Fluctuations in depositional conditions during the onset of severe climate events in Earth history predispose stratigraphic archives to hiatuses, often hindering complete reconstructions of paleoclimate events and their triggers. Several studies have proposed that a hiatus of unknown duration exists at the base of Oceanic Anoxic Event 2 (OAE2) in the North American Western Interior Basin at the base Turonian global boundary stratotype section and point (GSSP) in Pueblo, Colorado, which potentially influences integrated radioisotopic, biostratigraphic, and astrochronologic age models of the Cenomanian-Turonian boundary interval. To quantify the duration of this hiatus, refine the chronology of OAE2, and assess marine geochemical perturbations associated with the onset of the event, we present new 40Ar/39Ar dates from regional bentonites along with a new proximal-distal chemostratigraphic transect of the epeiric Western Interior Basin (WIB), including initial osmium isotope (Osi) and stable carbon isotope (δ13C) data. The new 40Ar/39Ar age determinations confirm and further constrain previous estimates of Cenomanian-Turonian boundary timing. Further, the regional chemostratigraphic synthesis demonstrates the conformity of the OAE2 successions correlated to Pueblo, shows that the duration of the lag between the onset of the Osi and δ13C excursions is ∼60 k.y., and thus constrains the magnitude of the pre-OAE2 hiatus in Pueblo to less than this value. The new astronomically tuned, conformable Osi record across the onset of OAE2 captures a geologically rapid onset of large igneous province volcanism, consistent with other records, such that the addition of CO2 to the ocean-atmosphere system may have driven changes in marine carbonate chemistry. Additionally, the refined chronostratigraphy of OAE2 and the Cenomanian-Turonian boundary in the central WIB improves correlation with other records, such as those in the Eagle Ford Group, Texas. The correlations highlight that discrepancies among OAE2 age models from globally distributed sections commonly stem from differing definitions of the event and uncertainties associated with astronomical tuning, in addition to stratigraphic preservation.

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