DYNAMIC LOCAL AND GLOBAL MARINE REDOX CONDITIONS DURING AN EARLY SILURIAN EXTINCTION (IREVIKEN EVENT): GEOCHEMICAL EVIDENCE FROM LAURENTIA

Repeated biotic crises have become the hallmark for the Silurian with at least three of the major marine turnover events being related to dramatic changes in global environmental conditions. Causal mechanism(s) linking these marine extinction events with positive δ¹³C excursions (CIE), paleoceanographic change, and climate remain poorly understood in the Silurian. Here we examine the Llandovery-Wenlock boundary Ireviken extinction event and associated CIE previously interpreted to reflect a major change in global oceanographic-climate state and enhanced organic carbon burial, respectively. New geochemical data presented here using both global and local paleoredox proxies (δ¹³C, δ³⁴S, and I/Ca) are from two widely separated sections in North America (Nevada and Tennessee) deposited on different margins of Laurentia. The high-resolution δ¹³C(carbonates and organics) and δ³⁴S (carbonate-associated sulfate and pyrite) data from both sections show parallel positive excursions indicative of perturbations of the global carbon and sulfur cycles. Numerical box modeling of δ¹³C and δ³⁴S data indicate these shifts can be generated by increases in the burial of organic carbon and pyrite in sediments deposited under euxinic (anoxic and sulfidic) conditions. Independently, I/Ca values point to locally anoxic bottom waters in the distal shelf/slope setting in Nevada before, during, and after the Ireviken CIE. While I/Ca values in the proximal shelf setting in Tennessee show relatively oxic waters during peak δ¹³C values and then bottom-water oxygen concentrations dropped throughout the remainder of the excursion. Our multiproxy data provide the first direct tests of models put forth for this Silurian biotic event and CIE that previously could only infer changes about marine redox. When the geochemical data for both global and local scale changes in marine redox are combined with paleontological data and evidence for eustatic sea-level rise point towards shoaling of euxinic waters onto the shelf as a driver for the Ireviken extinction event.