Dune Lake Holds Clues to Ancient Alaska
Dune Lake is a mile-long apostrophe in the boreal forest located about 25 miles southwest of Nenana. The lake, popular for its rainbow trout, is now luring scientists who have discovered that it has a story to tell about Alaska's past.
Dune Lake's name hints of its birth. At the height of the last Ice Age, glaciers in the Alaska Range pulverized rock into sand. More than 11,000 years ago, wind carried that sand northward, creating a field of dunes surrounding a bowl that slowly filled with water to become Dune Lake.
Scientists determined the age of Dune Lake by pulling up cores of muck from a 15-foot sediment layer on the lake bottom. This mass of goo is like gold to Jason Lynch, a graduate student at Duke University in North Carolina. While working on his Ph.D. degree, he spent many hours looking at Dune Lake's sediment cores, which scientists retrieve by jamming a pipe into the lake bottom. The cores from Dune Lake are packed with information about Alaska's past climate; each centimeter of gunk at represents 15 years of history. Trapped within that sediment are fossils of plant seeds, pollen released by trees and shrubs thousands of years ago, and tiny bits of charcoal.
Lynch looked past the other tidbits and focused on the charcoal fragments. If removed from the sample and spread out on a table, the charcoal looks like pepper. Those black specks reveal something about the fire history of Alaska's interior, especially when they are combined with pollen samples other researchers have studied.
Bruce Finney, Mary Edwards, and Nancy Bigelow of the University of Alaska Fairbanks used pollen samples from Dune Lake to determine that trees appeared there about 9,000 years ago. As rainfall increased after a period of drought, birch and white spruce were the first trees to colonize the area. According to Lynch's charcoal samples, big forest fires did not often sweep through the birch and spruce. But that changed a few thousand years later.
About 5,500 years ago, large wildfires burned in the Interior near Dune Lake. Lynch's samples from that era are black with charcoal fragments that wafted into the center of the lake from dense clouds of smoke. Lynch's results, when combined with climate data from that time, didn't initially make sense. Work done by Finney and others on ancient lake levels in the Interior showed that the climate then was relatively wet and cool, a period that should not have been ideal for wildfires.
The researchers found an answer to the puzzle when they traced the pollen record of the lake. Just before the large, frequent fires began near Dune Lake, black spruce moved into the area. The Interior was experiencing a wet, cool period, but the encroachment of resinous black spruce-the most combustible tree in Alaska-was enough to cloud the summers with smoke.
Dune Lake shows how climate change over the long term can affect forest fires. Warm summers punctuated with lightning prompts fires, but long-term climate changes, such as an increase of rain and snow, can result in changes of vegetation that may tip the fire balance in a different way.
