Arctic Redwood Fossils Are Clues to Ancient Climates

Bijal P. Trivedi
National Geographic Today
March 26, 2002

Axel Heiberg Island, at 82 degrees north and just a stone's throw from the North Pole, was once a great vacation spot—during the Eocene epoch, about 45 million years ago. Lush redwood forests, ferns, flowering plants, and a huge variety of animals, now extinct, once thrived here.

Hope Jahren, a geobiologist at Johns Hopkins University in Baltimore, Maryland, is using wood fossils from Axel Heiberg to discover prehistoric weather patterns that enabled this now bleak, cold, and dry desert to support such a rich array of life.

"I've always been enraptured with the idea that the Earth can change so dramatically," said Jahren "The Earth today is very different compared to how it was millions of years ago."

During the Eocene epoch, Axel Heiberg and much of northern Siberia and Alaska were covered in temperate forests with redwood-like trees called Metasequoias, similar to those now seen in Northern California.

The trees were between 30 and 40 meters tall (98 and 131 feet) and densely packed, providing a canopy for a plethora of ferns and flowers, said Jahren. The largest tree found had a diameter of three meters (ten feet). What remains of these ancient redwoods today is "rather extraordinary," said Jahren.

"These trees look like driftwood on the beach—they are dry and flaky, with almost no other alterations," said Jahren. Unlike these trees, ancient forests often become petrified through the steady infiltration of minerals over many years, which eventually replaces the wood tissue with stone.

Chemical Record

Because the wood is unadulterated, the tissues hold a chemical record of weather patterns during the period the tree lived. Jahren studies carbon, hydrogen, oxygen, and nitrogen because these elements are taken from the soil, water, and air and incorporated into the tissue of plants and animals.

Jahren and her colleague Leonel Silveira Lobo Sternberg of the University of Miami in Coral Gables, Florida, are examining chemically different forms, or isotopes, of oxygen in these ancient redwoods to reveal weather patterns during the Eocene period.

Oxygen that a plant uses, said Jahren, comes primarily from water. Determining the chemistry of that water could reveal exactly where it came from. Rain that arrives after traveling long distances over land has a very different chemical signature than rain that travels over the ocean or just very short distances, she explained.

The researchers' analysis of the oxygen content of the wood revealed "a bizarre absence of oxygen-18, the heavy isotope," said Jahren. Water contains both oxygen 16—the more common and lighter isotope—and the more rare oxygen 18. The analysis suggests that the water contained almost exclusively oxygen 16.

The study appeared a recent issue of GSA Today, a publication of the Geological Society of America.

Continued on Next Page >>




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