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

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.