Acceleration of thaw slump activity in glaciated landscapes of the Western Canadian Arctic

To evaluate the influence of climate and landscape factors on the size, distribution and growth dynamics of thaw slumps, we selected four impacted study areas in the western Arctic (figure 1). These regions span 5° of latitude, a gradient of approximately 5 °C in mean annual air temperature (MAAT), a 10 °C range in permafrost temperatures, and a two-fold difference (from ∼150 to 300 mm) in total annual precipitation (figure 1 and table 1). The intensively mapped study areas range in size from 600 to 3200 km², with study area sizes constrained by the imagery available. The most northerly study area, the Jesse Moraine on Banks Island, is a Low Arctic polar desert with the coldest and driest climate (MAAT = −14 ± 0.5 °C; MAP = 141.6 ± 5.0 mm) and a mean annual ground temperature around −10 to −12 °C (table 1). The Peel Plateau is the southernmost study area and is located in the high subarctic, where precipitation is twice as high (MAP = 305.2 ± 10.7 mm) and mean annual air temperature is 5 °C warmer than Banks Island. The Peel Plateau also has the highest ground temperatures, which range from −1 to about −3.0 °C (table 1). The climates of the Bluenose Moraine and Tuktoyaktuk Coastlands were intermediate with MAAT of between −12 and −10 °C, respectively, and MAP of ∼200 mm.

Air temperatures have increased over the past 5 decades in all study areas, with the largest increases occurring in winter (table 1). Rainfall has also increased across all of these study regions, but the greatest relative increase has occurred in the Jesse Moraine on Banks Island (table 1), which in the past has been referred to as a polar desert (Vincent 1982). All study regions were ice-covered during the last glacial maximum and contain thick layers of ice-rich permafrost and local ice-cored terrain, which occurs in association with hummocky moraine (Mackay 1971, St-Onge and McMartin 1999, Lacelle et al 2004, Lakeman and England 2012). The relative topographic relief and the density of lakes and streams vary among the four study areas. The Tuktoyaktuk Coastlands and Bluenose Moraine are lake-dominated environments, and the Peel Plateau and Jesse Moraine are fluvially incised landscapes. The Peel Plateau is characterized by the greatest relative relief and the Tuktoyaktuk Coastlands is the most topographically subdued landscape (table 1). Additional background on the study regions is provided in the supplementary materials.

To inventory thaw slumps and estimate rates of landscape change, we obtained greyscale airphotos and color satellite imagery from two time periods for each study region. Imagery had the following resolutions: Jesse Moraine (1960 = 2.4–2.5 m; 2004 = 0.6 m); Tuktoyaktuk Coastlands (1950 = 0.9 m; 2004 = 0.5 m); Bluenose Moraine (1952 = 0.9 m; 2006 = 10 m); and Peel Plateau (1950–1954 = 0.8–1.6 m; 2007–2008 = 0.6–10 m) (table S1). Active slumps were identified based on the presence of recently exposed sediments, poorly-developed vegetation, and a well-defined headwall (figure S1). All active slumps in each study area were digitized on-screen by viewing georeferenced imagery in ArcMap (2D; versions 10.0 and 10.1), Summit Evolution (3D; version 6.7), or DVP photogrammetry suite (3D; DVP-GS, Quebec, Canada). The mean annual rate of growth for each mapped slumps was calculated as follows:

$$\begin{eqnarray*}&&{\rm{Rate}}\;{\rm{of}}\;{\rm{slump}}\;{\rm{growth}}=\displaystyle \frac{\;{A}_{2}\;-\;{A}_{1}\;}{t\;},\end{eqnarray*}$$

where A₁ is the historical area (m²), A₂ is the modern area (m²), and t is the number of years between images. If multiple slumps coalesced into a single disturbance between the two time periods, they were treated as a single entity in first time period (A₁) by summing their areas. To explore the influence of slump size on average growth rate, we also calculated the proportional growth rate relative to modern slump size.

$$\begin{eqnarray*}&&{\rm{Proportional}}\;{\rm{growth}}\;{\rm{rate}}=\displaystyle \frac{\displaystyle \frac{({A}_{2}\;-\;{A}_{1})}{{A}_{2}}\;}{t}.\end{eqnarray*}$$

Slump areas were delineated using imagery with variable resolutions, but even the coarsest imagery (10 m) was suitable for mapping because the majority of disturbances we mapped were larger than 0.05 ha (figure S1). To account for differences in image resolution among regions we used minimum mapable areas that reflected the quality of imagery used (Lantz and Kokelj 2008). In the Bluenose Moraine and the Peel Plateau, slumps were mapped to the nearest 0.1 ha. In the Tuktoyaktuk Coastlands and the Jesse Moraine, slumps were mapped to the nearest 0.01 ha. To test for differences in slump growth rates among study regions, we used a one way analysis of variance (R, version 3.0.0, 2013-04-03, The R Foundation for Statistical Computing, www.r-project.org).

Parameters that describe regional environmental conditions were used as a basis to interpret differences in slump characteristics and their changes over time in the four study areas (table 1). We used Fulton's (1995) national-scale compilation to characterize the dominant surficial unit in each study area. Topographic characteristics (mean, standard deviation and range of elevations) were derived from digital elevation models published in the Canadian Digital Elevation Data resource (Government of Canada 2000). MAAT and mean annual precipitation (MAP) for each region were estimated data from WorldClim and the Commission for Environmental Cooperation (Hijmans et al 2005, Commission for Environmental Cooperation 2011a, 2011b). This dataset is a grid (resolution = 1 km²) of average monthly temperature and precipitation interpolated from weather station data (1950–2000). Changes in annual (November–October) and growing season (May–October) air temperatures between 1950 and 2005 were derived from NASA GISS maps of temperature change. These maps show gridded estimates of linear changes in temperature which are based on the least squares regression of temperatures from the GHCN v3 model (Hansen et al 2010, NASA GISS 2014). Data from the Bluenose Moraine area was not available from this model, so we used homogenized data from Environment Canada (2012) that combined temperature observations from two stations at Kugluktuk, NWT (∼175 km SE of the study region). We used least squares regression of annual and growing season temperatures from this dataset to estimate change between 1950 and 2005. To check for autocorrelation in this data we calculated the Durbin–Watson statistic. Changes in annual (November–October) and growing season (May–October) precipitation between 1950 and 2000 were derived from NASA GISS maps of changes in precipitation. These maps show gridded estimates of linear change based on least squares regression of precipitation data from the CRU TS 2.0 model (Mitchell et al 2004, NASA GISS 2014). Map outputs from NASS GISS do not provide significance tests for the trends reported.

Permafrost temperatures in each study area were estimated using the minimum reported ground temperatures from the closest available boreholes, or from regional compilations, which exist for the Tuktoyaktuk Coastlands (Burn and Kokelj 2009) and for the Peel Plateau (O'Neill et al 2015). Ground temperatures for the Jesse and Bluenose Moraines (Banks Island and mainland western Nunavut) were based on sparse data from a northern Canada ground temperature map reported in Smith and Burgess (2000) and for data from Johnson Point on Banks Island (Golder Associates 2012) (locations are noted in table 1).

Our mapping shows that retrogressive thaw slumping is an important driver of geomorphic change in environments in the western Canadian Arctic that are dominated by ice-rich morainal deposits (figures 1– 3). In accordance with our first hypothesis, the variation in slump size, density, and growth among our study regions was related to landscape factors including surficial geology, topography, ground-ice distribution and geomorphic setting. For example, the Jesse Moraine on Banks Island had the greatest number of slumps and the largest total slump area impacted, making it one of the most intensively impacted landscapes in the Canadian Arctic. The sensitivity of the ice-cored moraine on eastern Banks Island to thaw slumps is related to the widespread distribution of buried glacier ice in this area (Lakeman and England 2012), veneered by a thin till-layer, which can be less than 1 m thick.

Our data also show that ice-rich areas of greater relief promote the development of the large slumps. In the regions with greater relief (the Bluenose Moraine and Peel Plateau), topography promoted the downslope evacuation of thawed materials from the scar zone and the development of tall headwalls (Kokelj et al 2015) and perpetuated development of the largest disturbances. The Tuktoyaktuk Coastlands were characterized by relatively small lakeside slumps and the lowest proportional area impacted by disturbance. Slump sizes and densities in this region are partly limited by low relief and a greater proportion of lacustrine terrain that is not susceptible to thaw slumping (table 1).

Thaw slump development can have significant geomorphic and ecological impacts because this process rapidly degrades large volumes of ice-rich permafrost and effectively transports sediments and solutes from slopes to downstream environments (Lantuit et al 2012, Kokelj et al 2013, 2015). Increased slump activity in several of the regions we studied has been shown to exert major geomorphic and ecological impacts (Kokelj et al 2009a, 2013, 2015, Lantz et al 2009, Malone et al 2013, Thienpont et al 2013, Chin et al 2016). These impacts are likely to continue to intensify because sediment and solute yields increase nonlinearly as slumps enlarge, and once critical size thresholds are reached, several feedbacks intensify downslope sediment transfer and decrease the likelihood of rapid stabilization (Kokelj et al 2015, Lacelle et al 2015).

The observation that thaw slump activity is intensifying across the range of climates in the four study regions is consistent with our second hypothesis and indicates that this form of permafrost degradation is emerging as the dominant driver of geomorphic change in areas of ice-rich moraine in the western Canadian Arctic (figures 1– 3). There have been increases in the total number of slumps, mean area of slumps, and the proportion of terrain disturbed since the 1950s in all four of the regions we studied (table 2 and figure 3). This acceleration of thermokarst activity has occurred in conjunction with rising temperatures and increased snow and rainfall, suggesting that climatic factors are important influences. Significant winter warming has caused an increase in MAGT (Smith et al 2005) and an extension of the thaw season, which have likely both favoured thermokarst activity. Higher summer air temperatures can increase active-layer thickness (Zhang et al 1997, Hinkel et al 2001) and thaw near-surface ground ice, which leads to terrain instability. Warmer summer temperatures and radiative inputs are also correlated with ground ice ablation and variation in the rates of headwall retreat (Lewkowicz 1986), and previous observations of increased slump growth have been attributed to rising thaw season temperatures (Lantz and Kokelj 2008). However, our observation that long-term slump growth rates across the four study areas were strongly correlated with disturbance size, and not summer climate (figure 4) suggests that increasing temperatures are not the only driver of increased slump activity. The Jesse Moraine was characterized by the coldest air and permafrost temperatures as well as the lowest rate of summer warming; however, this region experienced the most dramatic intensification of slump activity (figure 3). Furthermore, when multi-decadal growth rates were normalized by slump size, there were no differences among regions with dramatically different summer climates (figure 4 and table S3). Climate trends also indicated that the increase in thaw season air temperatures have been substantially lower than winter warming (table 1). Although there is evidence that increasing permafrost temperatures, driven by rising air temperatures, can alter talik configuration and cause slump initiation along lakeshores (Kokelj et al 2009b), the rapidly warming, lake-rich Tuktoyaktuk Coastlands showed the lowest relative geomorphic change of all four regions studied. Taken together, this suggests that increasing air and permafrost temperatures were not the only drivers of the decadal-scale acceleration of thaw slump activity in the western Arctic.

Recent field studies in several parts of the Western Arctic indicate that increased rainfall is an important driver of the slump intensification we observed. Increased rainfall can make slopes unstable and increase landslides, gullying, and bank erosion (Cogley and McCann 1976, Lamoureux and Lafrenière 2009, Swanson 2012, Leibman et al 2014), all of which can increase the potential of thaw slump activity. The recent acceleration of slump activity on the Peel Plateau has been driven by a significant increase in the intensity of summer precipitation (Kokelj et al 2015). Investigation of sequential Landsat imagery and analysis detailed time series of slump sediment flux showed that rainfall events increased sediment removal from the slump scar zone and were an important driver of downslope debris tongue enlargement and slump perpetuation (Kokelj et al 2015). Summer season precipitation has increased substantially across all study regions, but the relative increase in rainfall has been greatest on Banks Island where the intensification of slumping has been most pronounced (table 1 and figure 3). The trend in increasing rainfall in this polar desert is supported by observations of recent gullying around the community of Sachs Harbor and the occurrence of thunderstorms and intense rainfall events previously unknown to Inuvialuit observers (Berkes and Jolly 2001, Jolly et al 2002). Our data suggest that rainfall-induced increases in slump activity are having profound geomorphic impacts on this fluvially-dominated, ice-cored polar desert landscape, despite the presence of cold, continuous permafrost. Additionally, the largest slumps, displacing the greatest volumes of sediment, were observed on the Peel Plateau, the wettest study area (figure 3 and table 1). Ultimately, it is likely that increased precipitation and air temperature are acting synergistically to intensify slump activity in the western Arctic. Increased rainfall can remove materials from the slump scar zone and perpetuate slump growth, while a longer thaw season and warmer temperatures can accelerate ablation and headwall retreat, and warmer winters may inhibit refreezing of scar zone materials.

Our analysis also shows that the response of permafrost landscapes to changing climate is strongly influenced by geomorphic setting, ground ice conditions, and paleogeography (Kokelj and Jorgenson 2013). Contrary to our third hypothesis, the ice-rich Jesse Moraine, with the coldest air and permafrost temperatures, experienced the greatest intensification of slump activity over the past 30 years; whereas, one of the warmest study areas, the Tuktoyaktuk Coastlands region, experienced the least amount of change (table 2 and figure 3). Several factors likely contribute to the sensitivity of the Jesse Moraine and the comparative stability of the Tuktoyaktuk Coastlands, including the lower proportion of terrain comprized of ice-rich moraine, and the lower topographic gradients in a lake-dominated environment. Because there are few streams, the potential for rainfall-induced fluvial erosion is minimal in the Tuktoyaktuk Coastlands. Furthermore, the massive ground ice deposits on the Tuktoyaktuk Coastlands are typically truncated 2–3 m below the terrain surface by a prominent thaw unconformity that developed during the Holocene thermal maximum (Burn 1997), which likely provides a thick protective layer. During this warm period, the Arctic coastline was up to 150 km north of its present location and the Laurentide Ice sheet had retreated several hundreds of kilometers eastward. The warm climate conditions in this region allowed colonization by spruce forest (Ritchie et al 1983), and likely the occurrence of forest fire that can result in deeper thaw (Mackay 1995), truncation of the near-surface massive ice, and widespread thermokarst activity (Rampton 1988, Murton 2001). The paleogeographic and terrain factors that characterize the Tuktoyaktuk Coastlands have made it more resilient to contemporary climate-driven thermokarst than the other areas examined in this study. The Jesse Moraine on Banks Island is extremely susceptible to slump intensification because extensive deposits of massive ground ice, likely consisting largely of buried glacier ice, are covered by a thin overburden of till (Lakeman and England 2012). The high density of smaller slumps that we mapped on Jesse Moraine, many of which were initiated recently, demonstrates that this region has emerged as one of the most dynamic thermokarst environments on the planet (figure 3 and table 2).