Subglacial Permafrost Research Articles

Subglacial deformation of permafrost beneath Wright Lower Glacier, Antarctica as revealed by direct observations and measurements from a subglacial tunnel

Subglacial processes influence the large-scale behaviour of glaciers and determine the geological processes that occur at the ice-substrate interface. In this paper we focus on subglacial deformation beneath a cold-based glacier resting on permafrost. We ask what are the drivers of subglacial permafrost deformation, how is strain distributed beneath glaciers with a permafrost substrate, how is strain transmitted from the glacier into the substrate, and how is deformed permafrost incorporated into the basal zone of the glacier? To address these questions a tunnel excavated in the margin of Wright Lower Glacier in the McMurdo Dry Valleys, Antarctica to observe the structure of the basal zone and bed of the glacier and to monitor basal ice deformation over a period of 4 years. The tunnel revealed a stacked sequence of stratified debris-bearing and clean ice layers, and thick rafts of frozen sediment. Isotopic and gas analyses reveal that despite the low local temperature of the basal ice (−18.5 °C), liquid water has played an important role in the formation of this basal sequence. The rafts of frozen sediment contain well preserved sedimentary structures and algae layers that show the material has been entrained from the underlying permafrost. Deformation measurements made with a combination of displacement transducers, strain arrays, and plumb-lines show that strain and motion within the basal zone are heterogeneous and that strain is transmitted through the permafrozen sediment and into intra-sediment ice layers. The distribution of strain in the basal zone produces a compound velocity profile resembling the deformation patterns inferred from deformation structures in Pleistocene permafrost and also resembles profiles associated with subglacial sediment deformation driven by hydrogeological processes. We conclude that ice-rich permafrost is readily deformed by glaciers and that there is no clear boundary between basal ice and deforming permafrost.

Climatic assessment of circum-Arctic permafrost zonation over the last 122 kyr

Permafrost is an important climate factor of the cold-region environments, known to evolve over a long timescale. Although direct observations are required for the exact determination of presence, this is difficult especially in remote areas. An indirect assessment of permafrost zonation can help in estimating the probable extents and providing an understanding of its spatiotemporal variability under different climate. In this paper, we adopt a previously defined indirect assessment method using freezing and thawing indices, which was revised and evaluated against new observation-based dataset. The revised method was applied to time-series of the circum-Arctic environment for the past 122 kyr constructed from a Greenland ice core, present-day reanalysis climatology, and a glacial isostatic adjustment model, and subsequently, examined in terms of the aggregation, persistence, and retreat of permafrost. Permafrost zones over exposed land varied between 9.11 million km2 (interglacial) and 26.7 million km2 (glacial); the maximum extent of subsea and subglacial permafrost was 7.7 million km2 and 0.5 million km2, respectively. The climatic sensitivity of its areal extent was examined for the meridional temperature gradient in the polar amplification (increase by 0.2 million km2 [°C/10°]−1 when warming occurs) and the global mean annual temperature (decrease by 4.3 million km2°C−1).

Subglacial permafrost dynamics and erosion inside subglacial channels driven by surface events in Svalbard

Abstract. Cold glacier beds, i.e., where the ice is frozen to its base, are widespread in polar regions. Common theories state that stable permafrost should exist under glacier beds on shorter timescales, varying from years to decades. Presently, only a few direct measurements of both subglacial permafrost and the processes influencing its thermal regime exist. Here, we present subglacial permafrost and active layer measurements obtained from within the basal drainage systems of two cold-based glaciers on Svalbard during the summer melt season. Temperature observations were obtained from subglacial sediment that was accessed through the drainage systems of the two glaciers in the previous winters. The temperature records cover the periods from spring to autumn in 2016 and 2019 at the glaciers Larsbreen and Tellbreen in central Svalbard. The ground temperature below Larsbreen indicates colder ground conditions, whereas the temperatures of the Tellbreen drainage system show considerably warmer conditions, close to the freezing point. We suggest the latter is due to the presence of liquid water all year round inside the Tellbreen drainage system. Both drainage systems investigated show an increase in subglacial sediment temperatures after the disappearance of snow bridges and the subsequent connection to surface meltwater supply at the start of the summer melt season. Temperature records show influence of sudden summer water supply events, when heavy melt and rain left their signatures on the thermal regime and the erosion of the glacier bed. Observed vertical erosion can reach up to 0.9 m d−1 at the base of basal drainage channels during summer. We also show that the thermal regime under the subglacial drainage systems is not stable during summer but experiences several freeze–thaw cycles driven by weather events. Our results show the direct importance of heavy melt events and rain on the thermal regime of subglacial permafrost and the erosion of the glacier bed in the vicinity of subglacial drainage channels. Increased precipitation and surface melt, as expected for future climate, will therefore likely lead to increased degradation of subglacial permafrost, as well as higher subglacial erosion of available sediment around the preferential hydrological paths. This in turn might have significant impacts on proglacial and fjord ecosystems due to increased sediment and nutrient input.

Pervasive cold ice within a temperate glacier – implications for glacier thermal regimes, sediment transport and foreland geomorphology

Abstract. This study suggests that cold-ice processes may be more widespread than previously assumed, even within temperate glacial systems. We present the first systematic mapping of cold ice at the snout of the temperate glacier Midtdalsbreen, an outlet of the Hardangerjøkulen icefield (Norway), from 43 line kilometres of ground-penetrating radar data. Results show a 40 m wide cold-ice zone within the majority of the glacier snout, where ice thickness is <10 m. We interpret ice to be cold-based across this zone, consistent with basal freeze-on processes involved in the deposition of moraines. We also find at least two zones of cold ice up to 15 m thick within the ablation area, occasionally extending to the glacier bed. There are two further zones of cold ice up to 30 m thick in the accumulation area, also extending to the glacier bed. Cold-ice zones in the ablation area tend to correspond to areas of the glacier that are covered by late-lying seasonal snow patches that reoccur over multiple years. Subglacial topography and the location of the freezing isotherm within the glacier and underlying subglacial strata likely influence the transport and supply of supraglacial debris and formation of controlled moraines. The wider implication of this study is the possibility that, with continued climate warming, temperate environments with primarily temperate glaciers could become polythermal in forthcoming decades with (i) persisting thinning and (ii) retreat to higher altitudes where subglacial permafrost could be and/or become more widespread. Adversely, the number and size of late-lying snow patches in ablation areas may decrease and thereby reduce the extent of cold ice, reinforcing the postulated change in the thermal regime.

Subglacial permafrost evidencing re-advance of the Greenland Ice Sheet over frozen ground

Greenland Ice Sheet (GrIS) covers an area of 1.7 million km2. It has been an important source of climate information and the air temperature history of Greenland is well known. However, the thermal history and temperature conditions of the Greenland bedrock are poorly known. There are only few records on the temperature of the proglacial bedrock and no records on bedrock temperature underneath the ice sheet. The Greenland Analogue Project (GAP) recently investigated hydrological, hydrogeological and geochemical processes in Kangerlussuaq, West Greenland. Because permafrost has a major hydrological impact in Arctic regions, the cryogenic structure of the bedrock was an important research topic. From previous studies it was already known that Kangerlussuaq is located within the zone of continuous permafrost. Temperature profiling in a new research borehole, extending horizontally 30 m underneath the ice sheet, revealed that permafrost is 350 m deep at the ice margin. This result raised the question how far the permafrost extends under the ice sheet? In order to investigate the thermal properties, we made a series of electromagnetic (EM) soundings at the ice margin area – on proglacial area and on the ice sheet – and detected, that subglacial permafrost extends at least 2 km from the ice margin to inland. We also observed a patchy unfrozen sediment layer between the ice and the frozen bedrock. Possible existence of subglacial sediments and their role in ice dynamics has been debated in many recent papers. Our successful campaign shows that geophysics can be used for bedrock investigations through thick ice, which is known to be challenging for electromagnetic methods. Our results provide the first direct evidence supporting the proposed Holocene ice re-advance over frozen ground, and contribute to the discussion on the rapid climate changes in past, to the future of the ice sheet under warming climate and hydrogeology at the ice margin.

Thermo-mechanical facies representative of fast and slow flowing ice sheets: the Weichselian ice sheet, a central west Poland case study

Abstract This study deals with an issue of thermo-mechanical facies, reflecting specific thermal and mechanical properties of the subglacial environment. The main objective of this study was to develop a model of glacitectionic deformation and its sedimentary record beneath fast and slow flowing ice sheets, based on investigations conducted in Wielkopolska (west central Poland). Sedimentary structures, mainly at the contact between subglacial tills and glacifluvial sediments, were recognized to delineate typical facies associations in a Weichselian glacigenic succession. Each association was interpreted as a record of the different depositional environments related to different subglacial conditions. Those investigations suggest the substratum was composed of frozen and dry, and wet and mobile spots, and four thermo-mechanical facies were distinguished: A – is representative of slower ice flow, dry and cold subglacial conditions, where driving stresses and normal effective pressure were high; B – is also related to slow ice flow and occurrence of cold subglacial permafrost, but with little amount of unfrozen water (however, higher than in facies A), with similar physical characteristics of the ice sheet as facies A; thermo-mechanical facies C and D represent wet and warm ice sole, with low normal effective pressure and driving stresses, thus lowering sediments’ shear strength and enabling high ice-flow velocities. We suggest that these facies have specific and non-random location, thereby revealing the relationship between subglacial thermo-mechanical conditions and ice sheet dynamics. Slow moving, cold-based ice occurred along ice sheet margins and inter-stream areas, whereas fast-moving, warm-based, well-lubricated ice, was typical of the axial parts of ice streams.

Mechanisms of basal ice formation in polar glaciers: An evaluation of the apron entrainment model

Previous studies of polar glaciers have argued that basal ice can form when these glaciers override and entrain ice marginal aprons that accumulate adjacent to steep ice cliffs. To test this idea, we have studied the morphology, structure, composition, and deformation of the apron and basal ice at the terminus of Victoria Upper Glacier in the McMurdo dry valleys, which are located on the western coast of the Ross Sea at 77°S in southern Victoria Land, Antarctica. Our results show that the apron has two structural elements: an inner element that consists of strongly foliated ice that has a steep up‐glacier dip, and an outer element that lacks a consistent foliation and has a down‐glacier, slope‐parallel dip. Although strain measurements show that the entire apron is deforming, the inner element is characterized by high strain rates, whereas relatively low rates of strain characterize the outer part of the apron. Co‐isotopic analyses of the ice, together with analysis of solute chemistry and sedimentary characteristics, show that the apron is compositionally different from the basal ice. Our observations show that aprons may become deformed and partially entrained by advancing glaciers. However, such an ice marginal process does not provide a satisfactory explanation for the origin of basal ice observed at the ice margin. Our interpretation of the origin of basal ice is that it is formed by subglacial processes, which are likely to include deformation and entrainment of subglacial permafrost.

The influence of basal processes on the dynamic behaviour of cold-based glaciers

There exists within glaciology a widely held assumption that basal sliding and bed deformation do not operate beneath cold-based ice and that their basal velocity is therefore zero, irrespective of bed conditions. Consequently, their ability to erode, entrain and transport sediment and thereby alter the landscape is assumed to be limited. Consequently, very little research has been focused towards describing and understanding the motion of cold-based ice-masses and our knowledge of their behaviour remains poor and the assumption of zero basal velocity, unchallenged. In this review paper, it is argued that this assumption is not universally applicable and that in certain circumstances, basal processes not only remain active at sub-freezing temperatures, but can significantly influence glacier motion. This is particularly the case in glaciers where sub-freezing basal thermal conditions coincide with the presence of fine grained, ice-rich subglacial sediments. Due to the lack of work undertaken on contemporary cold-based glaciers, much of the information used to support this argument is derived from field research in permafrost areas and on Quaternary glacial sediments, and from the laboratory testing of ice/sediment mixtures. It is concluded that the assumption of zero basal velocity beneath cold-based ice is overly simplistic and that in reality, the situation is likely to be much more complex. Work is therefore urgently required: firstly, to investigate the circumstances in which basal motion remains active at sub-freezing temperatures and to determine their influence on glacier motion and; secondly, to examine the likely extent of subglacial permafrost in both the contemporary and Quaternary glacial environment.

A relict landscape in the centre of Fennoscandian glaciation: Geomorphological evidence of minimal Quaternary glacial erosion

The Parkajoki area in northeastern Sweden is situated near the central area of Fennoscandian glaciation. Despite its location, the area is dominated by landforms induced by subaerial weathering and erosion processes, such as well-developed tors and associated saprolites, boulder fields, and boulder depressions. The glacial geomorphology is dominated by lateral and proglacial meltwater channels. Subglacial imprints indicative of thawed-bed conditions and reshaping by glacier sliding (e.g., fluting, drumlins, striae) are lacking. Hence, most of the landscape still exhibits a preglacial appearance. Because of its location, near the central area of glaciation, we attribute preservation to frozen-bed conditions of overriding ice sheets. The widespread distribution of well-developed tors and boulder fields, and the degree of chemical weathering of the bedrock, indicate that the area has been protected from glacial erosion during all glacial cycles since ice sheet initiation in the late Cenozoic. Unlike most other areas with tors in glaciated regions, the Parkajoki area is uniquely situated in the lowlands at 150–400 m.a.s.l. Moreover, this relict landscape is surrounded by glacial landscapes (including drumlins, ribbed moraine, and eskers) of varying age and at similar elevation. Hence, topographical reasons for this area being persistently cold-based cannot be invoked. By inference, we conclude that strain heat release never managed to cancel the initial subglacial permafrost conditions. We attribute this to divergent ice flow towards the convex outline of the ice sheet margin during deglaciations and to the relative roughness of the area compared to its surroundings. The implication is that to explain preservation throughout the Quaternary, all large-scale glaciations must have had a similar evolution concerning ice sheet configuration and internal dynamics as the last glaciation for which good constraints on evolution and outline are available.

Pleistocene permafrost of West Siberia as a deformable glacier bed

Subglacial permafrost is usually referred to as a factor impeding basal glacial processes, and the deformation of soft substrata in particular. In West Siberian, widespread glacial disturbances present in permafrost of Pleistocene age suggest that frozen sediments, if clayey and/or icy, can readily deform, thus translating basal glacial stress into sliding of the entire glacier/sediment complex along subglacial shear zones. Ductile deformations such as folds and diapirs are also widespread. The mode of deformation of frozen subglacial sediments is dependent on their lithology, ice content and temperature conditions. Signatures of former subglacial permafrost in currently thawed sediments are deduced from contrasting deformation behaviours of lithologically different sedimentary formations.