Arctic Impact Crater Lake Reveals Interglacial Cycles in Sediments





The coring equipment and other instrumentation was set up using a tripod over the hole in the ice. The scientists were able to extract a core of the topmost 8.5 meters of sediment.
The coring equipment and other instrumentation was set up using a tripod over the hole in the ice. The scientists were able to extract a core of the topmost 8.5 meters of sediment.

A University of Arkansas researcher and a team of international scientists have taken cores from the sediments of a Canadian Arctic lake and found an interglacial record indicating two ice-free periods that could pre-date the Holocene Epoch.



Sonja Hausmann, assistant professor of geosciences in the J. William Fulbright College of Arts and Sciences at the University of Arkansas, and her colleagues will report their preliminary findings at the American Geophysical Union meeting this week.



The researchers traveled by increasingly smaller planes, Ski-doos and finally sleds dragged on foot to arrive at the Pingualuit Crater, located in the Parc National des Pingualuit in northern Quebec. The crater formed about 1.4 million years ago as the result of a meteorite impact, and today it hosts a lake about 267 meters deep. Its unique setting – the lake has no surface connection to other surrounding water bodies – makes it a prime candidate for the study of lake sediments.



Scientists study lake sediments to determine environmental information beyond historical records. Hausmann studies diatoms, unicellular algae with shells of silica, which remain in the sediments. Diatoms make excellent bioindicators, Hausmann said, because the diatom community composition changes with environmental changes in acidity, climate, nutrient availability and lake circulation.



By examining relationships between modern diatom communities and their environment, Hausmann and her colleagues can reconstruct various historic environmental changes quantitatively.



However, most sediments of lakes in previously glaciated areas have limitations – they only date back to the last ice age.


“Glaciers are powerful. They polish everything,” Hausmann said. Glaciers typically carve out any sediments in a lake bed, meaning any record before the ice age is swept away.



However, the unique composition of the Pingualuit Crater Lake led Michel A. Bouchard to speculate in 1989 that the sediments beneath its icy exterior might have escaped glacial sculpting. So in May of this year, Hausmann and her colleagues donned parkas, hauled equipment on ski-doos and slogged through sub-zero temperatures for three weeks so they could core sediments and collect data from the lake.



They carefully carved squares of ice out to make a small hole for equipment, then began a series of investigations that included pulling up a core of the topmost 8.5 meters of sediment. An echosounder indicated that the lake bottom may have more than 100 meters of relatively fine-grained sediments altogether. During the time since the expedition, researchers have examined the physical, magnetic and sedimentological properties of the sediment core.



The sediment core contains mostly faintly laminated silts or sandy mud with frequent pebble-size rock fragments, which is typical of deposits found in water bodies covered by an ice sheet. Sandwiched in the middle of the faintly laminated silts and sandy mud, the researchers found two distinct and separate layers containing organically rich material that most likely date back well before the Holocene, representing earlier ice-free periods. The samples they found contain the remains of diatoms and other organic material, suggesting that they represent ice-free conditions and possibly interglacial periods.



“There are no paleolimnological studies of lakes that cover several warm periods in this area,” Hausmann said. The terrestrial record will be complementary to marine records or to long ice-core records from Greenland.



The international team of researchers in the field included Guillaume St-Onge; Reinhard Pienitz, principal investigator; Veli-Pekka Salonen of the University of Helsinki, Finland; and Richard Niederreiter, coring expert. Please visit http://www.cen.ulaval.ca/pingualuit/index.html for more information.

Methane Bubbling From Arctic Lakes, Now And At End Of Last Ice Age





UAF researcher Katey Walter lights a pocket of methane on a thermokarst lake in Siberia in March of 2007. Igniting the gas is a way to demonstrate, in the field, that it contains methane. (Credit: Photo by Sergey Zimov)
UAF researcher Katey Walter lights a pocket of methane on a thermokarst lake in Siberia in March of 2007. Igniting the gas is a way to demonstrate, in the field, that it contains methane. (Credit: Photo by Sergey Zimov)

A team of scientists led by a researcher at the University of Alaska Fairbanks has identified a new likely source of a spike in atmospheric methane coming out of the North during the end of the last ice age.



Methane bubbling from arctic lakes could have been responsible for up to 87 percent of that methane spike, said UAF researcher Katey Walter, lead author of a report printed in the Oct. 26 issue of Science. The findings could help scientists understand how current warming might affect atmospheric levels of methane, a gas that is thought to contribute to climate change.



“It tells us that this isn’t just something that is ongoing now. It would have been a positive feedback to climate warming then, as it is today,” said Walter. “We estimate that as much as 10 times the amount of methane that is currently in the atmosphere will come out of these lakes as permafrost thaws in the future. The timing of this emission is uncertain, but likely we are talking about a time frame of hundreds to thousands of years, if climate warming continues as projected.”



Ice cores from Greenland and Antarctica have shown that during the early Holocene Period–about 14,000 to 11,500 years ago–the levels of methane in the atmosphere rose significantly, Walter said. “They found that an unidentified northern source (of methane) appeared during that time.”



Previous hypotheses suggested that the increase came from gas hydrates or wetlands. This study’s findings indicate that methane bubbling from thermokarst lakes, which are formed when permafrost thaws rapidly, is likely a third and major source.



Walter’s research focused on areas of Siberia and Alaska that, during the last ice age, were dry grasslands atop ice-rich permafrost. As the climate warmed, Walter said, that permafrost thawed, forming thermokarst lakes.



“Lakes really flared up on this icy permafrost landscape, emitting huge amounts of methane,” she said.


As the permafrost around and under the lakes thaws, the organic material in it–dead plants and animals–can enter the lake bottom and become food for the bacteria that produce methane.



“All that carbon that had been locked up in the ground for thousands of years is converted to potent greenhouse gases: methane and carbon dioxide,” Walter said. Walter’s paper hypothesizes that methane from the lakes contributed 33 to 87 percent of the early Holocene methane increase.



To arrive at the hypothesis, Walter and her colleagues traveled to Siberia and northern Alaska to examine lakes that currently release methane. In addition, they gathered samples of permafrost and thawed them in the laboratory to study how much methane permafrost soil can produce immediately after thawing.



“We found that it produced a lot very quickly,” she said.



Finally, she and other researchers studied when existing lakes and lakes in the past formed and found that their formation coincided with the early Holocene Period northern methane spike.



“We came up with a new hypothesis,” she said. “Thermokarst lake formation is a source of atmospheric methane today, but it was even more important during early Holocene warming. This suggests that large releases from lakes may occur again in the future with global warming.”



Co-authors on the paper include Mary Edwards of the University of Southampton and the UAF College of Natural Science and Mathematics; Guido Grosse, an International Polar Year postdoctoral fellow with the UAF Geophysical Institute; Sergey Zimov of the Russian Academy of Sciences; and Terry Chapin of the UAF Institute of Arctic Biology. Funding was provided by the National Science Foundation, the Environmental Protection Agency and the National Aeronautics and Space Administration.