The occurrence and significance of C25HBI in Cenozoic saline lacustrine source rocks from the Western Qaidam Basin, NW China

Abstract

The saline lacustrine deposit of the Oligocene Lower Ganchaigou Formation is the main source rock for the Western Qaidam Basin, NW China. In this study, abundant highly branched isoprenoids with 25 carbon atoms (C25HBI) were detected in the upper section of the Lower Ganchaigou Formation. C25HBI is a biomarker for diatoms, and can provide information regarding biogeochemical processes during production and preservation of sedimentary organic matter. The carbon isotopic values of C25HBI in these source rocks were in the range of −18‰ to −20‰. The relative enrichment in 13C of C25HBI suggests that these isoprenoids were derived from diatom blooms. In this case, the diatoms used HCO3 − as an additional carbon source to dissolved CO2 in water column due to their high biological productivity. Conversely, the diatom blooms indicated an abundant nutrient supply to the environment and high primary productivity. Thus, the occurrence of 13C-enriched C25HBI could be an ideal marker for good source rocks. The decreased concentration of CO2 (aq) in the water column induced by diatom blooms can result in an enrichment in 13C of organic matter synthesized by primary producers. This may be a possible reason for the occurrence of 13C-enriched organic matter in the Cenozoic source rocks of the Western Qaidam Basin. Previous studies have shown that the high carbon number n-alkanes sourced from diatoms have no carbon preference. Hence, the sources of n-alkanes in regional source rocks are complex, and should be considered when discussing the sources of organic matter in future studies.