ALBERT

Description: Snow can act as an effective insulator, protecting the soil from cold winter atmospheric temperatures. The timing of first snow fall, and its thickness during the coldest months of the winter and the structure of the snowpack are important factors in determining the soil temperature drop over the winter. Although snow cover is often assesed late in the season, prior to melt, daily observations yield a basis for analyzing the effect of snowfall timing on soil temperature. Multiyear records are required to compare the effect of snowpack thickness.Surprisingly few observational records of this type are in the public domain for the Arctic. We present snow depths and soil temperature observation records up to 7 years in length and include sites on the North Slope of Alaska, at Ny Alesund, Spitsbergen and on Samoylov Island in the Lena Delta, Siberia. Vertical arrays of sensors were installed in and below the seasonally thawing layer of soil, from near the ground surface into the upper permafrost. At each site, the soil profile was characterized, and the depths and bulk densities of the soil horizons were measured. Snow depth was measured using either a snow probe, snow pit excavation or an automatically logged ultrasonic snow depth sensor. The observations cover a wide range of snow depths and soil temperatures: maximum annual snow depth at all sites ranged from less than 0.2 m to over 1.5 m in depth; the earliest soil freezing following snow fall (below 0.1 m) began on August 30 and some soil horizons unremained unfrozen until mid-December. The effect of an intermediate snow cover, under normal winter conditions, can be to delay freeze-back by at least 1 month at most depths. The combined effects of the exposed frost boil mineral soil, shallow snow depth and cold winter temperatures lead to extremely cold soil temperatures in the soil.

Description: Our aim is to measure and explain the seasonal changes in soil ice content in the frost boils of Galbraith Lake, Alaska. Instruments were installed in a frost boil to monitor the ground surface position and soil state over a period of 4 years. By comparing the subsidence and thaw rates, we calculate the soil ice content as a function of depth. Measured soil temperatures, liquid water contents and bulk apparent thermal conductivities are used to estimate latent heat production and release in the soil. The frost boil heaves during freezing and settles during thaw while the surrounding tundra heaves negligibly, but subsides measurably. Despite large changes in freezing rates from year to year, total heave and its distribution across the frost boil are similar between years. Winter air temperature and snow depth influence the freezing rate and ice distribution as a function of depth, but not the overall heave. This suggests that heave is controlled by water availability rather than the rate of heat removal from the soil. Areal ground subsidence rates between 2 and 5 cm/yr are due to the disappearance of ice at the base of the active layer, raising the possibility of ongoing thermokarst expansion around Galbraith Lake.

Description: The active layer is the most dynamic and sensitive subsurface component of the permafrost environment. The thickness of the active layer is expected to increase in a warmer climate. It has been widely documented that both the annual air temperature and permafrost temperatures are increasing. However, in many cases it does not appear to be true that the thickness of the active layer has increased substantially over the past decade. Part of this situation can be explained by short observational records and natural variability. Recorded air temperature observations exist for over 100 years, while active-layer thickness observations have only been recorded continuously over the past 15-20 years. After a few years of observations it was realized that another process was possibly masking the process of deeper active layer thaw. A new protocol strategy for measuring active-layer thickness was initiated by several research groups in northern Alaska. It quickly became clear that there was subsidence (thaw settlement) in the active layer and that this occurs in response to thawing of ice-rich soils at the base of the active layer or uppermost permafrost. The relevant questions of interest are: what is the typical annual amount of subsidence that can be expected, how long can this process of subsidence be sustained and what is the significance of this process? Increased active-layer thickness has hydrologic implications, as it increases subsurface storage and will both prolong drainage and freezing in the fall. If subsidence occurs during the thawing process, excess water is released for runoff and evapotranspiration. Long-term effects could involve changes in soil moisture, vegetation, and geomorphic processes. In low-lying coastal areas, the rate of subsidence could hypothetically eclipse sea level rise; and these areas could become inundated sooner than expected and become more susceptible to storm surges. All of this depends upon the distribution, horizontally and vertically, of the ice-rich layer at the base of the active layer or at the permafrost table. Presently, we have only a qualitative understanding of the distribution of this ice-rich layer and a handful of studies that report on the magnitude of subsidence at some specific locations. For this reason, the Circumpolar Active Layer Monitoring (CALM) program, beginning in 2004, has made subsidence measurements a priority. At the transition from the Brooks Range to the northern foothills in Alaska, Overduin and Kane (2006) monitored subsidence in the area of some frost boils between 2 and 5 cm/yr over a three year study period. The CALM programs pilot subsidence studies were also made in northern Alaska. Initial results, reported by Streletskiy et al. (2008), showed that subsidence averaging 12 and 13 cm occurred in 1 ha areas of the coastal plain and Brooks Range foothills, respectively, over a five year period between 2001 and 2006. If this magnitude of subsidence is sustained (given an appropriate climatic trend and ice-rich ground) over a few decades, it will significantly impact how the region looks and behaves.