Climatic changes in the northern South China Sea since the last glacial maximum

Abstract

The principal aim of this thesis is to reconstruct climatic changes in the open sea and coastal area of northern South China Sea (SCS) since the Last Glacial Maximum (LGM), and investigate its relationship with the East Asian monsoons, as well as insolation and sea level changes. The Sea Surface Temperature (SST) derived from long-chain alkenones in the open sea sediment core NS02G increased by 5°C and generally followed the sea level rise pattern from the LGM to mid Holocene. The SST kept increasing trend through the Holocene and reached the warmest state in the late Holocene. This warming trend is suggested to reflect the variability of winter SST and possibly relate to enhanced winter insolation.

In contrast to the core NS02G, the SSTs generated from coastal sediment cores NS01C and HKUV16 revealed warm mid Holocene and substantial cooling in the late Holocene. Distinct trends of Holocene SST in the open sea and coastal area of northern SCS suggest these two oceanographic settings are dominated by different climatic factors. Based on observations and paleo-SST reconstructions, the coastal area shows greater cooling in winter compared to the open sea of northern SCS, which is found mainly induced by the East Asian winter monsoon (EAWM). Subsequently, decreases in the coastal SSTs during the late Holocene were suggested to reflect intensified EAWM. The warming in the open sea of northern SCS and surrounding tropical oceans possibly contribute to the intensification of EAWM according to the coupling model of Asian monsoons.

The Average Chain Length (ACL), Carbon Preference Index (CPI) and magnetic susceptibility in the core NS02G decreased in similar pattern from the LGM to mid Holocene, suggesting less terrestrial material input when the sea level rose. During the late Holocene, n-alkanes-derived indices in both NS01C and NS02G cores were strongly modulated by the occurrence of n-alkanes characterized with even carbon predominance and maximum abundance at c_25, c_26 and c_27.

The alkenone-derived SSTs in the Mirs Bay of Hong Kong waters demonstrated a cooling trend over the last 400 years, particularly in the recent one century. The decrease of SST from the Little Ice Age (LIA) period to present in the Mirs Bay was suggested to relate to enhanced upwelling induced by stronger summer monsoon.

The application of Glycerol Dialkyl Glycerol Tetraethers (GDGTs) was explored in the core HKUV16 from the Pearl River Estuary and core T6 from the Mirs Bay. Coincidence between the variation of 〖TEX〗_86-temperature with ACL, CPI and magnetic susceptibility in the core HKUV16 suggests great influence of terrestrial material input on the 〖TEX〗_86-temperature estimation. Contrary to the cooling trend in alkenone-derived SST, TEX86-temperature of T6 increased by ~2°C in recent one century, suggesting different response of these two biomarker proxies to climatic changes in the coastal area.