Carbon isotopic shift and its cause at the Wuchiapingian–Changhsingian boundary in the Upper Permian at the Zhaojiaba section, South China: Evidences from multiple geochemical proxies

• The magnitude of δ13Corg negative shift at the W–C boundary is 3.0‰.
• Negative shift across the W–C boundary was caused by low primary productivity.
• The magnitude of δ13Corg negative shift at the P–Tr boundary is >1.64‰.

The Late Permian environmental change, connecting the Guadalupian–Lopingian (G–L) (Middle–Upper Permian) boundary mass extinction and the Permain–Triassic (P–Tr) boundary mass extinction, has attracted more and more attentions. A significant negative shift for carbon isotope had been found at the Wuchiapingian–Changhsingian (W–C) boundary in the Upper Permian recently. However, the cause(s) of this negative excursion is still unknown. To resolve this problem, we analyzed the bulk organic carbon isotope, total organic carbon (TOC) content, pyritic sulfur (Spy) content, major element concentrations, and molecular organic biomarkers in the Wujiaping and Dalong formations in the Upper Permian from the Zhaojiaba section in western Hubei province, South China. Our results show that (1) there was a significant negative excursion in organic carbon isotopes at the W–C boundary and again a negative excursion at the top of Changhsingian stage; (2) the significant negative excursion at the W–C boundary was probably a global signal and mainly caused by the low primary productivity; and (3) the negative carbon isotope excursion at the top of Changhsingian was probably caused by the Siberian Traps eruptions. A decline in oceanic primary productivity at the W–C boundary probably represents a disturbance of the marine food web, leading to a vulnerable ecosystem prior to the P–Tr boundary mass extinction.