Ancient wood points to arctic greenhouse

Chemical analyses of wood that grew in an ancient arctic forest suggest that the air there once was about twice as humid as it is now.

About 45 million years ago, forests of redwoods grew on what is now Axel Heiberg Island, a Maryland-size landmass off the northern coast of Canada. At some sites, wood from those trees is exquisitely preserved, apparently changed only by desiccation and slight compression by surrounding sediments. “It’s like driftwood,” says A. Hope Jahren, a geochemist at Johns Hopkins University in Baltimore.

By analyzing the ratios of oxygen isotopes and hydrogen isotopes in the wood’s cellulose, Jahren and Leonel S.L. Sternberg of the University of Miami could estimate how humid the ancient forest was. On average, they report in the May Geology, the forest’s atmosphere held about twice the water vapor found in the region today. The extra humidity would have provided a vital greenhouse effect, trapping outbound heat radiating from the ground during winter periods marked by 24-hour-a-day darkness.

Isotope ratios in a carbonate mineral infiltrating other wood samples suggest the region’s average annual temperature 45 million years ago was about 13C, says Jahren.

Overall, these environmental conditions match the springtime climate in today’s coastal forests of Oregon.


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