Cretaceous Super-Greenhouse Glaciers?

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Crocodiles roamed the Arctic and surface temperatures of the western tropical Atlantic Ocean climbed to 37 degrees Celsius (98 degrees Fahrenheit), compared with 29 degrees C (84 degrees Fahrenheit) today, during the Turonian period, between 93.5 million and 89.3 million years ago. Researchers have generally assumed that the Earth would have been ice-free during this Cretaceous super-greenhouse, but others have been intrigued by the possibility of hothouse glaciers, pointing to evidence such as erosion patterns in Antarctica and perplexing drops in sea level. A new analysis by André Bornemann and colleagues suggests that glaciers stretched across 50- to 60-percent of Antarctica some 91.2 million years ago, continuing for 200,000 years. The research team analyzed sediment samples from the Demerara Rise, under the tropical Atlantic Ocean. They used two methods. First, they looked at oxygen isotopes in tiny plankton called foraminifera that record the combined effects of sea level and temperature. Then, they used a second “paleothermometer” based on the fatty membranes of other marine organisms that change only in response to temperature to extract a sea level record, which can be related to continental ice. Studies of the long-ago climate might someday help researchers to better evaluate current events, Bornemann said. But, he emphasized that his study is unrelated to the current problem of global climate change, which is happening on a completely different, much more rapid time scale. This research appears in the 11 January 2008 issue of the journal Science.

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Scanning electron microscope images of Cretaceous planktic foraminifera, tiny sea creatures in the Demerara Rise. (A) Marginotruncana sinuosa. (B) A close-up of a lateral view of M. sinuosa. (C) Whiteinella baltica. (D) A close-up of the shell surface of Whiteinella baltica. The close-up images demonstrate the extraordinary preservation of the shells. Scale: white bars = 50 mu m.

[Image © Science]


 

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