PALEOCEANOGRAPHIC IMPLICATIONS OF RISING SEA LEVEL IN THE WESTERN INTERIOR SEAWAY THOUGH THE CENOMANIAN-TURONIAN BOUNDARY INTERVAL

The Western Interior Seaway (WIS) was a shallow epicontinental sea that connected the Arctic Basin and western Tethys. The uppermost Hartland Shale and lower Bridge Creek Limestone Members of the Greenhorn Formation record the history of transgression and peak flooding in the seaway as well as major oceanographic and environmental changes like Oceanic Anoxic Event 2 (OAE2) through the Cenomanian-Turonian (C/T) boundary interval. OAE2 is characterized by a positive carbon isotope excursion in organic and bulk carbonate representing a large scale perturbation of the global carbon cycle. The onset of OAE2 in the WIS is marked by a unique, abrupt oxygenation event as evidenced by abundant and diverse planktic and benthic foraminiferal assemblages and low TOC. The ‘Benthonic Zone’ at the base of the Bridge Creek Limestone records the rapid ventilation of the seafloor in the WIS; an event that coincides with the Plenus Cold Event in Europe. At multiple sites on Demerara Rise in the southern Caribbean, an abrupt positive Nd excursion is observed at the onset of OAE2, which might represent the incursion of a new intermediate water mass, potentially sourced from the WIS. Preliminary Nd data from fish debris in the Bridge Creek Limestone of Rock Canyon, CO record relatively radiogenic values (-5.5 to -0.1) in the latest Cenomanian, likely reflecting alteration of abundant volcanic ash. The WIS was a mixing chamber for Boreal water masses from the north and Tethyan water masses from the south. We hypothesize that a new intermediate water mass was created in the WIS through caballing as proposed by Hay et al. (1993) and exported into the Gulf of Mexico through the southern aperture of the seaway. As sea level rose and the southern sill of the WIS was breached, warm Tethyan waters rapidly pushed north into the WIS while cooler Boreal waters briefly invaded NW Europe. This rapid advance of Tethyan waters into the WIS also facilitated the short-lived formation and export of an intermediate water mass as recorded at Demerara Rise. To further test this, fish teeth and debris will be extracted for Nd analyses from bedding couplets of the Bridge Creek Limestone through the C/T boundary interval. Understanding the formation and export of an intermediate water mass may illuminate the role of the WIS in the spread of OAE2 and in Northern Hemisphere circulation.