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Spatial and Temporal Patterns of Thaw at CALM Sites on the North Slope of Alaska: 1995-2000
K. M. Hinkel and F. E. Nelson
Introduction The active layer refers to the upper zone above permafrost that experiences seasonal thawing and refreezing. Under a warming climate scenario predicted by general circulation models, the active layer would likely thicken. This would release water and carbon currently sequestered in the upper permafrost, and cause widespread differential ground subsidence, or thermokarst. The active layer thickness can be estimated using air or surface temperature as the forcing funciton. However, studies to date suggest that thickness varies spatially in response to local factors including winter snow cover pattern, summer rainfall, lateral variation in soil properties, and vegetation (Smith, 1975; Hinkel et al., 1993; Nelson et al., 1997a, 1997b). Currently, little is known about spatial variation in thaw depth as affected by primary landscape elements such as topographic depressions (basins, valleys) and highs (hills, ridges). Further, the impact of interannual varation in thaw depth is largely unknown at this landscape scale. The duel issues of spatial and temporal variability can best be assessed by repeated observation at permanent plots over long time periods. This entails collection of a 2-dimensional array of thaw depth at precise locations near the end of the thaw season. Such efforts are supplemented by measurement of air and ground surface temperature, and are most useful when combined with ancillary data such as soil and vegetation maps of the study area, soil moisture measurements, and snow cover patterns. This paper describes the results from a program of replicated measurements collected at seven 1 km2 grids over the period 1995-2000. Established in northern Alaska in the 1980s and 1990s (Walker and Walker, 1991; Walker, 1997), four of these surveyed and georeferenced grids on situated on the Arctic Coastal Plain physiographic province; the remaining three are in the Arctic Foothills. (See Figure 1) These grids form the core of the CALM (Circumpolar Active Layer Monitoring) program in northern Alaska. Characteristics of the seven sites are given in Table 1.
Each grid consists of a square array of surveyed permanent stakes separated by 100 m, yielding an array of 11 X 11 grid nodes across the 1 km2 study area. Thaw depth is determined by making replicate measurements at each of the 121 grid nodes using a 1-cm diameter graduated steel rod. Probing is done in mid to late August when thaw depths are near their end-of-season maximum.
End-of-season thaw depth were collected over the period 1995-2000, and a map was generated using the average thaw depth at each grid node over the period of record. This average thaw map is overlain on the DEM in Figures 2-8 to demonstrate the correlation between the thaw depth patterns and the primary terrain features, and the mean thaw depth for each year is shown in parentheses. The area without contours are blank nodes where measurements were not collected.
The lowermost maps in Figures 2-8 illustrates the pattern of thaw variability. We assume that thaw depth at a grid node responds only to air temperature forcing, but may vary across space due to local factors such as vegetation and soil properties. Thus, if in a particular year when the areally-averaged grid thaw depth (Zavg) is 50 cm and the node-specific thaw depth (Zi) is 55 cm, the normalized local thaw depth exceeds the grid average by 10% [(Zi - Zavg) / Zavg]. If there is greater thaw the following year, with an average of 60 cm for example, we would expect that grid node to have a thaw value of 66 cm if responding linearly to thermal forcing alone.
All other influencing factors being held constant, the node-specific thaw depth should respond relatively consistently. Over the 6-year period of record, the normalized thaw depth at a grid node may vary from a minimum of 12% to a maximum of 15%. In this case, the interannual node variability (INV) is 3%, which indicates a high degree of interannual consistency. Conversely, a grid node may demonstrate high thaw variability. In one year, the normalized thaw depth may be 20% greater than the mean (+20%), whereas it may be 30% less (-30%) in another year. Thus, the INV over the period of record is 50%, meaning that the node demonstrates a high degree of interannual variability in response to site-specific factors. We can calculate a grid-averaged INV, and map the INV to determine the degree of thaw consistency as it varies across the grid.
The first four sites are situated on the Arctic Coastal Plain. They are generally characterized by low relief, presence of thaw lakes and drained or partially drained thaw lake basins, and moist to wet acidic tundra.
The Atqasuk site is located near the Meade River on the inner Arctic Coastal Plain, about 100 km inland from the coast. The 1 km2 grid is situated on a sand dune field (Everett, 1980) developed on a 1% north-facing slope. A lake occupies a portion of the northeastern quadrant. The vegetation in the well-drained sandy higher slopes is wet sedge tundra.
This average end-of-season thaw depth for the period 1995-2000 is overlain on the DEM in Figure 2. At Atqasuk, relatively shallow thaw occurs on the well-drained sandy uplands. The thaw depth increases in the lake basins, with maximum values recorded in the wet margins of the lake. It appears that saturated soil conditions enhances heat flow to depth by increasing the bulk thermal conductivity of the upper substrate.
The pattern of INV demonstrates that, at Atqasuk, interannual measurements of thaw depth in the uplands are relatively consistent. By contrast, the lowlands surrounding the lake basin are characterized by high INV. Overall, the INV average for the grid is 28%. This variability map allows us to estimate the magnitude of thaw variation and to identify regions or grid nodes where thaw depth is not responding consistently to forcing. At Atqasuk, annual fluctuations in the lake level and soil moisture content probably causes the higher thaw variability around the lake margin. The isolated nodes of high INV in the uplands can result from three occurrences: (1) when ponding at the grid node occurs in some years by not others; (2) when there is a high degree of lateral thaw variability over short distances (<1 m); and (3) when the substrate is stone-rich, yielding erroneous probing estimates of thaw depth. The latter is particularly problematic in glacial tills at Foothill sites.
The Barrow 1 km2 grid is located about 5 km east of the village of Barrow on the outer Arctic Coastal Plain. The site is situated on reworked silts of marine origin. As shown in Figure 3a, the terrain is characterized by a drained lake basin or lagoon to the west (Central Marsh), and a polygonized upland to the east. These two regions are separated by a north-sound trending beach ridge composed of sandy gravels. The polygonized upland is wet sedge tundra with well-developed low-centered ice-wedge polygons in the northeast, and high-centered ice wedge polygons to the southeast.
Figure 3a shows the relation between average thaw depth and terrain, and demonstrate spatial uniformity across most of the grid. Active layer depths tend to be generally deeper on the west-facing beach ridge slope, with some regions showing extreme thaw depths. Furthermore, the average INV (27%) is similar to that at Atqasuk, and maximum variations are associated with those nodes of maximum thaw development.
A Vitel Hydra soil moisture probe was employed to measure the volumetric soil moisture content (%) at the grid nodes for the near-surface layer (0-7 cm). The pattern for two years is shown in Figure 3b, and clearly demonstrates the contrast between Central Marsh and the beach ridge, the low- and high-centered polygon regions, and the difference between a “normal” precipitation year (1996: 64 mm in summer) and extremely wet year (1997: 131 mm).
Snow depth has also been monitored regularly on the Barrow grid. A representative sample from spring 1998 is presented in Figure 3b. Prevailing winds in winter are from the east, which encourages the development of a drift to the lee of the beach ridge. The zones of maximum snow accumulation correlate exactly with the regions of maximum thaw depth. These drifts often persist several weeks after the snow cover has ablated from the surrounding tundra, and thus serve to inhibit soil thaw initiation. However, the insulating effect of deep snow retards heat loss in winter, so less energy is required to warm and thaw soil in spring. Temperature loggers installed at the base of the snow drift indicate minimum winter soil surface temperatures that are 7-10 C warmer than the soil temperatures in the non-drifted tundra (Hinkel, unpublished data). Thus, snow depth patterns, as affect by topography, has a major influence on the spatial patterns and magnitude of thaw.
The Betty Pingo and West Dock grids are located on the outer Arctic Coastal Plain within the Prudhoe Bay oil fields. The terrain is flat, polygonized, and is characterized by thaw lakes, drained thaw lake basins, and ponds (see Figures 4 and 5). The vegetation is moist and wet nonacidic tundra, and the soils are primarily Typic Aquorthels developed in alluvium.
The average thaw pattern and magnitude at Betty Pingo for the period of record is shown overlain on the DEM in Figure 4. Shallow thaw depths are found on the flat uplands separating the basins. There are two regions of enhanced thaw. One is clearly associated with the wet, low-lying basin. The second is apparent in the southeastern quadrant, where a west-facing slope exists between the higher ground and the ponded upland. Although snow depth measurements have not been collected from this site, it would appear that snow drifting at the break-in-slope has resulted in greater thaw depths. The pattern is similar to that observed at Barrow. Interannual node variability is maximized in these two regions, but the grid-averaged INV is relatively small (22%).
West Dock is located about 10 km from the Betty Pingo site and is very near the coast. The landscape, vegetation, and soils are similar. Two drained thaw-lake basins occupy the eastern half of the grid and are separated from the ponded polygonized upland to the west by a prominent beach ridge. The average thaw pattern for the period 1995-2000 isshown in Figure 5. Greater thaw depths are experienced in the wet drained lake basins and near large ponds, while the well-drained polygonized uplands experience generally shallow thaw. Higher INV values are associated with the surface depressions, but the grid average is relatively small at 18%.
At Coastal Plain sites, thaw depth distribution appears to be bimodal, with wet thaw-lake basins and lake margins experiencing significantly greater thaw than the uplands. This distribution is not revealed by the grid average or measurements of central tendency, but can be verified by separating measurements collected from the two regions for statistical comparison.
Because the distribution of the measurements collected in the basins and uplands are distinctly nonnormal, nonparametric techniques are employed. The Mann-Whitney test compares the equality of the sample medians, whereas the Kolmogorov-Smirnov test focuses on the sample distributions. Summary statistics and test results for the 1995-2000 grid node averages are shown in Table 2. They clearly demonstrate that at Atqasuk, West Dock and Betty Pingo, thaw depth in the basins is significantly greater then in the uplands. Further, a comparison on an annual basis would yield similar results.
The exception to this pattern is Barrow. Thaw depth in the upland is comparable to the upland at Atqasuk, but the basin (Central Marsh) does not experience significantly greater thaw depths. This may be explained by the fact that, although the soil is wet, there is no permanent standing water near the eastern margin of Central Marsh. By contrast, the other Coastal Plain sites have permanent bodies of water. An examination of the average thaw depths maps for these sites (Figures 2, 4 & 5) shows a general increase in thaw depth from the basin margins to the basin center. At Atqasuk and Betty Pingo in particular, lake margins are often covered by shallow standing water, resulting in average basin thaw depths 40% (Betty Pingo) and 95% (Atqasuk) deeper than observed in the uplands. Thus, it is likely that the local standing water depth exerts a strong influence on thaw depth.
Table 2: Summary statistics for upland and basin grid nodes at Coastal Plain sites. Nonparametric Mann-Whitney (M-W) and Kolmogorov-Smirnov (K-S) tests results reported as significant (s) or not significant (ns).
These sites are situated near the Dalton Highway in the Arctic Foothills physiographic province nearly due south of the Prudhoe Bay region. The area was deglaciated during the Holocene, and the surface deposits are largely glacial till with a discontinuous loess cover. The 1 km2 grids demonstrate more terrain variability than the sites on the Coastal Plain.
This grid is established along the flanks of a bedrock-cored hill adjacent to Toolik Lake (Figure 6). Moist acidic tundra has developed atop glacial tills (Auerbach et al., 1996). The vegetation and soils vary greatly with slope and aspect, and tend to be discontinuous.
Average thaw depth for the period of record is overlain on the DEM in Figure 6, which shows the view from the northeast. Thaw tends to be shallow near the base of the hill, and increases uphill on the north- and east-facing slopes to a maximum near the rounded hill top. The general pattern is best explained by the slope and aspect of the site; incident radiation is maximized at the hill top and decreases downslope to due terrain shading effects. Thus, topography exerts a primary influence on the general thaw pattern.
There is, however, a great deal of local interannual variation; the grid INV average of 41% is nearly twice that experienced at the Coastal Plain sites. Variation appears to be uniformly distributed across the grid, although higher magnitude INVs are found along the lake margin and in some of the wetter regions.
The Imnavait Creek site encompasses a north-south trending stream valley, the paired slopes, and a hill crest. The valley bottom is wet acidic tundra with a thick organic mat (Walker and Walker, 1996). The higher elevations are characterized by moist acidic tundra, but the slopes are broken by numerous water tracks that become more prominent downslope. These narrow zones contain shrub vegetation, especially dwarf willow.
The average pattern of thaw for the period 1995-2000 is shown in Figure 7, and no interannual pattern is readily discernible. Furthermore, average thaw does not appear to be related to topography, although there is a distinct tendency for patterns to be oriented downslope. The average INV is high at this site (37%) but no correlation with landscape features is apparent.
The Happy Valley site is situated on loess-covered, rolling hills with water tracks and a stream in the southeast corner. The vegetation is moist acidic tundra, with shrubs (willow) in the water tracks and riparian zone.
The average pattern of thaw, as demonstrated in Figure 8, is quite varied. The influence of the terrain on thaw depth is uniform, and deeper thaw appears to be associated with the water tracks. Enhanced thaw along water tracks is especially apparent in 1997, which was an abnormally wet year on the North Slope. The INV for the grid is high (average of 35%) and particularly pronounced at a number of the wetter grid nodes.
The grid averages for the six-year period of record are shown in Figure 9. Average thaw depth is minimized at Barrow, with a cold marine climate in summer, while the inland site of Atqasuk experiences intermediate values. Both West Dock and Betty Pingo experience substantially deeper thaw than the sites in the Foothills further to the south and at higher altitudes.
Note also that Coastal Plain sites are characterized by relatively small average INVs whereas the Foothills sites show substantially higher values. These results are consistent with those obtained by Nelson et al. (1998; 1999) and Gomersall and Hinkel (in press) using geostatistical methods to analyze thaw depth measurements collected using a nested hierarchical scheme. These studies demonstrated that most of the thaw variation at Foothills sites reflects the influence of tussocks on local heat flow. By contrast, thaw depth variation at Coastal Plain sites is due to the influence lake basins.
The temporal patterns for the period 1995-2000 at the Foothills sites are shown in Figure 10. Here, the dot represents the areally-averaged annual thaw depth, with bars extending one standard deviation in either direction; the triangles indicate the minimum and maximum thaw depths recorded on the grid.
Mean thaw depths appear quite consistent over the latter half of the decade. The exceptions are 1998, which was one of the warmest years on record on the North Slope of Alaska and northwestern North America (GSC, 1999). All sites experienced maximum average thaw in that year. By contrast, the summer of 2000 was cool, and minimum average thaw depths were recorded on all grids in that year. This demonstrates a regionally consistent response to air temperature forcing.
The plots in Figure 10 use the same scales. A cursory examination reveals that Coastal Plain sites have substantially greater dispersion around the mean value. Indeed, the average standard deviation for Coastal Plain sites, exclusive of Barrow, is nearly twice that of the Foothills sites. This reflects the influence of the bimodal distribution discussed earlier. As demonstrated in Table 2, the standard deviation in the Coastal Plain uplands is similar to that for the Foothills sites. Drained lake basins have substantially deeper thaw and much larger standard deviations, which inflate the measurements of central tendency when applied to the entire grid.
At Barrow, the record is somewhat longer. During the period 1962-68, replicate measurements of thaw depth were made at 19 plots by researchers with CRREL. These plots were reoccupied in 1991. As can be seen in Figure 11, the 1960s were characterized by a relatively deep active layer and significant interannual variation. No data is available for the period 1969-1990. The 1990s show a trend similar to that of the other CALM sites, but the pattern of gradual thaw deepening throughout the 1990s, peak thaw in 1998, and shallower thaw since that time, is more apparent. The mean annual thaw depth (Z) at each grid can be correlated to the accumulated degree days of thaw (ADDT) using the Stefan solution. Here, Z = b /ADDT, where b is a site dependent parameter (Hinkel and Nicholas, 1995). The ADDT was determined by using hourly surface temperature data, and summing the daily averages for the period beginning at thaw initiation and ending on the day of grid probing. Although well defined relations are apparent for some sites in Figure 12, others show no strong correlation (r2).
Six years of record permit several general conclusions: (1) Sites on the North Slope of Alaska respond consistently to forcing by air temperature on an interannual basis. All sites experienced maximum average thaw depth in 1998 and a minimum in 2000, consistent with the warmest and coolest summers during the period of record. (2) There is significant intrasite variation in thaw depth and near-surface soil moisture content within each 1 km2 grid, reflecting the impact of vegetation, substrate, snowcover dynamics, and terrain. (3) On the Coastal Plain, average thaw depth and thaw depth variation is significantly greater in drained thaw-lake basins, where soils are typically at or near saturation. This results in a bimodal distribution of thaw depths related to primary landscape elements. (4) Foothill sites demonstrate large spatial and node-specific interannual variability resulting from microtopography and temporal fluctuations of soil moisture content; this makes predictive mapping of thaw depth problematic at the scale and resolution of the grids. The spatial pattern of thaw depth across sites on the Coastal Plain is relatively consistent in the uplands. Thaw-lake basins and lake margins, however, exhibit more complex patterns attributable to fluctuating water levels.
This research is supported by NSF, grants OPP-9529783, 9732051 & 9911122 to KMH, and OPP-9612647, 9896238 & 9907534 to FEN. We are grateful to the Ukpeagvik Inupiat Corporation for administrative assistance and access to the Barrow Environmental Observatory.