Carbon cycling across the southern margin of Laurentia during the Late Ordovician

The Guttenberg Isotope Carbon Excursion (GICE), a positive carbon isotope excursion that occurs near the base of the Katian Stage, is thought to be a global event possibly related to Late Ordovician cooling. Documenting how much regional and global variability exists in carbon isotopic trends prior to and during the GICE is a critical aspect in understanding the implications of the excursion for interpreting changes in the global carbon cycle, paleoclimate, and chemostratigraphic correlation during the Ordovician. To investigate carbon isotopic trends along the southern margin of the Laurentian carbonate platform during the Late Ordovician, we measured bulk carbonate δ¹³C_carb and δ¹⁸O_carb as well as organic carbon δ¹³C_org values from four locations in Alabama. These sections are excellent study sites because they are well exposed, contain the regionally well-correlated Deicke and Millbrig K-bentonites, and are in a region where δ¹³C trends have not been studied. Carbonate δ¹³C_carb results from one section (i.e. Fort Payne) in northeastern Alabama record a 1.5‰ positive excursion above the Millbrig K-bentonite. We interpret the 1.5‰ positive excursion as a primary feature and identify it as the GICE. The characteristic GICE excursion is absent in the other study sections and we attribute this absence to the presence of unconformities (either non-depositional or erosional) in these sections. We find neither evidence for sea level control on δ¹³C trends nor conclusive evidence for δ¹³C_DIC gradients along the southeast margin of Laurentia's epeiric sea. Combined with the proximity to the Iapetus Ocean, these results suggest that carbon isotopic trends in Alabama are a reliable proxy for open ocean conditions.