-10 ka and the long eccentricity periodicities of-400 ka and 200 ka (a inferior respondence of ~400ka). In short, the paleoceanographical changes in the WPWP display the synchronous impact of the high-latitude icesheet forcing and the low-latitude tropical forcing.A comparison with the South China Sea (SCS) indicates that the deep sea water of the WPWP was well connected with both the southern and northern SCS during the Quaternary. While the surface water in the WPWP and the northern SCS was similar, the surface water of the southern SCS exchanged more frequently with the Northeast Indian surface water than with the WPWP water. Moreover, the comparison with the east Pacific indicates that the trend marked by high SST and a deep DOT in the western Pacific but low SST and a shallow DOT in the eastern Pacific became strengthened since the Mid-Pleistocene.