Wednesday, July 23, 2014

Frozen Solid - Permafrost Beneath the Tundra

It's hard to imagine that soils can be frozen for hundreds of years. But try to dig a hole into the Arctic tundra and you'll quickly find that the job is almost impossible due to permafrost; soil frozen solid.

Permafrost is technically defined as soil that is frozen for two or more consecutive years. It can be found throughout much of the North Slope, occurring at several inches to many feet below the soil surface. In Barrow, the depth to permafrost averages 18 inches (plus or minus), but does vary depending on landscape position (e.g., slope and aspect), water saturation, vegetation cover, and thickness of any moss or organic layer.
Today while working on the Barrow Environmental Observatory (BEO), it was all I could do to get a shovel into the ground to a depth of 6 to 8 inches. Having dug a hole, it was easy to see that the upper soil was largely composed of recent organic matter, with gray to black mineral soil deeper in the soil profile, and then frozen soil. A quick measurement with a temperature probe confirmed that soil temperatures were close to zero and considerably colder than that at depth. Sensors placed deep into the permafrost at our field site show that temperatures can be as low as -9 C and stay that temperature year round.

The layer of soil above permafrost is referred to as the active layer. This layer will continue to thaw and deepen throughout the season. By the end of September the active layer will be thicker than it is today and mark the true depth to permafrost.

One of the questions that the NGEE Arctic project is trying to understand is just how much rising temperatures will continue to thaw permafrost (or deepen the active layer) in the coming decades. This information needs to be incorporated into models if we are to predict changes to Arctic ecosystems in the future. Assuming that warmer temperatures will accelerate thaw depth, this may have significant consequences for the release of CO2 and CH4 from tundra landscapes. This can occur directly through an acceleration of microbial processes responsible for CO2 and CH4 production or indirectly through changes in landscape topography and water distribution. Our team is studying this latter dynamic by looking at CO2 and CH4 fluxes from low- and high-center polygons. High-center polygons form as a result of long-term changes in ice content due to rising temperature and they can have very different soil and vegetation characteristics than more common low-center polygons.

I'll show how we measure the flux of greenhouse gases from polygons on the tundra later this week...