Holocene climatic changes in coastal regions of the Northern China Sea

Abstract

East Asian Monsoon (EAM) greatly influences the oceanic conditions in the northern South China Sea (SCS), where sea surface temperature (SST) is thus of vital importance to explore the ocean-atmosphere-land interactions. Previous studies have shown decreased SST trend during the late-Holocene in this region. However, little is known about how the EAM variability affects SST changes in the northern SCS coast, due to scarcity of proxy-based reconstructions. Therefore, a new sediment core YJ, with high temporal resolution throughout the Holocene, is highly likely to provide new insight into the response of the northern SCS coastal conditions to the EAM variations. To this end, my master thesis focuses on core YJ to investigate organic lipid-based biomarkers (e.g. alkenones, n-alkanes, hopanoids and glycerol dialkyl glycerol tetraethers (GDGTs)). Generally, the -based SST record shows warm conditions prior to ~3 ka, and cold temperatures between ~3 ka and ~1.2 ka, and then large fluctuations (~5°C) over the past millennium. Meanwhile, alkenone contents are about 30~130ng/g over the investigated period, supporting the reliability of our -SST reconstruction. Compared to published records from open sea in the SCS, decreased SST is evident in core YJ between ~3 ka and ~1.2 ka, implying intensified coastal cooling driven by East Asian Winter Monsoon (EAWM). The inconsistency of SST in core YJ and other published records from the northern SCS coast indicates that site YJ is less influenced by the coastal upwelling induced by East Asian Summer Monsoon (EASM). n-Alkane and hopanoid proxies are used to explore allochthonous and aotuchthonous organic carbon sources. n-Alkane contents are relatively high at ca. 2.1 ka and 0.8 ka, when the Average Chain Length (ACL) are low. High n-alkane contents and low ACL values together imply the possible “algae/bacteria blossom”, which accounts for large fraction of short-chain n-alkanes. Moreover, lower Carbon Preference Index (CPI) values during these two periods indicate that in situ bacterial production may exert more impact than terrestrial plants. Interestingly, hopanoid contents exhibit an obvious anti-phase relationship with -SST, possibly due to i) negative response of bacterial growth to increased SST (that exceeds the optimum temperature) or/and ii) enhanced nutrient supplies from strengthened vertical mixing (driven by intensified EAWM). GDGTs are employed to further decipher coastal oceanic conditions and terrestrial organic matter input over the past two millennium. Branched and Isoprenoid Tetraethers index (BIT) is generally lower than 0.3, indicating the robustness of TEX86-SST reconstruction, which exhibits a cold period at ~500 yr BP (Little Ice Age, LIA). The opposite pattern between -SST and Archaeol and Caldarchaeol Ecometric index (ACE) demonstrates the cooccurrence of high SST and low salinity. In summary, decreased SST and increased hopanoid contents between 3 ka and 1.2 ka are probably related to intensified EAWM. Over the past millennium, SST record in sediment core YJ is consistent with northern hemisphere temperature, indicating less impact by EASM-induced coastal upwelling.