186-10 A Neritic Record of Cenomanian-Turonian Oceanic Anoxic Event 2 from Coastal Utah, U.S. Western Interior Seaway

Abstract:

The Upper Cretaceous Tropic Shale of southern Utah captures oceanographic changes that occurred along the shallow (<80-100 m) western margin of the Western Interior Seaway (WIS) during Oceanic Anoxic Event 2 (OAE2). To resolve these changes at high temporal resolution (<5 ka), this study reports planktic and benthic foraminiferal assemblages in relation to calcareous nannofossils, stable organic carbon isotope (d¹³C_org) chemostratigraphy, and organic biomarker datasets compiled from a composite 35-m outcrop and core section spanning ~1.09 myr.

The onset of OAE2, identified by a distinctive d¹³C_org signature, coincided with a rapid transgression and a brief interval of potential cooling as suggested by the abundance of nannofossil Biscutum constans. The tiny opportunistic planktic Guembelitra cenomana initially dominated surface waters and infaunal benthic species Neobulimina albertensis dominated at the seafloor. Other rare species of calcareous benthics, including Hoeglundina charlottae and Buliminella fabilis, demonstrate that this interval is correlative with the "Benthonic Zone" that marks an oxygenation event elsewhere in the WIS.

Epifaunal benthic Gavelinella dakotaensis proliferated as OAE2 intensified in the interval below Bentonite B during the late Cenomanian. The "Gavelinella acme" generally coincides with the widespread planktic “Heterohelix shift”, the latter likely triggered by high productivity and/or photic zone euxinia. However, G. dakotaensis likely records seafloor ventilation, proposed to be a function of caballing (downwelling and mixing) along a strong Boreal-Tethyan oceanographic front. Such frontal conditions appear to have alternated with euxinic conditions--marked by abundances of green sulfur bacteria biomarkers and peaks of Planoheterohelixand --related to stratification as suggested by the nannofossil species Eprolithus floralis and Cyclagelosphaera margerelii. The peak of OAE2 is marked by an abrupt shift back to Neobulimina dominance in benthic assemblages in the latest Cenomanian as the oceanic front waned and stratification again became established with the incursion of oxygen-poor Tethyan waters. Elevated Biscutum abundances suggest potential surface water cooling during the early Turonian as OAE2 waned. 

These results show the "Gavelinella acme" was longer-lived and Neobulimina dominance occurred earlier along the western margin of the WIS compared to studies across the seaway, suggesting that these bioevents were diachronous. The OAE2 interval in the WIS was characterized by dynamic changes in relative sea level and productivity, as well as by frontal mixing or stratification of Boreal and Tethyan-sourced water.

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