HIGH-RESOLUTION PAIRED ORGANIC AND CARBONATE CARBON ISOTOPE CHEMOSTRATIGRAPHY OF THE HANGENBERG EVENT FROM H28 AND H32 CORES IN SOUTHEAST IOWA

This study uses new high-resolution organic (δ¹³C_org) and carbonate (δ¹³C_carb) carbon isotope data from shelf deposits in Iowa to explore the Hangenberg Event, a mass extinction event and carbon isotope excursion spanning the Devonian-Carboniferous boundary. Biostratigraphic correlation of conodonts between the standard European section and strata in Iowa place the early and middle Hangenberg Crises in the siltstones and shales of the upper English River Formation (upper B. ultimus Zone) and the late Hangenberg Crisis in the limestone of the lower Louisiana Formation (lower-mid kockeli Zone). Our results show decoupling of the δ¹³C_carb and δ¹³C_org curves followed by a positive excursion in both carbonate and organic carbon isotopes. The curves decouple in the upper English River Formation, where δ¹³C_org values decrease and values of δ¹³C_carb increase simultaneously. Further investigation into the factors controlling the isotopic fractionation of carbon using Δ¹³C led us to conclude that the negative δ¹³C_org excursion is a global signal caused by the addition of chemoautotrophic organisms to the sediment. In the overlying Louisiana Formation δ¹³C_carb and δ¹³C_org values reach sustained peaks of +6‰ and -26‰, respectively. Calculated ƒ_org values increase in the Louisiana Formation, corresponding with the positive δ¹³C_carb excursion and indicating the most likely cause of the positive carbon isotope excursion is increased organic carbon burial. This study emphasizes the importance of high-resolution sampling, as it was a critical component of our ability to decipher short-term changes in the carbon cycle.