User:Macdonald-ross/Cenomanian–Turonian

At the Cenomanian–Turonian boundary in the Upper Cretaceous was quite a large extinction event. Ichthyosaurs and pliosaurs became extinct. So did 27 percent of marine invertebrates, including certain planktic and benthic foraminifera, molluscs, bivalves, dinoflagellates and calcareous nannofossils (microfossils).

The boundary is also known as the Bonarelli event. A one to two metre layer of thick black shale marks the boundary. It was first studied by Guido Bonarelli in 1891. It has interbedded black shale, chert and radiolarian sands. It spans a 400,000-year interval. Planktonic foraminifera do not exist in this Bonarelli level. The presence of radiolaria in this section shows there were nutrients.

One possible cause of this event is sub-oceanic volcanism. During that period, the rate of crustal production reached its highest level for 100 million years. This was mainly 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 sent 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. The resulting high levels of carbon burial would account for the black shale deposition in the ocean basins.