Sulfide isotope ratios as a method for examining environmental conditions leading up to the Permian-Triassic extinction

The Permian-Triassic (P-T) extinction that occurred 250 million years ago has an air of mystery surrounding it. One hypothesis for this event is widespread ocean anoxia. This study focuses on using sulfide isotope ratio values (δ³⁴S) as a method for investigating changes in the sulfur cycle leading up to the P-T boundary. The sample site for this study, Lingti 2, was deposited in the Neo-Tethys ocean, now located in the Spiti Valley region of Himichal Pradesh, India. The section consists of Permian black shale (Gungri Formation) and was sampled along strike from the base of the P-T Boundary to the base of the exposed shale at this location. Sulfide was extracted from these shales using chromium reduction. A Costech 4010 Elemental Analyzer (EA) and Isotope Ratio Mass Spectrometer (IRMS) were used to measure the δ³⁴S values in each sample. A chemostratigraphic profile of δ³⁴S and the variations in isotopic composition was evaluated in the context of redox state at the time of sulfide formation. Based on the idea that, under anoxic conditions, bacterial sulfate reducers fractionate sulfur isotopes (reducing ³²S preferentially over ³⁴S) and therefore depleting the ³²S (and enriching ³⁴S) in the surrounding pore waters. Episodic shifts in δ³⁴S were seen over time. These trends, coupled with degree of pyritization and iron speciation data, suggest periods of low sulfate with intervals euxinia prior to the P-T boundary. The low organic carbon content in these shales coupled with periodic "non-marine" S/C suggest that fluctuations in redox conditions combined with low organic matter content and reduced sulfate availability limited pyrite formation.