Sulfur and Carbon Geochemistry and the End Permian Mass Extinction

δ³⁴S from carbonate-associated sulfate coupled with δ¹³C (carbonate) from the Permo-Triassic (P-T) boundary at Curuk Dag, Turkey, reflect a combination of long-lived and episodic oceanographic factors that led to the largest biotic crisis of the Phanerozoic. A pronounced increase in average δ³⁴S values (from ~+25‰ to greater than +30‰ VCDT) and a decrease in average δ¹³C values (from +5‰ to +1‰ VPDB) across the P-T boundary supports a long-term carbon and sulfur cycle shift from organic carbon burial on land during the Permian to pyrite sulfur burial under euxinic conditions across the P-T boundary.

A detailed examination of the boundary interval reveals large negative fluctuations (~10‰) in δ³⁴S before the mass extinction horizon that are coincident with three- to five-fold increases in total sulfur. The pre-extinction negative excursions are interpreted to have resulted from the oxidation of pyrite during the extraction of CAS; this mechanism may explain similar negative excursions in the δ³⁴S of CAS reported from other P-T boundary sections. The increase in total sulfur has been reported from other P-T boundary sections and may indicate episodes of enhanced pyrite formation in shallow water carbonate settings due to the expansion of deep-ocean anoxia, possibly due to the introduction of volcanogenic reductants from the Siberian Traps.

At and above the mass extinction horizon, δ³⁴S increases dramatically, and δ³⁴S and δ¹³C exhibit negatively correlated fluctuations through a short stratigraphic interval. Here, the negative co-variation between δ³⁴S and δ¹³C indicates rapidly changing redox conditions in the upper ocean, attributed to multiple upward excursions of the chemocline separating surface oxic from deep anoxic/sulfidic (euxinic) water masses. The direct cause of the mass extinction was likely the change to euxinic conditions in the shallow ocean, which may have introduced toxic levels of CO₂ and H₂S to the atmosphere.