As the featureless expanse unfolds ahead of my snowmobile, the snow-covered tundra looks like a downy softy blanket—but it is not. Instead, the frozen surface feels more like stiff Styrofoam, jarring the ride. I absorb the bumps by holding my legs in a partial crouch. The loaded train of sleds that I’m towing leaves only a trail of shallow scratches in the hard snow.
This part of the world is arguably changing faster than most areas under a warmer climate, and it is essential to gather data on the subsurface to understand how permafrost is now releasing carbon that has been trapped for eons. Geophysicists have investigated the cryosphere for decades. However, borehole data from Alaska’s Arctic is lamentably sparse and making direct measurements of the subsurface is strikingly challenging. Today, modern instrumentation, computing power, and analysis software means that we are now, more than ever, gathering the data permafrost scientists need to predict the future of these systems.
This past spring I traveled along with a Ph.D. student to Alaska’s North Slope, where we met colleagues from the University of Alaska Fairbanks and the USGS. The main objective here was to make ground penetrating radar (GPR) and surface nuclear magnetic resonance (NMR) measurements of thawed sediments below lakes. The main event was a 300 km-long traverse by snowmobile, starting at the Toolik Field station and ending at the coast of the Arctic Ocean, during which we crossed dozens of lakes. The GPR revealed dipping sedimentary bedding structures in lake sediments, a clue that may help to understand the origin of the sharply variable bathymetry.
There are only a few ways to measure deeper into the thawed lake bottoms: In rare cases direct probing can detect the maximum extent of thaw, and recently airborne electromagnetic geophysics have proved effective for this purpose. In this case, we chose to use surface NMR since the unambiguous measurement of liquid water content gives us high confidence in interpreting the deepest extent of thaw. These measurements on drained lake basins, floating-ice lakes, and grounded ice lakes revealed differences in thaw depths, and in some cases, isolated thawed sediments deep below the terrestrial surface.
Using geophysics to measure properties of thawed permafrost teaches us about an important below-ground dimension of the cryosphere that is often out of view. With measurements like these, we hope to contribute to a new understanding of Arctic system change. Check out http://www.articlakeice.org to meet the rest of the team and find more information on Arctic lakes research.