Shift in the Mode of Carbon Cycling Recorded by Biomarkers and Carbon Isotopic Compositions in the Yanchang Formation, Ordos Basin: Autotrophy vs Heterotrophy.

Organic-rich shales and mudstones have long been investigated regarding the control of source, environment, climate, etc. on the enrichment of organic carbon. However, little is documented about how autotrophy and heterotrophy influence organic carbon cycling/export. Here, we show molecular and carbon isotopic compositional changes of the shale or mudstone source rocks from the Chang 3 to 7 members of the Yanchang Formation. The Chang 7 member source rocks have higher quality (480-500 mg/g) and total organic carbon (TOC) (15.3% on average) than other member source rocks; the sterane/hopane ratio and the δC of organic carbon and kerogen decrease from the Chang 3 to 7 members, but Δδ ([average δC of -C + -C] - [average δC of pristane + phytane]) increases, and no aryl isoprenoids and C aromatic carotenoids (e.g., isorenieratane) were observed. These low maturity biomarker features suggest that there were no water stratification, photic zone euxinia (PZE), and no obvious change in the organic matter source, and the water column is generally anoxic. A comparison of the δC of Pr and Ph with the δC of the -C17 and -C18 alkanes reveals a shift in the mode of carbon cycling/export (autotrophy versus heterotrophy) in the Yanchang Formation and that there was dominant heterotrophic bacterial activity or bacterial biomass in the Chang 7 member. The TOC spike in the Chang 7 member may result from boosted carbon cycling/export that improves organic carbon preservation than other members. Possible external forcings on the shift are abundant hydrothermal- or volcanic-derived metal salts as electron acceptors in the palaeowater, which is a reasonable explanation for enhanced heterotrophic bacterial activity. This finding improves our understanding of heterotrophic bacterial activity control on organic matter (OM) preservation and may be a significant supplement for understanding the ecological or environmental forcings in the Yanchang Formation, Ordos Basin.