MESOZOIC CLIMATES

Abstract

Compared to the present, the Mesozoic Earth was an alien world with a greenhouse climate, as we illustrate here by reference to Upper Triassic, Upper Jurassic, Lower and Upper Cretaceous palaeoclimate reconstructions generated on a General Circulation Model. Throughout the Mesozoic, dense forests grew close to both poles but experienced months-long daylight in warm summers, and months-long darkness in cold, sometimes snowy, winters. Ocean depths were warm (8 °C or more to the ocean floor) and reefs, with corals, grew 10° of latitude further north and south than at the present time. The whole Earth was warmer than now by several degrees centigrade, generating higher atmospheric humidity and a greatly enhanced hydrological cycle. Modelling suggests that much of the rainfall was predominantly convective in character, and often focused over the oceans and leaving major desert expanses on the continental areas. Polar ice-sheets are unlikely to have been present for most of the time because of the high summer temperatures achieved. An absence of major terrestrial ice-caps storing water enriched with 16O i s reflected in oxygen isotopic compositions in marine shells more negative than during later times when ice began to accumulate around the poles. Localized mountain glaciers cannot be ruled out, particularly over Antarctica during the Cretaceous. There is no convincing evidence of the sort of short-term, and large-scale, eustatic changes associated with major glaciation and deglaciation comparable with that of the Neogene. Model output does however suggest the possibility of sea-ice, particularly in the nearly enclosed Arctic seaway through some of the year, notably during the Cretaceous. Model results for the Jurassic suggest the possibility of upland ice-sheets during times corresponding with orbitally induced climatic minima. During the Triassic and Jurassic, the world was predominantly warm with at least four times present atmospheric CO2 and model outputs for evaporation and precipitation conform well with the known distributions of evaporites, calcretes and other climatically sensitive facies.