Geochemical evidence for expansion of marine euxinia during an early Silurian (Llandovery–Wenlock boundary) mass extinction

Abstract

Repeated biotic crises have become the hallmark for the Silurian with the three most significant marine turnover events being related to dramatic global environmental perturbations. Causal mechanisms linking these marine extinction events with positive carbon isotope (δ13C) excursions, paleoceanographic change, and climate remain poorly constrained. Here, we examine the positive δ13C excursion across the Llandovery/Wenlock boundary, and the associated Ireviken extinction event. This positive δ13C excursion has been interpreted to reflect a major change in global oceanographic-climate state and enhanced organic carbon burial. New geochemical data from two paleocean basins have been analyzed to determine local and global redox conditions using carbon- and sulfur-isotopes (δ34S), and iodine (I/Ca+Mg) proxies. The high-resolution δ13C and δ34S data from both sections show positive excursions indicative of global perturbations to each of these elemental cycles, with minimal temporal offsets between the two systems. Numerical box modeling of δ13C and δ34S data indicates that these isotopic shifts can be generated by significant increases in the burial of organic carbon and pyrite, which are most likely due to enhanced burial under euxinic (anoxic and sulfidic) conditions. Independently, I/(Ca+Mg) values point to locally anoxic bottom waters in the distal and deeper basinal setting in Nevada before, during, and after the Ireviken positive δ13C excursion. I/(Ca+Mg) values in the proximal shelf setting in Tennessee show relatively oxic waters during the onset of peak δ13C values, after which bottom-water oxygen concentrations dropped throughout the remainder of the excursion. This multiproxy paleoredox dataset provides the first direct evidence for local and global expansion of reducing marine conditions coincident with the Silurian biotic event and positive δ13C excursion. Integration of these geochemical data for local- and global-scale changes in marine redox conditions with the paleontological data and evidence for eustatic sea-level rise points toward a shoaling of anoxic and/or euxinic waters onto the shelf as a driver for the Ireviken extinction event.