Influence of oceanic anoxic events on the evolution of mid-Cretaceous radiolaria in the North Atlantic and western Tethys

Abstract

Mid-Cretaceous (Aptian-Turonian) extinction/radiation events of radiolaria from the North Atlantic and western Tethys are summarized in relation to Oceanic Anoxic Events (OAE) from investigations of radiolarian faunas, black shales and stable carbon isotopes. Extinction and radiation events of radiolaria can be observed from the late early Aptian, the earliest Albian, the latest Albian and the Cenomanian/Turonian boundary.These events correlate well with organic carbon-rich sediments that contain type II kerogen and show positive δ13C excursions. They correspond to the Oceanic Anoxic Events (OAE) in the early Aptian (OAE 1a, Selli Level), early Albian (OAE 1b), late Albian (OAE 1d) and latest Cenomanian (OAE 2, Bonarelli Level). Detailed investigations of the two most significant events, the Selli and the Bonarelli Level in the Umbria-Marche Basin of central Italy show that extinctions coincide with the first occurrence of marine black shales (type II kerogen) and the most prominent rise of δ13Corg values. Stepwise extinctions and continuous radiations precede and follow the events. Similar patterns are evident for the OAE 1b and 1d. Black shales dominated by terrestrial organic matter (type III kerogen e.g., OAE 1c) are not characterized by isotopic excursions or by radiolarian extinction/radiation events.Contemporaneous drowning events documented from carbonate platforms around the investigated basins provide a link to a model where flooding of lowlands and leaching of nutrients during a relative rise of sea-level lead to an increased productivity and an expansion of the oxygen minimum zone (OMZ). Because of the loss of deep habitats, deeper dwelling forms become extinct. Shallower dwelling radiolarians survive. A narrowing of the OMZ during highstands and relative sea-level falls together with a coupled decrease of the nutrient supply leads to the development of new niches and therefore to radiation.