Drained lake holds record
of ancient Alaska
By Ned Rozell
February 27, 2008
Not too long ago, a lake sprung a leak in the high country of
the Wrangell-St. Elias Mountains. The lake drained away, as glacier-dammed
lakes often do, but this lake was a bit different, and seems
to be telling a story about a warmer Alaska.
The lake, known as Iceberg Lake to people in McCarthy, about
50 airmails to the north, had been part of the landscape for
as long as people could remember. Pinched by glacial ice, the
three-mile-long, one-mile-wide lake on the northern boundary
of the Bagley Icefield was remote but notable enough that it
was the cover photo for a recent book about hiking Wrangell-St.
Elias National Park.
After holding water
for centuries, Iceberg Lake in the Wrangell-St. Elias Mountains
drained in 1999 and has lost its water every year since except
Photo by Mike Loso.
When McCarthy guide Richard Villa visited the area with a client
in the summer of 1999, he was stunned to see the lake, which
had lost much of its water. Villa later told Mike Loso, a Kennicott
resident part of the year and now a professor at Alaska Pacific
University in Anchorage.
Loso flew to the lake the next summer with Bob Anderson and Dan
Doak, scientists who also reside in McCarthy for part of the
year. The men saw a muddy lakebed where Iceberg Lake had sat
for so long. Streams of meltwater had cut though the mud, making
sharp canyons. Loso, Anderson, and Doak hiked into the gullies
and saw on the walls many layers of the former lake bottom. They
knew that each two layers of sediment-a thinner layer of fine-grained
deposits that settled in winter and coarser sand forced in with
summer runoff-represented a year in the life of the lake.
"We eyeballed these layers and said 'Wow, there's at least
1,000 of these things,'" Loso said.
Scientists often pull plugs of sediment from the bottom of lakes
to find an ancient record of pollen, dust, ash, and other things
that have drifted, or flowed in over the years, but their records
usually don't go back farther than the Little Ice Age, a cold
period from about 1600 to 1850 when many glaciers advanced. Those
glaciers plowed over most of the landscape, but Iceberg Lake
seemed to have escaped the gradual assault.
"(Iceberg Lake) is pinned between these two glaciers just
far enough away that it didn't get overrun by the Little Ice
Age (glaciers)," Loso said.
So instead of having a record of just the last few hundred years,
the floor of Iceberg Lake held a continuous record from 1998
back to A.D. 442, a span of more than 1,500 years.
That record is unique in that it seems to preserve a time of
warmer temperatures called the Medieval Warm Period that happened
before the Little Ice Age.
"It's the most recent time period warm enough to be comparable
to the present," Loso said.
When Loso and his colleagues used the thickness of layers (called
"varves") to interpret warmth in the area of Iceberg
Lake, they found that summer temperatures in that part of the
state were warmer in the late 20th century than they were during
the Medieval Warm Period.
Not only that, they saw that Iceberg Lake had never drained during
the Medieval Warm Period. Since the catastrophic leakage in 1999,
the lake has drained of meltwater every year except for 2001.
With such erratic behavior after centuries of stability, Iceberg
Lake might be saying that Alaska has been warmer recently than
it has been in a long, long time.
This column is provided
as a public service by the Geophysical
Institute, University of Alaska Fairbanks, in cooperation
with the UAF research community.
Contact the Editor
Ned Rozell [email@example.com]
is a science writer at the institute.
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