Thermokarst Lake and Shore Fen Development in Boreal Alaska

摘要

Patterns and rates of thermokarst lake and shore fen development were studied across three lowland ecoregions in the discontinuous permafrost zone of boreal Alaska. The organic-rich and fine-grained surficial deposits varied from glacio-lacustrine in the Copper River Basin, to abandoned floodplain cover deposits in the Tanana-Kuskokwim lowlands, to collapsed eolian silt and lacustrine deposits on the Koyukuk flats. Soils were ice rich and dominated by micro-lenticular, reticulate, ataxitic, and braided cryostructures within the top 2-3 m. Soil volumetric water contents generally ranged from 70 to 90%. Airphoto analysis showed that thermokarst lakes decreased from 18.7 to 17.8% in area over ～57 years and shore fens increased from 4.2 to 5.8% overall. A total of 76 lakes decreased, while 48 increased in size. Rates of shoreline change averaged -0.06 m/yr (lake contracted) where shore fens were continuous along the margins and averaged 0.12 m/yr (lake expanded) where fens were absent. Thermokarst dynamics varied due to multiple processes, including rapid expansion in ice-rich glacio-lacustrine deposits, rapid lateral drainage in one lake, widespread infilling by shore fens, and continuing thermokarst along the forest edge with replacement by shore fens instead of open water.