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

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 δ¹³C_org 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 δ¹³C_org 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 δ¹³C_org records across the KPB reveal a statistically significant correlation between δ¹³C_org and lithology, implying that local facies variation on the continental landscape controls δ¹³C_org values to some extent. In particular, δ¹³C_org values from coals, lignites, and carbonaceous shales are lower (0.5 – 1.0‰) than claystones and siltstones. We record a negative δ¹³C_org 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.