ABSTRACT: Thermal observations of permafrost growth at the Illisarvik drained lake site, Richards Island, Mackenzie Delta, N.W.T

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Abstract

n August 1918, 'Illisarvik Lake", located on a peninsula of northern Richards Island in the Mackenzie Delta, N.W.T. and on the verge of self-drainage by natural shoreline erosion, was artificially drained. The Illisarvik experiment, proposed by Dr. J.R. Mackay, is a unique long term investigation of permafrost growth under full scale natural conditions. The predrainage lake measured some 300 x 600 m, was 45 m from the sea coast at its closest point, with a mean lake surface 7 m above sea level and maximum water depths of 4.5 m. The freezeback of the thaw-bulb (talik) beneath the lake bed and associated permafrost processes (redistribution of moisture, ice segregation and frost heave, active layer growth, ice-wedge cracking) have been the subject of ongoing multidisciplinary research. Geothermal investigations focus on the monitoring of ground temperature cables, installed in hydraulically drilled boreholes in the lake bottom and surrounding shore, to delineate predrainage and postdrainage thermal conditions. Ten (10) cables were installed in April 1978 and 4 cables in August 1978, prior to drainage. In summer 1979, 10 more cables were installed to replace those which had been damaged during the predrainage ice-breakup in spring 1978, and to increase data coverage near the lake shore. A further 9 cables were installed in summer 1981 to increase data density in the vicinity of the warmest and deepest part of the talik. On average the cables contain 10 thermistor sensors and extend to depths of 20 to 30 m beneath the lake bed. Of the 33 cables installed by EPB, approximately half are currently operational. Ground temperature mesurements have been acquired whenever practicable, in general 3-4 times a year, and compiled into a continually updated data file. Monitoring of the ground temperature cables prior to drainage delineated a bowl-shaped talik extending to some 32 m beneath the lake bottom. Mean annual ground temperatures ranged from +3°C in the lake centre, to -3°C at the lakeshore and -7°C 250 m inland. In the deeper central part of the lake temperatures were negative beneath the talik, and temperature gradients were negative (down to the maximum depth of measurement of 90 m) averaging -50 mK/m within the permafrost section. The postdrainage monitoring revealed that by September 1984, 6 years after drainage, all sensors were below 0°C and hence permafrost by definition had formed beneath the lake bed. However there remains a large zone where temperatures are between -0.50°C and 0°C. In this zone pore waters are likely in the liquid state due to a gradual freezing point depression resulting from the increasing pressure generated by the freezeback of the talik. There is thus a core zone where latent heat of freezing remains to be liberated and permafrost processes are very dynamic.

Bibliography of Canadian Geomorphology