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

Abstract

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 δ18O of Site 1143 for the past 500ka is lighter by 0.5–1.0‰ in interglacial, and by 0.1–0.5‰ in glacial periods, than the Site 806B record from the Ontong Java Plateau. We infer the light glacial/interglacial G. ruber δ18O 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 δ18O values remained stable over the 3.3–2.5Ma period, the benthic Cibicidoides wuellerstorfi δ18O gradually became positive, leading to an obvious slope in their δ18O difference (Δδ18Ob−p). The stable glacial/interglacial G. ruber δ18O 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 δ18O. We interpret that the intensification of the East Asian monsoon winds coupled with the northern hemisphere glaciation 3.3–2.5Ma 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 2.5Ma, especially in the Quaternary period, the planktonic and benthic δ18O 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 δ18O difference (Δδ18Ob−p) indicates a period of steady fluctuations of the East Asian monsoon winds. The more positive values of Δδ18Ob−p probably suggest stronger East Asian winter monsoon during Quaternary glacials whereas the more negative values of Δδ18Ob−p may imply stronger East Asian summer monsoon during Quaternary interglacials.