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

The extent to which Paleozoic oceans differed from Neoproterozoic oceans, and the causal relationship (if any) between biological evolution and changing environmental conditions, is heavily debated. Here we report an expansive record of Paleozoic seafloor redox change from the upper Cambrian to Middle Devonian Road River Group of Yukon, Canada. This represents an ideal sedimentary succession to investigate long-term redox change, as it provides a nearly continuous record of deep-water marine sedimentation over ~110 million years of Paleozoic history. More than 1,100 shale samples from the Road River Group and other Paleozoic shale units worldwide were analyzed for major- and trace-element geochemistry, iron speciation, and total organic carbon content. Redox geochemical data indicate that bottom waters were broadly anoxic during the entirety of Road River Group deposition, but independent evidence from iron speciation and Mo/U ratios suggests that the biogeochemical nature of anoxia changed through time. Both in Yukon and in a global dataset, Ordovician through Early Devonian anoxic water columns were broadly ferruginous (non-sulfidic), with a transition towards more euxinic (sulfidic) water columns in the mid-Early Devonian (Pragian), coincident with the disappearance of graptolites. Thus, an ~80 million-year interval of the early Paleozoic was characterized by ferruginous bottom waters, similar to the Neoproterozoic redox geochemical record. Utilizing this new global shale compilation, trace metal-based inferences of the global extent of reducing conditions suggest that dynamic redox conditions characterized the Paleozoic, with relatively more oxygenated conditions in the late Cambrian-Early Ordovician and Mid-Late Devonian.