Stepwise and large-magnitude negative shift in δ¹³C_carb preceded the main marine mass extinction of the Permian-Triassic crisis interval

Large perturbations to the global carbon cycle occurred during the Permian-Triassic boundary mass extinction, the largest extinction event of the Phanerozoic Eon (542Ma to present). Controversy concerning the pattern and mechanism of variations in the marine carbonate carbon isotope record of the Permian-Triassic crisis interval (PTCI) and their relationship to the marine mass extinction has not been resolved to date. Herein, high-resolution carbonate carbon isotope profiles (δ¹³C_carb), accompanied by lithofacies, were generated for four sections with microbialite (Taiping, Zuodeng, Cili, and Chongyang) in South China to better constrain patterns and controls on δ¹³C_carb variation in the PTCI and to test hypotheses about the temporal relationship between perturbations to the global carbon cycle and the marine mass extinction event. All four study sections exhibit a stepwise negative shift in δ¹³C_carb during the Late Permian-Early Triassic, with the shift preceding the end-Permian crisis being larger (>3‰) than that following it (1-2‰). The pre-crisis shifts in δ¹³C_carb are widely correlatable and, hence, represent perturbations to the global carbon cycle. The comparatively smaller shifts following the crisis demonstrate that the marine mass extinction event itself had at most limited influence on the global carbon cycle, and that both Late Permian δ¹³C_carb shifts and the mass extinction must be attributed to some other cause. Their origin cannot be uniquely determined from C-isotopic data alone but appears to be most compatible with a mechanism based on episodic volcanism in combination with collapse of terrestrial ecosystems and soil erosion.