ALBERT

Description: Direct-Current (DC) electrical resistivity imaging has proven to be a suitable technique for a number of permafrost related questions. We present measurements from a high-arctic continuous, maritime permafrost site near Ny Alesund, Svalbard (Norway). The area under investigation features a great diversity of soil types and soil water contents. Sparse vegetation alternating with rock fields and exposed soil characterize the surface.A total of 25 different transects each of 47.5m length were investigated using a DC-Resistivity and Electrode Control System (RESECS) with 96 electrodes at a spacing of 0.5m in Wenner-alpha configuration. At three transects, fixed electrode arrays were installed and measured on a weekly basis in order to capture temporal changes. The study was conducted from August until mid-September, thus covering the period of maximum active layer thickness and the beginning of freeze-up.The specific resistivities at the surface ranged from less than 50 Ohm m in areas with damp clay to more than 1000 Ohm m in rock fields and on dry hill crests. In most cases, areas with such high resistivities only extended to depths of less than 1 m. From depths between 1m and 1.5m onwards, specific resistivities increased continuously, indicating the position of the freeze-thaw interface. This agrees well with thaw depths that were determined by point measurements along individual transects using a drill.The repeated measurements of the fixed electrode arrays displayed the most pronounced changes in the beginning of August, where decreases in specific resistivities of up to 40% over one week period were detected at depths between 1m and 2m. Afterwards, only insignificant changes were observed at these depths. This is interpreted to be the seasonal thawing of the active layer, which stagnates during in the second half of August. At depths less than 1m, both decreases and increases in specific resistivities were detected, most likely due to changes in the water content of the soil.

Description: Direct-Current (DC) electrical resistivity imaging has proven to be a suitable technique for a number of permafrost related questions. We present measurements from a high-arctic continuous, maritime permafrost site near Ny Alesund, Svalbard (Norway). The area under investigation features a great diversity of soil types and soil water contents. The surface is characterized by sparse vegetation alternating with rock fields and exposed soil.We investigated 25 different transects each of 47.5m length using a DC-Resistivity and Electrode Control System (RESECS) with 96 electrodes at a spacing of 0.5m in Wenner-alpha configuration. At three transects, fixed electrode arrays were installed and measured on a weekly basis in order to capture temporal changes. The study started in August, just before the active layer reached its maximum thickness, and extended until the beginning of freeze-up in mid-September.The specific resistivities at the surface ranged from less than 50$\Omega$m in areas with damp clay to more than 1000$\Omega$m in rock fields and on dry hill crests. In most cases, areas with such high resistivities only extended to depths of less than 1 m. From depths between 1m and 1.5m onwards, specific resistivities increased continuously, indicating the position of the freeze-thaw interface. This is in general agreement with thaw depths that were determined by point measurements along individual transects using a drill.The repeated measurements of the fixed electrode arrays displayed the most pronounced changes in the beginning of August, where up to a 50\% decrease in specific resistivity over a period of two weeks was measured at depths below 1 to 2m. This is interpreted to be the seasonal thawing of the active layer. A subsequent increase in specific resistivities at these depths until mid-September corresponding to the refreezing of the soil was only observed in some areas, which suggests spatial variations in the course of the refreezing process.

Description: In the arctic environment, the active layer (the thin layer of the soil above the permafrost, which annually thaws and freezes) is the zone where most hydrologic, biologic, ecologic, and geomorphic processes occur. The active layer is a dynamic region where soil properties (both hydraulic and thermal) rapidly change from the onset of snow melt through the freeze-back period. The rate of soil thaw is dependent upon a number of factors including soil material, soil moisture, and ice content. As these factors are variable with space and time, the position of the active layer is also both spatially and temporally variable. Knowing the condition of the soil (frozen, thawed, and position of the freeze/thaw interface) is fundamental to understanding and predicting the spatial and temporal energy and water fluxes. In this study, we incorporate a modified two-directional freeze-thaw algorithm into a spatially-distributed, process-based hydrologic model called TopoFlow. Written in IDL, TopoFlow was designed to handle the rapidly changing thermal and hydraulic soil properites that are ubiquitous throughout the Arctic. TopoFlow currently is able to simulate the major components of the water balance (precipitation, snow melt, evapotranspiration, groundwater flow, overland/channel flow) as well as some storage processes. The two-directional freeze-thaw algorithm is based upon the Stefan's equation. Physical properties of the soil (bulk density, porosity, mineral and organic content, water content) are specified as input variables. The algorithm is driven by surface temperature and temperature at a specified depth. The incorporated algorithm is tested in a small high-arctic watershed (Bayelva) located close to Ny-Alesund, Svalbard. Comparisions of simulated freeze/thaw interface (0°C isotherm), snow melt, evapotranspiration, and soil moisture content with field measurements will be presented.

Description: The landscape of the Barrow Peninsula in northern Alaska is thought to have formed over centuries to millennia, and is now dominated by ice-wedge polygonal tundra that spans drained thaw-lake basins and interstitial tundra. In nearby tundra regions, studies have identified a rapid increase in thermokarst formation (i.e., pits) over recent decades in response to climate warming, facilitating changes in polygonal tundra geomorphology. We assessed the future impact of 100 years of tundra geomorphic change on peak growing season carbon exchange in response to: (i) landscape succession associated with the thaw-lake cycle; and (ii) low, moderate, and extreme scenarios of thermokarst pit formation (10%, 30%, and 50%) reported for Alaskan arctic tundra sites. We developed a 30 × 30 m resolution tundra geomorphology map (overall accuracy:75%; Kappa:0.69) for our ~1800 km² study area composed of ten classes; drained slope, high center polygon, flat-center polygon, low center polygon, coalescent low center polygon, polygon trough, meadow, ponds, rivers, and lakes, to determine their spatial distribution across the Barrow Peninsula. Land-atmosphere CO2 and CH4 flux data were collected for the summers of 2006–2010 at eighty-two sites near Barrow, across the mapped classes. The developed geomorphic map was used for the regional assessment of carbon flux. Results indicate (i) at present during peak growing season on the Barrow Peninsula, CO2 uptake occurs at -902.3 106gC-CO2 day−1 (uncertainty using 95% CI is between −438.3 and −1366 106gC-CO2 day−1) and CH4 flux at 28.9 106gC-CH4 day−1(uncertainty using 95% CI is between 12.9 and 44.9 106gC-CH4 day−1), (ii) one century of future landscape change associated with the thaw-lake cycle only slightly alter CO2 and CH4 exchange, while (iii) moderate increases in thermokarst pits would strengthen both CO2 uptake (−166.9 106gC-CO2 day−1) and CH4 flux (2.8 106gC-CH4 day−1) with geomorphic change from low to high center polygons, cumulatively resulting in an estimated negative feedback to warming during peak growing season.