Poster

Recent circum-Arctic ice-wedge degradation and its hydrological impacts

Authors:
  • Melnikov Permafrost Institute Siberian Branch RAS, Yakutsk, Russia
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Abstract

Ice-wedges are common permafrost features formed over hundreds to thousands of years of repeated frost cracking and ice vein growth. We used field and remote sensing observations to assess changes in areas dominated by ice-wedges, and we simulated the effects of those changes on snow accumulation and runoff. We show that top melting of ice-wedges and subsequent ground subsidence has occurred at multiple sites in the North American and Russian Arctic. At most sites, melting ice-wedges have initially resulted in increased wetness contrast across the landscape, evident as increased surface water in the ice-wedge polygon troughs and somewhat drier polygon centers. Most areas are becoming more heterogeneous with wetter troughs, more small ponds (themokarst pits forming initially at ice-wedge intersections and then spreading along the troughs) and drier polygon centers. Some areas with initial good drainage, such as near creeks, lake margins, and in hilly terrain, high-centered polygons form an overall landscape drying due to a drying of both polygon centers and troughs. Unlike the multi-decadal warming observed in permafrost temperatures, the ice-wedge melting that we observed appeared as a sub-decadal response, even at locations with low mean annual permafrost temperatures (down to −14 °C). Gradual long-term air and permafrost warming combined with anomalously warm summers or deep snow winters preceded the onset of the ice-wedge melting. To assess hydrological impacts of ice-wedge melting, we simulated tundra water balance before and after melting. Our coupled hydrological and thermal model experiments applied over hypothetical polygon surfaces suggest that (1) ice-wedge melting that produces a connected trough-network reduces inundation and increases runoff, and that (2) changing patterns of snow distribution due to differential ground subsidence has a major control on ice-wedge polygon tundra water balance despite an identical snow water equivalent at the landscape-scale. These decimeter-scale geomorphic changes are expected to continue in permafrost regions dominated by ice-wedge polygons, with implications for land-atmosphere and land-ocean fluxes of water, carbon, and energy.

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Quantifying changes in thermokarst lake extent is of importance for understanding the permafrost-related carbon budget, including the potential release of carbon via lake expansion or sequestration as peat in drained lake basins. We used high spatial resolution remotely sensed imagery from 1950/51, 1978, and 2006/07 to quantify changes in thermokarst lakes for a 700 km2 area on the northern Seward Peninsula, Alaska. The number of water bodies larger than 0.1 ha increased over the entire observation period (666 to 737 or +10.7%); however, total surface area decreased (5,066 ha to 4,312 ha or −14.9%). This pattern can largely be explained by the formation of remnant ponds following partial drainage of larger water bodies. Thus, analysis of large lakes (>40 ha) shows a decrease of 24% and 26% in number and area, respectively, differing from lake changes reported from other continuous permafrost regions. Thermokarst lake expansion rates did not change substantially between 1950/51 and 1978 (0.35 m/yr) and 1978 and 2006/07 (0.39 m/yr). However, most lakes that drained did expand as a result of surface permafrost degradation before lateral drainage. Drainage rates over the observation period were stable (2.2 to 2.3 per year). Thus, analysis of decadal-scale, high spatial resolution imagery has shown that lake drainage in this region is triggered by lateral breaching and not subterranean infiltration. Future research should be directed toward better understanding thermokarst lake dynamics at high spatial and temporal resolution as these systems have implications for landscape-scale hydrology and carbon budgets in thermokarst lake-rich regions in the circum-Arctic.
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1] Even though the arctic zone of continuous permafrost has relatively cold mean annual air temperatures, we found an abrupt, large increase in the extent of permafrost degradation in northern Alaska since 1982, associated with record warm temperatures during 1989 – 1998. Our field studies revealed that the recent degradation has mainly occurred to massive wedges of ice that previously had been stable for 1000s of years. Analysis of airphotos from 1945, 1982, and 2001 revealed large increases in the area (0.5%, 0.6%, and 4.4% of area, respectively) and density (88, 128, and 1336 pits/km 2) of degrading ice wedges in two study areas on the arctic coastal plain. Spectral analysis across a broader landscape found that newly degraded, water-filled pits covered 3.8% of the land area. These results indicate that thermokarst potentially can affect 10– 30% of arctic lowland landscapes and severely alter tundra ecosystems even under scenarios of modest climate warming.
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1] Meteorological and soil temperature and moisture data for the period 1998–2005 are presented from a long term monitoring station in the central Lena River Delta at 72°N, 126°E. The investigation site, Samoylov Island, is situated in the zone of continuous permafrost and is characterized by wet polygonal tundra. The summer energy and water balance of the tundra was analyzed for the dry year 1999 and the wet year 2003. The summer water balance of the tundra was found to be mainly controlled by precipitation. The partitioning of the available energy was controlled by precipitation via the soil moisture regime, and by the synoptic weather conditions via radiation and the advection of maritime cold or continental warm air masses. In 2003, regular high precipitation resulted in a constant recharge of polygonal ponds. Of the available energy, 61% were partitioned into latent heat flux and 17% into ground heat flux; hence, the tundra behaved like a typical wetland. In 1999, low precipitation resulted in a loss of polygonal pond waters and a drying of upper soil layers. This led to lower latent heat flux (31% of available energy), higher ground heat flux (29%), and a considerably higher soil thaw depth compared to 2003. Surface and subsurface water flow had a significant influence on the tundra water balance. In 2003, the formation of new surface flow channels through thermo-erosion was observed, which is expected to have a strong and lasting influence on the hydrologic system of the tundra.
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Hydrological cycle intensification is an expected manifestation of a warming climate. We examine the quantitative significance of changes in freshwater fluxes across observational time series alongside those from a suite of coupled general circulation models for both the terrestrial pan-Arctic and Arctic Ocean. Trends in terrestrial fluxes from observations and GCMs are consistently positive. Significant trends are not present for all of the observations. Upward trends in the GCMs exhibit a higher statistical significance owing to lower inter-annual variability and relatively long time period examined. This fact limits our confidence in the robustness of the changes. Oceanic fluxes are more uncertain due primarily to the lack of long-term observations. Where available, marine flux estimates over recent decades suggest some decrease in saltwater inflow to the Barents Sea, implying a decrease in freshwater outflow. A decline in freshwater storage across the central Arctic Ocean and suggestions that large-scale circulation plays a dominant role in freshwater trends raise questions as to whether oceanic flows are intensifying. Although the oceanic freshwater fluxes are highly variable and consistent trends are difficult to verify, other components of the arctic freshwater cycle do show consistent positive trends over recent decades. This broad-scale increase in freshwater fluxes presents strong evidence that the arctic hydrological cycle is experiencing intensification.
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In the Arctic, surface hydrology plays an important role in controlling plant community composition and ecosystem processes such as land-atmosphere carbon and energy balance. Investigating how climate change in this region will affect surface hydrology and subsequent biotic, atmospheric, and climatic feedbacks could be key to understanding the future state of the Arctic and Earth systems. Improved methods for monitoring surface hydrology at large spatial scales are needed in the Arctic. Near Barrow, Alaska, a large-scale experiment with flooded, drained, and control treatment areas, each exceeding 9 ha, was initiated during summer 2008 following 3 years of monitoring under nonmanipulative conditions. Throughout the 2008 growing season, hyperspectral reflectance data were collected in the visible to near-infrared (IR) range using a 300 m long robotic tram system. Water table depth, surface water depth, and percent surface water cover were also measured. A spectral index (Normalized Difference Surface Water Index (NDSWI)) was developed using reflectance in the IR region (R1000 strong absorbance) and blue region (R460 poor absorbance). NDSWI was strongly correlated with both surface water depth and surface water cover, and was used to monitor spatial and temporal patterns of surface hydrology in the experimental treatment. Using 2002 and 2008 Quickbird satellite imagery, the index was also used to examine differences in NDSWI between experimental treatments. Using this approach, we demonstrate that the flooded treatment was significantly different from the other two treatments (drained and control) and that the new index can be used to monitor surface hydrology in arctic wetlands.
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Synthesis of river-monitoring data reveals that the average annual discharge of fresh water from the six largest Eurasian rivers to the Arctic Ocean increased by 7% from 1936 to 1999. The average annual rate of increase was 2.0 +/- 0.7 cubic kilometers per year. Consequently, average annual discharge from the six rivers is now about 128 cubic kilometers per year greater than it was when routine measurements of discharge began. Discharge was correlated with changes in both the North Atlantic Oscillation and global mean surface air temperature. The observed large-scale change in freshwater flux has potentially important implications for ocean circulation and climate.
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Ground ice is abundant in the upper permafrost throughout the Arctic and fundamentally affects terrain responses to climate warming. Ice wedges, which form near the surface and are the dominant type of massive ice in the Arctic, are particularly vulnerable to warming. Yet, processes controlling ice-wedge degradation and stabilization are poorly understood. Here we quantified ice-wedge volume and degradation rates, compared ground-ice characteristics and thermal regimes across a sequence of five degradation and stabilization stages, and evaluated biophysical feedbacks controlling permafrost stability near Prudhoe Bay, Alaska. Mean ice-wedge volume in the top 3 m of permafrost was 21%. Imagery from 1949 to 2012 showed thermokarst extent (area of water-filled troughs) was relatively small from 1949 (0.9%) to 1988 (1.5%), abruptly increased by 2004 (6.3%), and increased slightly by 2012 (7.5%). Mean annual surface temperatures varied by 4.9 °C among degradation and stabilization stages, and by 9.9 °C from polygon center to deep lake bottom. Mean thicknesses of the active layer, ice-poor transient layer, ice-rich intermediate layer, thermokarst-cave ice, and wedge ice varied substantially among stages. In early stages, thaw settlement caused water to impound in thermokarst troughs, creating positive feedbacks that increased net radiation, soil heat flux, and soil temperatures. Plant growth and organic-matter accumulation in the degraded troughs provided negative feedbacks that allowed ground ice to aggrade and heave the surface, thus reducing surface water depth and soil temperatures in later stages. The ground ice dynamics and ecological feedbacks greatly complicate efforts to assess permafrost responses to climate change.
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Arctic regions hold considerable reservoirs of soil organic carbon. However, most of this carbon is in a potential labile state, and expected changes in temperature and water availability could strongly affect the carbon balance of tundra ecosystems. Plant community composition and soil carbon are closely tied to microtopography and position relative to the water table. We evaluated CO2 fluxes and moss contribution to ecosystem photosynthesis in response to fine-scale topography across a drained lake bed in Barrow, Alaska, during two contrasting growing seasons. CO2 exchange was assessed through static chamber measurements in three vegetation classes distinguished by plant dominance and topographic position within low-centered polygons. Gross primary production (GPP) and ecosystem respiration (ER) were the lowest under high soil moisture conditions in 2006. ER responded more strongly to wet conditions, resulting in a larger summer sink in 2006 than in 2005 (64 vs. 17g CO2 m(-2), respectively). Microsites responded differently to contrasting weather conditions. Low elevation microsites presented a strong reduction in ER as a result of increased water availability. A maximum of 48% of daytime GPP and 33% of seasonal daytime GPP was contributed by moss on average across microtopographic positions. The interaction between fine-scale microtopography and variation in temperature and water availability can result in considerable differences in CO2 sink activity of the polygonal tundra.
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The landscape of the Barrow Peninsula in northern Alaska is thought to have formed over centuries to millennia, and is now dominated by ice-wedge polygonal tundra that spans drained thaw-lake basins and interstitial tundra. In nearby tundra regions, studies have identified a rapid increase in thermokarst formation (i.e. pits) over recent decades in response to climate warming, facilitating changes in polygonal tundra geomorphology. We assessed the future impact of 100 years of tundra geomorphic change on peak growing season carbon exchange in response to: (1) landscape succession associated with the thaw-lake cycle; and (2) low, moderate, and extreme scenarios of thermokarst pit formation (10, 30, and 50%) reported for Alaskan arctic tundra sites. We developed a 30 x 30m resolution tundra geomorphology map (overall accuracy:75%; Kappa:0.69) for our ~1800 km² study area composed of ten classes; drained slope, high-center polygon, flat-center polygon, low-center polygon, coalescent low-center polygon, polygon trough, meadow, ponds, rivers, and lakes, to determine their spatial distribution across the Barrow Peninsula. Land-atmosphere CO2 and CH4 flux data were collected for the summers of 2006-2010 at eighty-two sites near Barrow, across the mapped classes. The developed geomorphic map was used for the regional assessment of carbon flux. Results indicate (1) at present during peak growing season on the Barrow Peninsula, CO2 uptake occurs at -902.3 106gC-CO2day−1 (uncertainty using 95% CI is between -438.3 and-1366 106gC-CO2day−1) and CH4 flux at 28.9 106gC-CH4day−1(uncertainty using 95% CI is between 12.9 and 44.9 106gC-CH4day−1), (2) one century of future landscape change associated with the thaw-lake cycle only slightly alter CO2 and CH4 exchange, while (3) moderate increases in thermokarst pits would strengthen both CO2 uptake (-166.9 106gC-CO2day−1) and CH4 flux (2.8 106gC-CH4day−1) with geomorphic change from low to high-center polygons, cumulatively resulting in an estimated negative feedback to warming during peak growing season.This article is protected by copyright. All rights reserved.
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Many areas of the Arctic are simultaneously affected by rapid climate change and rapid industrial development. These areas are likely to increase in number and size as sea ice melts and abundant Arctic natural resources become more accessible. Documenting the changes that have already occurred is essential to inform management approaches in order to minimize the impacts of future activities. Here we determine the cumulative geoecological effects of 62 years (1949-2011) of infrastructure- and climate-related changes in the Prudhoe Bay Oilfield, the oldest and most extensive industrial complex in the Arctic, and an area with extensive ice-rich permafrost that is extraordinarily sensitive to climate change. We demonstrate that thermokarst has recently affected broad areas of the entire region, and that a sudden increase in the area affected began shortly after 1990 corresponding to a rapid rise in regional summer air temperatures and related permafrost temperatures. We also present a conceptual model that describes how infrastructure-related factors, including road dust and roadside flooding are contributing to more extensive thermokarst in areas adjacent to roads and gravel pads. We mapped the historical infrastructure changes for the Alaska North Slope oilfields for 10 dates from the initial oil discovery in 1968 to 2011. By 2010, over 34% of the intensively mapped area was affected by oil development. In addition, between 1990 and 2001, coincident with strong atmospheric warming during the 1990s, 19% of the remaining natural landscapes (excluding areas covered by infrastructure, lakes and river floodplains) exhibited expansion of thermokarst features resulting in more abundant small ponds, greater microrelief, more active lakeshore erosion and increased landscape and habitat heterogeneity. This transition to a new geoecological regime will have impacts to wildlife habitat, local residents and industry. This article is protected by copyright. All rights reserved.
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Circumpolar expansion of tall shrubs and trees into Arctic tundra is widely thought to be occurring as a result of recent climate warming, but little quantitative evidence exists for northern Siberia, which encompasses the world's largest forest-tundra ecotonal belt. We quantified changes in tall shrub and tree canopy cover in eleven, widely-distributed Siberian ecotonal landscapes by comparing very-high-resolution photography from the Cold War-era "Gambit" and "Corona" satellite surveillance systems (1965-1969) with modern imagery. We also analyzed within-landscape patterns of vegetation change to evaluate the susceptibility of different landscape components to tall shrub and tree increase. The total cover of tall shrubs and trees increased in nine of eleven ecotones. In northwest Siberia, alder (Alnus) shrubland cover increased 5.3 - 25.9% in five ecotones. In Taymyr and Yakutia, larch (Larix) cover increased 3.0 - 6.7% within three ecotones, but declined 16.8% at a fourth ecotone due to thaw of ice-rich permafrost. In Chukotka, the total cover of alder and dwarf pine (Pinus) increased 6.1% within one ecotone and was little-changed at a second ecotone. Within most landscapes, shrub and tree increase was linked to specific geomorphic settings, especially those with active disturbance regimes such as permafrost patterned-ground, floodplains, and colluvial hillslopes. Mean summer temperatures increased at most ecotones since the mid-1960s, but rates of shrub and tree canopy cover expansion were not strongly correlated with temperature trends and were better correlated with mean annual precipitation. We conclude that shrub and tree cover is increasing in tundra ecotones across most of northern Siberia, but rates of increase vary widely regionally and at the landscape-scale. Our results indicate that extensive changes can occur within decades in moist, shrub-dominated ecotones, as in northwest Siberia, while changes are likely to occur much more slowly in the highly continental, larch-dominated ecotones of central and eastern Siberia. This article is protected by copyright. All rights reserved.
Article
A methodology for adjusting the daily precipitation measured by the U.S. National Weather Service (NWS) 8-inch standard gauge for wind-induced undercatch, wetting loss, and trace amount of precipitation is provided. The application of the proposed adjustment procedures was made at 10 NWS climate stations in Alaska for 1982 and 1983. The results show the following: (1) Daily adjustment for wind-induced undercatch, wetting loss, and trace amount of precipitation increases the gauge-measured annual precipitation by 65- 800 mm for the 2 years (about 10 -140% of the gauge- measured yearly total) at the 10 stations in Alaska; (2) compared to wetting loss and trace amount of precipitation, wind-induced undercatch is the source of greatest error, although wetting loss and trace amount of precipitation are also significant systematic errors in the northern Alaska regions of low precipitation; (3) in the similar climate condition, the NWS 8-inch standard gauges with an Alter wind shield have a much lower adjustment for wind-induced undercatch than the unshielded gauges at nearby stations, and the unshielded gauges placed on the roof of the weather office building have a higher adjustment for wind-induced errors than those gauges mounted on the ground; (4) monthly adjustment factors (adjusted/measured precipitation) differ by station, and at an individual station by type of precipitation; (5) considerable intra-annual variation of the magnitude of the adjustments has been found in Alaska owing to the fluctuation of wind speed, air temperature, and frequency of snowfall. Using the constant correction factors (derived at a single intercomparison site) to the archived monthly precipitation records produces significant undercorrection of the wind-induced errors at high wind sites and overcorrecting of the errors at low wind sites. To avoid the undercorrection or overcorrection of the wind-induced errors, a constant correction factor should not be applied to gauge-measured snow data. Daily adjustments for systematic errors need to be applied to the archived precipitation data. It is expected that the adjustments will have an impact on climate monitoring.
Article
Daily soil temperature and thaw depth for the entire Arctic terrestrial drainage area are simulated using a one-dimensional heat transfer model with phase change. Analyses of temperature trends at the soil surface and at 2 m depth are presented for the 23-year time period 1980 through 2002. Soil warming is simulated for all permafrost regions, but is most pronounced (0.044°C/yr) at the surface in the continuous permafrost region. Trends for most recent years (1994–2002) are about three times higher. Active layer depth increases significantly for parts of Alaska and northern Canada, and southern and eastern Siberia. As assessed for the major river drainages, the most dramatic active layer deepening occurs in the Yenisey basin.
Article
Seasonal frozen states in the northern terrestrial cryosphere limit vegetation photosynthetic activities and evapotranspiration (ET) through cold temperature constraints to biological processes and chemical unavailability of water as a result of being frozen. Seasonal transitions of the landscape between predominantly frozen and thawed conditions are analogous to a biospheric and hydrological on/off switch, with marked differences in ET, vegetation productivity and other biological activity between largely dormant winter and active summer conditions. We investigated changes in freeze–thaw (FT) seasons and ET from 1983 to 2006 and their connections in the northern cryosphere by analyzing independent satellite remote sensing derived FT and ET records. Our findings show that the northern cryosphere (≥ 40 N) has experienced advancing (À2.5 days/decade; P = 0.005) and lengthening (3.5 days/decade; P = 0.007) non-frozen season trends over the 24-year period, coinciding with an upward trend (6.4 mm/year/decade; P = 0.014) in regional mean annual ET over the same period. Regional average timing of spring primary thaw and the annual non-frozen period are highly correlated with regional annual ET (|r| ≥ 0.75; P < 0.001), with corresponding impacts to annual ET of approximately 0.6 and 0.5% per day, respectively. The impact of primary fall freeze timing on ET is relatively minor compared with primary spring thaw timing. Earlier onset of the non-frozen season generally promotes annual ET in colder areas but appears to suppress summer ET by increasing drought stress in the southernmost parts of the domain where water supply is the leading constraint to ET. The cumulative effect of future freeze-thaw changes on ET in the region will largely depend on future changes of large-scale atmosphere circulations and rates of vegetation disturbance and adaptation to continued warming.
Article
Tundra ponds are a common type of wetland in the High Arctic. Their preservation is predicated upon ample water supply and storage to overcome evaporation losses. Two years of hydrological study of a cluster of ponds in a polar oasis of the Canadian Arctic showed the dominance of overland flow in the spring as an agent that recharged the pond storage. The freshet produced by snowmelt gave rise to extensive surface flow connections between the ponds and with their surrounding areas, but such flow connectivity lasted only about 2 weeks. After that, the ponds appeared to be separated from lateral drainage. Detailed mapping of the water and frost table positions together with water balance investigation, however, indicated the presence of subsurface flows between some ponds and with their adjacent slope. The flow magnitude was small relative to the vertical processes of evaporation and rainfall. Evaporation loss was mainly responsible for storage depletion, leading to a decline in pond level and shrinkage of open water area, unless major rain events restored the storage (as in 2006). It is postulated that climate warming could increase evaporation and active layer thickness to promote greater loss in surface water storage, or geomorphic processes could breach the pond rims, leading to the demise of ponds. Copyright © 2006 John Wiley & Sons, Ltd.
Article
The permafrost monitoring network in the polar regions of the Northern Hemisphere was enhanced during the International Polar Year (IPY), and new information on permafrost thermal state was collected for regions where there was little available. This augmented monitoring network is an important legacy of the IPY, as is the updated baseline of current permafrost conditions against which future changes may be measured. Within the Northern Hemisphere polar region, ground temperatures are currently being measured in about 575 boreholes in North America, the Nordic region and Russia. These show that in the discontinuous permafrost zone, permafrost temperatures fall within a narrow range, with the mean annual ground temperature (MAGT) at most sites being higher than −2°C. A greater range in MAGT is present within the continuous permafrost zone, from above −1°C at some locations to as low as −15°C. The latest results indicate that the permafrost warming which started two to three decades ago has generally continued into the IPY period. Warming rates are much smaller for permafrost already at temperatures close to 0°C compared with colder permafrost, especially for ice-rich permafrost where latent heat effects dominate the ground thermal regime. Colder permafrost sites are warming more rapidly. This improved knowledge about the permafrost thermal state and its dynamics is important for multidisciplinary polar research, but also for many of the 4 million people living in the Arctic. In particular, this knowledge is required for designing effective adaptation strategies for the local communities under warmer climatic conditions. Copyright © 2010 John Wiley & Sons, Ltd.
Article
This paper develops a three-step thaw model to assess the impact of predicted warming on an ice-rich polar desert landscape in the Canadian high Arctic. Air temperatures are established for two climate scenarios, showing mean annual increases of 4.9 and 6.5°C. This leads to a lengthening of the summer thaw season by up to 26days and increased thaw depths of 12–70cm, depending on the thermal properties of the soil. Subsidence of the ground surface is the primary landscape response to warming and is shown to be a function of the amount and type of ground ice in various cryostratigraphic units. In areas of pore ice and thin ice lenses with a low density of ice wedges, subsidence may be as much as 32cm. In areas with a high density of ice wedges, subsidence will be slightly higher at 34cm. Where massive ice is present, subsidence will be greater than 1m. Landscape response to new climate conditions can take up to 15years, and may be as long as 50years in certain cases.
Article
Spatial and temporal patterns of soil respiration rates and controlling factors were investigated in three wet arctic tundra systems. In situ summer season carbon dioxide fluxes were measured across a range of micro-topographic positions in tussock tundra, wet sedge tundra, and low-centre polygonal tundra, at two different latitudes on the Taimyr Peninsular, central Siberia. Measurements were carried out by means of a multi-channel gas exchange system operating in continuous-flow mode.Measured soil respiration rates ranged from 0.1 g CO2-C m−2 d−1 to 3.9 g CO2-C m−2 d−1 and rate differences between neighbouring sites in the micro-topography (microsites) were larger than those observed between different tundra systems. Statistical analysis identified position of the water table and soil temperature at shallow depths to be common controls of soil respiration rates across all microsites, with each of these two factors explaining high proportions of the observed variations.Modelling of the response of soil respiration to soil temperature and water table for individual microsites revealed systematic differences in the response to the controlling factors between wet and drier microsites. Wet microsites – with a water table position close to the soil surface during most of the summer – showed large soil respiration rate changes with fluctuations of the water table compared to drier microsites. Wet microsites also showed consistently higher temperature sensitivity and a steeper increase of temperature sensitivity with decreasing temperatures than drier sites. Overall, Q10 values ranged from 1.2 to 3.4. The concept of substrate availability for determining temperature sensitivity is applied to reconcile these systematic differences. The results highlight that soil respiration rates in wet tundra are foremost controlled by water table and only secondarily by soil temperature. Wet sites have a larger potential for changes in soil respiration rates under changing environmental conditions, compared to drier sites.It is concluded that understanding and forecasting gaseous carbon losses from arctic tundra soils and its implication for ecosystem-scale CO2 fluxes and soil organic matter dynamics require good knowledge about temporal and spatial patterns of soil water conditions. The water status of tundra soils can serve as a control on the temperature sensitivity of soil respiration.
Article
Leffingwell’s contraction-crack theory of ice-wedge polygons in permafrost has been examined from the point of view of mechanics. A nonlinear viscoelastic model of thermal stress in permafrost leads to results consistent with the theory within the limits of existing information on polygon dimensions, crack depths, temperature, and mechanical properties of ice and permafrost. Stresses that cause cracking are evidently generated not only by low temperature but also by rapid cooling. The size of the polygons can be explained in terms of the stress-perturbation due to a single crack and the distribution of mechanical flaws. The polygonal patterns can be classified according to whether or not the intersections are predominantly orthogonal. It is proposed that orthogonal polygons evolve by progressive subdivision, nonorthogonal ones by successive branching of cracks attaining high propagation velocities. Much of the discussion is general and applies directly to other types of contraction-crack polygons such as columnar basalt joints and mud cracks.
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The Ecology and Dynamics of Ice Wedge Degradation in High-Centre Polygonal Terrain in the Uplands of The Mackenzie Delta Region
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Steedman, A. E. The Ecology and Dynamics of Ice Wedge Degradation in High-Centre Polygonal Terrain in the Uplands of The Mackenzie Delta Region, Northwest Territories, Canada MS thesis, Univ. Victoria (2014).
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