Cenomanian-Turonian boundary event

The Cenomanian-Turonian boundary event, also known as the Cenomanian-Turonian extinction event, the Cenomanian-Turonian anoxic event, and referred to in Europe as the Bonarelli Event,^[1] was the latter of two anoxic extinction events early in the Cretaceous period. The event occurred approximately 91.5 ± 8.6 million years ago,^[2] and brought about the extinction of the Spinosauridae, Pliosauridae, and possibly Ichthyosauria; though coracoids of Maastrichtian age may belong to ichthyosaurs, indicating the survival of the group until the latest Cretaceous.^[3] Other animals lost some diversity as well. Although the cause is still uncertain, the result starved the Earth's oceans of oxygen for nearly half a million years, causing the extinction of approximately 27 percent of marine invertebrates.^[4] This global environmental disturbance increased atmospheric and oceanic temperatures. Boundary sediments show an enrichment of trace elements, and contain elevated δ13C values.^[5]

One possible cause was sub-oceanic volcanism occurring approximately 500,000 years earlier. During that period, the rate of crustal production reached its highest level for 100 million years. This was largely caused by the widespread melting of hot mantle plumes under the oceans at the base of the lithosphere. This resulted in the thickening of the oceanic crust in the Pacific and Indian Oceans. This volcanism would have produced large quantities of carbon dioxide into the atmosphere, leading to global warming. Within the oceans, the emission of SO₂, H₂S, CO₂, and halogens would have increased the acidity of the water, causing the dissolution of carbonate, and a further release of carbon dioxide. When the volcanic activity declined, this run-away greenhouse effect would have likely been put into reverse. The increased CO₂ content of the oceans could have increased organic productivity in the ocean surface waters. The consumption of this newly abundant organic life by aerobic bacteria would produce anoxia and mass extinction.^[4] The resulting elevated levels of carbon burial would account for the black shale deposition in the ocean basins.^[5]