Hothouse conditions were forced by massive emission of CO2 during the mid-Cretaceous superplume event that created huge submarine basalt plateaux and began the development of many island chains that litter the floor of the central Pacific. It was at this time that dinosaur-infested forests cloaked high latitudes, almost to both poles. Terrestrial evidence suggests that conditions cooled somewhat in the later Cretaceous, and sequence stratigraphy indicates cyclic sea-level fluctuations, ascribed by some to the development of Antarctic ice sheets. Resolving later Cretaceous global mean temperatures, and the ice-sheet question relies on oxygen isotopes from sea-floor sediments. These are now available with sufficient precision and resolution to show that hothouse conditions lasted a great deal longer than suspected (Huber, B.T. et al. 2002. Deep-sea paleotemperature record of extreme warmth during the Cretaceous. Geology, v. 30, p. 123-126).
The Pacific superplume’s maximum activity was over a period of 15 Ma from 125 to 110 Ma (Barremian and Aptian), although it lasted until the early Campanian (80 Ma). Contrary to the supposed magnitude of CO2 release by volcanism, heating reached a maximum from 94 to 80 Ma. Even at high southern latitudes, deep-ocean water remained at 14 to 19°C for these 14 Ma. Until the end of the Cretaceous it rarely fell below 10°C. The data rule out any circulation of cold, dense brines into the deep ocean basins from the formation of boreal sea ice, and consequently any influence by polar ice sheets. Sea level reached its highest during this period, almost certainly because the volume of the ocean basins shrank, being floored by young, warm, low-density crust formed by the superplume. Mid to late-Cretaceous flooding of the continental margins created uniquely favourable conditions for an explosive development of carbonate-secreting organisms of many kinds. Despite the burial of vast carbonate platforms, as well as thick boreal coal seams, these limestone “factories” seem incapable of having kept pace with greenhouse warming. Was CO2 the only means then of global warming?