Our Postdoc Sonya Antonova is a remote sensing specialist in the ongoing project PermaSAR. She analyses optical and radar satellite images to detect changes in the land surface. Prior to our expedition, Sonya had identified several locations in the Trail Valley Creek area where vegetation had disappeared. Such land disturbances take place when the ground moves significantly and rather rapidly, destroying or disrupting the topography and vegetation. The disturbances detected by Sonya are probably so-called active layer detachments. The active layer is the surface layer of the permafrost that thaws each year in summer. The soil beneath the active layer stays frozen all year round. Active layer detachments are shallow landslides, which occur in permafrost areas in response to high summer air temperature, rainfall events, or rapid melting of snow cover. With the climate warming strongly and rapidly throughout the Arctic, such land disturbances are bound to become a widespread phenomenon.
Field detectives find exciting land deformations
But to be sure what exactly we were seeing on the satellite images, we had to visit the sites on the ground. Field observations are indispensable when trying to understand signals from satellite imagery. And at every location Sonya had provided us, we could see exciting processes of land disturbance. Those sites were a real eye opener to me: while I had walked through this area on our way to the Trail Valley Creek research station, I had not seen the hidden “crawl” then. Clearly, the material on the slopes had moved rapidly. And this movement resulted in bare surfaces or new, different vegetation resettling on the freshly reworked material.
The drone takes a look over the disturbance site that is shaped like a horseshoe. A small area still has bare ground but lush green grass is already growing again on most of the disturbed soil. Source: Stephan Lange
At another site, vegetation had also disappeared but not due to an active layer detachment and a crawling of the upper soil downhill. Instead the area showed polygons. Polygons are extraordinary patterns in Arctic landscapes that form because of ice wedges in the soil. In the Trail Valley Creek area, the ice wedges in the soil melted and the soil collapsed. The newly formed depressions were filled with water, which shows up as disappearing vegetation on the satellite image.
How to remotely measure changes in the land surface
While it is nice to map these phenomena, it is be even better to estimate how much of the surface eroded. For that, we use two Digital Elevation Models (DEMs) from different years and calculate the difference between the elevations. The largest land disturbance we detected was about 100 by 100 m in size. In order to investigate such changes in the land surface, we need DEMs with very high resolutions. The higher the resolution, the better we can see details on the ground. Recently, two new DEMs with an unprecedented resolution were issued: the commercial global TanDEM-X DEM based on radar acquisitions with a resolution of 12 m; and the open-access pan-Arctic ArcticDEM based on optical stereo imagery with a resolution up to 2 m. Both DEMs used acquisitions from different years, making it possible to track the elevation changes in time. Moreover, an airborne laser scanner surveyed the area in 2016 and now, in 2018. This provides very accurate elevation information with a sub-meter resolution, and we will definitely compare the elevation models once the laser data from this year have been processed. You can read about our airborne laser scanning campaign here.
Good to know: ice wedges create polygonal patterns in the Arctic
Ice wedge polygons are one of the most common ground patterns in the Arctic. Individual polygons range from 5-50 m in diameter and form as the result of winter freezing and spring thawing. At temperatures below -15°C, the soil becomes so brittle that it cracks as it contracts in response to cold temperatures. Come spring, meltwater fills these cracks and subsequently freezes into ice wedges. Year after year, the cracks increase in depth and diameter, and so do the ice wedges. From the air, tundra patterned in this manner resembles a cracked, dried-up mud puddle. The striking geometric appearance of ice wedge polygons dominates the lowlands of arctic Canada.
Contributions by: Sonya Antonova
Edited by: Sina Muster