Evolution Of The East Asian Monsoon: Mineralogical And Sedimentologic Records In The South China Sea Since 20 Ma

Sediments from ODP Site 1143 in the southern South China Sea (SCS) and ODPSite 1146 in the northern SCS were used to trace sediment sources and obtain proxyrecords of past changes in the East Asian monsoon climate during the last 20 Ma based ona multi-proxy approach including a newly constructed quartz isolation procedure,determination of biogenic opal, identification of clay minerals by X-ray diffraction andgrain sizes measurements of extracted terrigenous materials, scanning electronmicroscope combined with energy dispersive X-ray spectrometer (SEM-EDS) andelements geochemical analysis, and other data acquired by previous studies in loess,eolian deposition in the Pacific, radiolarians, planktonic and benthic foraminifera, andblack carbon in the SCS. The major conclusions are as follows:(1) Analysis in geological setting and elements geochemistry results show that themost important source for sediments from ODP Site 1143 is Mekong River. However, anabrupt change of terrigenous Mass Accumulation Rate (MAR), median grain size, claymineral assemblage, index of degree of crystallinity of smectite and biogenic opal MARand content about 5.2 Ma indicates that the terrigenous source to the area of Site 1143was dominated by local sources at 10–5.2 Ma, largely from regions of crustal uplift andbasaltic eruption in Vietnam. However, since ~5.2 Ma, the sediment reaching ODP Site1143 has been dominated by flux from sources of the Mekong River drainage, especiallythe gorges that incise the edge of the Tibetan Plateau.(2) Terrigenous sediments from ODP Site 1146 were supplied by changed strengthof multiple sources and transport processes since 20 Ma. Grain-size data modeled by Endmember modeling method (EMM) indicated that eolian dust from the arid Asian inlandand fluvial input have contribute average 20% and 80% of total material to ODP Site1146, respectively. Moreover, about 75% fluvial input is from Pearl River and Taiwan,the other 25% from Luzon island. Further analysis in sedimentary environment, clayminerals and elements geochemistry combined with previous studies shows that fluvialquartz and feldspar come from Pearl river and Taiwan, smectite mainly from Luzon,kaolinite from Pearl river and illite and chlorite from Pearl river, Taiwan and the Yangtzeriver.(3) A simple model of the evolution of the East Asian summer monsoon based onthe clay/feldspar ratio, kaolinite/chlorite ratio and biogenic opal MAR has beenconstructed. We show two maximum strength periods of the summer monsoon at 8.5–7.6Ma and 7.1–6.2 Ma respectively. Subsequently there was a relatively stable periodbetween 6.2–3.5 Ma, continued intensification about 3.5–2.5 Ma;and last, a gradualweakening since 2.5 Ma. The monsoon has intensified again and shows remarkablyhigher-frequency variability since 1.0 Ma.(4) Remarkably high MARs of terrigenous material including quartz, feldspar,smectite and kaolinite, biogenic opal and carbonate between 17–15 Ma at ODP Site 1146suggest an intense tectonic movement around SCS at that time, which is maybe related tocessation of seafloor spreading in SCS. Furthermore, obvious change in degree ofcrystallinity of smectite and illite and REE geochemistry indicate that the sediments inputfrom Taiwan has strengthened since 3 Ma, which is possibly related to intensifiedmountain building of Taiwan since 3 Ma.(5) The eolian dust content and MARs modeled by EMM method, ratios of(illite+chlorite)/smectite, quartz plus feldspar proportion and the mean grain-size ofterrigenous materials at ODP Site 1146 were adopted as proxies for East Asian monsoonevolution. The consistent variation of these proxies since 20 Ma shows that the EastAsian winter monsoon intensity, drying degree in Asian inland and the intensity of wintermonsoon relative to summer monsoon have strengthened at 12?9 Ma, 8?4 Ma andespecially since 3 Ma. In comparison, summer monsoon was relatively stable between15?3 Ma, and possibly simultaneous intensified as winter monsoon at 3 Ma.(6) Simultaneous increase in sedimentation rates at ODP Sites 1143, 1146 and 1148,as well as in MAR of terrigenous materials, quartz, feldspar and clay minerals at ODPSite 1143 and 1148 at 3.5–2.5 Ma, may be the erosional response to both global climaticdeterioration and the strengthening of the East Asian summer monsoon after about 3–4Ma. The phased uplift of the Himalaya-Tibetan plateau may have played a significant rolein strengthening the Asian monsoon at 8 Ma and 3 Ma. However, the monsoonalenhancement is also maybe caused by the global cooling and ice buildup on Antarcticaand Arctic or the combined effect of Tibetan uplift and global cooling. Our researchresults appear to support the driving force of phased uplift of the Tibetan Plateau in thelong term evolution of Asian monsoon.