HR: 08:20h
AN: C11A-02 INVITED     [Abstracts]
TI: Reassessment of the Genesis of "Thaw Lakes" on the Arctic Coastal Plain in Northern Alaska
AU: * Shur, Y
EM: ffys@uaf.edu
AF: Civil and Environmental Engineering Department - University of Alaska Fairbanks, PO BOX 755900, Fairbanks, AK 99775 United States
AU: Jorgenson, M T
EM: tjorgenson@abrinc.com
AF: ABR, Inc, PO BOX 80410, Fairbanks, AK 99708 United States
AB: Oriented lakes and drained lake basins on the Arctic Coastal Plain in northern Alaska have been the subject of numerous studies for more than 50 years. From the beginning, the waterbodies have been described as "thaw lakes" and since then a thermokarst genesis for the lakes has been accepted without any quantitative analysis of the initial permafrost conditions and thaw-susceptibility of the upper permafrost. We initially sought to quantify ground ice changes in support of this concept through detailed permafrost and terrain studies in the northeastern NPRA, an area with thick deposits of loamy sand and abundant lakes. During 2001-2004 we conducted detailed terrain analyses that included field surveys, permafrost investigations, and photogrammetry. A terrain-unit approach was used to relate soil and ground ice properties to surficial deposits related to lake development. Cryogenic structures, ice volumes, and properties of upper permafrost were described from borehole cores taken from every stage of lake-basin development and in surrounding areas. Ground ice also was described and sampled at 20 exposures at lake and riverbanks. We classified stages of drained basin development and quantified their permafrost characteristics. The primary stage of lake development is usually described as degradation of ice-wedges at their intersections. A thaw bulb then develops under the deep water and the thaw lakes expand laterally through both mechanical and thermal erosion. Although we observed numerous ponds at ice-wedge crossings we did not observe later sequential stages of thaw lake development. Instead, we observed that initial shallow ponds were soon colonized by vegetation, which halted thermokarst. In addition, ice volumes and thaw settlement properties of soils were insufficient to allow thaw lake development. Under the standard concept of lake development, the formation of ice wedges raises the surface and allows the development of new thermokarst, and thus creates a "thaw lake cycle." Although we found that the ice-content of soils increased with age of lake-basin deposits, the ice content of lake-basin margins was insufficient to substantially raise the surface. The genesis of surficial deposit varies throughout the Arctic Coastal Plain and is closely related to soil properties, ice content of permafrost, and geomorphic processes. Consequently, lake distribution should be closely associated with ground ice conditions, but lakes are abundant on all deposits. Although true thermokarst lakes occur in some regions, such as the Colville River Delta, lower Brooks Foothills with thick loess deposits, and the Beaufort Sea coast from Barrow to Cape Halkett where thick marine silts are present, the majority of lakes on sandy deposits common across the coastal plain are not due to thermokarst. Instead, our analysis shows that lakes that formed during Holocene could not have resulted from the degradation of ground ice. Rather, these lakes formed simply by the accumulation of water in depressions.
DE: 9315 Arctic region
DE: 4546 Nearshore processes
DE: 1823 Frozen ground
SC: Cryosphere [C]
MN: 2004 AGU Fall Meeting