Abstract. The temperature proxies U37K′, LDI, TEX86H and RI-OH are derived from lipid biomarkers, namely long-chain alkenones from coccolithophorids, archaeal glycerol dialkyl glycerol tetraethers (GDGTs), long-chain diols ascribed tentatively to eustigmatophytes, and archaeal OH-GDGTs, respectively. The applicability of these proxies has been examined in the South China Sea (SCS), but most of these studies were focused on a single proxy and hence did not allow for a direct comparison between them. In this study, we investigated the above 4 proxies in the same set of surface sediment samples in the northern SCS shelf and related them to local sea surface temperature (SST), which allowed us to assess whether they represent certain seasons or are impaired by terrestrial influences, as well as to infer the preferred habitats of their source organisms. Terrestrial organic inputs appeared to have an impact on LDI, TEX86H and RI-OH proxies near the coast and lead to colder LDI and TEX86H derived temperatures, but a warmer RI-OH estimate in this region. After excluding samples influenced by terrestrial organic input, we found that LDI-derived temperature agreed well with annual SSTs, while U37K′ reflected mainly spring SST and both TEX86H and RI-OH indices were correlated with winter SST. The differential seasonal biases of these biomarker-derived temperatures observed here suggest that each biomarker’s source organism responds differently to regional marine environmental changes in an annual cycle. Specifically, marine eustigmatophytes are likely insensitive to nutrient variations and hence show the lowest seasonal variations, while coccolithophorids could have bloomed in late spring, when nutrient inputs by freshwater are usually highest. GDGT- and OH-GDGT-producing planktonic archaea likely thrive in winter, when conditions, such as relatively high nutrients levels, low light, and high concentrations of SPM in the upper water due to the enhancement of vertical mixing driven by a combination of surface cooling and strong winter monsoon winds, are favorable, although not clearly known.
Research on Technology of Drilling in High Temperature and High Pressure Gas Field in South China Sea
