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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.

One way to get water with these characteristics, said Jahren, is for that water to have traveled large distances over land. As water travels over land, she explained, the heavier oxygen is removed as it rains.

The only route allowing moisture laden air to travel thousands of kilometers over land before reaching Axel Heiberg would be across North America, possibly from the Gulf of Mexico, said Jahren. "This idea is compelling because it would supply water rich in oxygen 16 and supply warm air to this very northern region"—warm enough to nurture a forest.

Different Weather Patterns

Jahren finds this model of water transport intriguing "because this weather pattern is radically different from today." Current weather systems over North America tend to travel from west to east. In the Eocene epoch, a much warmer period when the poles were free of ice, weather systems could shift from south to north, said Jahren.

But there is another possible interpretation of Jahren's findings, cautioned Scott Wing, a paleontologist at the Smithsonian Institution in Washington, D.C.

"Water from snowfall also contains low quantities of oxygen 18, thus matching the water profile from the wood," he said. He suggested the possibility that snow, formed over the then ice-free Arctic Ocean, may have supplied the island with water. This would indicate that the northern regions were actually much colder than Jahren suggests.

"Isotope levels are very difficult to interpret, and there are lots of questions remaining," Wing said.

If Axel Heiberg were actually colder, it would imply that animals such as alligators, which were known to live at these latitudes, as well as plants must have been tolerant of the cold.

Whether Axel Heiberg actually received waters originating from equatorial regions is "still up for debate," said Wing.

But there are other questions left to answer. "These forests had four months of daylight and four months of complete darkness. Finding trees that could survive under these conditions is as flabbergasting as finding humans that live underwater," said Jahren.

Uncovering ancient weather patterns provides greater understanding of how ecosystems work, opening a window into the Earth's capabilities. It also offers new ideas about the kind of conditions that plants and animals might be able to survive in.

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