Isotropic thaw subsidence in undisturbed permafrost landscapes

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 40, , doi: /2013gl058295, 2013 Isotropic thaw subsidence in undisturbed permafrost landscapes Nikolay I. Shiklomanov, 1 Dmitry A. Streletskiy, 1 Jonathon D. Little, 2 and Frederick E. Nelson 3 Received 10 October 2013; revised 2 December 2013; accepted 4 December 2013; published 18 December [1] Observations in undisturbed terrain within some regions of the Arctic reveal limited correlation between increasing air temperature and the thickness of the seasonally thawed layer above ice-rich permafrost. Here we describe landscape-scale, thaw-induced subsidence lacking the topographic contrasts associated with thermokarst terrain. A high-resolution, 11 year record of temperature and vertical movement at the ground surface from contrasting physiographic regions of northern Alaska, obtained with differential global positioning systems technology, indicates that thaw of an ice-rich layer at the top of permafrost has produced decimeter-scale subsidence extending over the entire landscapes. Without specialized observation techniques the subsidence is not apparent to observers at the surface. This isotropic thaw subsidence explains the apparent stability of active layer thickness records from some landscapes of northern Alaska, despite warming near-surface air temperatures. Integrated over extensive regions, it may be responsible for thawing large volumes of carbon-rich substrate and could have negative impacts on infrastructure. Citation: Shiklomanov, N. I., D. A. Streletskiy, J. D. Little, and F. E. Nelson (2013), Isotropic thaw subsidence in undisturbed permafrost landscapes, Geophys. Res. Lett., 40, , doi: /2013gl Introduction [2] Recent reports document warming of permafrost [Romanovsky et al., 2010], increased flux of greenhouse gases in permafrost regions [Schuur et al., 2008], and damage to civil infrastructure induced by melting of ground ice [Streletskiy et al., 2012]. Particular attention has been focused on thermokarst terrain, localized systems of irregular thawinduced pits, mounds, bodies of standing water, steep-sided gullies, hillslope mass movements, and other collapse structures caused by differential subsidence accompanying thaw of icerich permafrost [Kokelj and Jorgenson, 2013]. Development of thermokarst terrain is often triggered by discrete, geographically constrained disturbance of vegetative cover or hydrological patterns [e.g., Shur and Jorgenson, 2007]. [3] The active layer, a layer of seasonally thawed material between the ground surface and the uppermost permafrost, plays a critical ecological role in permafrost regions by limiting the depth of most biological and hydrological activity in 1 Department of Geography, The George Washington University, Washington, D.C., USA. 2 Monroe Community College, Rochester, New York, USA. 3 Department of Geography, University of Delaware, Newark, Delaware, USA. the substrate. Increases in the thickness of the active layer can result in increased microbial activity [Tveit et al., 2013], release of CO 2 and CH 4 [Trucco et al., 2012], and loss of substrate-bearing strength [Streletskiy et al., 2012]. Despite its importance for cryospheric systems, long-term records of active layer thickness (ALT) in natural landscapes are few. The Circumpolar Active Layer Monitoring (CALM) network, created in the early 1990s, is currently producing ALT data at more than 200 sites in both polar regions and in several midlatitude mountain environments [Nelson et al., 2008; Shiklomanov et al., 2010a]. Although an increase in active layer thickness could be expected in response to climatic warming, data obtained from some CALM sites have not demonstrated consistent and progressively increasing values of ALT, despite warming near-surface air temperatures [Shiklomanov et al., 2010b]. [4] The apparent stability of ALT in many Arctic landscapes may, however, be illusory if thaw penetrates into an ice-rich layer underlying the long-term base of the active layer. Recent research demonstrates that permafrost and the active layer do not always operate as a simple two-layer system [Shur et al., 2005]. The apparent stability of ALT may be attributable to the presence in many permafrost regions of a transition layer, an ice-enriched horizon just below the base of the active layer that resists thaw owing to the large amounts of latent heat required to melt it. During warm summers, this ice-rich layer protects underlying permafrost from thaw and can create nonlinearities in the response of the permafrost system to climatic forcing [Nelson et al., 1998b; Hinkel and Nelson, 2003], including pronounced departures from the Stefan relation between thaw depth and accumulated degree day sums. Although the ice-rich character of the horizon slows its rate of degradation, progressive thaw under monotonic climate warming would lead to its destruction. Enrichment or degradation of ice within the transition layer are manifested through heave and subsidence at the ground surface, respectively, as ice lenses form and melt at the base of the active layer. To an observer measuring active layer thickness from the surface, thaw penetration into the ice-rich transition layer may not be apparent owing to thaw consolidation and net subsidence of the surface. [5] In this study we sought to determine if widespread, relatively homogeneous, decadal-scale thaw subsidence, possibly attributable to climatic change, is occurring in natural, undisturbed landscapes and, if so, to estimate its magnitude and evaluate its role in the response of permafrost to atmospheric forcing. Corresponding author: N. I. Shiklomanov, Department of Geography, The George Washington University, 1922 F Street NW, Washington, DC 20052, USA. (shiklom@gwu.edu) American Geophysical Union. All Rights Reserved /13/ /2013GL Site Descriptions [6] High-resolution field investigations designed to track interannual vertical movements associated with formation and ablation of ice near the permafrost table were begun in 6356

2 ranged from 0.44 to 0.58 m over the period. The frequency distribution of ALT values is distinctly bimodal, with larger values occurring in the lake basins [Hinkel and Nelson, 2003]. Several shallow holes drilled for installation of temperature-monitoring equipment revealed that the uppermost permafrost is extremely ice rich. Observations for this study were conducted within two adjoining 1 ha sections of the grid, defined by universal time meridian coordinates N, E and N, E. This subunit of the grid contains polygonized upland tundra, drained lake bottom, and an intervening narrow beach ridge. [8] CALM site U32A is a 1 ha area located at ( N, W) in the northern foothills of the Brooks Range elevation ranges from 240 to 245 m. The site lies atop a hill 1 km west of the Dalton Highway and slopes to the northwest at about 4. Previously designated Flux Plot 95-3, the site has been studied intensively [Walker and Bockheim, 1995; Nelson et al., 1997; Klene et al., 2001]. Detailed landscape, vegetation, and soil surveys characterize the U32A site as patchy, moist, nonacidic tundra vegetation, underlain by Pergelic cryaquolls. The surface is dotted by numerous frost boils. Observations of ALT have been made at the site each year, beginning in Mean values of ALT range from 0.23 to 0.61 m over the period. The uppermost permafrost is ice rich, as evidenced by shallow holes drilled to install thermal-monitoring equipment. Detailed cryostratigraphic and ice content measurements were not performed at either site. Figure 1. Locations of field sites in northern Alaska. Dashed line represents boundary between Arctic Coastal Plain and Arctic Foothills physiographic regions. the summer of 2001 and continued annually at two 1 ha CALM sites in the continuous permafrost zone of northern Alaska (Figure 1). The sites were selected to represent characteristic landscapes of the Arctic Coastal Plain and Arctic Foothills physiographic provinces of northern Alaska [Wahrhaftig, 1965]. [7] CALM site U5 is located on the coastal plain near the Prudhoe Bay oilfield s West Dock facility on the Beaufort Sea at ( N, W). This 1 1 km site consists of 121 observational nodes, spaced at 100 m intervals. The site has been in operation since 1993 under the name West Dock, and the results of active layer monitoring from it have been reported previously [Nelson et al., 1998a, 1998b, 1999; Hinkel and Nelson, 2003]. The grid consists of low relief created by the juxtaposition of tundra upland and drained thaw lakes. Elevation ranges from 2 to 4.5 m. Soils consist of Typic Aqorthels [Walker and Bockheim, 1995]. Vegetation is wet tundra in drained lake basins and moist nonacidic tundra on the uplands [Walker, 2003]. Well-developed low-centered ice wedge polygons are evident in the upland portions of the grid. The mean of 121 end-of-season active layer thickness observations for the grid 3. Methods [9] At each site, observations on the vertical position of the ground surface and thaw depth were performed annually at the end of the thawing season. Owing to logistical constraints, the exact date of observations varied from year to year between August 13 and August 21. Annual thaw depth and ground surface elevation observations were accompanied by continuous monitoring of air temperature. [10] Observations on the vertical position of the ground surface at the vegetation/organic soil layer interface were conducted at the end of the thawing season with highresolution Differential Global Positioning Systems (DGPS) equipment, using a four-stage nested sampling and analysis (NSA) design [Webster and Oliver, 1990] that provides full geographic representation of surface cover types and microtopographic elements within each sampling area [Nelson et al., 1999; Little, 2006]. NSA is an explicitly spatial implementation of nested analysis of variance. The hierarchy and spatial arrangement of sampling points is shown in Figure 2; computational details are provided in Nelson et al. [1999] and Little [2006]. Each site contained 32 observation points, arranged in four groups, with horizontal separations between sampling points of 1, 3, 10, and 30 m. An expanded version of the sampling design was used previously to assess the scale dependency of ALT in the North Slope, including the West Dock (U5) CALM grid [Nelson et al., 1999]. [11] ALT and DGPS observations conformed to the same sampling protocol in this study. Observations of vertical movements of the ground surface were accompanied by measurement of ALT at each sample point using a calibrated, 1 cm diameter steel rod to probe to the point of refusal, interpreted as the ice-bonded base of the active layer. 6357

3 Figure 2. (a) Dendrogram depicting four levels of nested sampling. (b) Spatial arrangement of sampling points at one of the primary centers, which are 30 m apart. Bifurcation at the four levels of the hierarchy provided eight sampling points at each primary station. Substation locations were established using randomly generated azimuths and distances of 10, 3, and 1 m. Total sample size at each of the U5 and U32A sites is 32. Further details are available in Nelson et al. [1999] and Little [2006]. [12] The elevation of the ground surface at each site was monitored annually in late August using the rapid static DGPS observation technique. During the initial phase of the observation program ( ) a Trimble 4000 base station receiver and a Trimble 4700 rover receiver (Trimble Navigation Ltd, Sunnyvale, California) were employed; details are provided elsewhere [Little et al., 2003; Little, 2006]. After 2005, equipment consisted of Trimble 5700 and Trimble R7 units. The three receiver models have the same accuracy but vary by design, power source, and datalogging times. Detailed specifications for each receiver model are available at The systematic error for determining absolute vertical position for Trimble 4700/5700/R7 receivers implementing rapid static DGPS surveys is estimated to be m for West Dock and m for Sagwon. The difference in systematic errors between sites is attributable to differences in distance from receivers to base station. [13] The rover antenna (Trimble Zephyr Geodetic) was mounted sequentially atop 32 platform targets installed at the nodes of the NSA design at each site. One foothills point (target) location was lost midway through the observation period owing to disturbance by an animal, and only the data from the 31 surviving locations are reported here. [14] The hollow, perforated cylindrical targets, composed of colorless Acrylite (Ridout Plastics Company, San Diego, California), facilitate transmission of natural light, air, and flowing water and minimize disturbances to vegetation and microclimatic parameters [Little et al., 2003]. Optical leveling with a theodolite was used to confirm the accuracy of the DGPS observations during the first 2 years of the study. DGPS measurements were made relative to a frost-defended benchmark at the coastal plain site and to a benchmark fixed in bedrock at the foothills site. Postprocessing of rapid static GPS surveys was performed using Trimble Geomatics Office software to obtain absolute elevations for each sampling point. Individual sampling points were averaged over each of the two sites. The relative changes in elevation position were calculated as differences in mean site elevation between subsequent years. [15] Each site was instrumented with a two-channel Hobo Pro miniature data logger (Onset Computer Corporation, Pocasset, Massachusetts) measuring air temperature continuously throughout the year. The data loggers employ thermistors to monitor temperatures from 50 C to +30 C, have an accuracy of ±0.2 C, and a precision of 0.02 C at the freezing point. Sensors were placed inside 5-gill radiation shields (R. M. Young Company, Traverse City, Michigan) and mounted atop aluminum tripods at 1.8 m above the ground surface. Air temperature was recorded at 2 h intervals. Bihourly readings were averaged to yield values of mean daily air temperature. The summer (thawing) index thawing degree days (DDT, C days) was computed by summing positive mean daily temperature over the course of the thawing season. DDT sums were calculated from the beginning of thawing until the date of thaw depth and DGPS observations. The dates of thaw depth and DGPS observationsvaryfromyeartoyearbyupto10days.the observation procedures followed to minimize the effects of ALT and DGPS measurements on tundra vegetation, and underlying ice-rich sediments are described in Nelson et al. [1998a] and Little et al. [2003], respectively. 4. Results [16] Both sites experienced net subsidence of the ground surface over the period of observation, reducing average site elevation by 0.09 m and 0.19 m, respectively, at the Arctic Coastal Plain and Arctic Foothills sites from 2001 to 2012 (Figure 3). As with ALT [Nelson et al., 1999] the scale of maximum variability of thaw subsidence is very different at the coastal plain and foothills sites. At U5 (West Dock) maximum variability occurs at separation distances of 30 m or more and is associated with differences between drained thaw lake bottoms and upland tundra. Conversely, maximum variability at the U32A site in the foothills occurred over much shorter distances (1 3 m), and is associated with variations in vegetation cover and the locations of frost boils. The same patterns hold for winter frost heave, which was monitored at the beginning of the thaw season (early June) during the first 2 years of the study [Little, 2006]. The variability patterns are similar to those found for ALT at sites on the coastal plain and in the foothills [Nelson et al., 1999]. [17] The magnitude of average winter heave exceeded that of summer subsidence in only 3 years at each site and by more than 0.01 m in only 1 year at each site. At a few times and point locations net annual heave was large relative to subsidence, primarily because thaw penetration was smaller than in other years. These shallow thaw events were attributable to low summer air temperature in 2005 and to dry soil conditions and a corresponding decrease in soil thermal conductivity as a result of low summer precipitation in The effect of soil moisture on heat propagation into the ground is most apparent at the foothills location (U32A, Figure 3b). Precipitation records from nearby snow telemetry (SNOWTEL) sites operated by the U.S. Natural Resources Conservation Service ( Alaska/alaska.html) indicate that the total amount of precipitation during the summer (June August) of 2007 was just 6358

4 Figure 3. ALT and average annual change in position of ground surface over period of observation. Degree days of thaw for summers from 2001 to 2012 are shown in the top parts of the diagrams. Disturbance of data loggers by animals created gaps in the U32A site temperature record in 2009 and mm. In contrast, the total summer precipitation during 2003, which experienced virtually the same thawing degree day accumulation as 2007 (728 in 2003 versus 725 in 2007), was 104 mm. In 2003, 0.03 m of subsidence was observed at the ground surface. Both the coastal plain and foothills sites showed maximum subsidence in 2004, owing to thaw penetration into the ice-rich transition layer during this unusually warm summer. 5. Discussion [18] Traditional methodology for ALT observations (e.g., mechanical probing and temperature records from thermistor strings) involves measurement from the ground surface and does not account explicitly for thaw penetration into the transition layer or for associated soil consolidation and net subsidence of the surface. The potential for discrepancy between total thaw penetration and ALT is, therefore, roughly proportional to the ice content of the transition layer. This has contributed to an impression that ALT may not follow climatic trends closely [e.g., Hinzman et al., 2005]. To account for ground subsidence in the active layer record, annual changes in the position of the ground surface relative to its level in 2001 were added to the active layer measurements produced by mechanical probing (Figure 4). This operation demonstrates a pronounced increase in thaw penetration over the period of measurement, while the ALT measurements alone show no such trend. [19] The depth to which thaw penetrates is linked to summer air temperature through the accumulated thawing degree days (DDT), the climatic index most commonly used as a measure of atmospheric forcing. The square root of DDT is usually used in analytic solutions to the Stefan problem of heat conduction with phase change [Andersland and Ladanyi, 2004]. To evaluate the thermal response of permafrost landscapes to climatic forcing, site-averaged annual ALT values were regressed on the square root of DDT, calculated from air temperature records obtained from each site, and accumulated from the date of snowmelt to the date of thaw depth and surface elevation measurements in late summer. Analysis was performed using probed values of ALT, as well as values corrected for changes in the vertical position of the ground surface. [20] Graphs of the square root of DDT versus ALT yield distinct, linear, landscape-specific relations, indicating Figure 4. Results from ordinary least squares linear regression of annual values of ALT (open symbols) and corrected ALT (crosses) on the square root of DDT at the Coastal Plain (U5) and Foothills (U32A) sites. Goodness of fitisimproved substantially at both sites through incorporation of DGPS data. All results are significant at p =

5 differences in the thermal sensitivity of landscapes to climatic forcing (Figure 4). The stronger relation in the coastal plain landscape is attributable to the lower spatial variability of surface and subsurface characteristics, including vegetation, organic layer thickness, and microtopography [Nelson et al., 1998a, 1999]. Accounting for changes in surface elevation attributable to subsidence provides substantial improvements in linear fit at both sites. It also contributes to changes in the slope of the regression line, indicating higher sensitivity of ice-rich Alaskan permafrost landscapes to climatic forcing than has been reported previously. [21] A distinct contrast exists between the widespread, gentle lowering of the surface reported here and the more pronounced and geographically differential subsidence forming thermokarst terrain, which is usually associated with discrete disturbance events at the ground surface. Without effective instrumentation and sampling designs, such movements would not be apparent to observers at the surface. Although subtle changes in surface elevation have been reported at point locations in permafrost environments [Nixon and Taylor, 1998; Overduin and Kane, 2006], their consistency within extensive landscape units has not previously been confirmed through direct field observation. We introduce the term isotropic thaw subsidence to distinguish between the slow, widespread, relatively homogeneous, and low-magnitude thaw-induced lowering of the land surface described here and the more geographically restricted, irregular, and relatively large-magnitude subsidence usually associated with thermokarst terrain. The term isotropic has a long history of use in theoretical geography to denote a region with relatively uniform properties and has been used previously with respect to subsidence of Earth materials [e.g., Whittaker and Reddish, 1989]. Isotropic thaw subsidence has the advantage of scale independence and can be applied to regions of varying size, ranging from the landscape scale described in this paper to the much larger alasses of unglaciated Siberia. 6. Conclusions [22] Results obtained in this study indicate that the entire natural landscapes underlain by ice-rich permafrost are subsiding slowly in response to warming of the atmospheric climate, without initiation by localized anthropogenic or geomorphic disturbances. Isotropic thaw subsidence is both slow (decimeters/decade) and geographically uniform relative to the development of many thermokarst features. Because it involves thaw consolidation of the soil column, isotropic subsidence may not be apparent to observers measuring active layer thickness and explains the apparent stability of ALT in areas underlain by ice-rich materials. Integrated over extensive regions, isotropic thaw subsidence may be responsible for thawing large volumes of carbon-rich substrate, leading to the release of very substantial amounts of CO 2 and CH 4 to the atmosphere and hydrosphere, and has potential for widespread negative impacts on existing infrastructure. [23] Remote-sensing technology, particularly interferometric synthetic aperture radar, may be capable of providing integrated assessments of isotropic thaw subsidence over large areas [Rykhus and Lu, 2008; Liu et al., 2010, 2012], although the detailed surface-based monitoring adopted by the CALM program and described in this paper is crucial for calibration and verification. Results from the sites described here and at other CALM sites in the Russian Arctic [Mazhitova and Kaverin, 2007] and northern Alaska [Shiklomanov et al., 2012] have prompted installation of equipment to monitor heave and subsidence at other CALM sites underlain by ice-rich permafrost [Shiklomanov et al., 2010a]. [24] Acknowledgments. This research was funded by the U.S. National Science Foundation (NSF) Office of Polar Programs under grants OPP and OPP to the University of Delaware and ARC and ARC to The George Washington University. Opinions, findings, conclusions, and recommendations expressed in this paper do not necessarily reflect the views of NSF. Mention of specific product names does not constitute endorsement by NSF. DGPS equipment and instruction in its use were provided by the UNAVCO facility (Boulder, CO). Data reported in this paper are available from the CALM program s web page We are grateful to two anonymous reviewers, whose insightful comments helped to sharpen the focus of the paper. [25] The Editor thanks two anonymous reviewers for their assistance in evaluating this paper. References Andersland, O. B., and B. Ladanyi (2004), Frozen Ground Engineering, Wiley, Hoboken, NJ. Hinkel, K. M., and F. E. Nelson (2003), Spatial and temporal patterns of active layer thickness at Circumpolar Active Layer Monitoring (CALM) sites in northern Alaska, , J. Geophys. Res., 108(D2), 8168, doi: /2001jd Hinzman, L., et al. (2005), Evidence and implications of recent climate change in northern Alaska and other Arctic regions, Clim. Change, 72, Klene, A. E., F. E. Nelson, and N. I. Shiklomanov (2001), The n-factor in natural landscapes: Variability of air and soil-surface temperatures, Kuparuk River Basin, Alaska, Arct. Antarct. Alp. Res., 33, Kokelj, S. V., and M. T. Jorgenson (2013), Advances in thermokarst research, Permafrost Periglacial Processes, 24, Little, J. D. (2006), Frost heave and thaw settlement in tundra environments: Applications of differential global positioning systems technology, M.S. Thesis, Department of Geography, University of Delaware, Newark, Del., 160 pp. Little, J. D., H. Sandall, M. T. Walegur, and F. E. Nelson (2003), Application of differential global positioning systems to monitor frost heave and thaw settlement in tundra environments, Permafrost Periglacial Processes, 14, Liu, L., K. Schaefer, T. Zhang, and J. Wahr (2012), Estimating average active layer thickness on the Alaskan North Slope from remotely sensed surface subsidence, J. Geophys. Res., 117, F01005, doi: / 2011JF Liu, L., T. Zhang, and J. Wahr (2010), InSAR measurements of surface deformation over permafrost on the North Slope of Alaska, J. Geophys. Res., 115, F03023, doi: /2009jf Mazhitova, G. G., and D. A. Kaverin (2007), Active layer and surface subsidence dynamics at Circumpolar Active Layer Monitoring (CALM) site in European North of Russia, Earth Cryology, XI(4), (in Russian). Nelson, F. E., N. I. Shiklomanov, G. R. Mueller, K. M. Hinkel, D. A. Walker, and J. G. Bockheim (1997), Estimating active-layer thickness over a large region: Kuparuk River basin, Alaska, USA, Arct. Alp. Res., 29(4), Nelson, F. E., K. M. Hinkel, N. I. Shiklomanov, G. R. Mueller, L. L. Miller, and D. A. Walker (1998a), Active-layer thickness in north-central Alaska: Systematic sampling, scale, and spatial autocorrelation, J. Geophys. Res., 103(D22), 28,963 28,973. Nelson, F. E., S. I. Outcalt, J. Brown, K. M. Hinkel, and N. I. Shiklomanov (1998b), Spatial and temporal attributes of the active-layer thickness record, Barrow, Alaska, U.S.A, in Proceedings of the Seventh International Conference on Permafrost, edited by. A.G. Lewkowicz, and M. Allard, pp , Centre d Etudes nordiques, Université Laval, Québec, Publication No. 57. Nelson, F. E., N. I. Shiklomanov, and G. R. Mueller (1999), Variability of active-layer thickness at multiple spatial scales, north-central Alaska, U.S.A, Arct. Antarct. Alp. Res., 31, Nelson, F. E., N. I. Shiklomanov, K. M. Hinkel, and J. Brown (2008), Decadal results from the Circumpolar Active Layer Monitoring (CALM) program, in Proceedings of the Ninth International Conference on Permafrost, edited by K. M. Hinkel and D. L. Kane, pp , University of Alaska Press, Fairbanks, Alaska. Nixon, F. M., and A. E. Taylor (1998), Regional active layer monitoring across the sporadic, discontinuous and continuous permafrost zones, 6360

6 Mackenzie Valley, northwestern Canada, in Proceedings of the Seventh International Conference on Permafrost, edited by. A. G. Lewkowicz, and M. Allard, pp , Centre d Etudes nordiques, Université Laval, Québec, Publication No. 57. Overduin, P. P., and D. L. Kane (2006), Frost boils and soil ice content: Field observations, Permafrost Periglacial Processes, 17, Romanovsky, V. E., S. L. Smith, and H. H. Christiansen (2010), Permafrost thermal state in the polar Northern Hemisphere during the International Polar Year : A synthesis, Permafrost Periglacial Processes, 21, Rykhus, R. P., and Z. Lu (2008), InSAR detects possible thaw settlement in the Alaskan Arctic Coastal Plain, Can. J. Remote Sens., 34, Schuur, E. A. G., et al. (2008), Vulnerability of permafrost carbon to climate change: Implications for the global carbon cycle, BioScience, 58, Shiklomanov, N. I., D. A. Streletskiy, and F. E. Nelson (2010a), Northern Hemisphere component of the global Circumpolar Active Layer Monitoring (CALM) program, in Proceedings of the Tenth International Conference on Permafrost, edited by K. M. Hinkel, pp , The Northern Publisher, Salekhard, Russia. Shiklomanov, N. I., D. A. Streletskiy, F. E. Nelson, R. D. Hollister, V. E. Romanovsky, C. E. Tweedie, J. G. Bockheim, and J. Brown (2010b), Decadal variations of active-layer thickness in moisture-controlled landscapes, Barrow Alaska, J. Geophys. Res., 115, G00I04, doi: / 2009JG Shiklomanov, N. I., D. A. Streletskiy, and F. E. Nelson (2012), Active layer thickness and thaw subsidence in permafrost terrain: Results from long-term observations near Barrow, Alaska. Abstract GC14A-04, presented at 2012 Fall Meeting, AGU, San Francisco, Calif., 3 7 Dec. Shur, Y. L., and M. T. Jorgenson (2007), Patterns of permafrost formation and degradation in relation to climate and ecosystems, Permafrost Periglacial Processes, 18(1), Shur, Y. L., K. M. Hinkel, and F. E. Nelson (2005), The transient layer: Implications for geocryology and global-change science, Permafrost Periglacial Processes, 16, Streletskiy, D. A., N. I. Shiklomanov, and F. E. Nelson (2012), Permafrost, infrastructure, and climate change: A GIS-based landscape approach to geotechnical modeling, Arct. Antarct. Alp. Res., 44, Trucco, C., E. A. G. Schuur, S. M. Natali, E. F. Belshe, R. Bracho, and J. Vogel (2012), Seven-year trends of CO 2 exchange in a tundra ecosystem affected by long-term permafrost thaw, J. Geophys. Res., 117, G02031, doi: /2011jg Tveit, A., R. Schwacke, M. M. Svenning, and T. Urich (2013), Organic carbon transformations in high-arctic peat soils: Key functions and microorganisms, ISME J., 7, , doi: /ismej Wahrhaftig, C. (1965), Physiographic Divisions of Alaska, U.S. Geological Survey Professional Paper 482, U.S. Geological Survey, Washington, D. C. Walker D. A. (2003), Hierarchic GIS: Process, pattern and scale for analysis of Arctic ecosystems (ITEX).Boulder, Colorado USA: National Snow and Ice Data Center, ( Walker, D. A., and J. G. Bockheim (1995), Site selection for the portable flux towers. ARCSS/LAII/Flux Study, June Summary of Field Activities. LAII Science Management Office, University of Alaska-Fairbanks. Webster, R., and M. A. Oliver (1990), Statistical Methods in Soil and Land Resource Survey, pp. 316, Oxford Univ. Press, New York. Whittaker, B. N., and D. J. Reddish (1989), Subsidence: Occurrence, prediction and control, in Developments in Geotechnical Engineering, pp. 528, Elsevier, Amsterdam. 6361