BrGDGTs sources in eastern China marginal seas and their constraints on seawater temperature reconstruction

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) serve as promising proxies for paleotemperature in both terrestrial and marine settings. However, their application in reconstructing seawater temperature in marginal seas is complicated by uncertainties arising from mixed land and marine sources, isomer effect, and variability in depth and seasonality. To explore these factors, we examined brGDGTs in surface sediments from the eastern China marginal seas (ECMS) and reanalyzed previously published brGDGT data from the ECMS and Chinese soils. Using a three-end-member mixing model based on source indicators of ∑IIIa/IIa and #Ringstetra, we found that the autochthonous fraction of sedimentary brGDGTs accounted for 59.6 ± 14.4 %, 47.8 ± 21.7 %, and 33.2 ± 17.7 % in the Bohai Sea (BS), Yellow Sea (YS) and East China Sea (ECS), respectively. For temperature reconstruction, the relationship with annual mean surface seawater temperature (SST) was better captured by the methylation index of 6-methyl brGDGTs (MBT’6Me) than by that of 5-methyl brGDGTs (MBT’5Me). This is due to the higher abundance of 6-methyl brGDGT producers compared to those producing 5-methyl isomers, resulting in the predominance of 6-methyl brGDGTs and thus a better response of MBT’6Me to SST changes. Additionally, brGDGTs in the ECMS were shown to reflect the annual mean rather than seasonal SST, as SST remains well above freezing throughout the year and supports continuous brGDGT production. In contrast to deep oceans, production in deep waters or sediments appears to have a negligible effect on sedimentary brGDGTs in the ECMS, as brGDGT-based indexes responded better to SST than to bottom seawater temperature (BWT). Furthermore, the methylation index of 6-methyl brGDGTs containing two rings (defined as MBT’6Me-c) emerged as a robust potential indicator for annual mean SST in the ECMS, showing improved correlation with SST and reduced residual mean error (RMSE) compared to MBT’6Me. The components (Ic, IIc’, and IIIc’) used to establish MBT’6Me-c are mainly derived from in-situ production rather than terrestrial input. Therefore, the index was less influenced by terrestrial inputs and is a reliable tool for SST reconstruction in the China marginal seas.