New research on Jakobshavn Isbrae, a tongue of ice extending out to sea from Greenland's west coast, shows that large, marine-calving glaciers don't just shrink rapidly in response to global warming, they also grow at a remarkable pace during periods of global cooling. Glaciers change.
Through an analysis of adjacent lake sediments and plant fossils, the researchers determined that the glacier, which retreated about 40 kilometers inland between 1850 and 2010, expanded outward at a similar pace about 200 years ago, during a time of cooler temperatures known as the Little Ice Age.
Jakobshavn Isbrae has been the focus of intense scientific interest because it is one of the world's fastest-flowing glaciers, releasing enormous quantities of Greenland's ice into the ocean. It is believed that changes in the rate at which icebergs calve off from the glacier could influence global sea level rise. The decline of Jakobshavn Isbrae between 1850 and 2010 has been documented, mostly recently through aerial photographs and satellite photographs.
"We know that Jakobshavn Isbrae has retreated at this incredible rate in recent years, and our study suggests that it advanced that fast, also," said Jason Briner, the associate professor of geology at the University of Buffalo, who led the research. "Our results support growing evidence that calving glaciers are particularly sensitive to climate change."
To reconstruct the glacier's advance from east to west during earlier, cooler years, Briner and his colleagues examined sediment samples from Glacial Lake Morten and Iceboom Lake, two glacier-fed lakes that sit along the glacier's path of expansion.
As Jakobshavn Isbrae expanded seaward, it reached Glacial Lake Morten first, damming one side of the lake with ice and filling the basin, previously a tundra-covered valley, with meltwater.
To pinpoint the time in history when this happened, the researchers counted annual layers of overlying glacial sediments and used radiocarbon dating to analyze plant fossils at the lake bottom (the last vestiges of the old tundra). The team's conclusion: Glacial Lake Morten formed between 1795 and 1800.
An analysis of sediment layers from the bottom of Iceboom Lake showed that Jakobshavn Isbrae reached Iceboom lake about 20 or 25 years later, around 1820.
Jakobshavn Isbrae's rate of expansion from Glacial Lake Morten to Iceboom Lake, as documented by the UB team, matched the glacier's rate of retreat between those two points. (Aerial imagery shows Iceboom Lake draining around 1965 and Glacial Lake Morten draining between 1986 and 1991.)
Citation: J.P. Briner, N.E. Young, E.K. Thomas, H.A.M. Stewart, S. Losee and S. Truex, 'Varve and radiocarbon dating support the rapid advance of Jakobshavn Isbræ during the Little Ice Age', Quaternary Science Reviews (Article in Press, Corrected Proof) doi:10.1016/j.quascirev.2011.05.017
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