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

New high-resolution organic (delta C-13(org)) and carbonate (delta C-13(carb)) 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 delta C-13(org) during the initiation of rising delta C-13(carb) values at the onset of the Hangenberg Event that ends prior to the onset of the final rise in delta C-13(carb) to values greater than +6.0% in the Louisiana Limestone. This negative excursion in delta C-13(org) 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 delta C-13(carb) and delta C-13(org) 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.