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In the Arctic, there are lakes where the bubbles will burn.

The bubbles are filled with methane gas. The flammable archaeal belches form as microbes feast on stores of long-frozen organic matter, freed up for decomposition thanks to permafrost thawing somewhere deep beneath the lakebed.

Graphic showing ERT transect of Bonanza Creek
Example of repeat electrical resistivity tomography transects and frost-probe observations, from the publication "Evidence for nonuniform permafrost degradation after fire in boreal landscapes." Burned, fire line, and unburned segments of the transect have distinct characteristics and a strong correlation with independent permafrost probe depth measurements.

In the winter, professor Katey Walter Anthony of the University of Alaska Fairbanks takes researchers to lakes like these to sweep away layers of snow from the frozen surface in search of opaque circles beneath clear ice: telltale signs of methane bubbles trying to escape.

In one video demonstration, some students crack the ice as others crouch at the ready with long, lit matches. As the match heads meet the escaped gas, flames shoot 10 feet high.

It’s dramatic stuff, but Anthony’s day-to-day work isn’t designed to entertain. The field test checks ice bubbles for methane - a greenhouse gas 25 times more potent than carbon dioxide. Monitoring methane as permafrost thaw quickens across the Arctic is important to understanding the impact of a warming climate.

“These lakes are emitting already tremendous amounts of methane,” Anthony explains in the video. “But when we look at how much carbon is in permafrost still frozen, and the potential for that permafrost to thaw in the future, we estimate that more than 10 times the amount of methane that’s right now in the atmosphere will come out of these lakes.”

That’s one reason researchers at the USGS Earth Resources Observation and Science (EROS) Center are working to investigate possible connections between changes to lake surfaces and the underground processes that produce methane beneath them.

Dr. Neal Pastick recently took part in a research trip to learn about the subsurface conditions around Big Trail Lake, a high-emission lake near Fairbanks, as well as to assess soil moisture, wetland conditions, and thaw depth at high latitudes. Pastick is an EROS contractor, lead author of a 2015 paper assessing the extent of and potential changes to Alaska’s near-surface permafrost, and a collaborator in ongoing EROS efforts to track high-latitude change.

“That would be a big thing: If we could establish a link between surface water dynamics and methane,” said EROS research physical scientist Bruce Wylie, a co-author on the 2015 study.

Color photo of Big Trail Lake, near Fairbanks, Alaska
Big Trail Lake near Fairbanks, Alaska is pictured in this photo from September of 2019. A collaborative research team representing multiple federal agencies and universities surveyed the lake's subsurface as part of an effort to understand the connections between belowground change and methane emissions.

Getting to that point will require a better understanding of the underground dynamics of thermokarst lakes, which form in the depressions created by thawing permafrost. Big Trail Lake was an ideal candidate, as it was formed around 50 years ago.

The ground team used electrical resistivity tomography (ERT) tools to get a high-resolution picture of permafrost depth at Big Trail, after an air campaign sponsored by NASA’s Arctic-Boreal Vulnerability Experiment (ABoVE) flew overhead to gather data on methane emissions.

 “We’re trying to characterize the distribution of deep thaw features beneath and adjacent to the lake, and then relate those features to measure greenhouse gas emissions and remotely-sensed estimates of lake expansion,” explained Pastick, who took part in the ground-based surveys with USGS geophysicists Dr. Burke Minsley and Dr. Stephanie James.

ERT surveys are especially useful in that regard, Neal said. Typically, scientists use metal probes, punching through soil to manually define permafrost depth. But that tried-and-true method comes with caveats. Depths between probes might differ. Rocks and compacted sand can fool even expert operators, and probes can miss “thaw bulbs” – thawing areas surrounded by permafrost – if those bulbs are beneath frozen ground. Probes can also miss features below a probe’s reach.

With ground-based ERT, researchers first place metal stakes in the ground every few meters and connect cables to lay out a transect that covers around 200 meters. The probes then send small electrical currents through the soil between the stakes to measure the resistivity between them. Generally, the greater the resistance, the more frozen the soil. What emerges is an unbroken, two-dimensional image of the subsurface, one that can “see” deeper and is more complete than traditional surveys can offer.

“It was really the only way to characterize the belowground features that were of interest,” said Minsley, whose group collected roughly 1,000 meters of ERT data, and has done similar work at more than 50 locations in Alaska in recent years. “You could go out and drill a borehole, but that’s a lot more destructive and time consuming, and you’re only getting information from a single location.  Geophysics gives us an efficient way to remotely detect subsurface permafrost properties.”

The teams also looked at soil and air temperatures and used nuclear magnetic resonance (NMR) tools to measure the water content of the frozen and unfrozen soils around Big Trail.

Color photo of research team near Faribanks, Alaska
A group of researchers from multiple agencies took part in a study of methane-emitting lakes near Fairbanks, Alaska in September of 2019. Pictured, from left to right, are Bruce Chapman (NASA), Liz Hoy (NASA/GST, Inc.), Nancy French (Michigan Tech University), Chengquan Huang (University of Maryland), Matt Macander (ABR Inc.), John Qu (George Mason University), Zhiliang Zhu (USGS), Burke Minsley (USGS), Neal Pastick (KBR/USGS contractor).

The initial results hold promise for researchers like Anthony, who rely on that sort of geophysical data to constrain their findings. Preliminary ERT results suggested that an undercutting thaw bulb may have formed on the eastern portion of the lake, revealing a possible source of enhanced methane production.

 “We may have provided a mechanistic solution for why they’re seeing hot spots where they’re seeing them,” Pastick said.

The next challenge for Pastick, Wylie, and Minsley will be to synthesize the data collected during the trip and incorporate it into what they’ve already learned about land change and permafrost in Alaska.

An Alaska-wide assessment of thermokarst lakes is in the works, one based on six decades of satellite data, aerial photography and other remotely-sensed data sources. That can serve as a roadmap as research teams continue to study methane and the impact of permafrost thaw.

“We’re looking to have a spatial database that says ‘this is how much of the land has transitioned to water,’ so we understand the footprint, the spatial extent and rates of change,” Pastick said. “Then we can start relating that to some of these methane emissions and subsurface changes.”



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