EARLY TRIASSIC SULFUR ISOTOPES: IMPLICATIONS FOR LINKING LAND, SEA AND AIR FOLLOWING THE END-PERMIAN MASS EXTINCTION

The End Permian mass extinction was the largest biotic crisis of the Phanerozoic with extinctions in both the marine and terrestrial realms. Recent sulfur isotopic data, geochemical modeling and biomarker research suggest that the mass extinction may have been caused by widespread shallow ocean euxinia (Newton et al., 2004; Grice et al., 2005; Kump et al., 2005; Marenco et al., 2006) during a prolonged period of global anoxia (Isozaki, 1997). The δ³⁴S composition of oceanic sulfate is a powerful indicator of the redox state of Earth’s oceans and atmosphere and thus presents a crucial test for mass extinction mechanisms involving changing redox conditions.

Based on low-resolution evaporite-based sulfur chemostratigraphy, the δ³⁴S composition of Earth’s oceans has been shown to have reached dramatically low values towards the end of the Permian, and to have increased to unusually high values during the Early Triassic, with a peak in the Spathian of about +30‰ CDT (Holser and Magritz, 1987). Recent sulfur isotopic studies of carbonate associated sulfate (CAS) have shown that δ³⁴S at the Permo-Triassic transition was highly variable before the Early Triassic δ³⁴S rise (Newton et al., 2004; Marenco et al., 2006).

New sulfur isotopic analyses of CAS from the western U.S. provide more insight into the δ³⁴S composition of Early Triassic oceanic sulfate. With little exception, the sulfur isotopic composition of CAS from limestones is always higher (up to 10‰) than coeval evaporites and dolostones. The CAS data from limestones reveal that δ³⁴S is variable throughout the Early Triassic, with variations ~5‰ within a given section. More importantly, contrary to what has been shown with evaporite-based analyses, CAS data from limestones demonstrate that δ³⁴S was already unusually high (around +30‰) during the Induan and reached a peak of about +35‰ and higher in the Spathian. The western U.S. δ³⁴S data shows that sulfur isotopic variability found at the Permo-Triassic boundary was not restricted to the mass extinction interval, but instead lasted throughout the Early Triassic. The elevated δ³⁴S values through the Early Triassic indicate that oceanic euxinia and atmospheric anoxia were prolonged phenomena that had drastic effects on life in both the marine and terrestrial realms.