Applications Of Organic Geochemical Proxies In Palaeo-environmental Research

Although the organic matter produced by living organisms only constitutes a very small fraction of sediment materials, it plays an important role in linking biosphere, hydrosphere, atmosphere and sediments, and records the palaeo-climatic and palaeo-environmental evolution.This thesis focused on the applications of organic geochemical proxies (TOC, TN,δ13CTOC,15N and biomarkers) in palaeo-climatic and palaeo-environmental research. It involves two case studies:1) We analyzed the δ13CTOC、TOC and biomarkers in the sediments of Xinyanggang Estuary in Yancheng, China. We found that the δ13C values of particulate organic matter (POM) and surface sediment decreased from the estuary mouth upstream to the head, indicating the reduced contributions from Spartina and marine phytoplankton to the POM and surface sediment and increased inputs from Phragmites. The C32/C30alkanol and cholesterol/sitosterol ratio decreased in the surface sediment, faithfully recording the variations in the contributions from Spartina and Phragmites. The combination of biomarker distribution and organic matter δ13C in the sediment can be used as indicators for sea water intrusion into the estuary/river and potentially paleo-sea level reconstruction.2) We analyzed the total organic carbon (TOC), total nitrogen (TN), bulk organic matter δ13C, and815N as well as magnetic index s-ratio in the sediment of Huguangyan Maar Lake in order to reconstruct lake paleo-productivity and paleo-environmental changes. Based on those data, we divide the Holocene into three stages. The early Holocene (10400～6100yr BP) is characterized by high TOC, high TN, high δ13CTOC and low δ15N as well as low s-ratio, reflecting a high primary productivity and a warm and wet climate; The lowest values of δ15N coupled with high TN during8500to7100yr BP might be attributed to the expansion of nitrogen-fixation cyanobacteria. During the mid-Holocene (6100-3600yr BP), TOC, TN and A13CTOC decreased while815N and s-ratio increased, suggesting a decreasing primary productivity and a cooler and drier climate. The late Holocene (3600yr BP to present) is the coolest and driest period of the Holocene, indicated by the constantly low TOC, TN and saturated s-ratio (-1). Such trend indicates gradually weakened summer monsoon driven by decreasing summer solar radiation at30°N through the Holocene. The δ13CTOC was low and δ15N was high before1000yr BP. However, their variations after1000yr BP are not consistent with the Holocene climate trend, and are probably a result of anthropogenic activity.