East Asian monsoon climate during the Late Pleistocene: high-resolution sediment records from the South China Sea

摘要

Based on the study of 10 sediment cores and 40core-top samples from the South China Sea (SCS) we obtainedproxy records of past changes in East Asian monsoon climate onmillennial to bidecadal time scales over the last 220,000 years.Climate proxies such as global sea level, estimates ofpaleotemperature, salinity, and nutrients in surface water,ventilation of deep water, paleowind strength, freshwater lids,fluvial and/or eolian sediment supply, and sediment winnowing onthe sea floor were derived from planktonic and benthic stable-isotope records, the distribution of siliciclastic grain sizes,planktonic foraminifera species, and the UK37 biomarker index.Four cores were AMS-14C-dated. Two different regimes ofmonsoon circulation dominated the SCS over the last two glacialcycles, being linked to the minima and maxima of NorthernHemisphere solar insolation. (1) Glacial stages led to a stableestuarine circulation and a strong O2-minimum layer via a closureof the Borneo sea strait. Strong northeast monsoon and coolsurface water occurred during winter, in part fed by an inflow fromthe north tip of Luzon. In contrast, summer temperatures were ashigh as during interglacials, hence the seasonality was strong. Lowwetness in subtropical South China was opposed to large riverinput from the emerged Sunda shelf, serving as glacial refuge fortropical forest. (2) Interglacials were marked by a strong inflow ofwarm water via the Borneo sea strait, intense upwelling southeastof Vietnam and continental wetness in China during summer,weaker northeast monsoon and high sea-surface temperaturesduring winter, i.e. low seasonality. On top of the long-termvariations we found millennial- to centennial-scale cold and dry,warm and humid spells during the Holocene,glacial Terminations Iand II, and Stage 3. The spells were coeval with publishedvariations in the Indian monsoon and probably, with the coldHeinrich and warm Dansgaard-Oeschger events recorded inGreenland ice cores, thus suggesting global climaticteleconnections. Holocene oscillations in the runoff from SounthChina centered around periodicities of 775 years,ascibed tosubharmonics of the 1500-year cycle in oceanic thermohalinecirculation. 102/84-year cycles are tentatively assigned to theGleissberg period of solar activity. Phase relationships amongvarious monsoon proxies near the onset of Termination IA suggestthat supper-monsoon rains and fluvial runoff from South Chinahad already intensified right after the last glacial maximum (LGM)insolation minimum, coeval with the start of Antarctic ice meltprior to theδ18O signals of global sea-level rise. Vice versa, thestrength of winter-monsoon winds decreased in short centennialsteps only 3000-4000 years later., along with the melt of glacialice sheets in the Northern Himisphere.