This study compares the relations between solar radiation and air and ground temperatures in the Quartermain Mountains of the McMurdo Dry Valleys of Antarctica with those in ice‐free Victoria Land and Arctic Canada. The surface offset is near 0°C at all sites in the Quartermain Mountains and other sites in coastal Victoria Land, whereas the thermal offset is near 0°C at shallow ice table depths (< 20 cm) and near 1°C for ice tables deeper than the depth of diurnal temperature variation. The surface and thermal offsets in Victoria Land differ markedly from those in Arctic Canada, which are generally characterised by a positive surface offset and a negative thermal offset. These important differences highlight the effects of a lack of vegetation, surface organic layer, snow cover and moisture content in near‐surface soils on the direction and magnitude of surface and thermal offsets. Summer ground surface temperatures in the Quartermain Mountains correlate strongly with incoming solar radiation. Based on measured ground surface temperatures and modelled potential incoming solar radiation, two zones with distinct ground surface temperatures are defined in the Quartermain Mountains: (i) perennially cryotic zones (PCZs) characterised by ground surface temperatures always below 0°C; and (ii) seasonally non‐cryotic zones (NCZs) characterised by ground surface temperatures > 0°C for at least a few hours. Soils in the PCZs experience water exchange through vapour diffusion, whereas soils in the NCZs contain features associated with liquid water activity, such as increased soil moisture and frozen ponds recharged by snow/glacier meltwater. Copyright © 2015 John Wiley & Sons, Ltd.

Ground ice is one of the most important and dynamic geologic components of permafrost; however, few studies have investigated the distribution and origin of ground ice in the McMurdo Dry Valleys of Antarctica. In this study, ice-bearing permafrost cores were collected from 18 sites in University Valley, a small hanging glacial valley in the Quartermain Mountains. Ground ice was found to be ubiquitous in the upper 2 m of permafrost soils, with excess ice contents reaching 93%, but ground ice conditions were not homogeneous. Ground ice content was variable within polygons and along the valley floor, decreasing in the centres of polygons and increasing in the shoulders of polygons towards the mouth of the valley. Ground ice also had different origins: vapour deposition, freezing of partially evaporated snow meltwater and buried glacier ice. The variability in the distribution and origin of ground ice can be attributed to ground surface temperature and moisture conditions, which separate the valley into distinct zones. Ground ice of vapour-deposition origin was predominantly situated in perennially cryotic zones, whereas ground ice formed by the freezing of evaporated snow meltwater was predominantly found in seasonally non-cryotic zones.

Increased upright vegetation growth (i.e. trees and shrubs) in northern environments can profoundly impact ground surface thermal conditions through winter warming (e.g. enhanced snow trapping) and summer cooling (e.g. increased shading). The debate over these opposite effects emphasizes the need to better constrain net temperature impacts of upright vegetation on soils in northern environments. We generate a series of simulations with a widely-used permafrost model to partition the absolute warming and cooling impacts of upright vegetation on ground surface temperatures for a variety of shading scenarios, climates and surficial materials types (i.e. bedrock, mineral and organic soils). These scenarios simulate annual temperature differences between the air and ground surface caused by upright vegetation to provide likely ranges for the net effects induced by vegetation. These simulations showed that ground surface temperature warming in the winter mostly overwhelmed ground surface cooling in the thawing season even when simulations included extreme shading effects. Constraining the simulations to current best estimates of the possible summer cooling impact of vegetation yielded a dominant winter warming signal for most snow depths and climate types. Differences in the magnitude of air-surface temperature offsets between sites underlain by bedrock, mineral and organic soil highlights the importance of considering differences in unfrozen moisture content in areas where the ground freezes and thaws seasonally. The results of this study suggest that the net ground surface temperature impacts of increased snow trapping by vegetation will far exceed cooling caused by enhanced shading following increases in tall vegetation in most northern environments.

Abstract. The cryostratigraphy of permafrost in ultraxerous environments is poorly known. In

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