Spatial heterogeneity in benthic foraminiferal assemblages track regional impacts of paleoenvironmental change across Cretaceous OAE2

The impact of global climate events on local ecosystems can vary
spatially. Understanding this heterogeneity can illuminate which
environments will be most impacted and the proximal drivers of ecosystem
responses. Cenomanian-Turonian marine deposits of the Western Interior
Seaway (WIS) record paleoceanographic changes associated with
transgression and the onset of Oceanic Anoxic Event 2 (OAE2). They provide
an ideal setting to study basinwide paleoecological responses during a
global perturbation. Here, we integrate benthic foraminiferal assemblages
from before, during, and after OAE2 via multivariate ordination analysis
to examine spatial patterns in faunal responses across the western United
States on a common scale and to interrogate a previously defined faunal
marker used for basin-wide correlation, the Benthonic Zone (BZ). We
identify oxygenation and organic matter quality as primary and secondary
controls of variation across the 10 stratigraphic records and use this
variation to infer paleoenvironmental changes. Stratigraphic trends
reveal, in contrast to previous studies, deoxygenation at the onset of
OAE2. They also reveal temporal patterns in oxygenation and productivity
consistent with the gradual northward migration of a southern water mass
into the WIS. This spatial heterogeneity hinders the use of the BZ in
biostratigraphy because assemblages change in response to diachronous
environmental change, and thus timing of the BZ with respect to OAE2 is
not consistent across the basin. Our study demonstrates that regional
processes can overshadow ecosystem responses to global events and
underscores the importance of considering how changes in the position of
water masses impact the expression of global biogeochemical perturbations.