Compound-specific carbon isotope study on the hydrocarbon biomarkers in lacustrine source rocks from Songliao Basin

Stable carbon isotopic composition of organic matter (δ¹³C_org) and compound-specific δ¹³C values of biomarkers from 15 lacustrine source rocks were analyzed to identify the original paleoenvironment and source organisms. The δ¹³C values of hopanes (δ¹³C_hop) ranged from ?68.7‰ to ?32‰ and exhibit strongly ¹³C-depleted values in the lower part of Member 1 of the Nenjiang Formation (K₂n¹, up to ?68.7‰), suggesting an origin from predominantly methanotrophic bacteria. ¹³C-enriched δ¹³C_Ga values and significantly ¹³C-depleted δ¹³C_hop in K₂n¹, which coincide with water stratification and an intermittent anoxic photic zone, represents a shallow chemocline. The presence of an intermittent anoxic photic zone, which means that the anoxia expanded into the euphotic zone, is beneficial for OM preservation and results in high values of TOC and HI in this section. However, the absence of gammacerane and ¹³C-enrichment of δ¹³C_hop in Member 2 of Nenjiang Formation (K₂n²) reflect a deeper chemocline, corresponding to relatively oxidizing conditions and low values of TOC and HI. Moreover, the negative correlation of TOC vs δ¹³C_org and HI vs δ¹³C_org reflects the control of OM formation by sedimentary environments rather than productivity in the water column. Thus, the depth of the chemocline not only controls the abundance of OM but also affects the development of the microbial community, such as chemoautotrophic bacteria in the deep chemocline and chemoautotrophic and methanotrophic bacteria in the shallow chemocline. Moreover, δ¹³C_Ga and δ¹³C values for 4-methyl steranes are related to water salinity, with a higher salinity accompanied by ¹³C-enrichment in gammacerane and 4-methyl steranes.