AN EXCEPTIONAL RECORD OF EARLY- TO MID-PALEOZOIC REDOX CHANGE FROM THE ROAD RIVER GROUP, YUKON, CANADA (Invited Presentation)

The early Paleozoic hosts many of the landmark events in animal evolution, including the most significant increase in animal diversity and ecological strategies (Great Ordovician Biodiversification Event), and one of the major mass extinctions. The causal relationship (if any) between these events and evolving environmental conditions—particularly the redox state of ancient oceans—is heavily debated. Here we build an extensive record of early- and mid-Paleozoic redox change from the upper Cambrian to Middle Devonian Road River Group of Yukon, Canada. The Road River Group represents an ideal sedimentary succession to investigate the long-term history of Paleozoic redox change because it provides a nearly continuous record of deep-water marine deposits, enabling the distinction of true signal from background fluctuations. Both the ichnological record and iron geochemical data suggest that the basin was dominantly anoxic for the entirety of Road River Group deposition. In the Ordovician and Silurian the anoxic water column was dominantly ferruginous (free ferrous iron), transitioning towards more sulfidic conditions in the Early to Middle Devonian. This anoxic background state was punctuated several times by oxygenated conditions sufficient for the establishment of widespread bioturbation: once in the late Katian, and following the early Sheinwoodian, Homerian, and late Ludlow positive carbon isotope excursions in the Silurian. Abundances of redox-sensitive trace metals in anoxic shale, generally understood to be indicative of global seafloor redox conditions, increase in Lower Devonian as compared to the Ordovician/Silurian strata, consistent with hypotheses of increased global oxygenation at that time. However in both the Road River Group and in new analyses of Ordovician-Devonian graptolitic shale collections worldwide, this trace metal increase is coupled to increasing dominance of sulfidic conditions, suggesting additional controls on metal abundances in anoxic shale than anoxic seafloor area alone. Overall, the Road River Group illustrates characteristics of both earlier Neoproterozoic oceans and later Mesozoic/Cenozoic oceans, and points towards a transitional ocean state in the Paleozoic.