Spatial Distribution Of Microbial Lipids In The Surface Sediments Of The South China Sea And The Implications For Paleoclimate Reconstruction

Lipid biomarkers,like long chain diols derived from marine algae and glycerol dialkyl glycerol tetrethers(GDGTs)derived from archaeal membrane lipids,were conducted to use as sea surface temperature(SST)indices to reconstruct paleotemperature,base on marine surface sediments in South China Sea(SCS).LDI(long chain diols)index based on long chain diols were well used to reconstruct paleotemperature in global ocean,while the work was not conducted in the SCS.The isoprenoid GDGTs(iGDGTs)derived TEX86(TetraEther indeX of tetraethers consisting of 86 carbon atoms)was confused to use in SCS,the meaning to reflect subsurface temperature or seasonal temperature was debated.In addition,the nearly discovered RIOH(Ring Index of OH-GDGTs)and OH-2/OHs(fractional abundance of OH-GDGTs)based on OH-GDGTs also well used to reconstruct in middle(low)latitude ocean,while in the SCS,is still need to be conducted.Therefore,we focused on the three kind of temperature indices among the water depth,in order to discuss the applicability in SCS,and further discuss the environmental factor which could influenced the indices.Multiple studies focused on the influence of water depth on the distribution of iGDGTs,while the carbonate compensation depth(CCD)was not studied so far.The results revealed that the distribution could be divided into three distinct zones: 0-200m;1000-3500 m and below the carbonate compensation depth(CCD).The linear relationship in shallower water,and the deep water showed similarly trend with the result in Mediterranean Sea,with relative more iGDGT-2 and less OH-GDGT-3.However,when the water depth deeper than 3500 m,the factional abundance of iGDGTs obviously changed,with an enhancement of 0.06 in TEX86 H was found below the CCD,equivalent to a bias of ca.4°C in paleotemperature.We purposed that the elevated ?CO2 and lower pH below the CCD leads to the presence of distinct microbial communities,contributing to the GDGT pool.Similarly,the CCD also influenced the OH-GDGT-0 and 1,except for the OH-GDGT-2,so that the RIOH index was influenced.The extraction methods of OH-GDGTs in the proxy were both including BlighDyer method and the ultrasonic method,whether the different methods could influence the RIOH and OH-2/OHs,need to be constraint.The results revealed that systematic difference were existed among the two methods,and the reconstructed temperature from Bligh-Dyer method displayed well linear relationship with the satellite temperature.So Bligh-Dyer method was being used to extract the OH-GDGTs.Subsequently,we conducted the relationship between the three indices with the satellite temperature,the result showed that(1)LDI index was not well used in SCS,followed we conducted the RDA analysis and the result showed that the LDI index may reflect the seasonal temperature due to that the summer temperature was higher than the growth temperature.(2)The TEX86 index was constrainted to use in the SCS,we also conducted the RDA analysis,and the result showed that the water depth may be the important factor to the TEX86 used in SCS;(3)The OH-2/OHs index was more reliable and better to use as a temperature index compared with the RIOH when analysis the OH-GDGTs,and we reconstructed the temperature formula in SCS,while more samples still needed to testify the applicability as a higher temperature index.