ABSTRACT: On the origin of aggradational ice in permafrost

CGRG Bibliography of Canadian Geomorphology
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Abstract

An investigation of the annual water balance of the near surface layers was conducted at eight sites near Mayo, Yukon Territory, to ascertain the origin of the ice-rich zone commonly found beneath the active layer in permafrost terrain. Water movement in frozen ground is principally driven by temperature-induced potential gradients, which move water downward into permafrost in summer and fall, and upward out of permafrost in winter. Less winter desiccation than summer infiltration occurs, due to the lower hydraulic conductivity at the prevailing temperatures. Overall, permafrost is a sink for atmospheric water. Direct measurements of the thermal and hydrological regimes at the sites were accompanied by measurements of soil heaving, which indicate water accumulation in particular soil layers. The hydrological processes were simulated on a short-term basis in PVC tubes filled with soil slurry. In addition, the tritium content of permafrost and the ground ice stratigraphy in the study area were examined for evidence of long-term moisture movement into permafrost. The results indicate that downward water movement into frozen ground occurs throughout the summer. During freeze-back, however, most water movement is upward, and results in frost heave. Up to 23 percent of the frost heave measured occurred within frozen soil, independent of the penetration of the frost front. Throughout the winter, upward water movement continued in the ground, driven by the soil temperature gradient. Overall, a mean maximum rate of water incorporation into permafrost of 3.5 mm year('-1) was detected. Tritium, derived from nuclear testing in the 1950s and 1960s, has infiltrated permafrost to depths of up to 50 cm. The distribution of tritium in permafrost at two sites irrigated with tritiated water in 1983 suggests that tritium infiltration is temperature-induced. The ground ice stratigraphy in the headwall of a thaw slump was studied. The 90 cm of segregated ice above a thaw unconformity, dated at 8870 (+OR-) 200 years BP, provides a mean minimum rate of permafrost aggradation of between 0.1 and 0.2 mm year('-1) over the past 8 ka.

Bibliography of Canadian Geomorphology