SYNCHRONOUS FACIES CHANGES AND QUANTIFIED HIATUSES AT THE ONSET OF MID-CRETACEOUS OCEANIC ANOXIC EVENT 2 IN THE WESTERN INTERIOR BASIN (N. AMERICA)

Arthur and colleagues in the 1970s were the first to recognize Mesozoic oceanic anoxic events based on positive carbon isotope (δ¹³C) excursions and show the utility of these major perturbations in the global carbon cycle as chemostratigraphic tools. Since then, other proxies for changes in ancient ocean chemistry, such as initial osmium isotope ratios (Os_i), have been implemented as high-resolution chemostratigraphic markers; they have potential to record the onset of submarine large igneous province (LIP) volcanism, such as that which occurred at the onset of Oceanic Anoxic Event 2 (OAE2) (~94 Ma).

Our recent work correlates high-resolution δ¹³C and Os_i chemostratigraphies from key Western Interior Basin (WIB) localities preserving OAE2 along a distal to proximal and depth transect. Although lithofacies from the different localities are distinct, each site records prominent excursions to unradiogenic Os_i compositions signaling enhanced submarine volcanism, which are followed by increased global organic carbon burial (as indicated by positive δ¹³C shift). Using a published astrochronologic time scale, the time lag between the onset of LIP volcanism before OAE2 and the carbon cycle response is quantified at 2-2.5 precession cycles (~40-50 kyr). The presence of this lag between Os_i and δ¹³C excursions in the Portland Core indicates that the hiatus in the uppermost Hartland Shale near the Cenomanian-Turonian Stage GSSP is relatively brief (<40 kyr) compared to the duration of OAE2.

Within the refined chronostratigraphic framework of bentonite stratigraphy, Os_i and δ¹³C chemostratigraphies, and biostratigraphy, changes in the depositional environment of the WIB can be evaluated at the highest temporal resolution to-date. Notably, carbonate content decreased synchronously in two intervals during the lag between Os_i and δ¹³C excursions in the WIB, and near Bentonite “A” where Os_i decreases subtly again. The compilation of datasets from a range of depositional settings demonstrates that carbonate sedimentation was suppressed in the Western Interior Seaway during these two intervals of OAE2. This suggests that the injection of large quantities of CO₂ to the ocean-atmosphere system from LIP volcanism altered marine carbonate chemistry or other factors governing carbonate production in the WIB.