Carbon isotopic composition of branched tetraether membrane lipids in a loess-paleosol sequence and its geochemical significance

Carbon isotopes of organic matter have been widely used in quantitative reconstruction of paleovegetation and paleoclimate since they are sensitive to vegetation and environmental changes. However, the effect of transformation of soil organic matter after burial remains unclear. The carbon isotopes of brGDGTs, which were likely produced by heterotrophic bacteria in soil or peat, provide an opportunity to fill this gap. In this study, we investigated the stable carbon isotopic composition of brGDGT-derived alkane spanning the last 60 kyr in the Lantian loess-paleosol sequence from the southern Chinese Loess Plateau, as well as its relationship with δ13C values of total organic carbon (TOC) and other established climatic proxies. The results show that the δ13C values of brGDGTs were slightly depleted (ca. 1-2‰) relative to TOC, and a good linear relationship between them was observed. These support a heterotrophic lifestyle of brGDGT-producing bacteria and suggest that δ13C values of brGDGTs may have potential use in paleovegetation reconstruction. The similar variations between δ13C values of brGDGTs and TOC in the soil core also suggest that the effect of soil organic matter transformation after burial may have limited impact on the environmental significance of δ13C values of TOC. In comparison with δ13C values of the TOC, the widely used proxy for past vegetation changes, the δ13C values of brGDGTs seem more specific and sensitive in the Lantian profile due to its specific origin and stable chemical structure.