Low Ice Content Research Articles

Analysis of taiga and tundra lake browning trends from 2002 to 2021 using MODIS data

Lakes in taiga and tundra regions may be silently undergoing changes due to global warming. One of those changes is browning in lake color. The browning interacts with the carbon cycle, ecosystem dynamics, and water quality in freshwater systems. However, spatiotemporal variabilities of browning in these regions have not been well documented. Using MODIS remote sensing reflectance at near ultraviolet wavelengths from 2002 to 2021 on the Google Earth Engine platform, we quantified long-term browning trends across 7616 lakes (larger than 10 km2) in taiga and tundra biomes. These lakes showed an overall decreased trend in browning (Theil-Sen Slope = 0.00015), with ∼36% of these lakes showing browning trends, and ∼1% of these lakes showing statistically significant (p-value <0.05) browning trends. The browning trends more likely occurred in small lakes in high latitude, low ground ice content regions, where air temperature increased and precipitation decreased. While temperature is projected to increase in response to climate change, our results provide one means to understand how biogeochemical cycles and ecological dynamics respond to climate change.

Influence of Climate Warming on the Ground Surface Stability over Permafrost along the Qinghai–Tibet Engineering Corridor

The warming climate has posed a serious threat on ground surface stability. In permafrost regions, ground surface instability may induce engineering and geological disasters, especially for the engineering corridor. It is difficult to evaluate ground surface stability over permafrost because the stability is influenced by various factors in permafrost regions. Many single index models cannot comprehensively evaluate the ground surface stability for permafrost. We, therefore, proposed an evaluation model considering different influential factors based on the trapezoidal fuzzy Analytical Hierarchy Process (AHP) method. And the ground surface stability was calculated and analyzed along the Qinghai–Tibet Engineering Corridor under three climate warming conditions (the slow climate warming, the medium climate warming and the rapid climate warming). The results show that the ground surface stability influential factors, including the mean annual ground temperature, the active layer thickness, and the volume ice content, will be greatly changed with the warming climate. By 2100, the percentage of high-temperature permafrost (−0.5 °C &lt; T ≤ 0 °C) will increase about 29.45% with rapid climate warming. The active layer thickness will have an average thickening rate of about 0.030 m/year. Most of the high ice content permafrost will change to low ice content permafrost. The ground surface stability, therefore, will be greatly changed with the warming climate along the Qinghai–Tibet Engineering Corridor. Compared to the present, the stable area will decrease about 5.28% by 2050 under the slow climate warming. And that is approximately 7.91% and 21.78% under the medium and rapid climate warming, respectively. While in year 2100, the decrement is obviously increased. The stable area will decrease about 11.22% under the slow climate warming and about 17.3% under the medium climate warming. The proportion of stable area, however, has an increasing trend under the rapid climate warming. This phenomenon is mainly caused by the warming climate which can lead to the permafrost being degraded to melting soil. The unstable area is mainly distributed near the Chumaer River high plain, Tuotuohe–Yanshiping, Wudaoliang, Tangula Mountains, and other high-temperature permafrost areas. This paper provides a reference for geological hazard prevention and engineering construction along the Qinghai–Tibet Engineering Corridor.

Response of active layer thickening to wildfire in the pan-Arctic region: Permafrost type and vegetation type influences

Northern high-latitude permafrost holds the largest soil carbon pool in the world. Understanding the responses of permafrost to wildfire is crucial for improving our ability to predict permafrost degradation and further carbon emissions. Recently, studies have demonstrated that wildfires in the pan-Arctic region induced the thickening of the active layer based on site or fire event observations. However, how this induced thickening is influenced by vegetation and permafrost types remains not fully understood due to the lack of wall-to-wall analysis. Therefore, this study employed remotely sensed fire data and modelled active layer thickness (ALT) to identify the fire-induced ALT change (ΔALT) for the pan-Arctic region, and the contributions of vegetation and permafrost were quantified using the random forest (RF) model. Our results showed that the average ΔALT and the sensitivity of ΔALT to burn severity both increased with decreasing ground ice content in permafrost. The largest values were detected in thick permafrost with low ground ice content. Regarding vegetation, the average and sensitivity of ΔALT in tundra were highest, followed by those in forest and shrub. When the individual environmental factors were all taken into account, the results showed that the contribution of vegetation types was much higher than that of permafrost types (20.2 % vs. 3.5 %). Our findings highlighted the importance of environmental factors in regulating the responses of permafrost to fire.

Performance and changes of high‐resolution (1 km) surface air temperature in Northern Hemisphere permafrost regions

AbstractSurface air temperatures are significant indicators of environmental and climatic change that affect a diverse set of physical systems including permafrost. Most temperature products, such as gridded or reanalysis data, are still at a relatively low spatial resolution, limiting the ability to simulate heterogeneous permafrost changes and leading to large uncertainties. Here we apply a downscaling method based on elevation to obtain high‐resolution surface air temperatures from the sixth Coupled Model Intercomparison Project in Northern Hemisphere permafrost regions. Root‐mean‐square errors and mean absolute errors after downscaling are reduced by 34 and 37%, respectively, relative to meteorological site data and gridded observations from the Climatic Research Unit. Compared to the downscaled surface air temperature data, nondownscaled model projections overestimate by 0.12–0.39°C in the discontinuous, isolated, and sporadic permafrost regions and underestimate up to 0.18°C in the continuous permafrost region under different emission scenarios. The warming rates in Northern Hemisphere permafrost regions were 0.093°C/10 year during the historical (1850–2014) period and are projected to be 0.22°C/10 year for SSP1‐2.6, 0.48°C/10 year for SSP2‐4.5, 0.75°C/10 year for SSP3‐7.0, and 0.95°C/10 year for SSP5‐8.5 during 2015–2100, which is 1.4–1.6 times the warming of nonpermafrost regions. Warming rates in high latitudes are 1.2–1.7 times higher than those in high‐elevation regions. Continuous permafrost regions' warming will be 1.2–1.4 times higher than in other permafrost regions. For permafrost with high ground ice content, warming will be 1.1 times greater than in permafrost regions with medium or low ground ice content.

Different responses of surface freeze and thaw phenology changes to warming among Arctic permafrost types

Arctic permafrost surface freeze–thaw (FT) changes related to warming could regulate the magnitude of global warming by altering the terrestrial carbon cycle and energy balances. This study investigated the sensitivity of surface FT changes to warming over Arctic permafrost regions by analyzing long-term changes in surface FT phenology from satellite remote sensing and meteorological variables from the climate data for the period from 1979 to 2017. Averaging over the entire Arctic permafrost regions, spring thawed date apparently advanced by −2.05 days decade−1, whereas autumn frozen date showed weak delaying trend of 0.83 days decade−1, implying the lengthening of the thawed season. Dividing the regions by permafrost types, advancing trends of thawed dates in continuous and high ice content permafrost areas (−2.57 and −2.70 days decade−1) were stronger than those over the discontinuous and low ice content permafrost areas (−1.61 and −1.73 days decade−1). The difference in changes in spring thawed dates between the regions is attributed to the difference in absolute magnitude of warming trends (e.g., 0.72 °C decade−1 for continuous vs. 0.44 °C decade−1 for discontinuous). However, the temperature sensitivity over discontinuous (low ice content) permafrost areas was 23% (10%) stronger than that over continuous (high ice content) permafrost areas for thawed date. In case of autumn, delaying trends of frozen dates were smaller over continuous and high ice content areas (0.69 and 0.74 days decade−1) than those over discontinuous and low ice content areas (1.01 and 0.88 days decade−1). This is mainly explained by the difference in temperature sensitivity (e.g., 1.57 days °C−1 for continuous vs. 2.18 days °C−1 for discontinuous) to warming between the regions rather than the difference in the absolute warming trends between the regions (e.g., 0.91 °C decade−1 for continuous vs. 0.51 °C decade−1 for discontinuous). The stronger temperature sensitivity of discontinuous and low ice content permafrost could be related to the lower demand of latent heat for the phase change of ground ice (or water). Overall, our results suggest that discontinuous and low ice content permafrost are more vulnerable to atmospheric warming. In addition to the magnitude of warming, the sensitivity to warming also needs to be considered when predicting permafrost FT changes.

Study of the defrosting behavior of concrete by using Taguchi-based grey relational analysis method

The frozen state of concrete in cold region has a significant influence on the determination of its performance. Scientific elimination of the influence of ice based on the comprehensive understanding of its defrosting behavior is significantly important. In this study, the defrosting behavior of the frozen concrete with different sizes, temperatures, and ice contents was researched by establishing a finite element model. The Taguchi–grey relational analysis (GRA) method was used to seek the numerical relationships among various factors. The simulated results show that the temperature field was highly consistent with the tested temperature history of concrete. The defrosting behavior of frozen concrete could be divided into three stages, and the time point at the completion of the Stage II was recommended as the defrosting time of concrete. Furthermore, temperature, size, and ice content affected the defrosting behavior of concrete by influencing the characteristics of different stages. The influence of ice on the determination of concrete performance could be easily eliminated at a higher temperature, a smaller size, and a lower ice content. Size played a more important role in the defrosting behavior of concrete than temperature and ice content. Eventually, a simplified equation for quickly estimating the defrosting time of concrete was proposed.

Conventional and UAV-Based Aerial Surveys for Long-Term Monitoring (1954–2020) of a Highly Active Rock Glacier in Austria

Rock glaciers are creep phenomena of mountain permafrost. Speed-up has been observed on several rock glaciers in recent years and attributed to climate change. Although rare, related long-term studies are nevertheless essential to bring a climate perspective to creep velocity changes. In the present study, we focused on changes both in the surface creep velocity and volume of the Leibnitzkopf rock glacier (Hohe Tauern Range, Austria) in the period 1954–2020. We applied 3D change detection using aerial images of both conventional (12 epochs between 1954 and 2018) and unmanned aerial vehicle (UAV)-based aerial surveys (2 epochs, 2019 and 2020), and combined this with ground and air temperature data. Photogrammetric processing (structure-from-motion, multi-view stereo) of the multi-temporal dataset resulted in high-resolution digital orthophotos/DOPs (5–50 cm spatial resolution) and digital elevation models/DEMs (10–50 cm grid spacing). Georeferencing was supported by five externally triangulated images from 2018, bi-temporal aerial triangulation of the image data relying on stable ground around the rock glacier, measured ground control points (2019 and 2020), and measured camera locations (PPK-GNSS) of the UAV flight in 2020. 2D displacement vectors based on the multi-temporal DOPs and/or DEMs were computed. Accuracy analyses were conducted based on geodetic measurements (2010–2020) and airborne laser scanning data (2009). Our analyses show high multi-annual and inter-annual creep velocity variabilities with maxima between 12 (1974–1981) and 576 cm/year (2019–2020), always detected in the same area of the rock glacier where surface disintegration was first observed in 2018. Our volume change analyses of the entire landform for the period 1954–2018 do not indicate any significant changes. This suggests little permafrost ice melt and/or general low ice content of the rock glacier. Analyses of the temperature data reveal a close relationship between higher temperatures and rock glacier acceleration despite the high probability of low ice content. This suggests that hydrogeological changes play an important role in the rock glacier system. The paper concludes with a summary of technical improvements and recommendations useful for rock glacier monitoring and a general view on the kinematic state of the Leibnitzkopf rock glacier.

Temperature Dependence of Unfrozen Water Quantity in Clay Soil with Different Moisture Content

We have investigated the relationship between the unfrozen quantity and total moisture content in clay soils in hygroscopic humidity region. To determine the unfrozen water quantity the method of continuous heat supply has been used (calorimetric method). Because crystallization heat of hygroscopic water in soil and that of volume water and different, the crystallization heat of soil water has been determined first during the experiment as a function of moisture and temperature, then it has been used to calculate the quantity of unfrozen water. It has been found that the dependence of unfrozen water quantity on total moisture content is more pronounced at low ice content and is not evident at all when it increases. In this case the dependence of unfrozen water quantity on total moisture content is of more complex pattern than that of the kind at high humidity. Besides the dependence is moistly evident at low ice content and becomes extinct at ice content increasing.

Transient modeling of the ground thermal conditions using satellite data in the Lena River delta, Siberia

Abstract. Permafrost is a sensitive element of the cryosphere, but operational monitoring of the ground thermal conditions on large spatial scales is still lacking. Here, we demonstrate a remote-sensing-based scheme that is capable of estimating the transient evolution of ground temperatures and active layer thickness by means of the ground thermal model CryoGrid 2. The scheme is applied to an area of approximately 16 000 km2 in the Lena River delta (LRD) in NE Siberia for a period of 14 years. The forcing data sets at 1 km spatial and weekly temporal resolution are synthesized from satellite products and fields of meteorological variables from the ERA-Interim reanalysis. To assign spatially distributed ground thermal properties, a stratigraphic classification based on geomorphological observations and mapping is constructed, which accounts for the large-scale patterns of sediment types, ground ice and surface properties in the Lena River delta. A comparison of the model forcing to in situ measurements on Samoylov Island in the southern part of the study area yields an acceptable agreement for the purpose of ground thermal modeling, for surface temperature, snow depth, and timing of the onset and termination of the winter snow cover. The model results are compared to observations of ground temperatures and thaw depths at nine sites in the Lena River delta, suggesting that thaw depths are in most cases reproduced to within 0.1 m or less and multi-year averages of ground temperatures within 1–2 °C. Comparison of monthly average temperatures at depths of 2–3 m in five boreholes yielded an RMSE of 1.1 °C and a bias of −0.9 °C for the model results. The highest ground temperatures are calculated for grid cells close to the main river channels in the south as well as areas with sandy sediments and low organic and ice contents in the central delta, where also the largest thaw depths occur. On the other hand, the lowest temperatures are modeled for the eastern part, which is an area with low surface temperatures and snow depths. The lowest thaw depths are modeled for Yedoma permafrost featuring very high ground ice and soil organic contents in the southern parts of the delta. The comparison to in situ observations indicates that transient ground temperature modeling forced by remote-sensing data is generally capable of estimating the thermal state of permafrost (TSP) and its time evolution in the Lena River delta. The approach could hence be a first step towards remote detection of ground thermal conditions and active layer thickness in permafrost areas.

Contrasting changes in surface waters and barrens over the past 60 years for a subarctic forest–tundra site in northern Manitoba based on remote sensing imagery

Intensified warming in the Arctic and Subarctic is resulting in a wide range of changes in the extent, productivity, and composition of aquatic and terrestrial ecosystems. Analysis of remote sensing imagery has documented regional changes in the number and area of ponds and lakes as well as expanding cover of shrubs and small trees in uplands. To better understand long-term changes across the edaphic gradient, we compared the number and area of water bodies and dry barrens (&gt;100 m2) between 1956 (aerial photographs) and 2008–2011 (high-resolution satellite images) for eight ∼25 km2 sites near Nejanilini Lake, Manitoba (59.559°N, 97.715°W). In the modern landscape, the number of water bodies and barrens were similar (1162 versus 1297, respectively), but water bodies were larger (mean 3.1 × 104 versus 681 m2, respectively) and represented 17% of surface area compared with 0.4% for barrens. Over the past 60 years, total surface area of water did not change significantly (16.7%–17.1%) despite a ∼30% decrease in numbers of small (&lt;1000 m2) water bodies. However, the number and area of barrens decreased (55% and 67%, respectively) across all size classes. These changes are consistent with Arctic greening in response to increasing temperature and precipitation. Loss of small water bodies suggests that wet tundra areas may be drying, which, if true, may have important implications for carbon balance. Our observations may be the result of changes in winter conditions in combination with low permafrost ice content in the region, in part explaining regional variations in responses to climate change.

Reorganization of vegetation, hydrology and soil carbon after permafrost degradation across heterogeneous boreal landscapes

The diversity of ecosystems across boreal landscapes, successional changes after disturbance and complicated permafrost histories, present enormous challenges for assessing how vegetation, water and soil carbon may respond to climate change in boreal regions. To address this complexity, we used a chronosequence approach to assess changes in vegetation composition, water storage and soil organic carbon (SOC) stocks along successional gradients within four landscapes: (1) rocky uplands on ice-poor hillside colluvium, (2) silty uplands on extremely ice-rich loess, (3) gravelly–sandy lowlands on ice-poor eolian sand and (4) peaty–silty lowlands on thick ice-rich peat deposits over reworked lowland loess. In rocky uplands, after fire permafrost thawed rapidly due to low ice contents, soils became well drained and SOC stocks decreased slightly. In silty uplands, after fire permafrost persisted, soils remained saturated and SOC decreased slightly. In gravelly–sandy lowlands where permafrost persisted in drier forest soils, loss of deeper permafrost around lakes has allowed recent widespread drainage of lakes that has exposed limnic material with high SOC to aerobic decomposition. In peaty–silty lowlands, 2–4 m of thaw settlement led to fragmented drainage patterns in isolated thermokarst bogs and flooding of soils, and surface soils accumulated new bog peat. We were not able to detect SOC changes in deeper soils, however, due to high variability. Complicated soil stratigraphy revealed that permafrost has repeatedly aggraded and degraded in all landscapes during the Holocene, although in silty uplands only the upper permafrost was affected. Overall, permafrost thaw has led to the reorganization of vegetation, water storage and flow paths, and patterns of SOC accumulation. However, changes have occurred over different timescales among landscapes: over decades in rocky uplands and gravelly–sandy lowlands in response to fire and lake drainage, over decades to centuries in peaty–silty lowlands with a legacy of complicated Holocene changes, and over centuries in silty uplands where ice-rich soil and ecological recovery protect permafrost.

Effects of Ice Content on the Thermal Erosion of Permafrost: Implications for Coastal and Fluvial Erosion

ABSTRACTThe effects of ice content and water flow on thawing and erosion of non‐cohesive permafrost banks were investigated through laboratory experiments. A critical ice content was identified as associated with turbulent flows (20% and 80% for Reynolds numbers of 15 900 and 12 700, respectively), above which thermal erosion results in ablation and a decrease in ice content increases ablation. Below the critical value, the ablation model over‐estimates the erosion rate as the removal of thawed sediments occurs episodically, possibly due to the imbrication of sand grains. The ablation model applies to rivers with non‐cohesive banks, high ice contents and subject to high‐velocity water flows. The model may explain differential erosion that results in massive ice layers in relief. At other sites, however, the retreat rate increases with ground ice content. This behaviour can be explained for sandy permafrost with a relatively low ice content and heterogeneous sand sizes and shapes subject to relatively low‐velocity water flow. These results apply only to perenially frozen sands, as even a small percentage of cohesive material would modify the relationships described. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Statistics and characteristics of permafrost and ground-ice distribution in the Northern Hemisphere

The recently published digital version of the International Permafrost Association (IPA) Circum-Arctic Map of Permafrost and Ground Ice Conditions (the IPA map), together with ancillary data sets of the global land cover characteristics data base and the Global Land One-kilometer Base Elevation data base, are used to investigate the distribution of permafrost and ground ice in the Northern Hemisphere. Our study indicates that permafrost underlies approximately 22.79×106 km2 or 23.9% of the exposed land area of the Northern Hemisphere. Permafrost extends from 26°N in the Himalayas to 84°N in northern Greenland. Approximately 70% of the permafrost is distributed between 45 and 67°N. Generally, permafrost with high ice content (>20% by volume) and relatively thick overburden cover (>5 to 10 m) is found at high latitudes, representing approximately 8.57% of the total permafrost area, or 2.02% of the exposed land area of the Northern Hemisphere. Permafrost with low ice content (<10% by volume with either thick or thin overburden cover) occurs mainly in mountainous regions and high plateaus, representing approximately 66.5% of the total permafrost area or 15.8% of the exposed land area. Approximately 62% of the permafrost of the Northern Hemisphere is found below 500 m a.s.l. and about 10% occurs above 3000 m a.s.l. Based on the IPA map categories, the estimated volume of ground ice in the Northern Hemisphere is between 5.63 and 15.12×103 km3, corresponding to ∼2–4 cm sea level equivalent. Based on alternative assumptions, the volume of ground ice may be between 11.37 and 36.55×103 km3, which corresponds to 3-10 cm sea-level equivalent.

Freezing and anoxia tolerance of slugs: a metabolic perspective

Freezing survival was assessed in three species of terrestrial slugs, a holarctic but native North American species, Deroceras laeve, and two species introduced from Europe, D. reticulatum and Arion circumscriptus. The introduced species showed very poor freezing survival. Supercooling points of the introduced species were quite high ( approximately -3 degrees C) and their freezing survival was very poor, limited to short-term freezing at -1.2 to -1.5 degrees C and low ice contents (23-44%). D. laeve showed a significant elevation of supercooling point between slugs collected in the autumn (-4.8 degrees C +/- 0.5) and those collected early in the spring (-3.1 degrees C +/- 0.4). This species also showed substantial freezing survival which was greater for spring-collected slugs (100% survival of 1 h freezing at -2 degrees C with an ice content of 65%) than for autumn animals (100% survival for 1 h at -1 degrees C with approximately 40% ice). Carbohydrate and amino acid responses to freezing and anoxia exposures were compared in the two Deroceras species. D. laeve showed a strong hyperglycemic response to freezing, a 100-fold increase in glucose levels that suggested that glucose may have a cryoprotective function in this species. D. reticulatum did not accumulate glucose and neither species produced glycerol or lactate. Both species showed typical responses to anoxia (aspartate and glutamate catabolism, alanine and succinate accumulation) and D. laeve also showed this pattern during freezing, suggesting a natural switch to anaerobiosis to support freezing survival.

Volcanic particle aggregation in explosive eruption columns. Part II: Numerical experiments

The goal of this paper is to determine the parameters that control the aggregation efficiency and the growth rate of volcanic particles within the eruption column. Numerical experiments are performed with the plume model ATHAM (Active Tracer High resolution Atmospheric Model). In this study we employ the parameterizations described in a companion paper (this issue). The presence of hydrometeors promotes the aggregation of ash particles, which strongly increases their fall velocities and thus their environmental impact. The tephra mass is about two orders of magnitude greater than that of hydrometeors during typical Plinian eruptions without interaction of external water. Ice is highly dominant in comparison to liquid water (> 99% by mass). This is caused by the fast column rise (> 100 m s − 1 on average) to very cold altitudes. Most particles occur in the form of frozen aggregates with low ice content. The collection efficiency is governed by the availability of hydrometeors acting as adhesives at the particles’ surface in our study, and wet ash particles have a higher sticking capacity than icy ones. Therefore, aggregation is fastest during the eruption within the column when limited regions of liquid water exist and when particle concentrations are very high (of the order of 10 5 cm − 3 ). Increased humidity in the background atmosphere generally leads to enhanced ice formation, but shows only a weak influence on the aggregation process. First sensitivity studies showed, however, a significant increase of the liquid water fraction when considering salinity effects. The availability of water or ice at the particles' surfaces is also governed by the surface properties, the porosity and permeability of ash, which are not well established to date. Particle growth is significantly enhanced for greater differences in the sizes and fall velocities among particles, as gravitational capture becomes more efficient. Our experiments indicate a major influence of the erupted particle size distribution. First sensitivity studies show that electrostatic forces result in a significant enhancement of aggregated particles. The present exploratory study provides new insights into the sensitivity of the ash aggregation process to a number of key parameters. Our results indicate the need of further constraining particle composition, size, porosity, permeability, and surface properties at low temperatures by in situ observations in the laboratory and in the field. In addition further research on electrostatic aggregation would be desirable.

Near‐surface ground ice in sediments of the Mackenzie Delta, Northwest Territories, Canada

AbstractThe ice content of near‐surface permafrost was determined at more than 70 sites in the Mackenzie Delta, Northwest Territories. Willow and alder communities growing on aggrading surfaces with well‐drained sandy silts and ‘warm’ ground temperatures were underlain by permafrost with low ice content. Spruce forests above the level of annual flooding, lakeside alder communities in the central and southern delta, and sedge wetlands in the northern delta with moist silty loam soils, low sedimentation rates and ‘cold’ ground temperatures were underlain by medium to high ice‐content permafrost. Beneath spruce forests, a layer of near‐surface ice accumulation 1 to 2 m thick was underlain by alluvium bonded by pore ice. These profiles indicate that contemporary conditions favour near‐surface ice accumulation. The preservation of ice‐poor permafrost at depth demonstrates the limited vertical accumulation of near‐surface segregated ice. In contrast, at lakeshore alder or sedge wetlands, segregated ice extended at least 2 m below the base of the active layer, indicating that permafrost aggraded in a saturated environment. Development of ground ice in the Mackenzie Delta may elevate alluvial surfaces, modify the flooding regime, and thereby influence soil conditions and ecological succession. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Triaxial constant stress and constant strain rate tests on ice-rich permafrost samples

A range of mechanical compression tests were conducted on frozen soil samples at temperatures close to the melting point of ice to determine both the creep and the strength properties under triaxial stress conditions. The samples were obtained from two rock glaciers in the Swiss Alps. For comparison, additional samples were prepared artificially for testing. The results showed that the minimum creep strain rate increases exponentially with increasing temperature and applied deviatoric stress. The applied strain rate is the main parameter influencing resistance. However, test results show that the micromechanical behaviour is similar for all tests and independent of the applied loading condition. Measurements of the volumetric strain showed that the sample composition has a major influence, so samples with high air void ratios resulted in continuous contraction (i.e., reduction of the sample volume), whereas samples with low volumetric air and ice contents demonstrated dilatant behaviour.Key words: permafrost, frozen soil, creep, strength, triaxial testing, self-healing.

Effects of volumetric ice content and strain rate on shear strength under triaxial conditions for frozen soil samples

AbstractA set of triaxial constant strain rate and constant stress tests was performed on artificially frozen soil samples to study the effect of the volumetric ice fraction, strain rate and also confining stress on the mobilized shear strength. In general, the peak shear strength increased with decreasing volumetric ice content and increasing strain rate. Pure ice samples, however, showed a peak shear strength that was higher than that for those containing about 80% ice by volume for a similar strain rate, although the loss in strength after this maximum was usually much more pronounced, reaching a lower large strain strength than that for frozen sands. The influence of the ice in samples with low ice contents was primarily noticeable at low strains, whereas the large strain behaviour was very similar to that of the unfrozen material. The tests showed the dependency of the mechanical failure and deformation mode of frozen soils on the loading conditions. It could be demonstrated further that large strains have a significant influence on the strength of frozen geo‐materials and therefore efforts should be made to establish in situ strain states when analysing the stability of frozen slopes. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Stratigraphic profiling with ground‐penetrating radar in permafrost: A review of possible analogs for Mars

We review our past and ongoing use of ground‐penetrating radar to investigate permafrost in Alaska and in the Dry Valleys of Antarctica. The results may be relevant to radar efforts on Mars because of arid conditions and the presence of ice. The pulses were centered at 50, 100, and 400 MHz. We interpret profiles from two sites in the eastern Taylor Valley to show glaciolacustrine and glaciofluvial stratigraphy. The maximum depth of stratigraphy profiled there was about 33 m. Near Fairbanks, Alaska, the depth of penetration at 50 MHz was near 80 m in marginally frozen and stratified alluvial sands. At the Fairbanks sites, supplementary drilling was required to differentiate between reflections from conductive bedrock, a graphitic schist, and those from the water table at depths of 20–25 m. At a site on the North Slope of Alaska, we profiled present and remnant freezing fronts in an alluvial floodplain. The relative permittivity at most sites ranged between about 4 and 5.5, which is consistent with dry conditions, the mineralogy, and low ice content. Weak interface reflectivity or the lack of further interfaces may have limited the interpretation of maximum penetration where no water table was present because signal absorption should have been low and scarce diffractions imply that scattering was weak. The interface reflectivities beneath Taylor Valley may be a function of only density contrasts, since free water, and possibly ice, is absent.

Verification of geophysical models in Alpine permafrost using borehole information

AbstractAt two permafrost sites in the Swiss Alps a range of geophysical methods were applied to model the structure of the subsurface. At both sites, borehole information was used to verify the quality of the model results. On the Murtèl-Corvatsch rock glacier (2700 m a.s.L; upper Engadine) a 58 m deep core drilling was performed in 1987. D. c resistivity measurements, refraction seismics, ground-penetrating radar (GPR) and gravimetric surveys allowed the shape of the permafrost table beneath the marked surface microtopography to be determined and the lateral extent of a deeper shear horizon to be established The validity of each method was verified by the borehole information (cores, density log and temperature). A coherent model of the rock-glacier structure was developed. At the Schilthorn (2970 m a.s.L; Bernese Oberland), it was not clear whether permafrost is in fact present. Various geophysical surveys (d.c. resistivity tomography, refraction seismics, GPR and EM-31) gave results that were not typical of permafrost environments. A 14 m percussion drilling revealed warm permafrost and a very low ice content. These geotechnical and geothermal data allowed reinterpretation of the geophysical results, improving modelling of ground conditions. The paper demonstrates that in the difficult terrain of Alpine permafrost, boreholes may be critical in calibration and verification of the results of geophysical methods. The most useful combinations of geophysical techniques proved to be (a) seismics with d.c. resistivity, and (b) gravimetry with GPR.