Environmental and Biotic Changes around the Cenomanian/Turonian Boundary (Late Cretaceous): Scenario of a Mass Extinction

With the extinction of approximately 26% of marine animal genera (e.g. foraminifers, nannoplankton, corals, echinoids, bivalves, ammonoids), the Cenomanian/Turonian boundary (CTB) is one of the ten most severe biotic crisis of life's history. The mid-Cretaceous was a time of transition in the ocean-climate system. Close attention to the precise timing and space extent of major proxies of global environmental changes leads to an integrative scenario for biotic changes during the Cenomanian. This mass extinction appears to result from the complex interplay of several environmental changes. Because of the widespread occurrence of organic-rich deposits and a carbon isotope excursion, a global oceanic anoxic event (OAE2) has been widely recognized as the primary cause of extinction at the CTB. However, anoxia occurs at the end of a long-term biotic crisis and its importance appears to be overestimated. Biotic changes before and near the CTB appears to be triggered mostly by the long-term global warming and flooding which spanned the entire Cenomanian. Indeed, this flooding which resulted in the highest highstand of the Mesozoic (probably triggered by increased spreading rate of mid-oceanic ridges) and the concurring global warming (probably triggered by an increased marine volcanism) led to the drowning of platforms, major changes in oceanic circulation, the poleward migration of warm surface waters, a more uniform climate, and a reduction of the equator-to-pole sea surface temperature gradient, among others. These changes contributed to the homogenization of marine habitats, to the decreasing endemism and diversity of organisms, to the poleward migration of tropical faunas, and thus to the Cenomanian/Turonian boundary mass extinction.