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Ancient clues from a frozen forest

Troy L. Péwé once discovered an interesting patch of woods near Ester, about nine miles east of Fairbanks. The spruce and birch trees of this forest were underground, sandwiched between layers of earth. Each tree was 125,000 years old.

Péwé, with the geology department at the University of Alaska Fairbanks from 1953 to 1965 and now with the department of geology at Arizona State University, found the forest when he worked for the U.S. Geological Survey in 1949. At the time, the U.S. government had assigned Péwé and other scientists to study permafrost. Péwé examined hillside cuts made by gold miners in Ester and found trees frozen between layers of loess. Loess, pronounced "luss," is silt produced from the grinding action of glaciers that has been picked up by winds and carried elsewhere.

Because the trees were buried about 45 feet below the present-day forest at Eva Creek, Péwé knew they were old. How old he didn't find out until 50 years later, after methods of finding the age of extremely old things had been developed. One of those methods, a system for determining the age of volcanic dust, proved particularly useful. Because pencil-thin layers of volcanic ash line the soil above and below the frozen forest, Péwé and others were able to get a rough estimate of the trees' age. Péwé said the frozen forest at Eva Creek thrived at a time that was up to 5 degrees Celsius warmer than it is today, when there was little-to-no permafrost.

Because the frozen forest is full of charred trees, Péwé suspects there were a lot of forest fires 125,000 years ago. Insect galleries carved into the bark of some of the frozen spruce indicate that the spruce bark beetle was also here then.

What preserved the Eva Creek frozen forest? During a cooling period, about 120,000 years ago, the Eva Creek trees died and were eventually covered with loess from dust storms that began on the Tanana Flats. The fine, powdery soil, the consistency of flour, was originally part of mountains in the Alaska Range before it was pulverized by the weight and force of glaciers and carried with melt water to the Tanana River. In the river, much of the silt settled in dry channels and riverbanks, where the wind picked it up and carried it somewhere else.

In an incredibly gradual process, loess coated the Eva Creek forest. The ancient trees froze as the climate became cold enough to produce permafrost. Eventually, miners in the 1940s unearthed the trees with hydraulic giants. Péwé found the trees between two layers of loess, one known as Gold Hill Loess the other as the Goldstream Formation.

Within the hills of Fairbanks are many other loess deposits, including one cut at the Parks Highway near Ester that Péwé says is the oldest and the thickest (202 feet deep) in Alaska. Years ago, he successfully lobbied to have the land purchased by the state so scientists could learn about the last period between ice ages. The Geophysical Institute recently took over that site as a study area.

What can we learn from the Eva Creek frozen forest and other sites like it? Péwé said that because the last period between ice ages was warmer than today, we may be able to predict the future by looking at the past.