FORAMINIFERAL RESPONSE TO THE ONSET OCEANIC ANOXIC EVENT 2 NEAR THE SOUTHERN APERTURE OF THE WESTERN INTERIOR SEAWAY

The Cenomanian/Turonian boundary (~94-93 Ma) is associated with intense and rapid changes to Earth’s climate, oceans and biosphere. Global temperatures increased, global sea level rose, and Oceanic Anoxic Event 2 (OAE2) developed and spread. OAEs are associated with increased marine productivity and the expansion of oxygen minimum zones, leading to the widespread deposition of organic-rich sediments and black shales. OAE2 represents a major perturbation to the marine carbon cycle as evidenced by the globally observed positive carbon isotope excursion, in both organic carbon and carbonates, that defines the event. Curiously, in the Western Interior Seaway (WIS), the onset of OAE2 is marked by a unique, abrupt oxygenation event (the ‘Benthonic Zone’) and low TOC. The WIS was a shallow epicontinental sea in the western interior of modern day US and Canada that connected the Tethys and Arctic Oceans. The seaway was a mixing chamber for Boreal water masses from the north and Tethyan water masses from the south. As sea level rose through this interval, there was increased communication between these two distinct water masses. This may have facilitated the creation of a new intermediate water mass through caballing, which was then exported into the Gulf of Mexico through the southern aperture of the seaway (Hay et al., 1993). The intense paleoenvironmental and paleoceanographic changes of this interval prompted extinction and biotic turnover in the water column. The Tokay Tongue of the Mancos Shale (Socorro Country, New Mexico) is an expanded section that records the onset of OAE2 and peak transgression in the seaway near its southern aperture. Here, we present a new offshore foraminiferal record to examine the biotic response to the rapid changes associated with OAE2 and rising sea level in the WIS. Preliminary foraminiferal assemblage data suggest that there are benthic foraminifera present below the ‘Benthonic Zone’, unlike correlative strata of the Hartland Shale found to the north. There is also evidence for boreal influence during the peak of OAE2 by the appearance of T. rainwateri. Although the intense ocean-climate-biosphere changes are well documented in the WIS and around the world, the effect of OAE2 coupled with oceanographic changes, like rising sea level, on foraminiferal communities is still poorly understood.