Neritic ecosystem response to Oceanic Anoxic Event 2 in the Cretaceous Western Interior Seaway, USA

Abstract

Cretaceous oceanic anoxic events (OAEs) were periods of geologically short (<1 million years) global change characterized by elevated temperatures, changes in ocean biogeochemistry, ecological turnover, and the global-scale deposition of black shales. After decades of OAE research, the intensity and spatiotemporal heterogeneity of ocean anoxia and its direct effects on marine ecology remain areas of active study. We present high-resolution organic geochemical and foraminiferal records from the western margin of the Western Interior Seaway (WIS) during the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2, ~94 Ma) that indicate reorganization of a neritic ecosystem in response to sea-level rise, and dynamic changes in redox conditions that were likely driven by enhanced marine productivity. A significant transgression prior to OAE2 decreased terrestrial organic matter input and led to enhanced productivity, anoxic bottom waters and sediments, and episodic photic zone euxinia. During the middle of OAE2, further enhanced productivity led to the “Heterohelix shift” in planktic foraminifera, the “Gavelinella acme” in benthic foraminifera, and deoxygenation in sediments, bottom waters, and the upper water column. The combined use of high-temporal resolution algal, bacterial, and terrestrial biomarkers, in addition to foraminiferal records, demonstrates the sensitivity of marine autotrophic and heterotrophic communities to sea-level rise and variable water column oxygenation in marginal marine settings during hyperthermal events. Furthermore, our results reveal the temporal and spatial heterogeneity of anoxia during OAE2 in the WIS. This study highlights widespread shallow marine feedbacks during global change that may affect neritic ecosystems under future warming scenarios.