| Abstract | Introduction
Permafrost is an important component of the landscape of the Mackenzie Valley NWT that influences both natural and socio-economic environments. Changes in the ground thermal regime and
active layer conditions in response to a changing climate can lead to alterations in drainage and ground stability which has implications for environmental and infrastructure integrity. The active layer responds to shorter term fluctuations in
climate compared to the thermal regime of deeper ground. Monitoring of two key cryospheric indicators (or essential climate variables), active layer thickness and permafrost thermal state allows an assessment of both inter annual variability and
longer-term change in permafrost conditions. The Geological Survey of Canada has maintained a permafrost monitoring network in the Mackenzie Valley (Figure 1) since the mid 1980s that consists of a suite of sites representing the range of
ecoclimatic conditions in the region. This paper describes the spatial variation in active layer thickness and permafrost thermal state and also documents the change that has occurred over time.
Study Sites and Instrumentation
The monitoring
transect extends from the tundra environments of the Tuktoyaktuk peninsula to the boreal forest in the south. The terrain consists of lacustrine, moraine, fluvial and deltaic sediments. Extensive peatlands are found in the southern portion of the
region where poor drainage resulted in thick accumulations of peat. Ice-rich fine-grained sediments such as lacustrine clays are common. Permafrost is continuous and several hundred metres thick on the Beaufort Coastal plain in the north and becomes
thin and sporadic in the southern NWT. Field sites (Figure 1) were selected to be representative of the vegetation and terrain conditions in the region. The active layer monitoring network was initiated in 1991 and originally consisted of 66 sites
with 45 still in operation. Thaw tubes are utilized to determine maximum summer thaw penetration and also maximum heave and subsidence of the ground surface. Details on instrumentation and site descriptions are available in Smith et al. (2009). Over
70 boreholes have been instrumented with thermistor cables connected to data loggers to measure ground temperatures to depths of 20 m. Some sites have been operational since the 1980s, but many were established between 2006 and 2008, during the
International Polar Year (Smith et al. 2010). At many sites, instrumentation has also been installed to measure air and ground surface temperature. More information on the instrumented sites along with recent data collected can be found in Smith et
al. (2015).
Current Conditions
Permafrost temperatures are above -2°C throughout a large portion of the region, especially within the discontinuous zone. Colder conditions are found within the continuous permafrost zone but permafrost
temperatures are highly variable ranging from -6 to -7°C in the tundra uplands to higher than -2°C in wet areas or where vegetation promotes snow accumulation. Active layer thickness (ALT) ranges from about 0.5 m in the north to greater than 1 m in
the south with ALT generally being less above treeline compared to that below treeline. Considerable spatial variability in ALT is observed particularly below treeline where high shrubs dominate and influence snow accumulation.
Temporal
Variability
Long-term records of permafrost temperature indicate permafrost has generally warmed in the Mackenzie Valley since the mid 1980s, which is consistent with increases in air temperature (Smith et al. 2010). Although this warming has
continued over the last decade, it has generally been at a lower rate. Since 2007, when many of the sites were established, permafrost temperatures have increased at most sites. Between 2007 and 2014, increases in permafrost temperature have ranged
from less than 0.1°C to 0.2°C in the discontinuous zone and from 0.2 to 0.5°C in the continuous zone (Smith et al. 2015). At warmer permafrost sites, especially where ground temperatures are close to 0°C and soils are ice-rich, latent heat effects
associated with phase change result in ground temperatures being less responsive to changes in climate (Smith et al. 2010). Greater interannual variability is observed in active layer records compared to ground temperature records and there has been
no pronounced trend in ALT over the 1991-2014 record. At many sites the maximum ALT occurred in 1998 which was one of the warmest years on record. ALT generally decreased following the 1998 peak, but there has been an increase in ALT at many sites
since 2005 which seems to coincide with a period of higher summer air temperatures. Although active layer development is influenced by summer air temperatures, the surface freezing index has decreased recently at some sites and the observed increase
in ALT may also be partly due to warmer winter conditions.
Summary
Data from the permafrost monitoring network has enabled characterization of spatial and temporal variability in active layer thickness and permafrost thermal state in the
Mackenzie Valley. Recent increases in ALT and permafrost temperature have been observed but the magnitude and rate of change varies spatially. The monitoring network generates essential information on permafrost in an important
transportation/transmission corridor, which can inform land management decisions, infrastructure planning and adaptation to a changing climate.
References
Smith, S.L., Chartrand, J., Duchesne, C. and Ednie, M., 2015. Report on 2014 field
activities and collection of ground thermal and active layer data in the Mackenzie Corridor, Northwest Territories, Geological Survey of Canada Open File 7935.
Smith, S.L., Riseborough, D.W., Nixon, F.M., Chartrand, J., Duchesne, C., and Ednie,
M. 2009. Data for Geological Survey of Canada active layer monitoring sites in the Mackenzie valley, N.W.T., Geological Survey of Canada Open File 6287.
Smith, S.L., Romanovsky, V.E., Lewkowicz, A.G., Burn, C.R., Allard, M., Clow, G.D.,
Yoshikawa, K., and Throop, J. 2010. Thermal state of permafrost in North America - A contribution to the International Polar Year. Permafrost and Periglacial Processes, 21: 117-135. |