DOES FOOLS GOLD TELLS THE TRUTH ABOUT ANOXIA? RETHINKING THE CAUSE OF THE LATE DEVONIAN MASS EXTINCTION USING SULFUR ISOTOPES IN CARBONATES

The Devonian Period is known for major biological and environmental changes on Earth, including the demise of reef communities during the mass extinction at the Frasnian-Famennian boundary (FFB). This biotic crisis is thought to have been caused by global anoxia based on the co-occurrence of one or two positive carbon isotope (δ¹³C) excursions during the major extinction pulses. Though anoxia can cause positive δ¹³C excursions to occur, other processes can affect δ¹³C trends (e.g., differences in the weathering of C-enriched rocks or alteration). Thus, a proxy independent of δ¹³C systematics is needed to confirm whether anoxia occurred before or during the FFB. Stable sulfur isotopic (δ³⁴S) trends are one such proxy for anoxia and elevated pyrite burial rates. δ³⁴S trends extracted from carbonate rocks (as carbonate-associated sulfate; CAS) are thought to record seawater δ³⁴S values. Sedimentary pyrite preserved in these same rocks can also be measured for, δ³⁴S to track changes in the sulfur cycle in parallel to the δ¹³C record.

This study reports new δ³⁴S trends measured from CAS and pyrite from a Late Devonian carbonate succession (Bactrian Mountain, SE NV) in the Great Basin region, USA. We aim to test whether paired δ¹³C and δ³⁴S trends resolve whether anoxia occurs before or during the FFB. We identify two positive δ¹³C excursions (1.5‰ and 1‰) above and below the FFB (possibly the Kellwasser Events). For the δ³⁴S record, only a single paired positive excursion is preserved in CAS and pyrite, 8‰ and 35‰, respectively. We interpret the lower δ¹³C excursion to be diagenetic in origin as these rocks are dolomitized with vein-filling carbonates, but the upper excursion occurs within well-preserved fossiliferous carbonates and post-dates the main pulses of the mass extinction. This occurrence, and preliminary correlations to nearby age-constrained sections, challenges the notion that anoxia was the main driver of this Late Devonian mass extinction and suggests other causes may have been more important than previously thought (e.g., cooling).