GEOCHEMICAL INVESTIGATION OF ENVIRONMENTAL CHANGES ASSOCIATED WITH THE EARLY SILURIAN IREVIKEN EXTINCTION EVENT

A major biotic crisis (the Ireviken event) occurred within the early Silurian, spanning the Llandovery-Wenlock boundary (~431 Ma), where there was an 80% reduction in biodiversity in both conodonts and graptolites, a 50% reduction in trilobites, and is associated with one of four major positive carbon isotope excursions during the Silurian. The extent and magnitude of the Ireviken event has been well documented through biostratigraphic and δ ¹³C studies, however, the models proposed to explain the cause of the extinction remains poorly constrained. For the first time this study has generated a coeval analysis of δ ³⁴S- carbonate-associated-sulfate and δ ¹³C_Carb to examine the global ocean paleoredox conditions of this event. This will test the hypothesis that the Ireviken event was the result of a proliferation of deep ocean anoxia and increased organic carbon burial due to a shift in deep water formation from the high latitudes to the low latitudes. This approach will also help further refine the primo and secondo ocean-climate state model put forward to best explain the oscillating sediment-types, biota, and δ¹³C values seen throughout the Silurian. The model shows how alternating between climate states can affectively change the site of deep water formation, disrupting ocean circulation, and thus induce deep ocean anoxia. However, this model makes the assumption of a deoxygenated deep ocean based on δ¹³C alone and lacks any empirical evidence for anoxia. Here we analyzed δ³⁴S due to its direct link with the long-term oxygen and carbon cycles through the process of pyrite formation. Anoxic and/or euxinic (sulfide-rich) events are often accompanied by enhanced burial of ³⁴S depleted pyrite and ¹³C depleted organic matter making δ³⁴S_CAS and δ¹³C_carb values positive. Previous studies have associated the Wayne formation’s Maddox member, Nashville, TN area, with the Ireviken event and recorded d¹³C values as heavy as 4 ‰. This study, on the same strata, has also been able to record comparable d¹³C values and d³⁴S_CAS values that document a global coeval positive shift (7 - 8 ‰ in magnitude) indicating widespread anoxia was prevalent at the time. In addition, we will present new data on I/Ca ratios in carbonates which can distinguish local oxygen conditions and further piece together the redox conditions of the Ireviken event.