Variations in microbial ecology during the Toarcian Oceanic Anoxic Event (Early Jurassic) in the Qiangtang Basin, Tibet: Evidence from biomarker and carbon isotopes

Abstract

Abstract Reduced animal diversity during mass extinctions is commonly associated with resurgence of microbial abundances. The Toarcian Oceanic Anoxic Event (T-OAE) was a critical interval characterized by dramatic perturbations to the environment and life in the oceans and on land. A comprehensive organic geochemical study was performed on 19 samples from the upper Pliensbachian-lower Toarcian interval of the Quse Formation across the T-OAE at the Bilong Co outcrop section (also known as Biluo Co) in the Qiangtang Basin. Bulk geochemical analysis [total organic carbon contents (TOC): 2.0–8.0% wt., S2: 11–37\u202fmg hydrocarbon/g bulk rock, Tmax: 435–441\u202f°C] consistently suggests a narrow thermal maturity range of the Quse shale samples. The δ13C of maltene (δ13Cmaltene) and kerogen (δ13Ckerogen) fractions generally agree with the previously published δ13Corg curves, demonstrating the global T-OAE negative carbon isotope excursion (N-CIE). The elemental composition suggests a predominance of Type II kerogen, indicating the kerogen composition casts minimal effect on carbon isotopic signals. Petrographic examination demonstrated that the kerogens are mainly composed of alginate and amorphous organic matter, suggesting that a dominant source of kerogen originated from primary producers such as marine photosynthetic species that recorded the carbon fixation isotopic signature, whereas the bulk maltene represents a mixture of diversified organisms. By comparing the offset between δ13Corg, δ13Cmaltene and δ13Ckerogen in the T-OAE interval, two phases separated by the nadir of δ13C were recognized: early N-CIE and late N-CIE. In general, Phanerozoic δ13C values for preserved maltene and bulk organic matter in sediments are equal to or more negative than syngenetic kerogen; in the early N-CIE, however, δ13Ckerogen is more negative than δ13Cmaltene. Such isotopic inversions were previously attributed to the proliferation of 13C-enriched cyanobacteria and heterotrophic bacteria, which have been reported in Precambrian and Permo-Triassic transitional records. Moreover, the corresponding low sterane/hopane values in early N-CIE indicate an ecological shift reflecting microbial response to catastrophic environmental change that caused the extinction and initiated ecosystem changes during the early stage of T-OAE.