High-resolution paired organic and carbonate carbon isotope chemostratigraphy of the Hangenberg Event from H-28 and H-32 cores in Southeast Iowa

This paper investigates the Hangenberg Event, a mass extinction event and perturbation to the global carbon cycle, which occurred near the Devonian-Carboniferous Boundary. Perturbations to the global carbon cycle are investigated using a ratio of carbon isotopes ¹³C¹²C (δ¹³C) and are presented in chronologic order, creating a carbon isotope curve. Here samples of organic carbon (δ¹³C_org) and carbonate carbon (δ¹³C_carb) were collected from marine deposits in southeastern Iowa and were used to examine possible causes of perturbations to the carbon cycle. A positive increase in carbon isotope values (also known as a positive excursion) was seen in limestone deposits of the Louisiana Formation. Typically positive excursions in carbonate carbon isotope curves are thought to be related to the burial of organic carbon and the data we collected indicates that the positive δ¹³C_carb excursion of the Hangenberg Event occurs simultaneously with increased burial of organic carbon.

Organic carbon and carbonate carbon isotope curves generally respond to the same triggers, creating the same curve shape. However, the small sampling interval we used gave us data showing that the curves begin shifting in different ways, or decouple. The δ¹³C_org curve becomes more negative, while the δ¹³C_carb curve becomes more positive. We hypothesize that this is a result of chemoautotrophic organisms being deposited in the sediment, driving organic carbon values to be more negative.