To arrive at a holistic picture of the climate of the transition, the researchers merged the plant data with physical information about the state of the atmosphere and ocean taken from chemical and isotopic information in the same sediments, and compared this to computer modelling of climate in the period.
"We can see that summer temperatures on land remained relatively warm throughout the Eocene/Oligocene transition, but that the period was marked by increasing seasonality," said Dr. Greenwood.
"Mean temperatures during the coldest month dropped by five degrees Celsius, to just above freezing," he said.
"This was probably not enough to create much in the way of continental ice on East Greenland," he said, "but it did wipe out palms and other subtropical trees such as swamp cypress. They were replaced by temperate climate trees such as spruces and hemlock."
The researcher said that, nonetheless, the middle period of the transition remained fairly warm. "Hickory and walnut were still present, but these became rare in the final stages," he said.
Although the march to a cooler world was gradual in northern latitudes, it was inevitable according to Dr. Greenwood.
"Changes in the earth's position in its orbit were leading a much greater seasonal range in radiation for polar regions and, overall, heat was becoming more concentrated in the tropics, largely due to a global drop in carbon dioxide levels in the atmosphere" he said.
|Contact: David Greenwood|
Natural Sciences and Engineering Research Council