THE BLACK SHALE RECORD OF THE ORDOVICIAN RADIATION: NEW INSIGHTS INTO REDOX CHANGE FROM THE ROAD RIVER GROUP OF YUKON, CANADA

The Ordovician radiation comprises the highest magnitude diversification of metazoan families in the fossil record, yet the catalyst(s) of this event remain contentious. Oxygen availability is often linked to the evolution and diversification of animals, and it is becoming increasingly clear that anoxia was common in early Paleozoic oceans. From this relatively reducing background state, carbonate proxy records suggest the ocean may have become more oxygenated coincident with the Ordovician radiation. Black shales are the other major proxy record recording ancient ocean redox conditions, and, to date, there have been no targeted shale geochemistry studies of the Ordovician radiation. Such studies would reveal the redox states of ocean basins during this time interval and provide important details on oxygen availability to marine ecosystems. The Road River Group, deposited in the Richardson Trough in Yukon, Canada, represents a nearly continuous deep-water marine succession of fine-grained sediments from the upper Cambrian to the Middle Devonian, which makes it an excellent unit to study early Paleozoic geochemical change. Our iron speciation results demonstrate that, throughout the Early and Middle Ordovician, local depositional conditions were largely anoxic and ferruginous with intermittent euxinia (anoxic and sulfide-dominated). Handheld XRF analyses of the redox-sensitive trace metals molybdenum and uranium support the iron speciation results, with substantial authigenic enrichments indicating that Early and Middle Ordovician black shales in the Richardson Trough were deposited under strongly reducing conditions. Thus, the Richardson Trough does not record local evidence of oxygenation during the Ordovician radiation, although further analyses of trace metal patterns will provide insight into the global redox landscape during this time.