The Silurian was a period characterized by rapidly fluctuating climate, multiple biotic turnover events, and eustatic sea-level variability. The Homerian Mulde biotic event (late Wenlock), ~428Ma, lies between the Sheinwoodian (early Wenlock) Ireviken and late Ludlow (mid- Ludfordian) Lau extinction events. It is characterized by a near complete extinction (95% loss) in graptolites, a severe drop (~80%) in diversity in conodonts, and a 50% loss in acritarchs (organic-walled phytoplankton groups), collectively known as the Wenlock ‘Big Crisis’. Previous studies have aligned the various Big Crisis extinctions to a double peaked carbon isotope excursion (CIE), known as the Mulde CIE, and inferred this to be a major change in the global carbon cycle. However, causal mechanisms that link the Mulde CIE to the ‘Big Crisis’ remain poorly constrained. In this thesis, two carbonate-bearing sequences connected to separate paleobasins, the shallow shelf carbonate sequence at McCrory Lane, TN and upper slope mixed siliclastic-marl sequence at Coal Canyon, Simpson Park Range, NV have been analyzed for multiproxy investigations. I present new δ13C, δ34S, I/(Ca+Mg), pyrite framboid, and carbonate microfacies data from these two successions spanning the Mulde CIE and Big Crisis interval. I document for the first time a positive ~10‰ shift in δ34SCAS during the Big Crisis extinction interval and first peak of the Mulde CIE. This suggests an expansion of reducing conditions globally; triggering enhanced organic matter and pyrite burial during a eustatic sea-level rise. At McCrory Lane, TN, local pyrite sulfur, I/(Ca+Mg), and microfacies fauna analysis support enhanced pyrite burial, and indicate the temporary contraction of reducing conditions during the Big Crisis survival and recovery intervals and protracted eustatic high stand. During the recovery interval of the Big Crisis the second Mulde CIE peak occurs and new δ34SCAS data suggest a second smaller expansion of reducing conditions in the late Wenlock oceans occurred. Interestingly, there is no documented no biotic turnover that occurs in concert with the second Mulde CIE peak suggesting a smaller and limited nature of this second expansion of marine reducing conditions. Statistical thin section analyses of fauna grain composition and niche replacement and near ubiquitous presence of pyrite framboids further support the notion that global oxygen content and bioavailability is still playing a major role in the evolution of biosphere and oceanic redox conditions through the mid-Paleozoic.
