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TOMKINS, JESSICA D.  Queen's University.
Lamoureux, Scott F.  Queen's University.

Mirror Lake, Northwest Territories (62 N, 128 W), is a weakly stratified, oligotrophic lake that receives meltwater and suspended sediment on a seasonal basis primarily from the partially glacierized southern portion of its catchment. This seasonal delivery of sediment, in addition to 137Cs measurements, suggests that the sedimentary record has an annual resolution (i.e., varved). Through image and microscopic analysis of thin sections, a varve chronology was developed and the resulting varve thickness record was compared to local meteorological data (Tungsten, 7 km from site) to determine hydrometeorological influences on varve formation during the past 35 years.

Analysis of the varve thickness and meteorological records indicates that the hydrological regime of the Mirror Lake catchment fluctuates on a decadal time scale between two recognizable modes. July temperature was the most influential climatic control over varve formation throughout the sedimentary record, although precipitation also played a role through the negative effect of thick snowpacks on glacier ablation during certain periods of the record. Varves formed in the 1970s reflect one mode of the hydrological system, where July mean temperature was the main control (r2 = 0.890, n = 11, p < 0.01) on varve thickness due to its positive influence on glacier ablation and average to below average snowpacks during this period. By contrast, varves formed in the 1980s reflect the second mode, whereby both summer temperature and snowfall were both significant influences on varve thickness. Observed mean July temperature increased 0.7 C between the 1970s and 1980s, while no other months showed substantial changes. Despite this temperature increase, above average snowfall during the 1980s weakened the July temperature signal in the sedimentary record. Thicker snowpacks are inferred to have delayed glacial melt, resulting in a stronger precipitation signal in the sedimentary record.

The decadal climatic variability determined at Mirror Lake was also evident through varve structure. Varves formed under the first hydroclimatic mode had only one silt unit while varves formed under the second mode had two prominent silt units. The lower silt unit of the second mode varves likely formed from spring nival melt or the first peak in glacial meltwater discharge in July, while the upper silt unit likely formed during peak melt in August. This upper unit was largely absent in years under the first hydrological regime mode. Therefore, varve structure in the Mirror Lake record reflected the dominant hydroclimatic processes operating to transport sediment to the lake to form varves, in addition to demonstrating an annual record of climatic variability.

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