The largest mass extinction since the advent of animals occurred during the Permian-Triassic (P-Tr) transition, ca. 252 Ma, and is commonly attributed to the eruption of the Siberian Traps large igneous province. However, the direct killing mechanism is still debated. In this study, we investigated marine redox conditions of the intermediate water column that most organisms inhabit with special attention to the time interval before the onset of the mass extinction. The carbon isotope composition of carbonate and organic carbon (delta C-13(carb) and delta C-13(org)) as well as the nitrogen isotope composition of bulk nitrogen (delta N-15) were analyzed in four P-Tr boundary sequences (Zhongli, Jianshi, Ganxi, and Chaotian sections), which record a transect from a shallow water carbonate platform to a deep water, lower ramp slope in South China. delta C-13(carb) shows a distinct negative shift in all sections and displays a clear, 2-4 parts per thousand, decreasing gradient accompanying an increase in water depth both before and after the mass extinction. A distinct negative shift in delta N-15 is observed in the shallow water Zhongli section, whereas a minor negative shift is present in the three deeper water sections. Before the mass extinction, the delta N-15 values from shallow water sections are higher than those from deeper waters. The low delta N-15 values close to 0 parts per thousand in deeper water sections suggest that microbial nitrogen fixation was the predominant source of biologically available nitrogen before the onset of the mass extinction. Thus, the water depth- dependent gradient in delta C-13(carb) and delta N-15 suggests that an oxygen-deficient intermediate water column was already present before the mass extinction. The uniform delta N-15 values around 0 parts per thousand accompanying the onset of the mass extinction reveal that anoxic intermediate waters expanded into shallow waters. Meanwhile, the distinct positive shift in delta C-13(org) observed in upper ramp slope sections, i.e., the Jianshi and Ganxi sections, suggests that a euxinic photic zone was at least episodically present in the earliest Triassic. The temporal coincidence between the expansion of intermediate water column anoxia and the onset of the P-Tr mass extinction supports the hypothesis that marine anoxia was a direct killing mechanism.
