Climates through the Mesozoic

The Late Triassic and Early Jurassic were times of heat and aridity. They also were times of marked seasonality; that is, well-defined seasons, strongly affected by the Pangaea continental mass. By the latter two-thirds of the Jurassic, however, as well as most of the Cretaceous, Earth is thought to have been without polar ice or glaciers on the northern parts of continents. This is quite beyond our own experience; now, glaciers occur at high latitudes at both poles, and the poles themselves are, for the present at least, covered in ice. The conclusion that there were no ice or glaciers above the Arctic and Antarctic Circles (latitudes 66° N and 66° S) is based largely upon the presence of warm climate indicators such as the fossils of warmth-loving plants and certain fish at high latitudes, and upon the absence of any evidence of continental glaciation from this time.

The absence of polar ice had an important consequence for climates: water that would have been bound up in ice and glaciers was instead in ocean basins. This in turn meant higher eustatic sea levels, which led to extensive epeiric seas. The increased abundance of water on the continents as well as in the ocean basins had a stabilizing effect on temperatures (because it decreased continental effects), and decreased the amount of seasonality experienced on the continents.

Continental climates are enormously variable, however, and in North America Upper Jurassic terrestrial deposits, features preserved in the rocks, such as oxidized sediments and calcium carbonate deposits, suggest that the Late Jurassic there was marked by seasonally arid conditions. So much for dinosaurs in steamy, swampy jungles!

Paleoclimates in the Cretaceous are somewhat better understood than those of the preceding periods. During the first half of the Cretaceous at least, global temperatures remained warm and equable. The poles continued to be ice-free, and the first half of the Cretaceous saw far less seasonality than we see today. This means that, although equatorial temperatures were approximately equivalent to those we experience today, the temperatures at the poles were somewhat warmer. Temperatures at the Cretaceous poles have been estimated at 0-15 °C, which means that the temperature differerence between the poles and the equator was only between 17 and 26 °C, considerably less than the ±41 °C of the modern Earth.

The first half of the Cretaceous was synergistic: tectonic activity, such as mountain building and sea-floor spreading caused an increase in atmospheric CO2 and a decrease in the volume of the ocean basins, which in turn increased the volume of epeiric seas. The seas thus stabilized climates already warmed by enhanced absorption of heat in the atmosphere.

Sound familiar? The Early to mid Cretaceous experienced the notorious "greenhouse" conditions that are currently of such concern today. Because several times in its past history, including in the Early Cretaceous, the Earth has "experimented" with greenhouse conditions, Earth history has a lot to offer to the dialog about global warming.

The last 30 million years of the Cretaceous produced a mild deterioration in the equable conditions of the mid Cretaceous. A pronounced withdrawal of the seas took place, and evidence exists of more pronounced seasonality.

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