Steroids in sediments from Zabuye Salt Lake, western Tibet: diagenetic, ecological or climatic signals?

Abstract

A 45 cm long core from Zabuye Salt Lake (Tibetan Plateau, S.W China) was studied to reveal the possible interference between diagenesis and climate signals. Steroids, including sterols and sterenes, dominate the soluble organic matter in these cores. The relative abundance of C27 sterol to the C29 sterols decreases with depth, resulting in a predominance of C29 sterols at the bottom section of this core. This change in the relative molecular distribution could be attributed to both environmental/ecological change and diagenetic complication of molecular signals. Sterols in the shallow sediments are relatively enriched in 13C compared to those from lower within the core. This enrichment is possibly associated either with environmental/climatic change (e.g., increase of salinity and global pCO2 level change etc.) or it could be attributed to the biogeochemical change of organic matter during early diagenesis. 4,4-dimethyl spirosterenes and their possible precursors, 4,4-dimethyl sterenes, constitute a major component of the apolar fraction of organic matter. δ13C values of the 4,4-dimethyl sterenes indicate that they are derived from phytoplanktonic algae rather than from bacteria. The δ13C values of the regular and spiro steroids differ by >2‰ suggesting either backbone arrangement of steroids might have involved isotopic fractionation or that these steroids are derived separately from different biological sources.