In terms of carbon footprint, though one hears a lot about carbon dioxide, it’s methane that wears the size 12 clodhopper. Methane is more effective at trapping heat in earth’s atmosphere than carbon dioxide and also contributes to the degradation of the ozone layer. When permafrost thaws, release of methane into the atmosphere from anaerobic decomposition contributes to climate warming, which subsequently causes more permafrost thaw – thus acting as an important feedback loop in global climate. Katey Walter Anthony (University of Alaska, Fairbanks) and an international team of colleagues are studying permafrost and thermokarst lakes to better understand how thawing permafrost and the subsequent release of methane is contributing to climate warming. Since 2008 Anthony and colleagues have worked on the interdisciplinary study in Cherskii and Yakutsk, Russia, all over Alaska, in western Canada, Greenland and Sweden describing the distribution of permafrost and the process of landscape evolution and gas escape as permafrost thaws.
“We have a pan-Arctic focus with the goal of understanding carbon release from permafrost. Not only are we describing the extent of thermokarst in Siberia, Alaska, western Canada and other regions of the Arctic, we’re looking at how thermokarst lakes develop and release methane in particular,” says Anthony.
Permafrost, soil at or below the freezing point of water for at least two years, is common at high latitudes. As the climate warms, however, permafrost thaws and forms an irregular landscape called thermokarst (the pitted nature of the surface resembles those developed in karst areas of limestone). In surface depressions, lakes form where massive ground ice melted. Permafrost contains vast reserves of carbon stored within a frozen framework that is released when permafrost thaws.
Anthony and colleagues are interested in yedoma, a specific type of permafrost that is particularly high in carbon and supersaturated with ice, about 50-90% by volume. Formed in unglaciated continental areas during the last ice age, yedoma is most prevalent in northeastern Siberia where it may be tens of meters thick. Thawing yedoma yields a significant source of atmospheric methane.
“Thermokarst lakes formed from the thawing of yedoma are very efficient at releasing carbon, in the form of methane, into the atmosphere,” Anthony explains. “As the ice melts, water and 46,000 year-old methane, CH4, are released. We are trying to quantify how much carbon is released as well as the variability in different regions.”
When headed to the field, Anthony and her co-investigator, Guido Grosse, first identify likely areas of permafrost exposures using satellite imagery. Ideal locations are usually along rivers where cut banks have excavated steep exposures that may be up to 50 m tall.
“These are the best places to work because we can see 60,000 years of history all at once. You can see the whole layered cake of ice and frozen soil in cross section! We can tease out a lot of information about past permafrost and climate,” Anthony says. “We have to be very careful when we sample to find a fresh cut that has not thawed in the recent past. The first part is just moving dirt with shovels and scrapers so we have to be very careful. We have to work quickly because the permafrost can thaw very quickly. We sample and describe different units with a focus on the amount of ice and carbon in representative layers. We can scale up. Studying broad exposures has some big advantages over permafrost coring, where our interpretation of an area is otherwise limited to what we find in 4cm diameter cores.”
Anthony does much of her methane field work during winter. And, while she says it’s no fun to wake up in -30 degree temperatures at field camps, winter work is easier in some ways. Lake ice provides an opportunity to map methane bubbles on thermokarst lakes. Coring permafrost requires the use of a permafrost drill, a gas-powered auger with a core barrel and drill bit at the end. Anthony says permafrost coring is often most easily accomplished in winter conditions when the permafrost is frozen solid. Samples can be quickly acquired from a snowmobile and there’s less chance of the core casing freezing up during the coring since it’s already cold.
Anthony’s team also prefers coring thermokarst lake sediments in the winter because they can use lake ice as a stable platform for field work. Sediments from the bottom of a lake can tell Anthony how old the lake is–some lakes developed at the end of the last ice age nearly 12,000 years ago, while others developed much later and have been expanding since.
Sometimes lake coring in summer is necessary. “Summer lake coring requires a huge amount of work. It’s very dirty. There are lots of mosquitoes. I have spent hours hammering a core barrel into the lake bed from a raft just to have nothing come up. It’s much easier in the winter when we can do it from the ice covering the lake. Then it requires much less gear and it’s stable,” says Anthony.
Anthony also maps lake methane bubbles during winter. Methane formed by microbes from thawing permafrost is released from lake bottoms in the form of bubbles all year long. In summer, bubbles rise to the top of the lake and burst, releasing almost pure methane into the atmosphere, but in the winter, lake ice forms a lid that traps methane bubbles.
“We use shovels to remove any fresh snow from the lake ice surface. What we find is really neat–the ice looks black and had beautiful white bubbles stacked on top of each other in place to place–much like the stars scattered across the night sky,” Anthony explains. “We map the distribution of the bubbles which get trapped, forming tall columns of methane. We can tell where the gas is coming from, how it clusters. We get a good spatial data set.”
Back in the lab Anthony sub-samples permafrost and lake cores for radiocarbon dates, a method that helps her and colleagues understand the history of permafrost formation across northern Siberia and Alaska.
In 2011, Anthony’s team, along with students and post doctoral candidates from the University of Alaska, Fairbanks, returned to Seward Peninsula and interior Alaska field sites to recover time-lapse cameras, temperature data loggers, and bubble traps, which record the rate of gas release. The team also worked in Cherskii, Russia. During that expedition, Anthony worked with three students and a postdoc who have sub-projects studying permafrost and peat along the Kolyma River.—Marcy Davis
Katey Walter Anthony's research is funded, in part, by NSF, NASA, and the Department of Energy