The development of the climate during the Cretaceous greenhouse interval is reviewed based on geological and paleontological records, geochemical proxy records for paleotemperature and atmospheric carbon-dioxide concentration (pCO 2), the production rate of oceanic crust, and the timing and scale of emplacement of large igneous provinces. Geological and paleontological evidence, and paleotemperature records indicate that the Early Cretaceous climate was relatively cool, possibly accompanied by the development of continental ice sheets. Subsequent warming reached a peak in the Turonian, when sea surface temperatures in equatorial and high-latitude regions exceeded 36°C and 20°C, respectively. The possibility of a maximum temperature above 36°C at the equator is inconsistent with the cirrus cloud negative-feedback hypothesis proposed for the modern ocean, which may indicate that the hypothesis is not valid for an ice-free greenhouse system. Although elevated levels of pCO 2 are thought to be responsible for this extreme warming, the timing of the pCO 2 maxima differs from the timing of oceanic volcanic activity, which emitted massive amounts of CO2 into the atmosphere, by ∼30 m.y.: volcanic activity peaked at ∼120 Ma, whereas pCO 2, temperature, and sea-level peaked at ∼90 Ma, indicating that the abiotic Mesozoic marine revolution was not a simple, single event. Moreover, the occurrence of intermittent cooling during the Late Cretaceous, coupled with sea-ice development in the Arctic Ocean, suggests that the mid-Cretaceous greenhouse system was capable of producing not only extreme warmth but also seasonal freezing. Although the Mesozoic marine revolution is assumed to have been triggered by the general warming that occurred during the Cretaceous, a more precise analysis of the timing and magnitude of biotic events is required to understand the paleoecosystem of this greenhouse period.
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