CHANGES TO THE CARBON AND SULFUR CYCLES DURING THE END-PERMIAN MASS EXTINCTION

The greatest mass extinction of the Phanerozoic occurred at the Permian/Triassic boundary (PTB, ~251 Ma). The mechanism for this extinction is extensively debated with a variety of external and internal triggers suggested. Several scenarios suggest widespread ocean anoxia during the late Permian. The possibility that the deep waters of the ocean also became rich in H₂S at this time has recently been suggested. The euxinic deep waters may have been periodically released to shelf environments which would locally alter the isotopic composition of the surface water sulfate. To test this we have obtained samples from two well studied marine sections which span the PTB, Meishan and Shangsi both located in Southern China. Carbonate Associated Sulfate (CAS) was extracted from the limestone for isotopic analysis and organic and inorganic carbon isotopes were measured. The δ ³⁴S of the pyrite has also been measured. The δ ³⁴SCAS obtained had several samples with extremely depleted ³⁴S values while Δ ¹³C also varied both before and across the PTB. Plots of δ ¹³Ccarb vs. Δ ¹³C have been shown to provide information about the fraction of carbon buried as organic (f) and the isotopic composition of the carbon being added to the system (δi). At Meishan the values for f range between 0.32 and 0.4 with δ i changing from -6.5‰ to -9‰ across the boundary. The Shangsi section has an f near unity before the boundary dropping to 0.43 across and into the Triassic. The δ i for Shangsi is -24‰ before the event, and -12.5‰ during and after. All of the carbon being buried was not organic, the extremely high values for f and low values for δ i indicate that the isotopic composition of the carbonate varied independently of oceanic carbon. This can be accomplished through changes to Δ ¹³C, such as those which have been measured in these sections. Similar high values for f and low δ i were observed when plotting δ ³⁴SCAS vs. Δ ³⁴S, suggesting that this system was also operating out of equilibrium during the end-Permian. From this data we have concluded that the deep water of the late Permian was euxinic and periodically upwelled into the photic zone either triggering or enhancing the end-Permian extinction.