The geneses of sedimentary organic matter with anomalous 13C-enriched isotopic composition in saline and freshwater lakes: A case study of lacustrine source rocks from Dongpu and Qikou sags, Bohai Bay Basin, eastern China

Abstract

The carbon isotopic composition of sedimentary organic matter (δ13COM) is notably important for the genetic identification of source rock and its products. The sapropelic organic matter (mostly plankton with high hydrogen index) is generally depleted in 13C as compared to humic organic matter (mainly higher plants with lower hydrogen index). However, some lacustrine sediments contain sapropelic source rocks with anomalous 13C-enriched isotopic ratios, especially reported in freshwater lakes. The purpose of this study is to discuss the geneses of lacustrine sedimentary organic matter characterized by anomalous 13C-enriched ratios in saline and freshwater sediments. To this end, we integrate the detailed organic geochemical data (including Rock-Eval analysis data, total organic carbon contents, total organic carbon isotopic compositions, biomarkers, and trace element concentrations) of 64 saline lake sediments and 39 freshwater lake sediments obtained from the Paleogene Shahejie Formation of Dongpu sag and Qikou sag, separately, in Bohai Bay Basin of eastern China. Both sags exist samples with high hydrogen index values (HI > 400 mg/g) and high total organic carbon contents (TOC > 2%), but anomalous 13C-enriched isotopic ratios (up to −21.1‰). In Qikou sag, the δ13COM values of sapropelic kerogen correlate positively with TOC contents. The anomalous enrichment of 13C in organic matter is associated with extremely high productivity of algae, such as algae bloom. In Dongpu sag, the enrichment of 13C in organic matter is attributed to the significant increase of drought and salinity. High salinity and aridity climate may lead to the abundance of C4 plants and Chlorobium as well as the enrichment of 13C in plankton.