C. R. Burn

Distribution and activity of ice wedges across the forest‐tundra transition, western Arctic Canada

Remote sensing, regional ground temperature and ground ice observations, and numerical simulation were used to investigate the size, distribution, and activity of ice wedges in fine-grained mineral and organic soils across the forest-tundra transition in uplands east of the Mackenzie Delta. In the northernmost dwarf-shrub tundra, ice wedge polygons cover up to 40% of the ground surface, with the wedges commonly exceeding 3 m in width. The largest ice wedges are in peatlands where thermal contraction cracking occurs more frequently than in nearby hummocky terrain with fine-grained soils. There are fewer ice wedges, rarely exceeding 2 m in width, in uplands to the south and none have been found in mineral soils of the tall-shrub tundra, although active ice wedges are found there throughout peatlands. In the spruce forest zone, small, relict ice wedges are restricted to peatlands. At tundra sites, winter temperatures at the top of permafrost are lower in organic than mineral soils because of the shallow permafrost table, occurrence of phase change at 0°C, and the relatively high thermal conductivity of icy peat. Due to these factors and the high coefficient of thermal contraction of frozen saturated peat, ice wedge cracking and growth is more common in peatlands than in mineral soil. However, the high latent heat content of saturated organic active layer soils may inhibit freezeback, particularly where thick snow accumulates, making the permafrost and the ice wedges in spruce forest polygonal peatlands susceptible to degradation following alteration of drainage or climate warming.

Surficial characteristics associated with the occurrence of permafrost near Mayo, Central Yukon Territory, Canada

Key criteria have been isolated for predicting the location of discontinuous permafrost in central Yukon Territory. Eighteen soil, vegetation, and topographic variables were documented at 60 sites in a 2500 km2 area, and analysed to establish their association with the occurrence of permafrost. The singular features most associated with permafrost were a thick surface organic layer and a hummocky microtopography. In combination, soil moisture content and organic-layer thickness were diagnostic of permafrost occurrence. The thickness of the organic layer, a horizon of low thermal diffusivity, governs attenuation of the summer temperature wave; soil moisture content supplies water for evapotranspiration, which reduces surface temperature. Variation in these indices accounted for 95% of variation in permafrost occurrence within the 60 sites. A diagnostic system to predict permafrost occurrence was constructed with these indices, and tested against field data from 60 different sites. The occurrence of permafrost in the second data set was predicted correctly at every site. The critical soil moisture content and organic-layer thickness were 25% (gravimetric) and 11 cm, repectively. Permafrost is present where these values are exceeded. The results indicate that the occurrence of permafrost in this portion of the discontinuous zone can be determined from relatively few variables.

The Mackenzie Delta: An Archetypal Permafrost Landscape

The Mackenzie Delta, in Canada’s western Arctic, is North America’s largest arctic delta. For over half the year the rivers and lakes of this vast alluvial plain are ice covered. Permafrost is ubiquitous in the delta and the surrounding landscape. Treeline traverses the delta, separating closed-canopy white spruce forests in southern parts from low shrub tundra and sedge wetlands at the Beaufort Sea coast. The extension of the delta northwards into the ocean is the net result of 128 Mt of sediment brought annually to the delta by Mackenzie and Peel Rivers, of which about two thirds are deposited offshore. The permafrost of the uplands adjacent to the delta is ice-rich, with numerous tabular bodies of almost pure ice that formed when the ground originally froze. Throughout the region the terrain surface is criss-crossed by networks of ice-wedge polygons, formed by water freezing in cracks opened by ground contraction during winter cooling. The world’s largest population of pingos – ice-cored, conical hills up to 50 m high – has developed in the sandy sediments of drained lakes in the area. These features form as permafrost aggrades in saturated lake sediments, and continual uplift of these little hills demonstrates the enormous forces that can be generated by ground freezing.

Composition and origin of a lithalsa related to lake-level recession and Holocene terrestrial emergence, Northwest Territories, Canada: Lithalsa composition and origin

Lithalsas of the Great Slave Lowland, Northwest Territories, occur within fine-grained glaciolacustrine, lacustrine, and alluvial deposits. Detailed investigations of a lithalsa revealed that it is composed of ice-rich sediments with ice lenses up to 0.2m thick below 4m depth. The observed ice accounted for about 2m of the 4m between the top of the lithalsa and adjacent terrain. The ice is isotopically similar to modern surface water, but enriched in δO relative to local precipitation. Total soluble cation concentrations are low in the basal, Shield-derived and unweathered glaciolacustrine sediments of the lithalsa. Higher concentrations in the overlying Holocene-aged lacustrine and alluvial deposits may be due to greater ion availability in Holocene surface waters. Increasing Cl and Na concentrations in clays at depth likely relate to exclusion and migration of these dissolved ions in pore water during ice lens formation though total soluble cations remain comparatively low. The lithalsa developed 700 to 300 cal yr BP. A conceptual model of lithalsa formation and landscape evolution illustrates that this feature and more than 1800 other lithalsas in the region have developed in association with Holocene terrestrial emergence following lake-level recession. Copyright

Old Crow Flats: Thermokarst Lakes in the Forest–Tundra Transition

Old Crow Flats, in northern Yukon, is a 5600 km2 Arctic wetland surrounded by mountains. It contains thousands of thermokarst lakes. The area was not glaciated during the Wisconsinan but was submerged by a glacial lake that drained catastrophically 15,000 years ago. Today the glacilacustrine plain is underlain by continuous permafrost and is within the forest–tundra ecotone of northern Yukon. Lakes cover approximately 35 % of the plain area. Many lakes have rectilinear shores and are oriented northeast–southwest or northwest–southeast where tundra vegetation dominates the ground cover. Where taiga and tall shrubs dominate the vegetation cover, lakeshores tend to be irregular. Drained lake basins are abundant in the Flats. Overlapping basins indicate that several generations of thermokarst lakes have formed and drained over the last 15,000 years. In tundra areas, drained basins generally have wet, depressed margins surrounding a slightly elevated centre. Ice wedge polygons are ubiquitous in the tundra and are often strikingly orthogonal near lakes and drained basins. The Flats are incised by Porcupine and Old Crow rivers which meander 20–50 m below the plain level. Effects of climatic warming on the Flats may threaten the traditional activities and food security of the Vuntut Gwich’in.

J. Ross Mackay, 1915–2014

John Ross Mackay, President of the Association of American Geographers in 1969–1970, died peacefully on 28 October 2014. With his passing, the geographical and permafrost research communities lost one of their foremost scientists. Ross was an exemplary field observer, an insightful theoretician, a prodigious author, and an esteemed mentor.