Carbon isotopic evidence for chemocline upward excursions during the end-Permian event

A negative shift in marine inorganic carbon-isotope composition (δ¹³C_carb) during the end-Permian mass extinction has been used as evidence for several different extinction mechanisms. Changes to the δ¹³C of organic matter and the difference between it and δ¹³C_carb (Δ¹³C = δ¹³C_carb - δ¹³C_org) have been examined at few locations, with conflicting interpretations. We examine the changes to both organic and inorganic carbon isotopes across the Permian-Triassic boundary at two marine sections from South China (Meishan and Shangsi) and compare these to data from other previously published sections. Through these analyses, we demonstrate that a decrease in Δ¹³C occurred during the extinction event throughout the Paleo-Tethys ocean. The extent and intensity of the decrease varies by location averaging a negative shift of ∼ 5‰. Several possibilities as to the cause of this shift exist including Siberian trap volcanism, a change in the terrestrial/marine organic carbon input to the system, or a change in the dominant marine biota brought about through environmental changes (such as widespread ocean anoxia/euxinia). The decrease in Δ¹³C observed at many of these sections across the event horizon is here interpreted to represent a shift from algae/cyanobacteria to less fractionating phototrophic sulfur bacteria in marine shelf environments resulting from upward excursions of the chemocline. These chemocline upward excursions would release euxinic water to the photic zone allowing phototrophic sulfur bacteria to thrive. The limited available biomarker data are consistent with this interpretation.