GSA 2020 Connects Online

Paper No. 178-3
Presentation Time: 10:35 AM

RECONSIDERING THE USE OF CONTINENTAL ORGANIC CARBON ISOTOPE RECORDS IN UNDERSTANDING CARBON CYCLING AT THE CRETACEOUS-PALEOGENE BOUNDARY


TOBIN, Thomas S., Geological Sciences, University of Alabama, 201 7th Avenue, Room 2003 Bevill Building, Tuscaloosa, AL 35487-0268, HONECK, Jacob W., Geological Sciences, University of Alabama, 201 7th Avenue, 2003 Bevill Building, Tuscaloosa, WA 35487, FENDLEY, Isabel, Department of Earth and Planetary Science, University of California, Berkeley, Berkeley, WA 94720, WEAVER, Lucas N., Department of Biology, University of Washington, 247 Life Sciences Building, Seattle, WA 98195, SPRAIN, Courtney J., Department of Geological Sciences, University of Florida, Williamson Hall, Gainseville, FL 32611, TUITE, Michael L., Jet Propulsion Laboratory, California Institute of Technology, Pasadena, WA 91109, FLANNERY, David M., School of Earth, Environmental and Biological Sciences, Queensland University of Technology (QUT), Brisbane, QLD 4000, Australia, MANS, Wade W., Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, WA 87131 and WILSON MANTILLA, Gregory P., Department of Biology, University of Washington, Life Sciences Bldg Rm 251, Seattle, WA 98195

The Cretaceous-Paleogene boundary (KPB) is synchronous with one of the largest mass extinctions, which is partially or entirely driven by the asteroid impact at Chicxulub, though debate about the relative importance of the Deccan Traps as a contributing factor to the mass extinction continues. Marine carbon isotope records across the KPB often record a relatively sudden and long-lasting (0.1 – 1 M.y.) negative shift in carbon isotope (δ13C) values immediately after the KPB that reflects a significant disturbance to carbon cycling associated with the mass extinction. In contrast, continental carbon isotope records across the KPB have been interpreted as showing a very short (< 10 k.y.) isotopic excursion. This inconsistency between the marine and continental records has complicated interpretations of global carbon cycling at this important time period in Earth history.

Here we present δ13Corg data from several sections spanning the KPB in the Hell Creek region of Montana (USA). In this region, the KPB has been recognized within a ‘impact claystone’ that contains direct evidence for bolide impact, and previous studies have reported a ~2.0‰ excursion in δ13Corg at the same stratigraphic level. However, the carbon isotope excursion is typically found in association with a coal layer that marks the contact between the Hell Creek and Fort Union formations, which may confound attempts to separate changes in local depositional regimes from global environmental changes. Our δ13Corg records across the KPB reveal a statistically significant correlation between δ13Corg and lithology, implying that local facies variation on the continental landscape controls δ13Corg values to some extent. In particular, δ13Corg values from coals, lignites, and carbonaceous shales are lower (0.5 – 1.0‰) than claystones and siltstones. We record a negative δ13Corg excursion associated with the KPB, but it is not statistically different from similar excursions associated with coals within the Fort Union Formation. Consequently, the proposed short-duration continental carbon isotope excursion at the KPB is at least in part the result of local lithological variation, rather than a global atmospheric signal. We advise caution when using this brief continental excursion to model or interpret carbon cycling associated with the KPB.