Microfossil and geochemical records reveal high-productivity paleoenvironments in the Cretaceous Western Interior Seaway during Oceanic Anoxic Event 2

Oceanic Anoxic Event 2 (OAE2; similar to 94.5 Ma) occurred in the latest Cenomanian and represents a perturbation to the global carbon cycle. The event is geochemically recorded in shales and limestones of the Western Interior Basin of North America and is additionally marked by foraminiferal bio-events (e.g., turnovers, extinctions). These bio-events are attributed to changing paleoceanographic conditions and circulation patterns in the Western Interior Seaway related to the onset of OAE2. Here we investigate the paleoenvironment near the southwestern edge of the seaway during OAE2 by integrating microfossil and geochemical records from the lower beds of the Tokay Tongue (Mancos Shale) at Carthage, New Mexico, USA. We demonstrate that this locality represents an expanded section of OAE2 based on temporal constraints from carbon isotope chemostratigraphy and the occurrence of regional marker ash-fall deposits (bentonites), limestones and other carbonate-rich beds. Prior to the onset of OAE2, a unique assemblage of benthic foraminiferal morphologies suggests the presence of a distinct water mass in the southwestern part of the seaway compared to coeval neritic and distal sites. Microfossil assemblages record the Benthonic Zone, a typical OAE2 bio-event, with some distinctions. The event is still identifiable and thereby useful in marking the earliest stages of OAE2. Early in OAE2, calcareous nannofossil and foraminiferal assemblages indicate intervals of high productivity. The dominance of biserial planktic foraminifera (Planoheterohelix spp.) suggests the development of photic zone euxinia with intensification of OAE2. During OAE2, epifaunal trochospiral benthic foraminifera (Gavelinella dakotaensis) suggest intervals of improved conditions related to food availability and seafloor ventilation. Later, as increased surface water productivity and subsequent food availability prevailed at the seafloor through the end of the OAE2 interval, benthic foraminifera were abundant and assemblages were dominated by infaunal, low oxygen tolerant species (Neobulimina albertensis), suggesting prevailing dysoxia. We attribute differences in the expression of bio-events and foraminiferal community composition at Carthage to the influence of freshwater runoff from the western margin that drove high-productivity conditions throughout OAE2.