Carbon isotopes of n-alkanes allow for estimation of the CO2 pressure in the Early Jurassic - A case study from lacustrine shale and cannel boghead in the Dachanggou Basin, Xinjiang, Northwest China

Abstract

The carbon isotope composition of sedimentary organic matter (OM) records abundant climate and environmental information, and a case from the Early Jurassic Badaowan Formation (northern Xinjiang) in Northwest China based on the integration of organic petrography, biomarkers, the carbon isotope composition of n-alkanes and the total organic carbon of coal and oil shale is presented. The results show that total organic carbon isotopes (δ13CTOC) are affected by the OM source composition, while carbon isotopes of n-alkanes show different variations. The fractionation of the two types of isotopes responds differently with varying precision to climate and environmental changes. The OM source is the main factor causing the fractionation of total organic carbon isotopes, and the fractionation of δ13C of n-alkanes reflects atmospheric CO2 pressure (pCO2). According to the covariation between the δ13C of n-alkanes in C3 plants and pCO2, the pCO2 during the deposition of oil shale and coal in the Lower Jurassic Badaowan Formation is restored. In the early stage of oil shale deposition, a higher pCO2 corresponds to a higher inertinite content, pointing to an increase in the fire frequency. Some of the lacustrine cannel boghead interbedded with oil shale is also correlated with a high pCO2, promoting biological productivity in lakes (including the surrounding hinterland) and the formation of lacustrine cannel boghead. The sedimentary archive variation in pCO2 exhibits periodicity, and lacustrine shale has the potential to act as a registering fluctuation of pCO2 at a higher resolution.