Controlling factors and environmental significance of BIT and δ13C of sedimentary GDGTs from the Pearl River Estuary, China over recent decades

The Pearl River Delta region has become one of the fastest developing regions in China in recent decades. As a result, large volumes of nutrients and carbon have been delivered to the Pearl River Estuary (PRE), causing eutrophication over the past century. In this study, four sediment cores were collected, they were analyzed for branched and isoprenoid tetraether lipids, total organic carbon content (TOC), total carbon to nitrogen (TOC/TON) ratio and delta C-13 values of crenarchaeol-derived biphytanes (delta(13)(CBP-Cren)) after ether cleavage of isoprenoid glycerol dialkyl glycerol tetraether lipids (iGDGT's). The purpose of this study is to analyze the controlling factors and environmental significance of the BIT index (a paleoproxy based on concentrations of branched and isoprenoid tetraether lipids) and delta C-13(Bp-Cren), values, and then to explore the possible responses of the BIT index and delta C-13(Bp-Cren) values to increasingly eutrophication (i.e., frequent outbreaks of algal blooms) occurring near the PRE in recent decades. The BIT index exhibits a decreasing trend in cores off the PRE (A8 and A9), indicating there was an increasing relative contribution of marine-derived organic carbon to the TOC content in the sediment. The delta C-13 values of all four core records exhibit decreasing trends toward the present day, exhibiting the influence of the Suess effect. However, the rates of delta C-13(Bp-Cren) reduction are lower in cores A8 and A9 that are close to the PRE, than those in cores that are more offshore (E501 and LE01, east of Hainan Island). In particular, the decrease in delta C-13(Bp-Cren) with time in cores A8 and A9 effectively stopped after the 1970s, but continued to occur in cores collected from east of Hainan Island. This may be mainly caused by the reduced riverine input of GDGT's and increasing eutrophication. The relative proportion of riverine Thaumarchaeota (with more negative delta C-13 values) decreased off the PRE, leading to the delta C-13(Bp-Cren) values of cores here being more positive. Furthermore, the declining trend of delta C-13(Bp-Cren) values (in response to the Suess effect) in cores A8 and A9 off the PRE may be partially offset by increasing water productivity. This study indicates that the BIT index and delta C-13(Bp-Cren) values in estuarine sediments were affected by variation in both riverine input of GDGT's and surface water productivity. As a result, the BIT index and delta C-13(Bp-Cren) in estuarine sediments may represent potential proxies for past changes in eutrophication in estuaries.