An examination and application of two geochemical proxies for environmental and climate reconstructions in northern South China Sea

Abstract

Two biomarkers, n-alkanes and alkenones, have been studied for the distribution of their relative concentrations spatially and temporally in sediments in the Pearl River Estuary. 59 modern sediments have been collected across the Pearl River Estuary and offshore Hong Kong, covering a wide range of environmental and sedimentary conditions. The relative abundances between long-chain (C27 to C33) terrestrial-derived n-alkanes, and short-chain (C18 to C22) n-alkanes which are ascribed to marine algal and bacterial production, have been analyzed as “terrestrial-to-marine n-alkane ratios”. Results show that this ratio follows a general gradient of increasing towards uplands. More detailed interpretation has revealed that this ratio is highly dependent on the proximity of the sampling site to the supply of terrestrial matters, i.e., to river mouths and lands. This finding provides fundamentals on the development of terrestrial-to-marine n-alkane ratio as a potential new proxy for terrestrial sediment flux.

The newly developed potential environmental and climate proxy, the relative abundances of n-alkane compounds, is applied on analysis of a sediment core drilled in coastal marine environment offshore Hong Kong, HKUV11, which can be dated back to c. 11,000 yr BP. Alkenone unsaturation ratios are also analyzed to reconstruct post-Younger Dryas change in sea surface temperature (SST). Records show a rapid warming of the ocean surface at the start of the Holocene epoch from c. 11,000 yr BP to c. 9000 yr BP, during when the SST researched its optimum, followed by a long period of more stabilized temperatures with the presence of some short-term cold events. The terrestrial-to-marine n-alkane ratio matches well not only with the alkenone SST data in this study, but also with the bulk organic carbon geochemical data and grain size analysis of HKUV11. This suggest that the n-alkane ratio is a reliable proxy for terrestrial sediment supply in this coastal and estuarine environment, as supported by both the field studies of its natural distribution across the environmental gradient, and the successful application on an actual sediment core.

The Holocene climate variability of coastal south China recovered by the HKUV11 sediment record has been compared with δ18O records from a Dongge Cave stalagmite, a Greenland ice core and the Northern Hemisphere summer insolation variability. Results show that late-Glacial to Holocene climate in coastal South China generally agrees with the trend in the distal high-latitude North Atlantic, suggesting a strong external driving force of global climate change by the NH summer insolation variability. A detailed comparison between the HKUV11 SST record and Dongge Cave Asian monsoon record reveals that the temperature in coastal south China is sensitively correlated to the strength of Asian monsoon. These results have important implications to how monsoonal winds and temperature are coupled together, and can provide insights to how they may interact under future climate change in this densely-populated region.