Biogeochemistry of sedimentary organic matter in the Yongjiang River estuary in the southern part of Hangzhou Bay, China, since the Late Pleistocene

A sediment core (YJK19-02) collected from the southern outlet of Hangzhou Bay near the Yongjiang River estuary in East China was analyzed for grain size, lignin, bulk elemental composition, stable carbon isotope, and rare earth elements (REEs) to determine the sources and diagenesis of sedimentary organic matter (OM) of the estuary and adjacent areas since the Late Pleistocene. delta 13C values (-24.80 parts per thousand--23.60 parts per thousand), total organic carbon/total nitrogen (TOC/TN) molar ratios (8.00-12.14), and light rare earth element/heavy rare earth element ratios (LREE/HREE=8.34-8.91) revealed the predominance of terrestrial sources of OM, mainly from the Changjiang (Yangtze) River. The lignin parameters of syringyl/vanillyl (S/V=0.20-0.73) and cinnamyl/vanillyl (C/V=0.03-0.19) ratios indicate the predominance of nonwoody angiosperms, and the vanillic acid/vanillin ratios [(Ad/Al)v=0.32-1.57] indicate medium to high degrees of lignin degradation. An increasing trend of Lambda (total lignin in mg/100-mg OC) values from ca. 14 500 a BP to ca. 11 000 a BP reflected the increase in temperature during the Late Pleistocene. However, a time lag effect of temperature on vegetation abundance was also revealed. The relatively higher and stable Lambda values correspond to the higher temperature during the mid-Holocene from ca. 8 500 a BP to ca. 4 500 a BP. Lambda values decreased from ca. 4 000 a BP to the present, corresponding to historical temperature fluctuations during this time. Our results show that the vegetation abundance in the Yongjiang River Basin since the Late Pleistocene was related to the temperature fluctuation duo to climate change.