ECOLOGICAL ANALYSES OF FORAMINIFERAL BIOEVENTS REVEAL SPATIAL VARIATION IN THE RESPONSE TO OAE2 (Invited Presentation)

Cenomanian-Turonian marine deposits of the Western Interior Seaway (WIS) exist across the western United States and record the dynamic paleoceanographic conditions associated with the Greenhorn transgression. Foraminiferal assemblages through the Cenomanian-Turonian boundary in the Great Plains region feature distinct bio-events, principally the ‘Benthonic Zone (BZ).’ The BZ, as defined by previous workers, coincides with the positive carbon isotope (δ¹³C) excursion that marks Oceanic Anoxic Event 2 (OAE2) in the WIS. The BZ is characterized by a rapid increase in the relative abundance and diversity of benthic foraminifera compared to planktonic foraminifera. Here we test the hypothesis that the BZ represents an ecological shift related to changing environmental conditions and is not simply a change in benthic foram abundance driven by factors such as changes in sedimentation rates or preservation. We integrate existing and new abundance counts of benthic forams from before, during, and after the BZ into a taxonomically standardized dataset that spans the WIS from a north-south transect from Montana to Texas and an east-west transect from Kansas to Utah. We use multivariate ordination analyses to summarize changes in morphotypes and taxonomic composition and to compare the primary axes of variation to independent environmental proxies. Preliminary analyses show a basin-wide shift in species composition coincident with the onset of the BZ; this shift is consistent in direction but varies in magnitude among sites. This shift is characterized by an increase in diversity and a decrease in dominance of Neobulimina albertensis, a low-oxygen indicator, and an increase in Tappanina laciniosa, a high productivity indicator. Further, the dominant morphotypes shift from infaunal to epifaunal into the BZ, although the expression differs among sites. Thus, we demonstrate that the BZ is an ecological response indicative of changing WIS environments and varies spatially with the relative intensity of the environmental change.