20 Years of active layer monitoring in the Mackenzie Valley, Northwest Territories

Abstract

The Mackenzie Valley landscape is characterized by widespread permafrost terrain ranging from continuous on the Beaufort coastal plain to sporadic discontinuous in the south. The Geological Survey of Canada has maintained an active layer monitoring network in the Mackenzie Valley and Delta since the early 1990s for determination of annual maximum thaw depth and ground movement. The network currently includes 45 functional sites established in areas of representative vegetation and surficial material. Ten of these contribute to the Circumpolar Active Layer Monitoring network (CALM). Thaw tubes are utilized to determine maximum annual thaw penetration and maximum heave and subsidence of the ground surface. For some sites probing is also done on established CALM grids to measure thaw penetration at a given time and soil moisture content are also determined at specific grid nodes. Most sites are equipped with instrument to measure air and near-surface ground temperature. Active layer thickness in the Mackenzie Valley ranges from less than 50 cm in the north to greater than 100 cm in the southern region. North of Tsiigehtchic, departures from the 2003-2012 mean active layer thickness indicate thicker active layers in the 1990s followed by a thinning of active layers to 2005, with increasing thickness to 2012. North of treeline, a direct relation between air temperature and active layer thickness is evident where active layers in warmer years (1993, 1998, 2006) are on average ~3-9 cm thicker than the 2003-2012 mean; whereas cooler years such as 2004 are associated with thinner (average ~4 cm) active layer for most sites. South of treeline, greater site to site variability is observed and is a consequence of the insulating effect of the vegetation in summer and variable snow cover in winter. The moderating effect of ground insulation for these sites is apparent with a dampening of the active layer response to variations in air temperature although some of the warmer (1998) and cooler (2004) years are still noticeable with active layer on average ~5 cm thicker and ~6 cm thinner for both years respectively. Since 2009 active layers have become thicker in all regions of the Mackenzie Valley. Changes in the active layer thickness influences surface stability through thaw settlement, frost heaving and water ponding. These effects also have implication for the load bearing capacity of the ground and, in some circumstance, slope stability. The monitoring network provides baseline information that is helpful for land use planning decisions, engineering design of infrastructure and for understanding the impacts of a changing climate on a permafrost environment.