EPISODES OF PERTURBATIONS OF CARBON AND NITROGEN CYCLES COUPLED WITH PERMO-TRIASSIC TERRESTRIAL MASS EXTINCTION IN NE PANGEA, BOGDA MOUNTAINS, NW CHINA

The timing, pattern, and causes of marine Permo-Triassic mass extinction (PTME) are well documented, but those of terrestrial PTME are poorly understood due to poor chrono- and biostratigraphic, geochemical, and geological constraints. Organic carbon (δ¹³C_org) and nitrogen (δ¹⁵N_org) isotopic compositions of kerogen were analyzed in in five high-resolution terrestrial sections across northern and southern Bogda Mountains over 90 km. The results show that prominent negative excursions of δ¹³C_org, from ~-23 to ~-31‰, occurred ~30 m above the last coal seam in the vicinity of Permo-Triassic boundary (PTB) in the five sections, concurrent with major positive shifts of δ¹⁵N_org from ~+5 to ~+7‰. Constrained by two U/Pb zircon ages of 250.32 ± 0.08 Ma at southern Taodonggou and 254.11 ± 0.05 Ma at northern Tarlong in the study area, those negative δ¹³C_org excursions in northwestern China can be correlated to the δ¹³C_org depletion near the PTB at terrestrial Chahe and marine Meishan sections in Southern China and, indirectly, inorganic δ¹³C records across PTB at marine Yunnan section in southern China. The δ¹³C_org chemostratigraphy locates a potential terrestrial PTB in northwestern China. Episodes of positive δ¹⁵N_org excursions synchronize with the negative δ¹³C_org excursions, suggesting intermittent CO₂ input in carbon cycle and enhanced nitrification and denitrification in intermittent aerobic nitrogen cycle during PTME. Abundant volcanic ash layers present right above the potential PTB, suggesting massive volcanic eruptions during the mass extinction interval. The coincidence of perturbation of carbon and nitrogen cycles, biotic extinction event, and volcanic eruptions suggest that PTME may result from CO₂ emissions from magma-coal interactions during flood magmatism in Siberian Trap.