HIGH-RESOLUTION FORAMINIFERAL AND STABLE ISOTOPE RECORD FROM THE TOKAY TONGUE (MANCOS SHALE) THROUGH THE ONSET OF OCEANIC ANOXIC EVENT 2

The Cenomanian-Turonian (C-T) boundary (~94-93 Mya) is associated with major changes in Earth’s climate and ocean, including warming of global temperatures, rising sea level, and the onset of Oceanic Anoxic Event 2 (OAE2). This globally observed event is defined by a positive carbon isotope (δ¹³C) excursion in both organic carbon and carbonate and represents a major perturbation to the carbon cycle. These changes affected the marine biosphere and resulted in extinction and turnover events in the water column. The Western Interior Seaway (WIS) was a shallow epicontinental sea that occupied a foreland basin in western North America through this interval. Previous microfossil studies across the WIS have defined patterns of biotic and oceanographic change, such as the ‘Benthonic Zone’ (Eicher & Worstell, 1970) and the ‘Heterohelix shift’ (Leckie, 1998), that reflect responses to environmental change on the seafloor and in the plankton. Here, to better understand the biotic response to rapid oceanographic and environmental changes linked to OAE2 at the southern aperture of the WIS, we present a high-resolution foraminiferal assemblage record and bulk organic carbon δ¹³C chemostratigraphy from central New Mexico. The Tokay Tongue of the Mancos Shale in central New Mexico preserves an expanded stratigraphic record of OAE2 for high-resolution reconstruction of oceanographic and environmental changes through the event. We collected 175 samples from below and through the C-T boundary from the Tokay Tongue in Carthage, NM. Samples were crushed, soaked, boiled, and washed over a 63-micron sieve, then dried and picked for population analysis. Bulk powdered samples were analyzed for stable isotopes using standard methodology at Northwestern University; the results facilitate chemostratigraphic correlation with published d¹³C records from other WIS sites. Preliminary foraminiferal assemblage data show that the record captures bio-events like the ‘Benthonic Zone,’ but at times differs from correlative strata in the deeper, central part of the WIS, which may suggest the presence of a distinct water mass at this site. This record can be compared to other high-resolution studies to better understand the effect of rapid environmental changes associated with OAE2 and rising sea level on foraminiferal communities in the WIS.