High-centered Polygons Research Articles

Active-Layer Thickness Measurements Using a Handheld Penetrometer at Boreal and Tundra Sites in Eastern Siberia

Active-layer thickness (ALT) is one of the most robust measures used to assess the impact of climate change on terrestrial permafrost. Testing of a handheld dynamic cone penetrometer showed that it was capable of measuring ALT with the same level of accuracy as conventional methods in boreal and tundra sites in eastern Siberia. The penetrometer also characterised the vertical structure of ground hardness within the active layer. The vertical profile of penetrometer measurements corresponded closely with soil plasticity and the liquid limit in high-centred polygons produced by thermokarst subsidence in dry grassland areas at a boreal site at Churapcha. The ALT was markedly deeper (>70 cm) at gravelly slope points adjacent to a wet tundra plain (<50 cm) in a CALM grid (R8) at Tiksi. Overall, the penetrometer is considered to provide an accurate and informative proxy for rapidly assessing the spatial heterogeneity and interannual changes in ALT. Copyright © 2016 John Wiley & Sons, Ltd.

Isotopic insights into methane production, oxidation, and emissions in Arctic polygon tundra.

Arctic wetlands are currently net sources of atmospheric CH4 . Due to their complex biogeochemical controls and high spatial and temporal variability, current net CH4 emissions and gross CH4 processes have been difficult to quantify, and their predicted responses to climate change remain uncertain. We investigated CH4 production, oxidation, and surface emissions in Arctic polygon tundra, across a wet-to-dry permafrost degradation gradient from low-centered (intact) to flat- and high-centered (degraded) polygons. From 3 microtopographic positions (polygon centers, rims, and troughs) along the permafrost degradation gradient, we measured surface CH4 and CO2 fluxes, concentrations and stable isotope compositions of CH4 and DIC at three depths in the soil, and soil moisture and temperature. More degraded sites had lower CH4 emissions, a different primary methanogenic pathway, and greater CH4 oxidation than did intact permafrost sites, to a greater degree than soil moisture or temperature could explain. Surface CH4 flux decreased from 64nmolm(-2) s(-1) in intact polygons to 7nmolm(-2) s(-1) in degraded polygons, and stable isotope signatures of CH4 and DIC showed that acetate cleavage dominated CH4 production in low-centered polygons, while CO2 reduction was the primary pathway in degraded polygons. We see evidence that differences in water flow and vegetation between intact and degraded polygons contributed to these observations. In contrast to many previous studies, these findings document a mechanism whereby permafrost degradation can lead to local decreases in tundra CH4 emissions.

Vegetation composition and shrub extent on the Yukon coast, Canada, are strongly linked to ice-wedge polygon degradation

Changing environmental and geomorphological conditions are resulting in vegetation change in ice-wedge polygons in Arctic tundra. However, we do not yet know how microscale vegetation patterns relate to individual environmental and geomorphological parameters. This work aims at examining these relations in polygonal terrain. We analysed composition and cover of vascular plant taxa and surface height, active layer depth, soil temperature, carbon and nitrogen content, pH and electrical conductivity in four polygon mires located on the Yukon coast. We found that vascular plant species composition and cover correlates best with relative surface height. Ridges of low-centred polygons and raised centres of high-centred polygons support the growth of mesic and wetland species (e.g., Betula glandulosa , Salix pulchra , S. reticulata , Rubus chamaemorus , various ericaceous dwarf shrubs, Eriophorum vaginatum , Poa arctica ). Wetland and aquatic plant species (e.g., E. angustifolium , Carex aquatilis , C. chordorrhiza , Pedicularis sudetica ) grow in low-lying centres of polygons and in troughs between polygons. We also found a relationship between vascular plant species composition and substrate characteristics such as pH, electrical conductivity and total organic carbon, although the individual influence of these parameters could not be determined because of their correlation with relative surface height. Our findings stress the regulatory role of microtopography and substrate in vegetation dynamics of polygonal terrain. Ongoing warming in this region will lead to changes to polygonal terrain through permafrost degradation and subsequent conversion of low-centred into high-centred polygons. Our results indicate that shrubs, particularly Betula glandulosa and heath species, have the potential to expand most. Keywords: Ice-wedge polygon mires; western Canadian Arctic; modern vegetation; microtopography; permafrost. (Published: 17 June 2016) To access the supplementary material for this article, please see the supplementary files in the column to the right (under Article Tools). Citation: Polar Research 2016, 35 , 27489, http://dx.doi.org/10.3402/polar.v35.27489

Geochemical drivers of organic matter decomposition in arctic tundra soils

Climate change is warming tundra ecosystems in the Arctic, resulting in the decomposition of previously-frozen soil organic matter (SOM) and release of carbon (C) to the atmosphere; however, the processes that control SOM decomposition and C emissions remain highly uncertain. In this study, we evaluate geochemical factors that influence microbial production of carbon dioxide (CO2) and methane (CH4) in the seasonally-thawed active layer of interstitial polygonal tundra near Barrow, Alaska. We report spatial and seasonal patterns of dissolved gases in relation to the geochemical properties of Fe and organic C in soil and soil solution, as determined using spectroscopic and chromatographic techniques. The chemical composition of soil water collected during the annual thaw season varied significantly with depth. Soil water in the middle of the active layer contained abundant Fe(III), and aromatic-C and low-molecular-weight organic acids derived from SOM decomposition. At these depths, CH4 was positively correlated with the ratio of Fe(III) to total Fe in waterlogged transitional and low-centered polygons but negatively correlated in the drier flat- and high-centered polygons. These observations contradict the expectation that CH4 would be uniformly low where Fe(III) was high due to inhibition of methanogenesis by Fe(III)-reduction reactions. Our results suggest that vertically-stratified Fe redox reactions influence respiration/fermentation of SOM and production of substrates (e.g., low-molecular-weight organic acids) for methanogenesis, but that these effects vary with soil moisture. We infer that geochemical differences induced by water saturation dictate microbial products of SOM decomposition, and Fe geochemistry is an important factor regulating methanogenesis in anoxic tundra soils.

Polygonal tundra geomorphological change in response to warming alters future CO2 and CH4 flux on the Barrow Peninsula.

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: (i) landscape succession associated with the thaw-lake cycle; and (ii) low, moderate, and extreme scenarios of thermokarst pit formation (10%, 30%, and 50%) reported for Alaskan arctic tundra sites. We developed a 30 × 30 m 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 (i) at present during peak growing season on the Barrow Peninsula, CO2 uptake occurs at -902.3 10(6) gC-CO2 day(-1) (uncertainty using 95% CI is between -438.3 and -1366 10(6) gC-CO2 day(-1)) and CH4 flux at 28.9 10(6) gC-CH4 day(-1) (uncertainty using 95% CI is between 12.9 and 44.9 10(6) gC-CH4 day(-1)), (ii) one century of future landscape change associated with the thaw-lake cycle only slightly alter CO2 and CH4 exchange, while (iii) moderate increases in thermokarst pits would strengthen both CO2 uptake (-166.9 10(6) gC-CO2 day(-1)) and CH4 flux (2.8 10(6) gC-CH4 day(-1)) with geomorphic change from low to high center polygons, cumulatively resulting in an estimated negative feedback to warming during peak growing season.

Modeling challenges for predicting hydrologic response to degrading permafrost

The fate of the approximately 1,700 billion metric tons of carbon (Tarnocai et al. 2009) currently frozen in permafrost affected regions of the Arctic and subarctic is highly uncertain (IPCC 2007), primarily because of the potential for topographic evolution and resulting drainage network reorganization as permafrost degrades and massive ground ice contained in ice-rich permafrost soils melts. Computer modeling is a key tool in untangling these complex feedbacks to understand the evolution of the Arctic and subarctic landscapes and the potential feedbacks with the global climate system. Some of the challenges associated with modeling the hydrologic system in and around degrading permafrost are discussed in this essay. Modeling requirements depend very strongly on the spatial resolution of the model. Two different classes can be identified, depending on whether microtopography is explicitly resolved or incorporated into the model through a subgrid parameterization. The focus here is on the computational challenges associated with microtopography-resolving models using hydrologic response of polygon mires as an example. In such microtopography-resolving models, horizontal grid spacing on the order of 0.25mwould typically be required. Although highand low-centered ice wedge polygons have been identified as important controls on Arctic surface hydrology (e.g. Liljedahl et al. 2012) and evolution from lowto highcentered polygon landscapes is expected as Arctic temperatures increase (Jorgenson et al. 2006), as far as we are aware, there is no existing computer code that represents the full range of processes required to model the co-evolution of surface topography, active layer, and permafrost at the microtopography-resolving scale. For microtopography-resolving models of hydrology in permafrost landscapes, it is convenient to partition the large number of coupled processes into four critical sets: subsurface thermal/hydrology, surface thermal processes, mechanical deformation, and overland flow processes. However, it is important to recognize that the partitioning is somewhat arbitrary and that multiple tightly coupled processes exist within each set.

Microbial biomass and community structure changes along a soil development chronosequence near Lake Wellman, southern Victoria Land

AbstractFour pedons on each of four drift sheets in the Lake Wellman area of the Darwin Mountains were sampled for chemical and microbial analyses. The four drifts, Hatherton, Britannia, Danum, and Isca, ranged from early Holocene (10 ka) to mid-Quaternary (c. 900 ka). The soil properties of weathering stage, salt stage, and depths of staining, visible salts, ghosts, and coherence increase with drift age. The landforms contain primarily high-centred polygons with windblown snow in the troughs. The soils are dominantly complexes of Typic Haplorthels and Typic Haploturbels. The soils were dry and alkaline with low levels of organic carbon, nitrogen and phosphorus. Electrical conductivity was high accompanied by high levels of water soluble anions and cations (especially calcium and sulphate in older soils). Soil microbial biomass, measured as phospholipid fatty acids, and numbers of culturable heterotrophic microbes, were low, with highest levels detected in less developed soils from the Hatherton drift. The microbial community structure of the Hatherton soil also differed from that of the Britannia, Danum and Isca soils. Ordination revealed the soil microbial community structure was influenced by soil development and organic carbon.

Polygon pattern geomorphometry on Svalbard (Norway) and western Utopia Planitia (Mars) using high-resolution stereo remote-sensing data

Polygonal systems formed by thermal contraction cracking are complex landscape features widespread in terrestrial periglacial regions. The manner in which cracking occurs is controlled by various environmental factors and determines dimension, shape, and orientation of polygons. Analogous small-scale features are ubiquitous in Martian mid- and high-latitudes, and they are also inferred to originate from thermal contraction cracking. We studied the geomorphometry of polygonally-patterned ground on Svalbard to draw a terrestrial analogy to small-scale polygonal structures in scalloped terrain in Martian mid-latitudes. We performed a comparative quantitative terrain analysis based on high-resolution stereo remote-sensing data (HRSC-AX and HiRISE) in combination with terrestrial field data and multivariate statistics to determine the relationship of polygon geomorphometry to local environmental conditions. Results show that polygonal structures on Svalbard and in Utopia Planitia on Mars are similar with respect to their size and shape. A comparable thermal contraction cracking genesis is likely. Polygon evolution, however, is strongly related to regional and local landscape dynamics. Individual polygon dimensions and orthogonality vary according to age, thermal contraction cracking activity, and local subsurface conditions. Based on these findings, the effects of specific past and current environmental conditions on polygon formation on Mars must be considered. On both Earth and Mars, the smallest polygons represent young, recently-active low-centered polygons that formed in fine-grained ice-rich material. Small, low-centered Martian polygons show the closest analogy to terrestrial low-centered ice-wedge polygons. The formation of composite wedges could have occurred as a result of local geomorphological conditions during past Martian orbital configurations. Larger polygons reflect past climate conditions on both Earth and Mars. The present degradation of these polygons depends on relief and topographical situation. On Svalbard the thawing of ice wedges degrades high-centered polygons; in contrast, the present appearance of polygons in Utopia Planitia is primarily the result of contemporary dry degradation processes.

Modeling vapor diffusion within cold and dry supraglacial tills of Antarctica: Implications for the preservation of ancient ice

We modeled water–vapor diffusion within Mullins till, a relatively dry supraglacial till in southern Victoria Land, Antarctica, that rests directly on Mullins Glacier, purportedly one of the oldest alpine glaciers in the world. Like most supraglacial tills in cold-desert environments, Mullins till contains three characteristic facies: a weathered facies representing the oxidation of iron-bearing minerals and the physical disintegration of surface rocks; a sand-wedge facies representing the episodic infill of thermal cracks associated with contraction-crack polygons; and an underlying fresh facies representing the addition of englacial debris (sourced from rockfall) as overlying ice sublimes. Using a one-dimensional model for Fickian vapor diffusion through porous media, we show that the rate of subsurface ice sublimation varies by ~ 5.5% beneath till facies and that over timescales of 10 5 years diffusion through “porous” sand wedges contributes to the development of deep troughs surrounding high-centered polygons. Applying site-specific meteorological data collected over a four-year period, we show that ice loss at the stagnant terminus of Mullins Glacier is ~ 6.6 × 10 −5 m a −1, a value that (although low and assuming an ice thickness of ~ 150 m) is consistent with complete ice loss under current environmental forcing in ~ 2.5 Ma. Our sensitivity tests indicate that the vast majority of sublimation occurs during the summer months. Calculated summertime losses drop to zero with either a reduction in soil and ice surface temperatures of ~ 6.4 °C or an increase in atmospheric relative humidity from 44% to 75%, both of which could arise from an increase in cloud cover over Mullins Glacier. Sublimation responses to meteorological forcing are not uniform across Mullins Glacier. A summer increase in soil temperature of 2 °C results in negligible change in ice sublimation at Mullins terminus, but a 27% increase in ice loss in upper Mullins Valley. The key factor is the thickness of Mullins till, which is greater near the glacier terminus. For till thicknesses exceeding ~ 25 cm, non-linear variations in soil temperature result in downward vapor fluxes, capable of producing thin, cm-scale lenses of secondary pore that cap the surface of buried glacier ice. This downward vapor flow, sourced from modern snowfall and/or elevated atmospheric relative humidity, is one of the key factors that enable long-term preservation of buried glacier ice. Overall, our results highlight the subtle relations among changes in till texture, till thickness, and meteorological forcing on the rate of subsurface ice loss and provide insight into the plausible range of conditions under which multi-million-year-old ice can exist beneath thin supraglacial tills, <~50-cm thick, in southern Victoria Land, Antarctica.

Paleoecological Evidence for Transitions between Contrasting Landforms in a Polygon-Patterned High Arctic Wetland

The formation of many arctic wetlands is associated with the occurrence of polygon-patterned permafrost. Existing scenarios to describe and explain surface landforms in arctic wetlands (low-center and high-center polygons and polygon ponds) invoke competing hypotheses: a cyclic succession (the thaw-lake hypothesis) or a linear succession (terrestrialization). Both hypotheses infer the predictable development of polygon-patterned wetlands over millennia. However, very few studies have applied paleoecological techniques to reconstruct long-term succession in tundra wetlands and thereby test the validity of existing hypotheses. This paper uses the paleoecological record of diatoms to investigate long-term development of individual polygons in a High Arctic wetland. Two landform processes were examined: (1) the millennial-scale development of a polygon-pond, and (2) the transition from low-center to erosive high-center polygons. Diatom assemblages were quantified from habitats associated with contrasting landforms in the present-day landscape, and used as an analog to reconstruct past transitions between polygon types. On the basis of this evidence, the paleoecological record does not support either of the existing models describing the predictable succession of polygon landforms in an arctic wetland. Our results indicate a need for greater paleoecological understanding, in combination with in situ observations in present-day geomorphology, in order to identify patterns of polygon wetland development and elucidate the long-term drivers of these landform transitions.

The Relationship Between Soil Moisture and NDVI Near Barrow, Alaska

Ice-wedge polygons and drained thaw lake basins cover large portions of the Barrow Peninsula of Alaska. These landforms lead to variations in topography in the relatively flat coastal plain. These variations in topography, while small, lead to large variations in soil moisture, which have a substantial impact on carbon, water, and energy fluxes in these systems. In this study, we hypothesize that the spatial patterns of soil moisture control variations in vegetation production, and hence remotely sensed greenness. We investigate the relationship between soil moisture and a remotely sensed measure of vegetation greenness, the normalized difference vegetation index (NDVI). Soil moisture was measured in the summer of 2000 in the fetch of an eddy flux tower (0.5 km2 footprint), and NDVI data were collected using an aerial digital multispectral camera system. Results indicate that the relationship between soil moisture and NDVI was significant in areas dominated by high-centered polygons and troughs, and where the general slope of the terrain promoted the redistribution of soil moisture. However, in areas with little to no relief, the correlation between soil moisture and NDVI was not significant.

Spatial distribution of near surface soil moisture and its relationship to microtopography in the Alaskan Arctic coastal plain

The Arctic coastal plain of Alaska is characterized by marked heterogeneity in microtopography and above ground vegetation productivity at a variety of scales. This heterogeneity may be expected to lead to large variations in near surface soil moisture and have a substantial impact on measured and modeled fluxes of carbon and water. In this study, we hypothesized that microtopography was the primary control over the spatial patterns of near surface soil moisture. Near surface soil moisture measurements were collected in the summers of 2000, 2001, 2002 and 2003 in the fetch of an eddy flux tower (0.5 km2). Results confirmed the expected relationship between intra- and inter-seasonal variations in near surface soil moisture and variations in precipitation. However, over two time periods, near surface soil moisture increased without corresponding measured precipitation inputs and this was attributed to fog and dew, which are difficult to measure, and/or the melting of the active layer. Spatial variations in near surface soil moisture are largely controlled by microtopography in areas characterized by high centered polygons and troughs. In areas without large variations in microtopography, macrotopography, in the form of drained thaw lakes, has a substantial control over near surface soil moisture.

Ground ice in the Northern Foothills, northern Victoria Land, Antarctica

AbstractThis progress report classifies the different types of ground-ice bodies that occur in the Northern Foothills, northern Victoria Land, Antarctica. Oxygen isotope variations are presented, but interpretation is kept to a minimum pending further investigations. Surface ice, as distinct from moving glacier ice, occurs in the form of widespread buried (‘dead’) glacier ice lying beneath ablation (sublimation) till, together with perennial lake ice, snow banks and icing-blister ice.’Dry’ permafrost is uncommon, and interstitial ice is usually present at the base of the active layer and in the near-surface permafrost. This probably reflects the supply of moisture from the Ross Sea and limited sublimation under today’s climate. Intrusive ice occurs as layers within perennial lake-ice covers and gives rise to small icing blisters. Small ice wedges found beneath the furrows of high-centered polygons appear to agree with the model of sublimation-till development proposed by Marchant and others (2002).

Formation of patterned ground and sublimation till over Miocene glacier ice in Beacon Valley, southern Victoria Land, Antarctica

A thin glacial diamicton, informally termed Granite drift, occupies the floor of central Beacon Valley in southern Victoria Land, Antarctica. This drift is 40 Ar/ 39 Ar analyses of presumed in situ ash-fall deposits that occur within Granite drift. At odds with the great age of this ice are high-centered polygons that cut Granite drift. If polygon development has reworked and retransported ash-fall deposits, then they are untenable as chronostratigraphic markers and cannot be used to place a minimum age on the underlying glacier ice. Our results show that the surface of Granite drift is stable at polygon centers and that enclosed ash-fall deposits can be used to define the age of underlying glacier ice. In our model for patterned-ground development, active regions lie only above polygon troughs, where enhanced sublimation of underlying ice outlines high-centered polygons. The rate of sublimation is influenced by the development of porous gravel-and-cobble lag deposits that form above thermal-contraction cracks in the underlying ice. A negative feedback associated with the development of secondary-ice lenses at the base of polygon troughs prevents runaway ice loss. Secondary-ice lenses contrast markedly with glacial ice by lying on a δD versus δ 18 O slope of 5 rather than a precipitation slope of 8 and by possessing a strongly negative deuterium excess. The latter indicates that secondary-ice lenses likely formed by melting, downward percolation, and subsequent refreezing of snow trapped preferentially in deep polygon troughs. The internal stratigraphy of Granite drift is related to the formation of surface polygons and surrounding troughs. The drift is composed of two facies: A nonweathered, matrix-supported diamicton that contains >25% striated clasts in the >16 mm fraction and a weathered, clast-supported diamicton with varnished and wind-faceted gravels and cobbles. The weathered facies is a coarse-grained lag of Granite drift that occurs at the base of polygon troughs and in lenses within the nonweathered facies. The concentration of cosmogenic 3 He in dolerite cobbles from two profiles through the nonweathered drift facies exhibits steadily decreasing values and shows the drift to have formed by sublimation of underlying ice. These profile patterns and the 3 He surface-exposure ages of 1.18 ± 0.08 Ma and 0.18 ± 0.01 Ma atop these profiles indicate that churning of clasts by cryoturbation has not occurred at these sites in at least the past 10 5 and 10 6 yr. Although Granite drift is stable at polygon centers, low-frequency slump events occur at the margin of active polygons. Slumping, together with weathering of surface clasts, creates the large range of cosmogenic-nuclide surface-exposure ages observed for Granite drift. Maximum rates of sublimation near active thermal-contraction cracks, calculated by using the two 3 He depth profiles, range from 5 m/m.y. to 90 m/m.y. Sublimation rates are likely highest immediately following major slump events and decrease thereafter to values well below our maximum estimates. Nevertheless, these rates are orders of magnitude lower than those computed on theoretical grounds. During eruptions of the nearby McMurdo Group volcanic centers, ash-fall debris collects at the surface of Granite drift, either in open thermal-contraction cracks or in deep troughs that lie above contraction cracks; these deposits subsequently lower passively as the underlying glacier ice sublimes. The fact that some regions of Granite drift have escaped modification by patterned ground for at least 8.1 Ma indicates long-term geomorphic stability of individual polygons. Once established, polygon toughs likely persist for as long as 10 5 –10 6 yr. Our model of patterned-ground formation, which applies to the hyperarid, cold-desert, polar climate of Antarctica, may also apply to similar-sized polygons on Mars that occur over buried ice in Utopia Planitia.

Holocene Climate Effects on the Development of a Peatland on the Tuktoyaktuk Peninsula, Northwest Territories

Six cores were collected from a small peatland on the Tuktoyaktuk Peninsula, yielding up to 1.8 m of organic material and 1.2 m of ice-rich material. Stratigraphic relationships, radiocarbon dating, and pollen, plant macrofossil, and isotopic analyses are used to reconstruct the developmental history of the peatland. Organic matter began to accumulate in the basin about 7200 yr B.P., during a period when the climate is thought to have been warmer than present. Initially the permafrost table was probably below the bottom of the basin, and an open-water mineral wetland with emergent and submergent aquatic vegetation occupied the site. Enriched δ18O values from ice samples confirm the existence of an open water body with a water balance strongly affected by evaporation. Transformation to a graminoid fen peatland was underway by 6300 yr B.P. Permafrost and ice began to form in the peatland at this time. Low-centered polygons probably began to develop by 4700 yr B.P. and subsequently evolved into the high-centered polygons which characterise the southwestern part of the peatland today. Peat accumulation in the polygons has been extremely slow for the last 4000 years. The eastern, wetter part of the basin has largely remained unchanged since its inception about 7000 yr B.P. Development of the peatland may be linked to permafrost formation that was controlled by regional climate changes during the Holocene.

Airphoto Analysis of Winter Seismic Disturbance in Northeastern Alaska

Airphoto interpretation was used to quantify the extent of disturbance caused by seismic exploration on the 60,000 ha coastal plain of the Arctic National Wildlife Refuge during the winters of 1984 and 1985. The relationships of vegetation type, trail location and traffic pattern to the amount of disturbance were investigated. Approximately 20% of the seismic trails were photographed at 1:6000 scale, using color infrared film. Ground data collected at 194 sites were used to develop a photo interpretation key describing the photo signatures of seven vegetation types and four disturbance levels. Vegetation types and disturbance levels were determined for 4914 circles of 3 mm diameter on the aerial photos (18 m ground distance). Fourteen percent of the points were interpreted as having no disturbance (level 0), 57% had level 1 disturbance (low), 27% had level 2 (medium) and 2% had level 3 (high). Wet or partially vegetated areas were the least susceptible to disturbance. Vegetation types with mounds, tussocks, hummocks or high-centered polygons and dryas terraces were more heavily disturbed. Camp move trails and overlapping seismic and camp move trails created in 1984 caused more disturbance than other trail types due to multiple passes of vehicles over narrow trails. U.S. Fish and Wildlife Service monitors were more successful at minimizing disturbance the second year by requesting that vehicle operators avoid multiple passes on the same trail, sensitive vegetation types and areas of low snow cover.Key words: airphoto analysis, winter seismic exploration, seismic trails, vegetation disturbance, traffic patterns, Alaska, Arctic National Wildlife Refuge, arctic coastal plain

Some Typical Landforms of Permafrost at Yakutia (Siberia), Barrow (Alaska) and Mackenzie Delta (Canada)

Scientific expeditions in permafrost regions were carried out at Yakutia, Siberia in the middle of August, 1972 and at Barrow, Alaska and Tuktoyaktuk, Mackenzie Delta in the summers of 1974 and 1977.I Yakutia, SiberiaField observations were conducted on permafrost around Lake Surdakh, which is located 250km northeast from Yakutsk, Siberia. Lake Surdakh is an alas, which is a depression formed by a thermokarst development in which local deep thawing has taken place in permafrost masses accompanied by the melting of ground ice. The shape of the alas is nearly round and the diameter of Lake Surdakh is about 4km. The lake is surrounded from all sides by a larch forest. The height of the basin cliffs is about 20m. At some places along the cliffs underground ices are exposed. The top of the ices is 1.5m below the top of the cliffs and the ices sink into the ground in a wedge-like shape. Observations were made as to the exposed ice, the soil around the exposed ice and the forest above the ground, and also on the some alases in the vicinity.II Barrow and TukutoyaktukField observations were conducted on high center polygons, pingos, ice wedges and exposed massive ground ice, all of which showed characteristic land feature of the tundra area composed of permafrost.a. High center polygonSpreading gregariously, high center polygons formed an orthogonal net pattern near Footprint Creek, 3km southwest of NARL, Barrow. Each mound was 5-10m in width and 50-100cm in height. A crack ran along the trough between two neighboring mounds. The surface at the raised center of a mound was covered with vegetations including willow, birch and crowberry. Core samples showed the structures of layers; namely, the upper layer of 5-10cm in thickness was composed of mixture of roots, organic material, and silt, while the layer underneath was composed almost of ice-richpeat, which had such an extremely high water content as amounts to 1000-2000%, the ice being uniformly contained not in the form of ice lenses. Meanwhile, as for the trough, its surface was covered with gramineae or formed puddles, while the layer above the depth of 40-50cm was composed of silty soil and contained distinct ice lenses, whereas the layer below this depth was composed of ice, which was polycrystalline with ice crystals several cm in length and almost stretching vertically. Polygons studied by the authors exemplified the type of ice-wedge polygons.

A new Clitocybe from the Alaskan Arctic

A new species of Clitocybe is described from the Alaskan Arctic. This agaric grows on low-center polygon rims, high-center polygons, and on beach ridges. Drawings of microscopic features and macrophotographs are included.

Nitrogen Fixation in Arctic Coastal Tundra in Relation to Vegetation and Micro-Relief

A study to estimate nitrogen fixation input in arctic coastal tundra was carried out using the acetylene reduction assay. Areal estimation was attempted by high intensity sampling over a limited area of tundra containing both high-centred and low-centred polygons with their corresponding variations in micro-vegetation. The highest average rates of acetylene reduction were obtained from cores in damp interpolygonal troughs (10.50 µmoles ethylene/m²-hr) where mats of the blue-green alga Nostoc were abundant. Wet moss-algal associations in hydric meadows showed high nitrogenase activity (average 6.86 µmoles ethylene/m²-hr) and dry high-centred polygons were comparatively inactive (2.80 µmoles ethylene/m²-hr). The lichens Peltigera sp. and Stereocaulon sp. were the most active nitrogen fixers in the drier tundra. Nitrogen fixation increased with rising temperature with a measured Q10 for Nostoc commune of 3.7.