BENTHIC FORAMINIFERA MORPHOLOGY TRACK DYNAMIC SHIFT IN OXYGENATION DURING OAE2 IN THE SOUTHWESTERN WESTERN INTERIOR SEAWAY

The Late Cenomanian experienced high global temperatures, sea-level rise, and increased primary productivity, triggering Oceanic Anoxic Event 2 (OAE2; ~94 Ma). The Western Interior Seaway (WIS), a shallow epicontinental sea in the western U.S., records OAE2 in formations including the Mancos Shale. Microfossil data from the WIS reveal foraminiferal bio-events marked by shifts in benthic foraminifera abundance. These unicellular marine protists, which build calcium carbonate tests, respond to food availability and oxygenation shifts during OAE2. One response is morphological change in test shape and size. While assemblage responses to OAE2 are well documented, intraspecific morphometric changes remain understudied.

This study examines test morphometrics in two dominant benthic OAE2 taxa, Neobulimina albertensis and Gavelinella dakotensis, linked to key bio-events: the Benthonic Zone, Gavelinella Acme, and Neobulimina Dominance. Foraminifera from Mancos Shale sections at Carthage, New Mexico, and Lohali Point, Arizona, were analyzed. The shape of N. albertensis was quantified via Fourier shape analysis, while G. dakotensis porosity was assessed using pore-to-total area ratios. Multivariate analyses integrating morphometric and geochemical data reveal environmental drivers of test variation in both taxa, while stratigraphic interpretations reveal temporal changes in test morphology.

Results suggest asynchronous bottom water and pore water oxygenation shifts due to transgression-driven water mass changes, alongside intraspecific morphological shifts revealing insights into the paleoecology of these key benthic species. Organic matter burial and decay could have driven pore water anoxia before bottom water anoxia. Integrating established and novel methods enhances interpretations of southwestern WIS oxygenation during OAE2. Morphometric approaches may help identify foraminiferal bio-events at sites with low benthic abundances where relative abundance cannot be used to identify bio-events. As OMZs expand with climate change, understanding Cretaceous OAEs provides insight into future climate states. This research highlights foraminifera morphometrics as a valuable tool for studying hypoxic events in the past, present, and future.