Chemoautotrophy as the driver of decoupled organic and carbonate carbon isotope records at the onset of the Hangenberg (Devonian-Carboniferous Boundary) Oceanic Anoxic Event

Abstract

New high-resolution organic (δ13Corg) and carbonate (δ13Ccarb) carbon isotope data from middle shelf deposits in southeastern Iowa demonstrate decoupled signals during the onset of the Hangenberg Event and across the Devonian-Carboniferous Boundary. High-resolution sampling captures a transient negative excursion in δ13Corg during the initiation of rising δ13Ccarb values at the onset of the Hangenberg Event that ends prior to the onset of the final rise in δ13Ccarb to values greater than +6.0‰ in the Louisiana Limestone. This negative excursion in δ13Corg is coincident with a significant increase in Total Organic Carbon (TOC) content in the underlying English River Formation, which likely corresponds to the well-known Hangenberg Black Shale of the classical European sections. The complex behavior of the carbon isotope record recovered here, combined with recently published geochemical data from classical European sections, demonstrate that a succession of geochemical events took place during the initiation of this global biogeochemical event that include a negative excursion in both δ13Ccarb and δ13Corg prior to the major positive carbon isotope excursion, and that the role of organic carbon burial in this Devonian Oceanic Anoxic Event (OAE) extends well beyond the depositional interval of the Hangenberg Black Shale.