Although the distribution of mono-, di-and trimethylated 2-methyl-2-(4,8,12-Trimethyltrideeyl) chromans (MTTCs) is well understood as an indicator of water salinity, their origin and formation mechanism are still ambiguous and under debate. In this paper, abnormally high levels of MTTCs were detected in Cenozoic saline lacustrine source rocks from the Western Qaidam Basin. Using a two-step column chromatography method, the MTTCs and naphthalenes were separated from other aromatic compounds and concentrated in one fraction, so that the stable carbon isotope compositions of these compounds could be accurately measured. Similar carbon isotope ratios for the mono-, di-and trimethylated 2-methyl-2-(4,8,12-Trimethyltrideeyl) chromans in a given sample suggest the MTTCs may share the same biological source(s). The MTTCs from the Western Qaidam Basin have similar carbon isotope compositions to primary producer-derived pristane and phytane. However, the  13 C values for the MTTCs showed significant differences with other primary producers-derived biomarkers (e.g. C 27 and C 29 steranes), probably indicating a distinct microalgae source from specific niches for the MTTC compounds. We speculate that the MTTCs distribution patterns may be controlled by the water chemistry dynamics in niches, which, in turn, is affected by hypersaline bottom water.

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 (C 25 HBI) were detected in the upper section of the Lower Ganchaigou Formation. C 25 HBI 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 C 25 HBI in these source rocks were in the range of -18‰ to -20‰. The relative enrichment in 13 C of C 25 HBI suggests that these isoprenoids were derived from diatom blooms. In this case, the diatoms used HCO 3 -as an additional carbon source to dissolved CO 2 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 13 C-enriched C 25 HBI could be an ideal marker for good source rocks. The decreased concentration of CO 2 (aq) in the water column induced by diatom blooms can result in an enrichment in 13 C of organic matter synthesized by primary producers. This may be a possible reason for the occurrence of 13 C-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.