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...
