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Ancient trees discredit drought

Researchers say stumps can't prove climate change

Max Torbenson of the University of Minnesota marks the position of a tree stump submerged off the northeast shore of Lake Winnipeg that died between 3,000 and 4,000 years ago.  Photos by Scott St. George / Winnipeg Free Press

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Max Torbenson of the University of Minnesota marks the position of a tree stump submerged off the northeast shore of Lake Winnipeg that died between 3,000 and 4,000 years ago. Photos by Scott St. George / Winnipeg Free Press

The remains of ancient trees found below the surface of Lake Winnipeg refute the idea Manitoba's largest lake experienced radically lower levels in recent times.

Samples taken from tree stumps in the water near Lake Winnipeg's northeastern shore date back 2,880 to 4,150 years, according to newly published research by University of Minnesota geographer Prof. Scott St. George and graduate student Max Torbenson.

This has policy implications, as geographers believed the trees were only 300 to 500 years old, a split second in the geological scale of time.

Based on previous research by Erik Nielsen of the Manitoba Geological Survey, stumps found below the surface of the lake were thought to be the remains of trees that grew around the year 1650.

It was believed Lake Winnipeg suffered from a severe drought that allowed the boreal forest to encroach below what is now the waterline.

In 2011 and 2012, former Manitoban St. George and Torbenson took a pair of float plane trips to the Spider Islands area of Lake Winnipeg to investigate tamarack stumps found near the shore.

They undertook a broader sample of submerged tree stumps that revealed a much older date range for the trees. They lived at a time when Lake Winnipeg was a very different body of water with a very different footprint, St. George said.

"When we thought the trees were 300 or 400 years old, we thought this was a period when the level of the lake was a lot lower," St. George said in a telephone interview last week, after his research was published in the Holocene, a scientific journal.

"Now that we've discovered these stumps are actually 3,000 to 4,000 years old, we don't think they can be evidence of ancient climate change."

Lake Winnipeg is a remnant of glacial Lake Agassiz, which covered much of Manitoba until about 8,000 years ago, when a large portion of its water drained into Hudson Bay. Lake Winnipeg didn't begin resembling its modern form until about 2,500 years ago, when two previously independent lakes around what are now the northern and southern basins joined together.

A big influence on the changing shape of the lake is a geological phenomenon known as isostatic rebound, or the gradual lifting of the land following the melting of the glaciers.

When ice covered most of Manitoba, its weight depressed the surface of the province. After the glaciers disappeared, the surface has been rising -- but not at the same rate everywhere in the province.

"The direction of the rebound is not exactly north-south. There's an angle to it," St. George explained.

The rebound is the likely explanation for the presence of forests in what is now the northeast edge of Lake Winnipeg. St. George said this is valuable to know, as the belief a severe drought took place hundreds of years ago had modern policy considerations.

"Those stumps are not evidence of an ancient drought," he said. "That's important, because the level of Lake Winnipeg has so many implications for hydroelectric power, for resources and for fisheries."

bartley.kives@freepress.mb.ca

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