Relationships between carbon isotope evolution and variation of microbes during the Permian–Triassic transition at Meishan Section, South China

Abstract

This paper investigates kerogen carbon isotopes, the difference between carbonate and kerogen carbon isotopes (Δ13Ccarb-kero = δ 13Ccarb − δ 13Ckero) and the difference between carbonate and n-C19 alkane compound-specific carbon isotopes (Δ13Ccarb-n-C19 = δ 13Ccarb − δ 13Cn-C19) during the Permian–Triassic transition at Meishan, South China. The results show that kerogen carbon isotopes underwent both gradual and sharp shifts in beds 23–25 and 26–29, respectively. The differences between carbonate and organic carbon isotopes, both the Δ13Ccarb-kero and Δ13Ccarb-n-C19, which are mainly affected by CO2-fixing enzyme and pCO2, oscillated frequently during the Permian–Triassic transition. Both the variations of Δ13Ccarb-n-C19 and Δ13Ccarb-kero coupled with the alternation between cyanobacteria and green sulfur bacteria indicated by biomarkers. The episodic low values of Δ13Ccarb-n-C19 corresponded to episodic blooms of green sulfur bacteria, while the episodic high values of Δ13Ccarb-n-C19 corresponded to episodic blooms of cyanobacteria. The relationships between the variation of carbon isotopes and biota show that the microbes which flourished after the extinction of macroorganism affected the carbon isotope fractionation greatly. Combining the carbon isotope compositions and the pattern of size variation of the conodont Neogondolella, this paper supposes that anoxia of the photic zone at bed 24 was episodic and it would be caused by the degradation of terrigenous organic matters by sulfate reducing bacteria in the upper water column. Considered together with results from previous research, the high resolution variation of the biogeochemistry presents the sequence of the important geo-events during the Permian–Triassic crisis.