Reconstructing the southern South China Sea upper water column structure since the Last Glacial Maximum : Implications for the East Asian winter monsoon development

摘要

Upper water column dynamics in the southern South China Sea were reconstructed in order to track changes in the activity of the East Asian winter monsoon (EAWM) since the Last Glacial Maximum. We used the difference in the stable oxygen isotopes (Δδ18O) and Mg/Ca-based temperatures (ΔT) of surface-dwelling (G. ruber) and thermocline-dwelling (P. obliquiloculata) planktonic foraminifera and the temperature difference between alkenone- and P. obliquiloculata Mg/Ca-based temperatures to estimate the upper ocean thermal gradient at International Marine Past Global Change Study (IMAGES) core MD01-2390. Estimates of the upper ocean thermal gradient were used to reconstruct mixed layer dynamics. We find that our Δδ18O estimates are biased by changes in salinity and, thus, do not display a true upper ocean thermal gradient. The ΔT of G. ruber and P. obliquiloculata as well as the alkenone and P. obliquiloculata suggest increased surface water mixing during the late glacial, likely due to enhanced EAWM winds. Surface water mixing was weaker during the late Holocene, indicating a weaker influence of winter monsoon winds. The weakest winter monsoon activity occurred between 6.5 ka and 2.5 ka. Inferred EAWM changes since the Last Glacial Maximum coincide with EAWM changes as recorded in Chinese loess sediments. We find that the intensity of the EAWM and the East Asian summer monsoon show an inverse behavior during the last glacial and deglaciation but covaried during the middle to late Holocene.