麻豆传媒

Thaw lakes- widespread thermokarst features in lowland ice-rich permafrost- have bathymetry that depends on depth of ice-rich substrate, talik (thaw bulb) shape, lake generation (later-generation lakes form in drained basins of earlier lakes), lake age and climate conditions over varying time scales. To investigate plan view morphology of early generation and later generation lakes, air photos and measurements of plan view characteristics are used to determine relationships between lake size, ellipticity, and ground ice environment at sites in NW Alaska and N Yukon. A numerical model of heat transfer, phase change and thaw-subsidence beneath a non-expanding lake in cross-section is developed to explore the effect of shallow and deep ground ice environments on thaw lake bathymetry. Results show lakes develop basins > 10 m deep over 5000-8000 y in deep, ice-rich material, and shallow basins where ground ice is localized near the surface. A more comprehensive numerical model of heat transfer and phase change, thaw-subsidence and mass wasting at lake margins is developed to (i) explore thaw lake bathymetry in deep and shallow ground ice; (ii) compare model bathymetry against natural lakes; and (iii) test the sensitivity of thaw lake bathymetry and bank retreat to climate change. In simulations of lake expansion in a deep ground ice environment, modeled lakes expand at 0.26 m/yr and have margins inclined 20-30^o, consistent with lakes in northwestern Alaska where permafrost is rich in syngenetic ground ice to >80 m. In shallow ground ice, modeled lakes expand 0.04 m/yr, have gently inclined margins (< 10^o) and are < 4 m deep, consistent with the bathymetry of natural lakes on the Yukon Coastal Plain in northwestern Canada where ground ice is concentrated in the upper few metres of permafrost. Using the model to estimate methane emission from expanding thaw lakes in Siberia suggests the predicted increases in mean annual temperature of 0.3^oC/decade in the coming century will accelerate lake expansion and resultant CH4 release by 2-3x, with a time lag of 75 to 100 years between onset of warming and the maximum acceleration. The magnitude of accelerated expansion of methane emissions (to >5 Tg y^-1 for Siberia after 100 y) may be a significant feedback to warming.

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Supervisor: Lawrence Plug