Development of the East Asian monsoon and Northern Hemisphere glaciation: oxygen isotope records from the South China Sea

Oxygen isotope records of the surface-dwelling foraminifer Globigerinoides ruber from ODP Site 1143 in the southern South China Sea (SCS) are presented for the last 5Ma. The G. ruber delta(18)O of Site 1143 for the past 500 ka is lighter by 0.5-1.0parts per thousand in interglacial, and by 0.1-0.5parts per thousand in glacial periods, than the Site 806B record from the Ontong Java Plateau. We infer the light glacial/interglacial G. ruber delta(18)O at Site 1143, compared with the open western Pacific, to have been caused by stronger monsoon-induced precipitation over the SCS. While glacial/interglacial planktonic delta(18)O values remained stable over the 3.3-2.5 Ma period, the benthic Cibicidoides wuellerstorfi delta(18)O gradually became positive, leading to an obvious slope in their delta(18)O difference (Deltadelta(18)O(b-p)). The stable glacial/interglacial G. ruber delta(18)O over this period is probably caused by the decrease of sea surface salinity, which counteracted the effects of global ice volume and sea surface temperature on the G. ruber delta(18)O. We interpret that the intensification of the East Asian monsoon winds coupled with the northern hemisphere glaciation 3.3-2.5 Ma ago likely brought frequent and strong precipitation over the SCS and/or caused large-scale intrusions of Borneo alongshore low-salinity waters to the southern SCS, which greatly freshened the SCS and decreased its salinity. After 2.5 Ma, especially in the Quaternary period, the planktonic and benthic delta(18)O show similar variations over glacial/interglacial cycles in responding to the waxing and waning of the northern hemisphere continental ice sheet, and their relatively stable delta(18)O difference (Deltadelta(18)O(b-p)) indicates a period of steady fluctuations of the East Asian monsoon winds. The more positive values of Deltadelta(18)O(b-p) probably suggest stronger East Asian winter monsoon during Quaternary glacials whereas the more negative values of Deltadelta(18)O(b-p) may imply stronger East Asian summer monsoon during Quaternary interglacials. (C) 2004 Elsevier Ltd. All rights reserved.