The Cenomanian–Turonian boundary (90.4 Ma) represents a major period of worldwide environmental disturbance. The physical manifestations of this are: elevated atmospheric and oceanic temperatures; a significant sea-level transgression; and a period of widespread anoxia, leading to the formation of oceanic black shales, and the extinction of 26% of all genera. Elevated δ13C values and enrichment of trace elements in Cenomanian–Turonian boundary sediments, combined with a reduction in 87Sr/86Sr, also imply a severe environmental perturbation. At this time oceanic crustal production rates reached their highest level of the last 100 million years. This was principally caused by extensive melting of hot mantle plumes at the base of the oceanic lithosphere, and the development of vast areas (up to 1×106 km2) of thickened oceanic crust in the Pacific and Indian Oceans. The anomalous volcanism associated with the formation of these oceanic plateaux may have been responsible for the environmental disturbances c. 90 Ma. These eruptions would also have resulted in the emission of large quantities of CO2 into the atmosphere, leading to global warming. Additionally, the emission of SO2, H2S, CO2 and halogens into the oceans would have made seawater more acidic resulting in the dissolution of carbonate, and further release of CO2 This run-away greenhouse effect was probably put into reverse, by the decline of the anomalous volcanic activity, and by increased (CO2-driven) productivity in oceanic surface waters,leading to increased organic carbon burial, black shale deposition, anoxia and mass extinction in the ocean basins

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