Evidence for the expansion of anoxia during the Smithian from a quantitative interpretation of paired C-isotopes

Abstract The Smithian negative C-isotope excursion was one of the major perturbations of the global carbon cycle following the end-Permian mass extinction event. Profound oceanographic and biological changes occurred during the Smithian, but the mechanism driving the negative C-isotope excursion and its links with marine environmental and biotic changes remain poorly constrained. Here, we use high-resolution paired C-isotope records from the early Smithian to the early Spathian in the Jiarong section, South China, to investigate the origin of the carbon cycle perturbation and its relationship with contemporaneous environmental changes. The paired C-isotope data reveal parallel negative excursions in both δ13Ccarb and δ13Corg during the Smithian that can be correlated globally. The global Smithian negative C-isotope excursion shows a good first-order correspondence with global transgression and ocean anoxia. The results of numerical box modeling suggest that the Smithian negative C-isotope excursion could have been generated by increased organic carbon oxidation in response to the upward movement of anoxic bottom waters during transgression. The oxidative decay of organic carbon is primarily fueled by the consumption of oxygen and sulfate in the atmosphere–ocean system. Hence, we use the model to quantify the oxidants consumption rate that could be required to simulate the global Smithian negative C-isotope excursion through organic carbon oxidation. The modeling results show that the organic carbon oxidation during the Smithian could have generated a high demand for oxidants in the ocean, leading to the expansion of anoxia. Our study provides quantitative constraints on the causal link between the Smithian negative C-isotope excursion and widespread anoxia. The nadir of the Smithian negative C-isotope excursion coincided with the severe loss of biodiversity, suggesting that the expansion of anoxia in response to the transgressive upward movement of anoxic waters may have driven the late Smithian extinction. Hence, it could be one of the causes of the prolonged Early Triassic biotic recovery.