INVESTIGATING THE ROLE OF MARINE REDOX CONDITIONS IN AN EARLY SILURIAN MASS EXTINCTION (IREVIKEN EVENT): A DEEPER WATER PERSPECTIVE

The Ireviken carbon isotope excursion (CIE) was a large-magnitude positive shift in δ¹³C values that is recorded in early Silurian (~432 Ma; Llandovery-Wenlock boundary) strata globally. Associated with this CIE is a marine mass extinction, the Ireviken Event (murchisoni Event in graptolites), approximately 80% of conodont and graptolite fauna went extinct, along with a ~50% reductions in trilobites and brachiopods, as well as a decline in chitinozoans and acritarchs. Previous paleoredox studies of carbonate sequences throughout the Ireviken CIE and extinction event have shown evidence for widespread euxinic (anoxic + sulfidic) conditions in the Silurian oceans as a causal mechanism. This study investigates two localities that preserve deeper basinal mudstone/shale sequences to further test this redox-based hypothesis: the Baltic Basin (Latvia) and the Cape Phillips Basin (Bathurst Island, Arctic Canada). The Ireviken CIE has been previously documented at both localities. We utilized a set of local paleoredox proxies for both sites (δ³⁴S_pyr, [Mo, V, U, Mn], Fe-speciation) to constrain local marine water column conditions. Additionally, we analyzed samples from both sections for ε²⁰⁵Tl, a global paleoredox proxy for Mn-oxide burial. Our local proxy records indicate both deeper water sections were reducing throughout the Ireviken CIE interval. Our new Tl isotope data suggest the global oceans were more reducing prior to the CIE, followed by a brief oxic event during the rising limb of the CIE, and then a return to more reducing conditions globally during peak CIE and falling limb values. This study presents new insights and constraints on paleoredox conditions during this interval of the early Silurian suggesting a more complex marine redox history associated with the Ireviken extinction event and CIE.