Evidence of negative carbon isotope excursions and intense wildfire activity in Upper Triassic coal deposits from Junggar Basin, China

The Late Triassic extinction across the Norian–Rhaetian boundary (NRB) resulted in significant faunal turnover in both terrestrial and marine realms. It is associated with negative carbon isotope excursions (NCIEs) worldwide in the marine realm. However, a paucity of coeval terrestrial NCIE records and geochemical data limits our understanding of the causes of such carbon perturbations. Here, we report significant NCIEs in coal-bearing strata from Junggar Basin (NW China), synchronous with NCIEs in the marine Norian–Rhaetian sections. High-resolution δ13C, maceral and mercury (Hg) concentration data recorded in coal across the NRB provide direct evidence for a relationship between NCIEs, wildfire events and volcanism. The contemporaneous occurrence of terrestrial NCIEs with the lowest inertinite content suggests a decoupling of NCIEs from intense wildfire events, and shows that NCIEs could occur in more humid environments. The terrestrial sediments also record anomalously high Hg coinciding with high content of inertinite, suggesting the occurrence of widespread wildfires. Volcanic eruptions significantly elevate global temperatures by releasing massive amounts of CO2, increasing upper tropospheric water vapour and intensifying thunderstorm activity, ultimately enhancing wildfire frequency through increased lightning probability. The decoupling of NCIEs from anomalies in Hg enrichment and wildfire events suggests different sources or deposition rates for C and Hg; the anomalies in Hg and high inertinite content could be related to widespread wildfires associated with volcanic activity, while the NCIEs can be attributed to methane release or accelerated chemical weathering, possibly caused by global warming due to palaeoclimate feedback from intense volcanic activity.