Phytoplankton dynamics and nitrogen cycling during Oceanic Anoxic Event 2 (Cenomanian/Turonian) in the upwelling zone of the NE proto-North Atlantic

The Cenomanian-Turonian (Late Cretaceous) climate warming was closely coupled to profound perturbations of biogeochemical cycles and ecosystems. The occurrence of organic matter-rich sediments across various depositional environments of the proto-North Atlantic hereby marks severe oxygen-deficient conditions, culminating in Oceanic Anoxic Event (OAE 2) at the Cenomanian/Turonian boundary. Here we combine bulk, isotope and molecular geochemical techniques to characterize trends in organic matter accumulation and its relationship to biogeochemical cycling (nitrogen, carbon) and marine phytoplankton community shifts at the Tarfaya shelf, situated at the northeastern margin of the proto-north Atlantic. Cenomanian to lower Turonian sediments generally contain significant abundances of well-preserved marine organic matter. Substantial enrichment of well-preserved marine organic matter, however, occurred throughout the middle-upper Cenomanian and lower Turonian and peaked during OAE 2. Organic matter enrichment can be linked to a shift towards increased biomass contributions from prokaryotes (bloom-forming cyanobacteria), whose proliferation occurred in relation to rising sea water temperatures that reached its maximum during OAE 2. A late Cenomanian/early Turonian sea level highstand further promoted accumulation and preservation of marine organic matter at the Tarfaya shelf. Eukaryotic phytoplankton groups, such as rhodophyceae and pelagophyceae, also benefited from high sea water temperatures and proliferated during OAE 2, while haptophyceae, bacillariophyceae and chlorophyceae were more abundant during the early, middle and early-late Cenomanian (pre-OAE 2) times. Due to their differential ammonium and nitrate demand, phytoplankton community shifts were further modulated by the abundance of reduced and oxidized nitrogen species. Negative δ¹⁵N values indicate that ¹⁵N-depleted ammonium was the dominant nitrogen species in the proto-North Atlantic throughout the entire Cenomanian and the early Turonian. Isotopically depleted ammonium was provided to marine primary producers by a microbial loop (regenerative nutrient cycle) and by upwelling either from anoxic bottom waters or from expanded oxygen minimum zones. Molecular markers further indicate that intense diazotrophy was limited to the OAE 2 interval and occurred in conjunction to profound changes within the eukaryotic phytoplankton community, subsequently to the Plenus Cold Event.