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New high-resolution organic (δ13C org) and carbonate (δ13C 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 δ13C org during the initiation of rising δ13C carb values at the onset of the Hangenberg Event that ends prior to the onset of the final rise in δ13C carb to values greater than +6.0‰ in the Louisiana Limestone. This negative excursion in δ13C 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 δ13C carb and δ13C 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. • First high-resolution paired δ13C record from the Hangenberg in North America. • Demonstration of apparent decoupling of carbon records at the onset of Hangenberg. • First identification of Hangenberg Black Shale equivalent in study area [ABSTRACT FROM AUTHOR]

The tristate area of Iowa, Illinois, and Missouri contains some of the best-exposed Mississippian strata in the world, including the type area for the Mississippian subsystem, across a broad carbonate platform known as the Burlington shelf. Strata have been mapped as thinnest along the central middle shelf and thickening both up-ramp and down-ramp, forming a complex dumbbelllike stratigraphic pattern rather than a simple clinoform geometry thinning into the basin. Additionally, two significant hiatuses at the Devonian-Carboniferous boundary and Kinderhookian-Osagean boundary greatly complicate stratigraphic correlations across the region. As a result, the precise temporal relationships between strata deposited across the region remain uncertain. Two large biogeochemical events occurred during this interval that provide facies-independent chronostratigraphic tools: the Hangenberg event, which marks the Devonian-Carboniferous boundary, and the Kinderhookian-Osagean boundary event. To target these events, we collected 66 conodont samples and 1005 carbonate carbon isotope samples from three cores and three outcrops and integrated the results with existing data from key facies/depth transitions across the Burlington shelf. Our new data demonstrate a complex relationship among complementary stratigraphic thicknesses, where the Devonian-Carboniferous boundary interval is thin or absent in the up-ramp innershelf setting and preserved in a significantly expanded interval in the central to distal middle-shelf deposits of southeast Iowa and northeast Missouri. However, the overlying Kinderhookian-Osagean boundary interval is not preserved in this down-ramp setting but is preserved in significantly expanded strata in the up-ramp inner-shelf setting of central Iowa. [ABSTRACT FROM AUTHOR]