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Analyses of trees and other organic material buried in a riverbank near Lake Superior’s northwestern shore shed new light on how much and when the lake level varied soon after the end of the last ice age.
Researchers have long known that the water level in Lake Superior has fluctuated, but pinning down the dates of those variations has been tough, says Matthew Boyd, a paleoecologist at LakeheadUniversity in Thunder Bay, Ontario. Many techniques that scientists have used to try and estimate the age of beaches, dunes, and other features that denote ancient lake levels aren’t accurate, he notes. Now, Boyd and colleague James T. Teller of the University of Manitoba in Winnipeg have unearthed new clues about the lake’s history, they reported Monday in Houston at the annual meeting of the Geological Society of America.
The researchers found those clues in an 11.5-meter-tall riverbank along the lower reaches of the KaministiquiaRiver, which flows into Lake Superior at Thunder Bay. The lowest 2.5 meters of exposed sediment is sandy and arranged in rippled layers, a sign the material was deposited by flowing river water, says Boyd. This stratum is capped with a 2- to 10-millimeter-thick layer of leaves, moss, wood and other organic matter, indicating water flow had slowed, enabling the material to accumulate. Carbon-dating samples of this material indicate that it was deposited about 8,900 years ago. In some places at the site, small spruce trees alive at that time — previously growing onshore but suddenly standing in water — were buried intact.
A 3-meter-thick band of sediment that overlies the organic material is arranged in thin, flat layers, or varves — a sure sign that the material was laid down in still waters. That, in turn, indicates that water level in Lake Superior had risen to flood the area, Boyd says. Water level rose as the land at the eastern end of the lake — which serves as the overflow outlet — slowly rebounded. This lifting was in response to the melting of the ice sheet that had weighed down the region during the ice age.
Before the water level in Lake Superior rose about 8,900 years ago, the site where the spruce forest now is buried sat several kilometers from the lake. So, Boyd notes, lake level must have risen many meters during the episode recorded in the varved stratum. The number of varves in the 3-meter-thick band suggests that water level remained relatively high for more than 400 years before dropping again.