REGIONAL ACTIVE LAYER MONITORING ACROSS THE SPORADIC, DISCONTINUOUS AND CONTINUOUS PERMAFROST ZONES, MACKENZIE VALLEY, NORTHWESTERN CANADA F. Mark Nixon 1, Alan E. Taylor 2 1. Geological Survey of Canada, 601 Booth St., Ottawa, ON, K1A 0E8 e-mail: mnixon@gsc.nrcan.gc.ca 2. ASL Environmental Sciences Inc., 1986 Mills Road, Sidney, BC, V8L 5Y3 e-mail: altaylor@kcorp.com Abstract Fifty-eight sites have been established along a 1200 km transect in the Mackenzie Valley to monitor processes linking climate, climate change, permafrost and the active layer. Annual maximum thaw penetration and surface movement are measured relative to thaw tubes anchored in permafrost. Active layer thickness, calculated from thaw penetration and surface movement, varies more with local soil properties, vegetation and microclimate than with regional atmospheric climate. While thaw penetration has increased at most sites over the last 4-6 years, this increase is not always reflected by an increase in active layer thickness because of thaw settlement. Air and shallow ground temperatures are measured every 2-6 hours at many sites. Air thawing degreedays (DD) are up to three times greater than ground thawing DD, an effect of surface vegetation and snow cover. Larger air thawing DD values are required in boreal forest than in the tundra to achieve similar active layer thicknesses. Introduction Climate, more particularly soil-climate, has changed significantly in the Mackenzie Valley and Delta. Mackay (1975) cites several lines of evidence that indicate a 3 C increase in air temperatures from the late 1800Õs to the mid-1900õs, followed by a 2 C decrease. Since the 1970Õs, an increase greater than 1 C has been recorded (Skinner and Maxwell, 1994). For a scenario of CO 2 doubling, general circulation models project a substantial increase in temperature, particularly in winter (Boer et al., 1992). A feature of permafrost that has responded significantly to past climate change is the thickness of the active layer (Mackay, 1975; 1976). Changes in active layer thickness influence surface stability through thaw settlement, frost heave and bearing capacity. Slope stability is also affected (Aylsworth and Egginton, 1994). Besides these geotechnical characteristics, changes in active layer thickness affect hydrology (Hinzman and Kane, 1992), soil moisture (Edlund et al., 1989) and nutrient availability (Waelbroeck et al., 1997) and subsequently impact the ecology of an area through modified vegetation. Beginning in 1990, a 1200 km transect was established along the Mackenzie Valley, Northwestern Canada to quantify the active layer and particularly to monitor the long-term (decadal) change in active layer character in an absolute and statistically robust manner. The transect trends northwesterly, starting in sporadic permafrost, crossing the discontinuous permafrost zone and terminating in continuous permafrost at the Beaufort Sea coast (Figure 1). In this paper, we present data from these sites with some preliminary interpretation. The transect is part of CALM (Circumpolar Active Layer Monitoring) program, an initiative of the International Permafrost Association (IPA) to archive statistical active layer thicknesses throughout the polar regions over many years (Nelson and Brown, 1997). Data will be submitted annually to the Global Geocryological Database of IPA (Barry et al., 1996). Location and site selection Starting in 1990, 58 sites were established in natural, undisturbed areas along the Mackenzie Valley (Figure 1). Observations of general site conditions, vegetation and forest types, and spring snow depths were made to characterize the local environment when choosing each site (Nixon and Taylor, 1994; Nixon et al., 1995; for site descriptions, see International Permafrost F. Mark Nixon, Alan E. Taylor 815
Figure 1. Location of thaw-depth monitoring stations in the Mackenzie Valley, Northwest Territories, Canada; (inset) in Mackenzie Delta area. Association, 1998). In the regional perspective, the transect crosses several ecoregions (Ecological Stratification Working Group, 1995), permafrost boundaries (Heginbottom et al., 1995) and a variety of forest types (Canada Department of Indian and Northern Affairs, 1974) from the boreal forest to the tundra. Instrumentation Maximum annual thaw penetration and maximum heave and subsidence of the ground surface are measured using a modified version of a frost or thaw tube (TT) (Rickard and Brown, 1972; Mackay, 1973), described in Nixon et al, (1995). Over half these sites are also instrumented with automatic air and ground temperature loggers (from 1993; see Fig. 5 in Nixon et al., 1995). Automatic loggers are used to record temperatures every 2 to 6 hours for a year or more. Additional instrumentation is installed south of Mountain River (91TT20, 65 40'29"N): 5 sites have probe arrays to measure a temperature-electric potential profile through the active layer (Hinkel et al., 1997) and additional 1.5-5 m ground temperature cables or probes extending through the active layer into permafrost. An active 816 The 7th International Permafrost Conference
The outer tube of the TT assembly is terminated in permafrost well below the active layer. Each summer, measurements of current thaw penetration, ground level, and maximum heave or subsidence are made relative to this tube as a stable reference level. From these, we derive two quantities for the preceding summer: (1) the maximum thaw penetration, independent of the ground surface; (2) calculated maximum active layer thickness that is assumed to coincide in time with the maximum subsidence of the surface. Note that maximum seasonal thaw is measured, not seasonal thaw progression, as the sites are visited only briefly each year. Figure 2. Maximum active layer thickness from thaw tube measurements for several years, versus latitude. layer probing grid is established at 7 sites so far (CALM, Nelson and Brown, 1997). ACCURACY OF THAW TUBES In a limited study under laboratory conditions, Dolecki (1994) reported the TT frost table was within 2 cm of the thermal frost table and of that measured by probing. During annual summer site visits to the field, the ice horizon in the TT is compared to several probings within a metre of the TT, and commonly falls within the range of probing and is rarely more than a few centimetres beyond. Measurements of thaw penetration (TT) and surface movement (scriber) are made to 1 mm, although due to several mechanical factors, the accuracy of measurements is about 2 cm. Results The active layer thicknesses (definition 2, above) for each complete year of record range from 38 cm to 182 cm and appear to vary from site-to-site as a result of a complex interplay between site-specific and regional factors (Figure 2). Some of the scatter may arise also from the single-point nature of a TT measurement that may be an extreme of the sample distribution for the local area (F. Nelson, personal communication 1997). At some sites, the CALM frost probing grids (Nelson and Brown, 1997) will allow us eventually to place TT values in relation to a statistical mean for each locality. Across this transect, regression of active layer thickness on latitude appears uncorrelated (r 2 =0.19, after known anomalous sites were removed). Stratifying sites by published forest classification (Canada Department of Indian and Northern Affairs, 1974) provides low cor- Figure 3. Year-to-year variability of the maximum thaw penetration at selected sites, from south to north in sequence of latitude. F. Mark Nixon, Alan E. Taylor 817
thaw penetration from installation to 1995 (mean increase, 2.5 cm/year). A decrease in thaw penetration in 1996 at sites north of Norman Wells corresponds to the lowest summer mean temperature for this area in more than 4 years, with August and September being about 2 C lower than normal (Canadian Meteorological Centre, 1996). Changes in active layer thicknesses since installation show a strong correlation with changes in measured thaw penetration (r 2 =0.73, Figure 4) but generally do not show an equivalent increase over the same period, again because of the effect of thaw consolidation. Figure 4. Change in thaw penetration versus change in active layer thickness, since installation. relation, largely because there are few sites in any one class in this regional transect. Maximum annual thaw penetration (definition 1, above) is plotted as a bar chart in sequence of latitude in Figure 3. Records that may contain a component of heave, or that are incomplete, have been discarded. The choice of thaw penetration (rather than active layer thickness) for comparison of interannual thaw simplifies the interpretation by eliminating such local complexities as ground-ice melting and soil compaction. Over 75% of sites show a steady increase in maximum Figure 5. Multiseasonal (1994-1996) mean active layer thickness, as measured by the thaw tube network, versus square root of mean air and ground thawing degree-days. THAWING DEGREE-DAYS Degree-days (DD) were calculated from paired air and ground temperature data for both thawing and freezing seasons. The calculation was made relative to the soil freezing point identified from the temperature of the "zero curtain" on the temperature-time data (Taylor, 1995). Logger failures or radiation shield destruction by wind or animals results in fewer data being available for the air DD calculation. Figure 5 shows the relationship between mean active layer thickness, as recorded by the thaw tubes, and the square root of mean air and ground thawing DD, as calculated from the temperature loggers; in both cases, the means are those of the 1994-1996 annual values. Air thawing DD at sites in the tundra of the Mackenzie Delta are less than those in the boreal forest of the Mackenzie Valley, consistent with the climatological gradient southwards, while active layer thicknesses in both regions are similar. Data clustering in the multi-year means (Figure 5) suggests larger air thawing DD values are required in boreal forest than in the tundra to achieve similar active layer thicknesses. This reflects the insulating effect of the thick moss and surface vegetation, and the additional thawing energy required to melt the thicker snowpack in the Valley, compared to the Delta. Measurements in late winter over 4 or 5 seasons at many sites shows that snow cover is deeper in the boreal forest (57 +/- 11 cm) than on the tundra (30 +/- 25 cm). Some additional data scatter in the ground thawing DD (Figure 5) arises from variability in depth of the shallow ground temperature sensor from site to site, because of the difficulty in determining the "ground surface" in the complex moss-peat-mineral soil profile. Otherwise, a similar clustering distinguishing Valley and Delta data is preserved, suggesting that soil conditions in the Valley are less favourable to thaw development than in the Delta. This may arise from higher ice contents within the surficial organic soils of the boreal forest, and different soil properties. 818 The 7th International Permafrost Conference
Discussion Active layer thickness varies between sites as a result of a complex interaction of local and regional factors. The discrepancy between annual thaw penetration and active layer thickness at a monitoring site is not unexpected. The response of active layer thickness to increasing thaw penetration from year-to-year will depend on a number of highly variable characteristics of the soil column involved, such as excess ice volume in near-surface permafrost, grain size and soil structures that control thaw consolidation. Assigning the difference between thaw penetration and active layer thickness at a site to settling is supported by the close correspondence to an independent measure of subsidence. This illustrates the interesting fact that increasing thaw penetration from year-to-year will not result necessarily in the same increase in active layer thickness. The DD plot (Figure 5) generalizes the influence of the melting spring snow cover, vegetation and insulating thick moss and peat (and possibly higher soil water content and associated latent heat) of the boreal forest, compared to the tundra, in suppressing the growth of the active layer. Snow, vegetation, organics and soils appear to buffer the thermal effect of the southward climatological gradient. Ground thawing DD are less than the air thawing DD, for these same reasons. The scatter reflects variability of site/soil environmental conditions expected for a transect crossing the sporadic to continuous permafrost zones. Conclusions (1) Along a 1200 km transect of the Mackenzie Valley, maximum annual thaw penetration increases from installation (1990-1992) to 1995, at a mean rate of 2.5 cm/year at over 75% of the sites. A decrease in thaw penetration in 1996 at sites north of Norman Wells corresponds to a cooler late summer. (2) This steady increase in maximum thaw penetration is not reflected in a similar increase in active layer thickness because of the variation in thaw consolidation among sites. (3) Active layer thickness appears to vary from site-tosite due to site-specific and regional factors. (4) Thaw tubes, as described in this report, provide an inexpensive annual record of both maximum thaw penetration and active layer thickness suitable for multiyear comparison. However, thaw tubes do not provide information on local variability, and any particular tube in the network reported here may be in an anomalous situation relative to a mean active layer thickness derived from extensive sampling (e.g., probing). (5) Larger air thawing degree-day values are required in the boreal forest than in the tundra to achieve similar active layer thicknesses, due to the insulating effect of surface vegetation, and the generally deeper snowpack in the Mackenzie Valley, compared to the Delta. Ground thawing degree-days suggest that soil conditions in the boreal forest are also less favourable to thaw development than in the tundra, probably due to higher ice contents within active layer soils of the boreal forest, and different soil properties. Acknowledgments Logistic support was provided by the Polar Continental Shelf Project, Natural Resources Canada, Tuktoyaktuk; the Inuvik Research Centre, Aurora Research Institute; Water Survey of Canada, Environment Canada in Fort Simpson and Inuvik, and Department of Northern Affairs, Fort Simpson and Norman Wells. Funding was provided by the Geological Survey of Canada under the Green Plan and the Panel on Energy Research and Development. Generous help and advice was provided by many individuals and communities in the Mackenzie Valley and Delta. The project was originally conceived by Paul Egginton, Geological Survey of Canada. Three reviews have contributed to the manuscript. References Aylsworth, J. M. and Egginton, P. A. (1994). Sensitivity of slopes to climate change. In Cohen, S.J. (ed.), Mackenzie Basin Impact Study, Interim report #2. Proceedings of the sixth biennial AES/DIAND meeting on northern climate & mid-study workshop of the Mackenzie basin impact study, Environment Canada, pp. 278-283. Barry, R., Brown, J., Clark, M. and Hanson, C. (1996). Global geocryological database: Circumpolar active layer permafrost system (CAPS). Frozen Ground, 20, 6-7. Boer, G. J., McFarlane, N. A. and Lazare, M. (1992). Greenhouse gas-induced climate change simulated with the CCC second-generation general circulation model. Journal of Climate, 5, 1045-1077. Canada Department of Indian and Northern Affairs (1974). Vegetation types of the Mackenzie corridor. Environmental- Social Committee, Northern Pipelines, report 73-46. 85 p. Canadian Meteorological Centre (1996). Canadian Climate Summary, 1, April to October, Atmospheric Environment Service, Environment Canada, Downsview, Ontario. F. Mark Nixon, Alan E. Taylor 819
Aylsworth, J. M. and Egginton, P. A. (1994). Sensitivity of slopes to climate change. In Cohen, S.J. (ed.), Mackenzie Basin Impact Study, Interim report #2. Proceedings of the sixth biennial AES/DIAND meeting on northern climate & mid-study workshop of the Mackenzie basin impact study, Environment Canada, pp. 278-283. Barry, R., Brown, J., Clark, M. and Hanson, C. (1996). Global geocryological database: Circumpolar active layer permafrost system (CAPS). Frozen Ground, 20, 6-7. Boer, G. J., McFarlane, N. A. and Lazare, M. (1992). Greenhouse gas-induced climate change simulated with the CCC second-generation general circulation model. Journal of Climate, 5, 1045-1077. Canada Department of Indian and Northern Affairs (1974). Vegetation types of the Mackenzie corridor. Environmental- Social Committee, Northern Pipelines, report 73-46. 85 p. Canadian Meteorological Centre (1996). Canadian Climate Summary, 1, April to October, Atmospheric Environment Service, Environment Canada, Downsview, Ontario. Dolecki, J. L. (1994). The Accuracy of a Thaw Tube in Determining the Maximum Depth of Thaw in Permafrost Regions. BSc thesis, Department of Geological Sciences, Queens University, Kingston (31 pp.). Ecological Stratification Working Group (1995). A National Ecological Framework for Canada. Centre for Land and Biological Resources Research, Research Branch, Agriculture and Agri-Food Canada and Ecozone Analysis Branch, State of the Environment Directorate, Environment Canada, Ottawa/Hull. Edlund, S. A., Alt, B. T. and Young, K. L. (1989). Interaction of climate, vegetation, and soil hydrology at Hot Weather Creek, Fosheim Peninsula, Ellesmere Island, Northwest Territories. In Current Research 1989-D. Geological Survey of Canada, Ottawa, pp.125-133. Heginbottom, J. A., Dubreuil, M. A. and Harker, P. A. (1995). Canada -Permafrost. In National Atlas of Canada, 5th edition, National Atlas Information Service, Natural Resources Canada, MCR 4177, scale 1:7 500 000. Hinkel, K. M., Taylor, A. E. and Outcalt, S. I. (1997). Seasonal patterns of coupled flow in the active layer at three sites in northwest North America. Canadian Journal of Earth Sciences, 34, 667-678. Hinzman, L. D. and Kane, D. L. (1992). Potential response of an Arctic watershed during a period of global warming. Journal of Geophysical Research, 97, D3, 2811-2820. International Permafrost Association (1998). CAPS (Circumpolar active layer Permafrost system). Database CD-ROM. In preparation. Mackay, J. R. (1973). A frost tube for the determination of freezing in the active layer above permafrost. Canadian Geotechnical Journal, 10, 392-396. Mackay, J. R. (1975). The stability of permafrost and recent climatic change in the Mackenzie Valley, N.W.T. In Report of Activities, Part B, Geological Survey of Canada, Paper 75-1B, pp. 173-176. Mackay, J. R. (1976). Ice-wedges as indicators of recent climatic change, western Arctic coast. In Current Research 1976-A, Geological Survey of Canada, pp. 233-234. Nelson, F. E. and Brown, J. (1997). Global change and permafrost: Circumpolar active layer monitoring network (CALM). Frozen Ground, 21, 21-24. Nixon, F. M. and Taylor, A. E. (1994). Active layer monitoring in natural environments, Mackenzie Valley, Northwest Territories. In Current Research 1994-B, Geological Survey of Canada, pp. 27-34. Nixon, F. M., Taylor, A. E., Allen, V. S. and Wright, F. (1995). Active layer monitoring in natural environments, lower Mackenzie Valley, Northwest Territories. In Current Research 1995-B, Geological Survey of Canada, pp. 99-108. Rickard, W. and Brown, J. (1972). The performance of a frosttube for the determination of soil freezing and thawing depths. Soil Science, 113, 149-154. Skinner, W. and Maxwell, B. (1994). Climate patterns, trends and scenarios in the Arctic. In Mackenzie Basin Impact Study, Interim report #2. Proceedings of the sixth biennial AES/DIAND meeting on northern climate & mid-study workshop of the Mackenzie basin impact study, Environment Canada, pp.125-137. Taylor, A. E. (1995). Field measurements of n-factors for natural forest areas, Mackenzie Valley, Northwest Territories. In Current Research 1995-B, Geological Survey of Canada, pp. 89-98. Waelbroeck, C., Monfray, P, Oechel, W. C., Hastings, S. and Vourlitis, G. (1997). The impact of permafrost thawing on the carbon dynamics of tundra. Geophysical Research Letters, 24, 229-232. 820 The 7th International Permafrost Conference