NITROGEN ISOTOPES TELL A TALE OF HOT AND COLD CLIMATE BLACK SHALES IN THE FAMENNIAN APPALACHIAN BASIN

From global hothouse conditions at its base to temperate latitude glaciation at its conclusion, the Famennian stage of the Late Devonian witnessed dramatic changes in the Earth’s climate. Abundant organic-rich facies of the Famennian Appalachian Basin contain a record of the impact of the reoxygenation of the atmosphere/ocean system on marine biogeochemical cycles. With respect to the marine nitrogen cycle, the increasing depth of the oxygenated mixed layer in the Famennian Appalachian Basin resulted in increased water column nitrification/denitrification and a consequent enrichment in sediment δ¹⁵N. We report a ~7‰ enrichment in δ¹⁵N_total from the base of the Famennian in the Lower Huron Shale where evidence points to low pO₂ and a warm climate regime to the the top of the Cleveland Shale where there is abundant evidence for nearby continental glaciation and a cool climate regime. The significance of our finding is that while climate does not exert the primary control on black shale deposition, the climate-driven redox state of the water column is reflected in the bulk N isotope composition of the rock.