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Access of surface meltwater to beds of sub-freezing glaciers: Preliminary insights
Abstract
Sufficiently deep water-filled fractures can penetrate even cold ice-sheet ice, but glaciogenic stresses are typically smaller than needed to propagate water-filled fractures that are less than a few tens of meters deep, as shown by our simplified analytical treatment based on analogous models of magmatic processes. However, water-filled fractures are inferred to reach the bed of Greenland through >1 km of ice and then collapse to form moulins, which are observed. Supraglacial lakes appear especially important among possible crack 'nucleation' mechanisms, because lakes can warm ice, supply water, and increase the pressure driving water flow and ice cracking.
... [21][22][23]. Previous theoretical work suggests that surface fractures attain the bed of an ice sheet provided there is sufficient water available to maintain a full fracture [24][25][26][27][28] . Here we consider the dimensions of observed hairline surface fractures and supraglacial streams, and develop a model of hydrofracture propagation in https://doi.org/10.1038/s41561-023-01208-0 ...
... step the model calculates the change in water level in the fracture, and then the new propagation depth. Refreezing by ice accretion onto the fracture walls at each level in the fracture is finally calculated following Alley et al. 26 , but here we apply observed temperature profiles and the duration for which that level has been submerged. Fractures in which the accreted ice reaches the full fracture width are likely to become blocked, preventing further propagation. ...
... Alternative fracture propagation models 27,28,32,35 are qualitatively consistent with van der Veen 25 : specifically, dry fractures cannot penetrate to the bed, while water-filled fractures can (with the exception of shallow, narrow water-filled fractures considered by Alley et al. 26 , which cannot reach the bed). Providing that a water level near to the surface is required to propagate the fracture, then the process enabling that propagation is unlikely to greatly affect our results presented here, since we find that the rate and depth of propagation is still critically limited by water supply. ...
Dramatic supraglacial lake drainage events in Greenland and Antarctica are enabled by rapid hydrofracture propagation through ice over 1 km thick. Here we present a slower mode of hydrofracture, where hairline surface fractures intersect supraglacial streams, and hypothesize that penetration depth is critically limited by water supply and englacial refreezing. We develop a model of stream-fed hydrofracture, and find that under most conditions in Greenland, 2-cm-wide fractures can penetrate hundreds of metres before freezing closed. Conditions for full-depth hydrofracture are more restricted, requiring larger meltwater channels and/or warm englacial conditions. Given the abundance of streams and surface fractures across Greenland and Antarctica’s expanding ablation zones, we propose that stream-driven hydrofractures are ubiquitous—even where distant from supraglacial lakes and crevasse fields. This intriguing process remains undetectable by current satellite remote sensing, yet has two major impacts that warrant further investigation. First, by driving widespread cryohydrologic warming at depths far greater than surface crevassing, it explains a consistent cold bias in modelled englacial thermal profiles. Second, the associated reduction in ice viscosity and increased damage accumulation act to enhance the vulnerability of ice sheets and shelves to dynamic instability as supraglacial drainage networks expand inland to higher elevations.
... Water supply is limited by a combination of factors: (i) the short length of fracture, typically less than a few meters, underlying the stream; (ii) supraglacial channels are often choked with snow in early summer, which impedes water flow; and (iii) the hydraulic head (stream depth) driving water into an underlying fracture is typically <2 m, whereas supraglacial lakes commonly attain depths >5 m [20][21][22]. Previous theoretical work suggests that surface fractures can reach the bed of an ice sheet provided there is sufficient water available to maintain a full fracture [23][24][25][26]. Here we consider the dimensions of observed hairline surface fractures and supraglacial streams, and develop a model of hydrofracture propagation in cold ice where meltwater supply is limited. ...
... We assume the fracture intersects a supraglacial stream with semi-circular cross section (radius r c ). The model employs van der Veen's [23] linear elastic fracture mechanics and Toricelli's equation for water leakage [27], and is supplemented by relationships from Alley et al. [24]. We also consider two end-member cases of feedback between water flow and fracture aperture enlargement by viscous heat dissipation. ...
... Hence, a good estimate for propagation rate should lie between these two limits. In both cases, occlusion of the fracture by ice freezing onto the walls follows Alley et al. [24]. ...
Dramatic supraglacial lake drainage events in Greenland and Antarctica are enabled by rapid hydrofracture propagation through >1 km ice. Here, we present a slower mode of hydrofracture, where hairline surface fractures intersect supraglacial streams, and hypothesise that fracture penetration depth is critically limited by water supply and englacial refreezing. We apply a model of stream-fed hydrofracture to the Greenland Ice Sheet and find that, under most conditions, 2-cm-wide fractures can penetrate hundreds of metres before freezing closed. Full-depth hydrofracture is more restricted, requiring relatively large meltwater channels and/or warm englacial conditions. Given the abundance of streams and surface fractures across Greenland and Antarctica's expanding ablation zones, stream-driven hydrofractures are likely ubiquitous even where distant from supraglacial lakes and crevasses. While lake-driven hydrofracture is now argued to have minimal long-term dynamic impacts, this intriguing stream-fed hydrofracturing has two important consequences. First, by driving widespread cryohydrologic warming at depths far greater than surface crevassing, it explains a cold bias in modelled englacial thermal profiles. Second, the associated decrease in ice viscosity, and increased damage accumulation, will enhance the vulnerability of ice sheets to dynamic instability under climate warming. This process is likely undetectable by remote sensing, and warrants more detailed modelling and field investigations.
... However, the effect of stress concentration is comparatively small in highly crevassed areas (van der Veen, 1998a;Benn and Evans, 2010). The presence of water in a crevasse can also have a large influence over the depth to which the fracture can penetrate (Weertman, 1973;Robin, 1974;van der Veen, 1998a;Boon and Sharp, 2003;Alley et al., 2005;Fountain et al., 2005a, b;Benn et al., 2007;Benn and Evans, 2010). In a water-filled crevasse, the pressure exerted by the weight of the water (as water is slightly more dense than ice) acts in the same direction as the deviatoric tensile stress, forcing the crevasse walls apart. ...
... In a water-filled crevasse, the pressure exerted by the weight of the water (as water is slightly more dense than ice) acts in the same direction as the deviatoric tensile stress, forcing the crevasse walls apart. As the crevasse propagates deeper the pressure exerted by the increasing water column (provided there is a sufficient supply of water) enables the fracture to penetrate further, eventually reaching the bed (van der Veen, 1998a(van der Veen, , 2007Alley et al., 2005;Benn et al., 2007Benn et al., , 2009Benn and Evans, 2010). Crevasse propagation to the bed has important implications for meltwater drainage in Arctic glaciers (e.g. ...
... Fountain et al., 2005a, b;Benn et al., 2009;Gulley, 2009), ice sheets (e.g. Alley et al., 2005;Das et al., 2008;Doyle et al., 2013), and for ice shelf break-up (e.g. . ...
This study examines how longitudinal foliation develops in glaciers and ice sheets in a
wide range of topographic, climatic, and dynamic settings, at a variety of spatial scales.
Study locations include four valley glaciers in Svalbard (Austre Brøggerbreen, Midtre
Lovénbreen, Austre Lovénbreen, and Pedersenbreen), a valley glacier in Canada
(Sermilik Glacier), and seven outlet glaciers in Antarctica (Hatherton Glacier, Taylor
Glacier, Ferrar Glacier, Lambert Glacier, Recovery Glacier, Byrd Glacier, and Pine Island
Glacier). Detailed structural mapping of the valley glaciers from satellite imagery and
field-based measurements were used to document the formation of longitudinal
foliation in small-scale ice masses. These findings were ‘up-scaled’ and applied to much
larger glaciers and ice streams. Longitudinal foliation develops in concentrated bands
at flow unit boundaries as a result of enhanced simple shear. However, longitudinal
foliation is not directly observable from satellite imagery at the surface of larger-scale
valley glaciers. The longitudinal structures visible at the surface of larger-scale glaciers
form at flow-unit boundaries and are composed of bands of steeply dipping
longitudinal foliation; however, they appear as individual linear features on satellite
imagery as a result of the comparatively low spatial resolution of the imagery. The
persistence of flowlines in the Antarctic Ice Sheet through areas of crevassing and net
ablation (blue-ice areas) suggests that they are the surface representation of a three-dimensional structure. Flowlines are therefore inferred to be the surface expression of
flow-unit boundaries composed of bands of steeply dipping longitudinal foliation. The
survival and deformation of flowlines in areas of ice flow stagnation indicates that
flowlines form in their initiation zones and not along their entire length. Furthermore,
these ice stagnation areas indicate that flowlines record past ice dynamics and
switches in ice flow.
... The inland migration of surface melt observed across the Greenland Ice Sheet over recent decades is unprecedented in the observational era, and controls on whether emerging surface meltwater can access the bed locally are not fully understood (Culberg et al., 2021;MacFerrin et al., 2019). Although observational evidence (Chudley et al., 2019;Das et al., 2008;Doyle et al., 2013;Stevens et al., 2015;Tedesco et al., 2013) and theoretical work (Alley et al., 2005;Krawczynski et al., 2009;van der Veen, 2007) support the ability of water stored in supraglacial lakes to hydro-fracture to the bed through thick ice, the likelihood of lake drainage via hydro-fracture at any particular location is controlled by the near-surface stress state rather than by water volume alone (Stevens et al., 2015). It has been hypothesized that an inland migration of surface melt that can access the bed could destabilize significant regions of the ice sheet (Alley et al., 2005;Leeson et al., 2015); however, it is currently unclear how vulnerable inland ice is to hydro-fracture. ...
... Although observational evidence (Chudley et al., 2019;Das et al., 2008;Doyle et al., 2013;Stevens et al., 2015;Tedesco et al., 2013) and theoretical work (Alley et al., 2005;Krawczynski et al., 2009;van der Veen, 2007) support the ability of water stored in supraglacial lakes to hydro-fracture to the bed through thick ice, the likelihood of lake drainage via hydro-fracture at any particular location is controlled by the near-surface stress state rather than by water volume alone (Stevens et al., 2015). It has been hypothesized that an inland migration of surface melt that can access the bed could destabilize significant regions of the ice sheet (Alley et al., 2005;Leeson et al., 2015); however, it is currently unclear how vulnerable inland ice is to hydro-fracture. The vulnerability of the inland ice-sheet to increases in surface-meltwater-induced sliding depends, in part, on the ability of surface-to-bed meltwater pathways to develop in regions of nascent lake formation Poinar et al., 2015). ...
Supraglacial lakes have been observed to drain within hours of each other, leading to the hypothesis that stress transmission following one drainage may be sufficient to induce hydro‐fracture‐driven drainages of other nearby lakes. However, available observations characterizing drainage‐induced stress perturbations have been insufficient to evaluate this hypothesis. Here, we use ice‐sheet surface‐displacement observations from a dense global positioning system array deployed in the Greenland Ice Sheet ablation zone to investigate elastic stress transmission between three neighboring supraglacial lake basins. We find that drainage of a central lake can place neighboring basins in either tensional or compressional stress relative to their hydro‐fracture scarp orientations, either promoting or inhibiting hydro‐fracture initiation beneath those lakes. For two lakes located within our array that drain close in time, we identify tensional surface stresses caused by ice‐sheet uplift due to basal‐cavity opening as the physical explanation for these lakes' temporally clustered hydro‐fracture‐driven drainages and frequent triggering behavior. However, lake‐drainage‐induced stresses in the up‐flowline direction remain low beyond the margins of the drained lakes. This short stress‐coupling length scale is consistent with idealized lake‐drainage scenarios for a range of lake volumes and ice‐sheet thicknesses. Thus, on elastic timescales, our observations and idealized‐model results support a stress‐transmission hypothesis for inducing hydro‐fracture‐driven drainage of lakes located within the region of basal cavity opening produced by the initial drainage, but refute this hypothesis for distal lakes.
... How this meltwater is extracted and focused remains an unresolved question. It has also been proposed that water-filled through-going fractures (crevasses) reach bedrock and connect with the subglacial drainage system (Van der Veen, 2007;Alley et al., 1988;Weertman, 1971), which therefore influences the dynamics of ice flow by lubrication (Kamb, 2001;Engelhardt and Kamb, 1997), and this causes ice flow to speed up (Zwally et al., 2002). ...
... Although hydrofracturing and crevasse formation (Melton et al., 2022;Van der Veen, 2007) may be the most efficient mechanism of permeability development in glaciers, microcracks and shearing may be a smaller-scale mechanism. It has also been suggested that meltwater flow through hydrofractures (crevasses) to the basal region of an ice mass is analogous to magmatic processes (Alley et al., 1988;Weertman, 1971), and this can influence glacial dynamics. Other than summer meltwater, which drains through moulins and crevasses (Van der Veen, 2007;Zwally et al., 2002), what other processes drive the subglacial water along the bedrock topography in the interior of an ice mass? ...
In frozen cylinders composed of deuterium ice (Tm+3.8 ∘C) and 10 % water ice (Tm0 ∘C), it is possible to track melt pathways produced by increasing the temperature during deformation. Raising the temperature to +2 ∘C produces water (H2O) which combines with the D2O ice to form mixtures of HDO. As a consequence of deformation, HDO and H2O meltwater are expelled along conjugate shear bands and as compactional melt segregations. Melt segregations are also associated with high-porosity networks related to the location of transient reaction fronts where the passage of melt-enriched fluids is controlled by the localized ductile yielding and lowering of the effective viscosity. Accompanying the softening, the meltwater also changes and weakens the crystallographic fabric development of the ice. Our observations suggest meltwater-enriched compaction and shear band initiation provide instabilities and the driving force for an enhancement of permeability in terrestrial ice sheets and glaciers.
... How this meltwater is extracted and focused remains an unresolved question. It has also been proposed that water-filled through-going fractures (crevasses) reach bedrock and connect with the subglacial drainage system (Van der Veen, 2007;Alley et al., 1988;Weertman, 1971), which therefore influences the dynamics of ice flow by lubrication (Kamb, 2001;Engelhardt and Kamb, 1997), and this causes ice flow to speed up (Zwally et al., 2002). ...
... Although hydrofracturing and crevasse formation (Melton et al., 2022;Van der Veen, 2007) may be the most efficient mechanism of permeability development in glaciers, microcracks and shearing may be a smaller-scale mechanism. It has also been suggested that meltwater flow through hydrofractures (crevasses) to the basal region of an ice mass is analogous to magmatic processes (Alley et al., 1988;Weertman, 1971), and this can influence glacial dynamics. Other than summer meltwater, which drains through moulins and crevasses (Van der Veen, 2007;Zwally et al., 2002), what other processes drive the subglacial water along the bedrock topography in the interior of an ice mass? ...
In frozen cylinders composed of deuterium ice (T m + 3.8 • C) and 10 % water ice (T m 0 • C), it is possible to track melt pathways produced by increasing the temperature during deformation. Raising the temperature to +2 • C produces water (H 2 O) which combines with the D 2 O ice to form mixtures of HDO. As a consequence of deformation, HDO and H 2 O meltwater are expelled along conjugate shear bands and as compactional melt segregations. Melt segrega-tions are also associated with high-porosity networks related to the location of transient reaction fronts where the passage of melt-enriched fluids is controlled by the localized ductile yielding and lowering of the effective viscosity. Accompanying the softening, the meltwater also changes and weakens the crystallographic fabric development of the ice. Our observations suggest meltwater-enriched compaction and shear band initiation provide instabilities and the driving force for an enhancement of permeability in terrestrial ice sheets and glaciers.
... Similarly, surface and basal crevasses generally are primarily opening-mode cracks. If driven by sufficient internal water pressure, as from a surface lake (Alley et al. 2005b), opening-mode cracks can connect from the surface to the base of grounded as well as floating ice. The internal pressure of ice, however, largely prevents mode I air-filled cracks or water-filled basal crevasses from penetrating the full ice thickness. ...
... Failure by percolation connection of lakes breaking through farther from the front might lead to ice-shelf loss, but may instead be a mechanism to prevent near-frontal meltwater wedging (Robel & Banwell 2019; also see Bell et al. 2017, Trusel et al. 2022. The role of inland drainage was explored earlier by Parizek et al. (2010); they showed that initial warming of a long-cold environment is more likely to remove an ice shelf by meltwater wedging than is subsequent warming, because sufficiently warm systems develop drainage systems inland through moulins [holes that drain water from the surface to the bed (Alley et al. 2005b)] that rapidly remove surface meltwater, generally to the ocean, thus reducing or eliminating filled crevasses at the ice front. ...
Uncertainty about sea-level rise is dominated by uncertainty about iceberg calving, mass loss from glaciers or ice sheets by fracturing. Review of the rapidly growing calving literature leads to a few overarching hypotheses. Almost all calving occurs near or just downglacier of a location where ice flows into an environment more favorable for calving, so the calving rate is controlled primarily by flow to the ice margin rather than by fracturing. Calving can be classified into five regimes, which tend to be persistent, predictable, and insensitive to small perturbations in flow velocity, ice characteristics, or environmental forcing; these regimes can be studied instrumentally. Sufficiently large perturbations may cause sometimes-rapid transitions between regimes or between calving and noncalving behavior, during which fracturing may control the rate of calving. Regime transitions underlie the largest uncertainties in sea-level rise projections, but with few, important exceptions, have not been observed instrumentally. This is especially true of the most important regime transitions for sea-level rise. Process-based models informed by studies of ongoing calving, and assimilation of deep-time paleoclimatic data, may help reduce uncertainties about regime transitions. Failure to include calving accurately in predictive models could lead to large underestimates of warming-induced sea-level rise. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 51 is May 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
... In western Greenland, lake-draining moulins are formed or re-activated by meltwater amassed in supraglacial lakes (Das et al., 2008;Tedesco et al., 2013;Stevens et al., 2015); other moulins likely form by the interaction of supraglacial streams with crevasses, absent lakes (McGrath et al., 2011;Smith et al., 2015;Koziol et al., 2017). In either case, formation of a moulin requires formation of a fracture, which generally requires an extensional strain-rate regime (Alley et al., 2005;Smith et al., 2015). Remote-sensing surveys over western Greenland, however, have found that supraglacial lakes generally occupy areas where the annual mean strain rates are compressional (Joughin et al., 2013;Poinar et al., 2015;Andrews, 2015;Catania et al., 2008). ...
... This work is motivated by the apparent contradiction that moulins, which require extension to form or activate, are often located in or near lake basins, where ice flow is generally compressional (Alley et al., 2005;Das et al., 2008;Krawczynski et al., 2009). This pattern suggests that strainrate transients -perturbation events on the scale of hours to days -are responsible for moulin formation or re-activation (Stevens et al., 2015). ...
A leading hypothesis for the mechanism of fast supraglacial lake drainages is that transient extensional stresses briefly allow crevassing in otherwise compressional ice flow regimes. Lake water can then hydrofracture a crevasse to the base of the ice sheet, and river inputs can maintain this connection as a moulin. If future ice sheet models are to accurately represent moulins, we must understand their formation processes, timescales, and locations. Here, we use remote-sensing velocity products to constrain the relationship between strain rates and lake drainages across ∼ 1600 km2 in Pâkitsoq, western Greenland, between 2002–2019. We find significantly more extensional background strain rates at moulins associated with fast-draining lakes than at slow-draining or non-draining lake moulins. We test whether moulins in more extensional background settings drain their lakes earlier, but we find insignificant correlation. To investigate the frequency at which strain-rate transients are associated with fast lake drainage, we examined Landsat-derived strain rates over 16 and 32 d periods at moulins associated with 240 fast-lake-drainage events over 18 years. A low signal-to-noise ratio, the presence of water, and the multi-week repeat cycle obscured any resolution of the hypothesized transient strain rates. Our results support the hypothesis that transient strain rates drive fast lake drainages. However, the current generation of ice sheet velocity products, even when stacked across hundreds of fast lake drainages, cannot resolve these transients. Thus, observational progress in understanding lake drainage initiation will rely on field-based tools such as GPS networks and photogrammetry.
... 2018). Indeed, given that water-filled crevasses can often propagate downwards without limit (by overcoming the lithostatic stress in the ice) (Fountain and Walder, 1998;van der Veen, 2007), the calving margin itself will be located approximately where these crevasses reach the lake or sea level, assuming of course that there is a free connection between the crevasse and sea or lake (Alley et al., 2005;Benn et al., 2007b;Sugiyama et al., 2011). ...
... On the other hand, where velocity gradients are small near the grounding line, crevasses will only propagate to shallow depths which will allow the terminus to become buoyant and form a floating ice tongue for a lake-terminating glacier, or an ice shelf for a tidewater glacier (e.g. Warren et al., 2001;Boyce et al., 2007;Trüssel et al., 2013;. Therefore, velocity gradients, combined with the ice thickness relative to water depth, will determine the maximum possible extent of the calving glacier terminus and how this extent changes in both time and space (Alley et al., 2005;Benn et al., 2007a;Nick et al., 2010;Shapero et al., 2016). ...
Proglacial lakes are becoming ubiquitous at the termini of many glaciers worldwide, leading to increased glacier mass loss and terminus retreat due to the influence of these proglacial lakes on ice dynamics. However, despite the highly dynamic nature and relative insensitivity to climate of many lake-terminating glaciers, an understanding of the key processes forcing their behaviour is lacking. As a result, it is difficult at present to accurately assess and predict how these glaciers may respond in the future. A novel method to address this difficulty, however, is through the use of repeat uncrewed aerial vehicle (UAV) imagery, which can provide high to ultra-high resolution (cm-dm scale) imagery of the ice surface at varying spatial and temporal scales, depending on the needs of the study, although its use as a tool for investigating the dynamics of lake-terminating glaciers is so far limited. This research utilised ultra-high resolution repeat UAV imagery to provide insights into the changing dynamics of Fjallsjökull, a large lake-terminating glacier in southeast Iceland, across the 2019 and 2021 summer melt seasons. The findings indicate that the overall dynamics of the glacier are controlled by the ~120 m deep bedrock channel under the study region, which is causing the glacier to flow faster as it enters deeper water, leading to increased ice acceleration, thinning and retreat, with the glacier being decoupled from the local climate as a result. Such a close correspondence between ice velocity and surface thinning suggests the implementation of the positive feedback mechanism dynamic thinning in this region of Fjallsjökull, with such heightened rates of surface thinning and frontal retreat likely to continue in the future until the glacier recedes out of the bedrock channel into shallower water. Within this overall pattern, however, more localised, short-term changes in glacier dynamics were also observed, with these likely being forced primarily by subaqueous melting at the waterline, rather than the specific bedrock topography. Finally, supraglacial lake drainage may also be important for forcing sub-daily (e.g. hourly) increases in velocity, although further work is required to quantify its influence more accurately. As a result, these findings clearly indicate the complex nature of the calving process, as well as the dynamics of calving glaciers in general, highlighting the need for continued monitoring of lake-terminating glaciers at varying spatial and temporal scales in order to better understand and predict how they may respond in future.
... The presence of water in a crevasse also has a large influence over the depth to which the fracture can penetrate, resulting from the density difference between ice and water (Alley et al., 2005;Benn, Warren, & Mottram, 2007;Boon & Sharp, 2003;Robin, 1974;van der Veen, 1998avan der Veen, , 2007Weertman, 1973). In a water-filled crevasse, the pressure exerted by the weight of the water column acts in the same direction as the deviatoric tensile stress, forcing the crevasse walls apart by a process known as "hydrofracturing." ...
... In a water-filled crevasse, the pressure exerted by the weight of the water column acts in the same direction as the deviatoric tensile stress, forcing the crevasse walls apart by a process known as "hydrofracturing." As the crevasse propagates deeper, the pressure exerted by the increasing water column (provided there is a sufficient supply of water) enables the fracture to penetrate further, eventually reaching the bed (Alley et al., 2005;Benn, Warren, & Mottram, 2007;Benn et al., 2009;van der Veen, 1998avan der Veen, , 2007. In this situation, an additional term can be added to Equation 9 to account for the effect that water has on crevasse penetration depth: ...
The aims of this review are to: (a) describe and interpret structures in valley glaciers in relation to strain history; and (b) to explore how these structures inform our understanding of the kinematics of large ice masses, and a wide range of other aspects of glaciology. Structures in glaciers give insight as to how ice deforms at the macroscopic and larger scale. Structures also provide information concerning the deformation history of ice masses over centuries and millennia. From a geological perspective, glaciers can be considered to be models of rock deformation, but with rates of change that are measurable on a human time‐scale. However, structural assemblages in glaciers are commonly complex, and unraveling them to determine the deformation history is challenging; it thus requires the approach of the structural geologist. A wide range of structures are present in valley glaciers: (a) primary structures include sedimentary stratification and various veins; (b) secondary structures that are the result of brittle and ductile deformation include crevasses, faults, crevasse traces, foliation, folds, and boudinage structures. Some of these structures, notably crevasses, relate well to measured strain‐rates, but to explain ductile structures analysis of cumulative strain is required. Some structures occur in all glaciers irrespective of size, and they are therefore recognizable in ice streams and ice shelves. Structural approaches have wide (but as yet under‐developed potential) application to other sub‐disciplines of glaciology, notably glacier hydrology, debris entrainment and transfer, landform development, microbiological investigations, and in the interpretation of glacier‐like features on Mars.
... These features efficiently deliver water to the bed and are kept open due to frictional heat from the flowing water, meaning that they might deform or even close during periods without discharge (i.e., winter). Where supraglacial lakes occur, the lake water can propagate through hydrofracturing to the bed, with ice thicknesses up to 1 km (Alley et al., 2005). ...
... Therefore, a more likely source is the drainage of supraglacial lakes. Drainage of supraglacial lakes is possible through hydrofracturing, even on ice more than 1 km thick (Alley et al., 2005;Das et al., 2008). Supraglacial lakes are common in the ablation area of the western Greenland ice sheet (Chu, 2014) in a zone at least 100 km wide (van den Broeke et al., 2008). ...
Ice sheets are disintegrating due to global warming. One factor controlling ice-sheet behavior is the processes active beneath the ice sheet. In particular, processes connected to glacial meltwater drainage are essential to understand ice-sheets behavior in a warming climate. Investigating sediments and geomorphology of drainage systems below ice sheets is complicated; however, formerly glaciated regions are easily accessible. These regions display landforms and sediments formed by the processes at the ice-sheet bed.
Glacial landforms were mapped in the south Swedish uplands, an area that makes up a large part of the former south-central part of the Scandinavian Ice Sheet. This region was deglaciated during the Bølling-Allerød warm period, before the Younger Dryas cold event. During the Bølling-Allerød, large amounts of meltwater were derived from ice sheets to the world's oceans.
In the form of detailed digital elevation models, new datasets have made it possible to map formerly glaciated regions in unprecedented detail and pinpoint locations for detailed sedimentological work.
The map produced is the first comprehensive inventory of glacial geomorphology produced for the south Swedish uplands. This mapping discovered several new features that are added to the plethora of landforms already known, including radial hummocks tracts interpreted to be tunnel valleys, glaciofluvial meltwater corridors, and a new V-shaped hummock referred to as murtoos.
Hummock tracts within the area demonstrate a heterogeneous hummock morphology. As previously mapped, a lobate band of hummock tracts can be traced through southern Sweden. However, the hummock tracts also display a clear radial pattern of hummock corridors associated with ice flow. Based on the geomorphological and sedimentological analysis, the radial pattern of hummock corridors are interpreted as tunnel valleys or glaciofluvial meltwater corridors and is suggested to reflect strong meltwater activity at the bed of the ice sheet.
The V-shaped hummocks (murtoos) are argued to be a new and distinct subglacial landform with a morphology related to overall ice flow. Based on ice-sheet scale distribution, geomorphological analysis, and sedimentological studies, a formational model is hypothesized. The model is driven by variations in the subglacial hydrological system connected to repeated influx from supraglacial meltwater to the ice-sheet bed within the distributed system.
Tunnel valleys, glaciofluvial corridors, and murtoos are all proposed to be formed in the subglacial hydrological system. The formation of these landforms indicates intense melting at the ice-sheet surface, and this is clearly associated with times of climate warming. The landform connection can be illustrated as a times-transgressive landform system, where murtoos are suggested to form first, followed by TVs, GFCs, and finally, eskers.
... Several factors were investigated to determine if one or more environmental conditions or lake characteristics would trigger a rapid drainage after a certain threshold or under certain conditions. An idea that has often been modeled (Alley et al., 2005;Arnold et al., 2014;Banwell et al., 2013;Banwell et al., 2016;Koziol et al., 2017;Tsai and Rice, 2010;van der Veen, 2007) but also recently contradicted by observations (Fitzpatrick et al., 2013;Williamson et al., 2018) is the idea that each lake has a specific volume limit, above which the pressure from the water causes a hydrofracture-induced drainage. In Fig. 4, we 170 investigated nine often-draining lakes to see at which volumes they drained and what the maximum volume was that they reached in seasons when they did not drain. ...
Supraglacial lakes are known to undergo rapid drainages in which the contents of the lake are drained through ice hydrofracture to the glacier bed, typically within several hours. Despite the impact of this sudden loss of meltwater from the glacier, the conditions leading up to a rapid drainage are not fully understood. In this study, the spatial and temporal variability of rapid drainages was evaluated over two major glaciers in Northeast Greenland: Zachariæ Isstrøm and Nioghalvfjerdsfjorden (79N Glacier). Over the 2016–2022 summer melt seasons, each supraglacial lake was tracked via Sentinel-2 optical imagery to find the occurrence of any rapid drainages. The spatial distribution of rapid drainages as well as the seasonal timings were then evaluated against several other factors, such as ice strain rate, elevation, lake volume and seasonal surface temperature. It was found that the drainage patterns of individual lakes varied substantially, with some lakes having drained only a couple times and others nearly every year in the observed time frame. Furthermore, some lakes tended to drain around the same week in the melt seasons when they did rapidly drain, while others had a more sporadic drainage timing. Similarly, certain clusters of lakes tend to drain in similar time frames when they do drain, whereas it was found that most lakes did not follow a drainage tendency based on physical location. However, the phenomenon of chain drainages, in which more than one neighboring lake drains nearly simultaneously, was observed several times. While it was seen that drainages tend to occur later with higher elevations, little correlation was found between the occurrence of rapid drainages and the other investigated factors. It appears several conditions would need to be filled to allow for a rapid drainage to occur, particularly the existence of a crevasse within the lake boundaries.
... Closed by boundary conditions zero ice thickness at the ice front and global mass conservation, the nonlinear diffusion model yields a selfsimilar solution for the ice thickness (Halfar 1981;Huppert 1982;Hindmarsh 1990;Nye 2000;Bueler et al. 2005). During the summer months (June-September) the ice-bed interface of the GrIS becomes partially lubricated by surface meltwater which penetrates the ice column via surface-to-bed pathways (Alley et al. 2005;Das et al. 2008). These spatially-finite injections of meltwater reduce friction between the ice and bed below, leading to acceleration of the overlying ice (Joughin et al. 2013;Stevens et al. 2016). ...
We present a theoretical investigation into the dynamics of a viscous gravity current subjected to spatially-finite lubrication (i.e., a `slippery patch'). The work is motivated by grounded ice sheets flowing across patches of basal meltwater which reduce the ice-bed frictional coupling, causing perturbations enhancing ice motion, with implications for increased ice flux into the ocean and sea level rise. The flow is characterized by transitions between shear- and extension-dominated dynamics, which necessitates boundary-layer solutions at the transition points. We develop a depth-integrated analytical model of Newtonian flow which concisely reveals fundamental relationships between ice sheet geometry (thickness, surface slope, and slippery patch length) and the magnitude and spatial extent of resulting horizontal deviatoric stresses. This reduced-order analytical model shows good quantitative agreement with numerical simulations using 2-D Newtonian Stokes equations, which are further extended to the case of a non-Newtonian flow. From the reduced-order model, we rationalize that the slippery patch-induced stress perturbations are exponentially-decaying functions of distance upstream away from the patch onset. We also show that the amplitude of the perturbation scales linearly with the surface slope and patch length while the decay lengthscale scales linearly with ice thickness. These fundamental relationships have implications for the response of the Greenland Ice Sheet to the inland expansion of basal meltwater presence over the coming warming decades.
... Therefore, it is plausible that the vertical expansion of crevasses within Duiya Glacier might be substantial because of high subglacial water pressure. In addition, water can counteract the ice creep that causes crevasses to close within the glacier by exerting pressure on both sides of the crevasses, thereby allowing them to expand (Alley et al. 2005;van der Veen 2007). We also discovered numerous supraglacial lakes on Duiya Glacier ( Figure 11). ...
This study investigates the effects of glacial lakes on the flow velocities of Duiya Glacier, southern Tibetan Plateau, by employing satellite Synthetic Aperture Radar (SAR) Pixel-Offset-Tracking Small-Baseline-Subset (PO-SBAS) technology. Duiya Glacier, which terminates in a lake, exhibits approximately 10 m/yr higher horizontal speed and a higher melting rate with a vertical Non-Surface-Parallel-Flow (nSPF) velocity of – 7 m/yr than those of its land-terminating counterpart – West Duosangpu Glacier. Notably, Duiya Glacier experiences significant seasonal velocity fluctuations with accelerated flow between May and October. By integrating glacier geometry, changes in glacier boundaries and the extent of proglacial lakes, the distribution of supraglacial lakes, and climatic variables, we reveal that proglacial and supraglacial lakes play a crucial role in increasing the flow velocities of Duiya Glacier. Duiya Glacier flows faster because of increased subglacial water pressure resulting from water influx from these lakes. This phenomenon becomes conspicuously evident during May – October, when increased meltwater due to increased temperatures and precipitation further elevates the subglacial water pressure. Our method highlights the potential for understanding the impact of glacial lakes on glacier movements at a large scale, leveraging the capabilities of satellite SAR PO-SBAS technology for continuous, wide-scale, high temporal resolution, and 3D velocity monitoring.
... GNSS data from 2019 also reveals that the drainage started after the seasonal acceleration had started. This could act as a drainage event trigger if the stresses are already close to a critical threshold, as an increase in strain rate acts as an additional displacement boundary condition, as was also suggested by Alley et al. (2005). ...
Supraglacial lake drainage through fractures delivers vast amounts of water to the ice sheet base on timescales of hours. This study is concerned with the mechanisms of supraglacial lake drainage and how a particular area of Nioghalvfjerdsbræ with a lake of a volume up to 1.23 · 108 m3. We found extensive fracture fields being formed and vertical displacement across the fracture faces in some instances. The fractures are accommodated with triangular gullies, in the order of 10’s m’s, into which water is flowing still weeks after the main lake drainage, but also instances in which the water level rises over the surface end of summer. These gullies are sometimes reactivated in subsequent years and their size at the surface remains unchanged over some years, which is in agreement with viscoelastic modelling. Using ice-penetrating radar, we find englacial, three-dimensional features originating from the drainage, changing over years but remaining detectable even years after their formation. The drained water forms a blister underneath the lake, which is released over several weeks. In this area, no lakes existed before an increase in atmospheric temperatures in the mid-1990s as we demonstrate using reanalysis data. It is transformed from lake-free to frequent, abrupt drainage delivering massive amounts of lubricant and freshwater at the seaward margin.
... 3 Moreover, natural occurrences of hydraulic fracturing can arise from magmatic intrusions 4 and water-driven vertical crevassing in glaciers. 5,6 To optimize production and maintain environmental sustainability, 7 a comprehensive understanding of the physical mechanisms governing fracture and extraction is essential. ...
We introduce an innovative instrument designed to investigate fluid-induced fractures under mixed loading conditions, including uniaxial tension and shear stress, in gels and similar soft materials. Equipped with sensors for measuring force, torque, and fluid pressure, the device is tailored for compatibility with x-ray tomography scanners, enabling non-invasive 3D analysis of crack geometries. To showcase its capabilities, we conducted a study examining crack-front segmentation in a hydrogel subjected to air pressure and a combination of tension and shear stress.
... However, significant uncertainty remains in meltwater runoff estimates as a result of the lack of knowledge of heterogeneous infiltration and refreezing processes within the snow and firn layers (Pfeffer and Humphrey, 1996) as well as the depths to which meltwater can descend beneath the ice sheet surface . If the increasing seasonal surface melting trend continues (Franco et al., 2013;Noël et al., 2021), perennial firn aquifer formation and expansion may increase the possibility of crevasse deepening via meltwater-induced hydrofracturing (Alley et al., 2005;van der Veen, 2007), especially if crevasse fields expand into perennial firn aquifer areas as a result of accelerated ice flow . Meltwater-induced hydrofracturing is an important component of supraglacial lake drainage during the melting season Stevens et al., 2015), leading to at least temporary localized accelerated ice flow velocities (Zwally et al., 2002;Joughin et al., 2013;Moon et al., 2014) as well as ice discharge from outlet glaciers (Chudley et al., 2019) and mass balance changes . ...
Perennial firn aquifers are subsurface meltwater reservoirs consisting of a meters-thick water-saturated firn layer that can form on spatial scales as large as tens of kilometers. They have been observed within the percolation facies of glaciated regions experiencing intense seasonal surface melting and high snow accumulation. Widespread perennial firn aquifers have been identified within the Greenland Ice Sheet (GrIS) via field expeditions, airborne ice-penetrating radar surveys, and satellite microwave sensors. In contrast, ice slabs are nearly continuous ice layers that can also form on spatial scales as large as tens of kilometers as a result of surface and subsurface water-saturated snow and firn layers sequentially refreezing following multiple melting seasons. They have been observed within the percolation facies of glaciated regions experiencing intense seasonal surface melting but in areas where snow accumulation is at least 25 % lower as compared to perennial firn aquifer areas. Widespread ice slabs have recently been identified within the GrIS via field expeditions and airborne ice-penetrating radar surveys, specifically in areas where perennial firn aquifers typically do not form. However, ice slabs have yet to be identified from space. Together, these two ice sheet features represent distinct, but related, sub-facies within the broader percolation facies of the GrIS that can be defined primarily by differences in snow accumulation, which influences the englacial hydrology and thermal characteristics of firn layers at depth.
Here, for the first time, we use enhanced-resolution vertically polarized L-band brightness temperature (TVB) imagery (2015–2019) generated using observations collected over the GrIS by NASA's Soil Moisture Active Passive (SMAP) satellite to map perennial firn aquifer and ice slab areas together as a continuous englacial hydrological system. We use an empirical algorithm previously developed to map the extent of Greenland's perennial firn aquifers via fitting exponentially decreasing temporal L-band signatures to a set of sigmoidal curves. This algorithm is recalibrated to also map the extent of ice slab areas using airborne ice-penetrating radar surveys collected by NASA's Operation IceBridge (OIB) campaigns (2010–2017). Our SMAP-derived maps show that between 2015 and 2019, perennial firn aquifer areas extended over 64 000 km2, and ice slab areas extended over 76 000 km2. Combined together, these sub-facies are the equivalent of 24 % of the percolation facies of the GrIS. As Greenland's climate continues to warm, seasonal surface melting will increase in extent, intensity, and duration. Quantifying the possible rapid expansion of these sub-facies using satellite L-band microwave radiometry has significant implications for understanding ice-sheet-wide variability in englacial hydrology that may drive meltwater-induced hydrofracturing and accelerated ice flow as well as high-elevation meltwater runoff that can impact the mass balance and stability of the GrIS.
... Crevasses are an important pathway for the transfer of water to the subglacial environment of glaciers and ice sheets, while water itself can drive the propagation of crevasses via hydrofracture (Alley et al., 2005;Krawczynski et al., 2009;van der Veen, 2007;Weertman, 1973). Once full-depth hydrofracture has occurred, water flow forms an efficient route for continued meltwater delivery to the bed in the form of moulins. ...
... Crevasses are an important pathway for the transfer of water to the subglacial environment of glaciers and ice sheets, while water itself can drive the propagation of crevasses via hydrofracture (Alley et al., 2005;Krawczynski et al., 2009;van der Veen, 2007;Weertman, 1973). Once full-depth hydrofracture has occurred, water flow forms an efficient route for continued meltwater delivery to the bed in the form of moulins. ...
Surface crevasses on the Greenland Ice Sheet (GrIS) capture nearly half of the seasonal runoff, yet their role in transferring meltwater to the bed has received little attention relative to that of supraglacial lakes and moulins. Here, we present observations of crevasse ponding and investigate controls on their hydrological behavior at a fast‐moving, marine‐terminating sector of the GrIS. We map surface meltwater, crevasses, and surface‐parallel stress across a ∼2,700 km² region using satellite data and contemporaneous uncrewed aerial vehicle (UAV) surveys. From 2017 to 2019 an average of 26% of the crevassed area exhibited ponding at locations that remained persistent between years despite rapid advection. We find that the spatial distribution of ponded crevasses does not relate to previously proposed controls on the distribution of supraglacial lakes (elevation and topography) or crevasses (von Mises stress thresholds), suggesting the operation of some other physical control(s). Ponded crevasse fields were preferentially located in regions of compressive surface‐parallel mean stress, which we interpret to result from the hydraulic isolation of these systems. This contrasts with unponded crevasse fields, which we suggest are readily able to transport meltwater into the wider supraglacial and englacial network. UAV observations show that ponded crevasses can drain episodically and rapidly, likely through hydrofracture. We therefore propose that the surface stress regime influences a spatially heterogeneous transfer of meltwater through crevasses to the bed of ice sheets, with consequences for processes, such as subglacial drainage and the heating of ice via latent heat release by refreezing meltwater.
... Glacier acceleration enhances crevassing, thus increasing the area of direct meltwater transfer to the ice-bed interface. Intense melt events not only promote opening of new moulins through meltenlargement of existing crevasses but also drive hydrofracturing, a mechanism through which water-filled crevasses or supraglacial meltwater ponds abruptly propagate downwards (Alley et al., 2005;van der Veen, 2007;Das et al., 2008). Supraglacial lake drainage is a widespread mechanism for moulin activation in the upper ablation zone and hence may contribute significantly to subglacial reorganization (Bingham et al., 2003;Andrews et al., 2018;Hoffman et al., 2018). ...
By regulating the amount, the timing, and the location of meltwater supply to the glacier bed, supraglacial hydrology potentially exerts a major control on the evolution of the subglacial drainage system, which in turn modulates ice velocity. Yet the configuration of the supraglacial hydrological system has received only little attention in numerical models of subglacial hydrology so far. Here we apply the two-dimensional subglacial hydrology model GlaDS (Glacier Drainage System model) to a Svalbard glacier basin with the aim of investigating how the spatial distribution of meltwater recharge affects the characteristics of the basal drainage system. We design four experiments with various degrees of complexity in the way that meltwater is delivered to the subglacial drainage model. Our results show significant differences between experiments in the early summer transition from distributed to channelized drainage, with discrete recharge at moulins favouring channelization at higher elevations and driving overall lower water pressures. Otherwise, we find that water input configuration only poorly influences subglacial hydrology, which instead is controlled primarily by subglacial topography. All experiments fail to develop channels of sufficient efficiency to substantially reduce summertime water pressures, which we attribute to small surface gradients and short melt seasons. The findings of our study are potentially applicable to most Svalbard tidewater glaciers with similar topography and low meltwater recharge. The absence of efficient channelization implies that the dynamics of tidewater glaciers in the Svalbard archipelago may be sensitive to future long-term trends in meltwater supply.
... Much of what we know about the locations, timings and magnitudes of rapid lake drainage events comes from the analysis of optical satellite imagery (Box and Ski, 2007;McMillan et al., 2007;Sneed and Hamilton, 2007;Leeson et al., 2013;Moussavi et al., 2016;Pope et al., 2016;Williamson et al., 2018) although studies have recently begun using optical imagery from drones (Chudley et al., 2019) and airborne and satellite radar data (Miles et al., 2017;Schröder et al., 2020). Conventional understanding is that rapid lake drainages are confined to the summer and may be driven by active in situ hydrofracture through the lake bottom triggered by increased lake volume (Alley et al., 2005;van der Veen, 2007;Krawczynski et al., 2009;Arnold et al., 2014;Clason et al., 2015) and/or by passive fracture in response to perturbations in ice sheet flow induced by surface meltwater initially tapping the bed via nearby moulins (Stevens et al., 2015;Chudley et al., 2019). In this understanding, lakes completely or partially drain during the summer then freeze during the winter, either freezing through completely or maintaining a liquid water core Koenig et al., 2015;Miles et al., 2017;Law et al., 2020). ...
Surface lakes on the Greenland Ice Sheet play a key role in its surface mass balance, hydrology and biogeochemistry. They often drain rapidly in the summer via hydrofracture, which delivers lake water to the ice sheet base over timescales of hours to days and then can allow meltwater to reach the base for the rest of the summer. Rapid lake drainage, therefore, influences subglacial drainage evolution; water pressures; ice flow; biogeochemical activity; and ultimately the delivery of water, sediments and nutrients to the ocean. It has generally been assumed that rapid lake drainage events are confined to the summer, as this is typically when observations are made using satellite optical imagery. Here we develop a method to quantify backscatter changes in satellite radar imagery, which we use to document the drainage of six different lakes during three winters (2014/15, 2015/16 and 2016/17) in fast-flowing parts of the Greenland Ice Sheet. Analysis of optical imagery from before and after the three winters supports the radar-based evidence for winter lake drainage events and also provides estimates of lake drainage volumes, which range between 0.000046 ± 0.000017 and 0.0200 ± 0.002817 km3. For three of the events, optical imagery allows repeat photoclinometry (shape from shading) calculations to be made showing mean vertical collapse of the lake surfaces ranging between 1.21 ± 1.61 and 7.25 ± 1.61 m and drainage volumes of 0.002 ± 0.002968 to 0.044 ± 0.009858 km3. For one of these three, time-stamped ArcticDEM strips allow for DEM differencing, which demonstrates a mean collapse depth of 2.17 ± 0.28 m across the lake area. The findings show that lake drainage can occur in the winter in the absence of active surface melt and notable ice flow acceleration, which may have important implications for subglacial hydrology and biogeochemical processes.
... Meltwater in surface crevasses can apply hydraulic pressure on crevasse walls and promote crevasse growth deeper into the glacier, a process that is often referred to as hydrofracture. Under certain conditions, water-filled crevasses can penetrate through the full thickness of the glacier leading to iceberg calving [3][4][5] and enhance basal sliding by altering subglacial hydrology [6][7][8]. In fact, hydrofracture occurs at tens of thousands of sites on the Greenland ice sheet each summer, and its occurrence is increasing with atmospheric warming [9]. ...
Hydraulic fracture (or hydrofracture) can promote the propagation of meltwater-filled surface crevasses in glaciers and, in some cases, lead to full-depth penetration that can enhance basal sliding and iceberg calving. Here, we propose a novel poro-damage phase field model for hydrofracturing of glacier crevasses, wherein the crevasse is represented by a nonlocal damage zone and the effect of hydrostatic pressure due to surface meltwater is incorporated based on Biot’s poroelasticity theory. We find that the elastic strain energy decomposition scheme of Lo et al. (2019) with an appropriate fracture energy threshold can adequately represent the asymmetric tensile-compressive fracture behavior of glacier ice subjected to self-gravity loading. We assessed the performance of the model against analytical linear elastic fracture mechanics solutions by comparing their predictions of maximum crevasse penetration depth. The model simulates both surface crevasse propagation in the interior region of the glacier, as well as cliff failure in the terminus region. The excellent performance of the proposed model for air/water-filled surface crevasses in idealized land- and marine-terminating grounded glaciers illustrates its applicability to studying the dynamic response of glaciers to atmospheric warming.
... Examples from western Greenland show that water delivery to the bed uses moulins but J o u r n a l P r e -p r o o f also occurs as supraglacial lakes drain to the bed through hydrofracturing (Das et al., 2008). The drainage of supraglacial lakes is possible even on thick ice-sheets, <1km (Alley et al., 2005), and supraglacial lakes are common throughout the ablation area of western Greenland (Chu, 2014; Selmes et al., 2011), and the zone of supraglacial lakes can be at least 100 km wide (van den Broeke et al., 2008). These lake drainages have been shown to occur annually or perhaps several times per year (Chudley et al., 2019;Leeson et al., 2013;McMillan et al., 2007;Selmes et al., 2011), and can last with concomitant elevated water pressures for at least hours to days (Box and Ski, 2007). ...
Knowledge about processes beneath ice sheets, and in particular the processes connected to subglacial hydrology, is crucial for an understanding of ice sheets and how they react in a warming climate. Recently, v-shaped subglacial landforms (murtoos) have been found in those parts of the former Fennoscandian Ice Sheet where rapid ice-margin retreat occurred. Based on their geomorphology and distribution, murtoos have been suggested to form where the bed experienced high influxes of meltwater. Here, we investigate the sedimentology and internal structure of murtoos at four localities in southern Sweden to better understand murtoo genesis. The excavated murtoos consist of heterogenous diamict showing reasonably strong fabrics interbedded with sorted sediments. Sediments show signs of ductile deformation and lquefaction. We interpret these landforms as subglacial landforms created by till deposition and sedimentation from meltwater with subsequent deformation. Cross-cutting relationships and inter-bedding of sorted sediments suggest a stepwise formation including periodic deformation events. We propose a model that is based on a dynamic subglacial meltwater system. We suggest that the subglacial environment is within the distributed system where the bed receives meltwater from repeated influxes of supraglacially derived meltwater. The processes suggested in this model of formation are strikingly similar to the character of glaciological and hydrological dynamics observed on the Greenland ice sheet today.
... These lakes have been monitored by satellites for lake evolution and lake depth (Box and Ski, 2007;Sneed and Hamilton, 2007;Pope et al., 2016). Supraglacial lakes deliver surface melt water to the icebed interface via hydrofracture, moulins, and crevasse drainage, resulting in local and regional acceleration (Alley et al., 2005;Das et al., 2008;Krawczynski et al., 2009;Colgan et al., 2011;Sole et al., 2011;Smith et al., 2015;Stevens et al., 2015). The impact of infiltrating surface melt water on basal friction, inducing an ice dynamic response has been well documented (Iken and Bindschadler, 1986;Zwally et al., 2002;Meierbachtol et al., 2013;Doyle et al., 2014;Willis et al., 2015;Chu et al., 2016). ...
Supraglacial lakes over the Greenland Ice Sheet can demonstrate multi-model drainage states. Lakes can demonstrate incomplete drainage, where residual melt can become buried under ice and snow and survive throughout the winter. We evaluate atmospheric factors that influence the propensity for the formation of buried lakes over the ice sheet. We examine the spatial and temporal occurrence and behavior of buried lakes over the Jakobshavn Isbrae and Zachariae Isstrøm outlet basins and assess the magnitude of insolation necessary to preserve melt water using a numerical lake model from 2009 to 2012. Buried lakes tend to occur at higher elevations within the ablation zone and those present at elevations > 1000 m tend to reoccur over several seasons. Lakes without buried water are relatively small (∼1 km²), whereas lakes with buried water are larger (∼6–10 km²). Lake area is correlated with the number of seasons sub-surface water persists. Buried lakes are relatively deep and associated with complex supraglacial channel networks. Winter stored water could be a precursor to the formation of supraglacial channels. Simulations of the insulation potential of accumulated snow and ice on the surface of lakes indicate substantial regional differences and inter-annual variability. With the possibility of inland migration of supraglacial lakes, buried lakes could be important in the evolution of ablation/percolation zone hydrology.
... Conventional understanding is that rapid lake drainages are confined to the summer. They may be driven by active in-situ hydrofracture through the lake bottom triggered by rising water levels in the lake (Alley et al., 2005;van der Veen, 2007;Krawczynski et al., 2009;Arnold 45 et al., 2014;Clason et al., 2015) and/or by passive fracture in response to perturbations in ice sheet flow induced by surface meltwater initially tapping the bed via nearby moulins (Stevens et al., 2015;Chudley et al., 2019). Conventional understanding is that lakes that completely or partially drain during the summer then freeze during the winter, opening of crevasses and the generation of moulins maintaining a liquid water core Koenig et al., 2015;Miles et al., 2017;Law et al., forthcoming 2020). ...
Abstract. Surface lakes on the Greenland Ice Sheet play a key role in its surface mass balance, hydrology, and biogeochemistry. They often drain rapidly in the summer via hydrofracture, which immediately delivers lake water to the ice sheet base over timescales of hours to days and then allows meltwater to reach the base for the rest of the summer. Rapid lake drainage, therefore, influences subglacial drainage evolution, water pressures, ice flow, biogeochemical activity, and ultimately the delivery of water, sediments and nutrients to the ocean. It is assumed that rapid lake drainage events are confined to the summer, as this is when all observations to date have been made. Here we develop a method to quantify backscatter changes in satellite radar imagery, which we use to document the drainage of six different lakes during three winters in fast flowing parts of the Greenland Ice Sheet. Analysis of optical imagery from before and after the three winters supports the radar-based evidence for winter lake drainage events and also provides estimates of lake drainage volumes, which range between 0.000046 and 0.0202 km3. For three of the events, optical imagery allows photoclinometry (shape from shading) calculations to be made showing mean vertical collapse of the lake surfaces ranging between 4.04 m and 7.25 m, and drainage volumes of 0.004 km3 to 0.049 km3. The findings show that background winter ice motion can trigger rapid lake drainage, which may have important implications for subglacial hydrology and biogeochemical processes.
... The ocean circulation under the ice shelf is strongly linked to the forcing provided by the ice shelf and the bathymetry (Williams et al., 2002). However, the presence of sea ice, indirectly modulates the stress on sea ice by reducing the amplitude of ocean swell (Thomson and Rogers, 2014), and the freezing atmospheric temperatures promote the changes in ice shelf morphology (Alley et al., 2005;Scambos et al., 2003). The rate of melting and freezing at the interface between the ice shelf and the ocean can be characterized by the magnitude of the interaction between the ice shelf and the ocean (Holland et al., 2008). ...
The Amery ice shelf (AIS) dynamics and mass balance play key role to decipher changes in the global climate scenario. The spatio-temporal changes in morphology of the AIS were studied into a number of transects at 5 km uniform intervals using multi-dated Moderate Resolution Imaging Spectro-radiometer (MODIS) satellite data (2001–2016) of the austral summer months (January–March). Past ice shelf extents have been reconstructed and future ice shelf extents were estimated for 5- and 10-year time periods. The rate changes of AIS extent were estimated using the linear regression analysis and cross-validated with the coefficient of determination (R²) and root-mean-square error (RMSE) methods. Further, the changes in shelf extent were linked to prevailing factors viz. mass changes, Southern Annular Mode (SAM) index, and ocean-air temperatures. The study reveals that the AIS extent has been prograded at the rate of 994 m/year with an average 14.5 km increase in the areal extents during 2001–2016, as compared to the year 2001, whereas, the maximum advancement in ice shelf extent was recorded during the 2006–2016 period. Based on the linear regression analysis, the predicted ice shelf extents (i.e., the summer 2021 and 2016) show progradation in all the transects. About 52% of transects exhibit ±200 m RMSE values, indicating better agreement between the estimated and satellite-based ice-shelf position. The recent changes (2017–2019) observed in the ice shelf are cross validated with predicted ice self-extent rates. The eastern part of Mackenzie Bay to Ingrid Christensen coast recorded advancement in the ice shelf extents and mass which is the feedback of positive SAM along with a decrease in the temperatures (air temperature and sea surface temperature). The present study demonstrates that the combined use of satellite imagery and statistical techniques can be useful in quantifying and predicting ice shelf morphological variability.
... Lake drainages have previously been proposed to exploit preexisting moulins (2,3). Evidence suggests that water entering the subglacial system through preexisting moulins can trigger hydrofracture by inducing localized acceleration and hence a transient extensional flow regime in a "precursor" event (3,30). However, there is no evidence of precursory acceleration or surface-to-bed connection at Lake 028. ...
Supraglacial lake drainage events influence Greenland Ice Sheet dynamics on hourly to interannual timescales. However, direct observations are rare, and, to date, no in situ studies exist from fast-flowing sectors of the ice sheet. Here, we present observations of a rapid lake drainage event at Store Glacier, west Greenland, in 2018. The drainage event transported 4.8 × 10 ⁶ m ³ of meltwater to the glacier bed in ∼5 h, reducing the lake to a third of its original volume. During drainage, the local ice surface rose by 0.55 m, and surface velocity increased from 2.0 m⋅d ⁻¹ to 5.3 m⋅d ⁻¹ . Dynamic responses were greatest ∼4 km downstream from the lake, which we interpret as an area of transient water storage constrained by basal topography. Drainage initiated, without any precursory trigger, when the lake expanded and reactivated a preexisting fracture that had been responsible for a drainage event 1 y earlier. Since formation, this fracture had advected ∼500 m from the lake’s deepest point, meaning the lake did not fully drain. Partial drainage events have previously been assumed to occur slowly via lake overtopping, with a comparatively small dynamic influence. In contrast, our findings show that partial drainage events can be caused by hydrofracture, producing new hydrological connections that continue to concentrate the supply of surface meltwater to the bed of the ice sheet throughout the melt season. Our findings therefore indicate that the quantity and resultant dynamic influence of rapid lake drainages are likely being underestimated.
... The ice-front pressure difference generates tensile ice stresses that tend to open nearly vertical surface crevasses behind the cliff. These may reach the bed if meltwater filled (Alley et al., 2005), but if well drained, extend downward a maximum of ~55%-60% of the subaerial cliff height in the limit of zero tensile ice strength (Fig. DR4A), at which point there is a transition to compression (Nye, 1957). (In crevasse fields such as those observed on Helheim's surface [ Fig. DR2], penetration depths of air-filled crevasses are reduced somewhat [van der Veen, 1998] by stress interactions.) ...
Retrogressive slumping could accelerate sea-level rise if ice-sheet retreat generates ice cliffs much taller than observed today. The tallest ice cliffs, which extend roughly 100 m above sea level, calve only after ice-flow processes thin the ice to near flotation. Above some ice-cliff height limit, the stress state in ice will satisfy the material-failure criterion, resulting in faster brittle failure. New terrestrial radar data from Helheim Glacier, Greenland, suggest that taller subaerial cliffs are prone to failure by slumping, unloading submarine ice to allow buoyancy-driven full-thickness calving. Full-Stokes diagnostic modeling shows that the threshold cliff height for slumping is likely slightly above 100 m in many cases, and roughly twice that (145-285 m) in mechanically competent ice under well-drained or low-melt conditions.
... Moulins are formed due to melting, by fastflowing water, of a previously opened preferential flow pathway such as a crevasse. Surface fractures (or crevasses) may be extended to the ice sheet bed by "hydrofracture"-meltwaterinduced downward propagation of pre-existing fractures (Weertman, 1972;van der Veen, 1998;Alley et al., 2005;Krawczynski et al., 2009). ...
Coupling between runoff, hydrology, basal motion, and mass loss (“hydrology-dynamics”) is a critical component of the Greenland Ice Sheet system. Despite considerable research effort, the mechanisms by which runoff influences ice dynamics and the net long-term (decadal and longer) dynamical effect of variations in the timing and magnitude of runoff delivery to the bed remain a subject of debate. We synthesise key research into land-terminating ice sheet hydrology-dynamics, in order to reconcile several apparent contradictions that have recently arisen as understanding of the topic has developed. We suggest that meltwater interaction with subglacial channels, cavities, and deforming subglacial sediment modulates ice flow variability. Increasing surface runoff supply to the bed induces cavity expansion and sediment deformation, leading to early-melt season ice flow acceleration. In the ablation area, drainage of water at times of low runoff from high-pressure subglacial environments toward more efficient drainage pathways is thought to result in reductions in water pressure, ice-bed separation and sediment deformation, causing net slow-down on annual to decadal time-scales (ice flow self-regulation), despite increasing surface melt. Further inland, thicker ice, small surface gradients and reduced runoff suppress efficient drainage development, and a small net increase in both summer and winter ice flow is observed. Predicting ice motion across land-terminating sectors of the ice sheet over the twenty-first century is confounded by inadequate understanding of the processes and feedbacks between runoff and subglacial motion. However, if runoff supply increases, we suggest that ice flow in marginal regions will continue to decrease on annual and longer timescales, principally due to (i) increasing drainage system efficiency in marginal areas, (ii) progressive depression of basal water pressure, and (iii) thinning-induced lowering of driving stresses. At higher elevations, we suggest that minor year-on-year ice flow acceleration will continue and extend further into the interior where self-regulation mechanisms cannot operate and if surface-to-bed meltwater connections form. Based on current understanding, we expect that ice flow deceleration due to the seasonal development of efficient drainage beneath the land-terminating margins of the Greenland Ice Sheet will continue to regulate its future mass loss.
... At the bottom of these lakes high pressure can cause hydro-fracturing, as described by e.g. Alley et al. (2005a), Veen (2007), and Das et al. (2008), where the resulting cracks can form connections to the base through up to 1000 m thick ice. These connections are called moulins and they are the most important mechanism for rapid water transport to the base of the ice. ...
Subglacial hydrology is a key component in ice sheet dynamics and controls the sliding of ice sheets. Modelling the integrated system between ice dynamics and subglacial hydrology is essential for understanding current changes in the system and projecting future evolution of ice sheets and their contribution to sea level rise. The recent acceleration of mass loss of the Greenland ice sheet can be largely attributed to dynamic thinning at the ice margin, where hydrologic processes play a significant role in the speed-up of outlet glaciers. Models of subglacial hydrology recently have progressed to incorporate multiple components of the drainage system and are able to represent observed seasonal evolution of an efficient drainage system during the melt season, but the application of models on a continental scale remains a challenge. This doctoral thesis analyzes different approaches to model the subglacial hydrology and its interaction with the ice flow in respect to their ability to be applied to large domains. Two different models are developed and analyzed. A balance flux model coupled to the ice dynamics model SICOPOLIS is used to study the effect of subglacial water on the Eurasian ice sheet, applied to the simulation of future sea level contribution of Greenland where it reveals that the effect of subglacial discharge on submarine melting is comparable to increased ocean warming. Additionally, this model is utilized in the study of subglacial lakes at Recovery Glacier, Antarctica. The second model is an equivalent aquifer model which describes the water flow in a porous layer adapted to exhibit the properties of the complex drainage system. The evolution of the system is achieved by locally adjusting the transmissivity. It is shown that this approach leads to realistic pressure and discharge distributions which compare well with more sophisticated models, while keeping computational costs low.
... In contrast, the CV groups (6 and 7) that are defined by isolated ponded regions that are distributed over a larger area at higher elevations where the ice is thicker, are less likely to instantaneously drain large volumes of water directly to the bed (Figure 2A). The capacity to propagate water-filled fractures to the bed is dependent on the rate of meltwater input into the fractures, which controls the degree to which hydrostatic pressure can overcome fracture tip closure (van der Veen, 1998;Van der Veen, 2007;Alley et al., 2005). Though the water-filled crevasses in this study are closely spaced, there is sufficient tensile stress over these features to propagate fractures to the bed (Lampkin et al., 2013). ...
The Greenland Ice Sheet has experienced accelerated mass loss over the last couple decades, in part due to destabilization of marine-terminating outlet glaciers. Retreat and acceleration of outlet glaciers coincides with atmospheric and oceanic warming resulting in a significant contribution to sea-level rise. The relative role of surface meltwater production, runoff and infiltration on the dynamics of these systems is not well-understood. To assess how surface meltwater impacts shear margin dynamics and regional ice flow of outlet glaciers, we investigate the impact of basal lubrication of Jakobshavn Isbræ shear margins due to drainage from water-filled crevasses. We map the areal extent of inundated crevasses during summer (May–August) from 2000 to 2012 using satellite imagery and determined an increasing trend in the total areal extent over this time interval. We use a numerical ice flow model to quantify the potential impact of weakened shear margins due to surface melt derived basal lubrication on regional flow velocities. Ice flow velocities 10 km from the lateral margins of Jakobshavn were amplified by as much as 20%, resulting in an increase of ~0.6 Gt yr⁻¹ in ice-mass discharge through the shear margins into the ice stream. Under future warming scenarios with increased surface melt ponding, simulations indicate up to a 30% increase in extra-marginal ice flow. We conclude that surface meltwater will likely play an important role in the evolving dynamics of glacier shear margins and the future mass flux through Greenland's major marine-terminating outlet glaciers.
... While the hairline fractures could perhaps move some water, to permit much water to move meters through cold ice they would need to be large enough that water moves quickly and does not instantaneously refreeze. For example, a 1 mm wide crack in ice that is −10 • C freezes shut in about 45 s (Alley et al., 2005;Eq. 8). ...
To assess the influence of various heat transfer processes on the thermal structure of near-surface ice in Greenland's ablation zone, we compare in situ measurements with thermal modeling experiments. A total of seven temperature strings were installed at three different field sites, each with between 17 and 32 sensors and extending up to 21 m below the ice surface. In one string, temperatures were measured every 30 min, and the record is continuous for more than 3 years. We use these measured ice temperatures to constrain our modeling experiments, focusing on four isolated processes and assessing the relative importance of each for the near-surface ice temperature: (1) the moving boundary of an ablating surface, (2) thermal insulation by snow, (3) radiative energy input, and (4) subsurface ice temperature gradients below the seasonally active near-surface layer. In addition to these four processes, transient heating events were observed in two of the temperature strings. Despite no observations of meltwater pathways to the subsurface, these heating events are likely the refreezing of liquid water below 5–10 m of cold ice. Together with subsurface refreezing, the five heat transfer mechanisms presented here account for measured differences of up to 3 ∘C between the mean annual air temperature and the ice temperature at the depth where annual temperature variability is dissipated. Thus, in Greenland's ablation zone, the mean annual air temperature is not a reliable predictor of the near-surface ice temperature, as is commonly assumed.
The mechanical behaviour of ice is a complex phenomenon that is influenced by various factors, such as temperature, loading conditions, and structural geometry. To accurately predict the response of ice structures and estimate ice loads, appropriate models are required. In this article, we have reviewed several widely known material models for ice, including elastic, viscoelastic, plastic, damage, and fracture models. Elastic models are simple and easy to use, but they do not account for the time-dependent behaviour of ice. Viscoelastic models, on the other hand, can predict the evolution of damage and failure in ice structures but can be computationally ex-pensive. Plastic models can simulate the ductile behaviour of ice under high stress but do not account for damage and fracture. Damage models can simulate the evolution of damage and failure in ice structures but can also be computationally expensive. Fracture models can simulate the brittle behaviour of ice and predict crack propagation but require accurate input data. In practice, a combination of models is often used to account for different aspects of ice behaviour. With the advances in computer technology and simulation techniques, it is be-coming increasingly possible to simulate more complex ice structures and loading conditions. This could lead to the development of more accurate and efficient ice models that can be used for a wider range of applications, such as predicting the behaviour of ice structures in response to climate change. The effects of climate change on the behaviour of ice and the resulting impact on infrastructure are a growing concern. Therefore, the development of more accurate and efficient ice models is critical for the sustainable development of these regions.
This article presents a numerical solution of the one-dimensional Stefan problem with two phase transitions, which is implemented on a non-uniform grid. The system of equations is written in a general form, i.e. it includes not only conductive, but also convective and dissipative terms. The problem is solved numerically by the front-fixing method on a non-uniform grid using an implicit finite-difference scheme, which is implemented by the sweep method. This algorithm can also be used to create more complex mathematical models of heat and mass transfer, as well as to describe glacial and subglacial processes. The mathematical apparatus proposed in the article was used to solve a specific problem of water freezing in a glacial crevasse. The presence and progression of crevasses, in turn, is a demonstrative factor indicating the dynamic activity of the glacier. Crevasses formed in one way or another can not only expand, but also decrease in size until they completely disappear. One of the reasons for their closure is the freezing of near-surface meltwater in the crevasse. Such a process was observed on glaciers near Mirny and Novolazarevskaya stations (East Antarctica). This process is modeled as an example of solving the Stefan problem. It is believed that all media are homogeneous and isotropic. The temperature of the water in the crevasse corresponds to the melting temperature of the ice. Modeling has shown that for the coastal part of the cold Antarctic glacier with an average temperature of –10°C and below, crevasses 5–10 cm of width freeze in less than a week. Wider ones freeze a little longer. 30 cm wide crevasses close in about two to three weeks, depending on the temperature of the glacier.
Fluid-filled fracture propagation is a complex problem that is ubiquitous in geosciences, from controlling magma propagation beneath volcanoes to water transport in glaciers. Using scaled analog experiments, we characterized the internal flow inside a propagating flux-driven fracture and determined the relationship between flow and fracture evolution. Different flow conditions were created by varying the viscosity and flux (Q) of a Newtonian fluid injected into an elastic solid. Using particle image velocimetry, we measured the fluid velocity inside the propagating fracture and mapped the flow across the crack plane. We characterized the internal flow behavior with the Reynolds number (Re) and explored Re values spanning five orders of magnitude, representing very different internal force balances. The overall fracture tip propagation velocity is a simple linear function of Q, whereas the internal velocity, and Re, may be vastly different for a given Q. We identified four flow regimes—viscous, inertial, transitional, and turbulent—and produced viscous and inertial regimes experimentally. Both flow regimes exhibit a characteristic flow pattern of a high-velocity central jet that develops into two circulating vortices on either side. However, they exhibit the opposite behavior in response to changing Q: the jet length increases with Q in the inertial regime, yet decreases in the viscous regime. Spatially variable, circulating flow is vastly different from the common assumption of unidirectional fracture flow and has strong implications for the mixing efficiency and heat transfer processes in volcanic and glacial applications.
In Southeast Greenland, summer melt and high winter snowfall rates give rise to firn aquifers: vast stores of meltwater buried beneath the ice-sheet surface. Previous detailed studies of a single Greenland firn aquifer site suggest that the water drains into crevasses, but this is not known at a regional scale. We develop and use a tool in Ghub, an online gateway of shared datasets, tools and supercomputing resources for glaciology, to identify crevasses from elevation data collected by NASA's Airborne Topographic Mapper across 29000 km ² of Southeast Greenland. We find crevasses within 3 km of the previously mapped downglacier boundary of the firn aquifer at 20 of 25 flightline crossings. Our data suggest that crevasses widen until they reach the downglacier boundary of the firn aquifer, implying that crevasses collect firn-aquifer water, but we did not find this trend with statistical significance. The median crevasse width, 27 meters, implies an aspect ratio consistent with the crevasses reaching the bed. Our results support the idea that most water in Southeast Greenland firn aquifers drains through crevasses. Less common fates are discharge at the ice-sheet surface (3 of 25 sites) and refreezing at the aquifer bottom (1 of 25 sites).
A conceptual model, based on field observations and assumed physics of a perennial firn aquifer near Helheim Glacier (southeast Greenland), is evaluated via steady-state 2-D simulation of liquid water flow and energy transport with phase change. The simulation approach allows natural representation of flow and energy advection and conduction that occur in vertical meltwater recharge through the unsaturated zone and in lateral flow within the saturated aquifer. Agreement between measured and simulated aquifer geometry, temperature, and recharge and discharge rates confirms that the conceptual field-data-based description of the aquifer is consistent with the primary physical processes of groundwater flow, energy transport and phase change. Factors that are found to control simulated aquifer configuration include surface temperature, meltwater recharge rate, residual total-water saturation and capillary fringe thickness. Simulation analyses indicate that the size of perennial firn aquifers depends primarily on recharge rates from surface snowmelt. Results also imply that the recent aquifer expansion, likely due to a warming climate, may eventually produce lakes on the ice-sheet surface that would affect the surface energy balance.
Nearly all meltwater from glaciers and ice sheets is routed englacially through moulins. Therefore, the geometry and evolution of moulins has the potential to influence subglacial water pressure variations, ice motion, and the runoff hydrograph delivered to the ocean. We develop the Moulin Shape (MouSh) model, a time-evolving model of moulin geometry. MouSh models ice deformation around a moulin using both viscous and elastic rheologies and melting within the moulin through heat dissipation from turbulent water flow, both above and below the water line. We force MouSh with idealized and realistic surface melt inputs. Our results show that, under realistic surface melt inputs, variations in surface melt change the geometry of a moulin by approximately 10 % daily and over 100 % seasonally. These size variations cause observable differences in moulin water storage capacity and moulin water levels compared to a static, cylindrical moulin. Our results suggest that moulins are important storage reservoirs for meltwater, with storage capacity and water levels varying over multiple timescales. Implementing realistic moulin geometry within subglacial hydrologic models may therefore improve the representation of subglacial pressures, especially over seasonal periods or in regions where overburden pressures are high.
Marine-terminating glaciers lose mass through melting and iceberg calving, and we find that meltwater drainage systems influence calving timing at Helheim Glacier, a tidewater glacier in East Greenland. Meltwater feeds a buoyant subglacial discharge plume at the terminus of Helheim Glacier, which rises along the glacial front and surfaces through the mélange. Here, we use high-resolution satellite and time-lapse imagery to observe the surface expression of this meltwater plume and how plume timing and location compare with that of calving and supraglacial meltwater pooling from 2011 to 2019. The plume consistently appeared at the central terminus even as the glacier advanced and retreated, fed by a well-established channelized drainage system with connections to supraglacial water. All full-thickness calving episodes, both tabular and non-tabular, were separated from the surfacing plume by either time or by space. We hypothesize that variability in subglacial hydrology and basal coupling drive this inverse relationship between subglacial discharge plumes and full-thickness calving. Surfacing plumes likely indicate a low-pressure subglacial drainage system and grounded terminus, while full-thickness calving occurrence reflects a terminus at or close to flotation. Our records of plume appearance and full-thickness calving therefore represent proxies for the grounding state of Helheim Glacier through time.
Perennial firn aquifers are subsurface meltwater reservoirs consisting of a meters-thick water-saturated firn layer that can form on spatial scales as large as tens of kilometers. They have been observed within the percolation facies of glaciated regions experiencing intense seasonal surface melting and high snow accumulation. Widespread perennial firn aquifers have been identified within the Greenland Ice Sheet (GrIS) via field expeditions, airborne ice-penetrating radar surveys, and satellite microwave sensors. In contrast, ice slabs are nearly-continuous ice layers that can also form on spatial scales as large as tens of kilometers as a result of surface and subsurface water-saturated snow and firn layers sequentially refreezing following multiple melting seasons. They have been observed within the percolation facies of glaciated regions experiencing intense seasonal surface melting, but in areas where snow accumulation is at least 25% lower as compared to perennial firn aquifer areas. Widespread ice slabs have recently been identified within the GrIS via field expeditions and airborne ice-penetrating radar surveys, specifically in areas where perennial firn aquifers typically do not form. However, ice slabs have yet to be identified from space. Together, these two ice sheet features represent distinct, but related, sub-facies within the broader percolation facies of the GrIS that can be defined primarily by differences in snow accumulation, which influences the englacial hydrology and thermal characteristics of firn layers at depth.
Here, for the first time, we use enhanced-resolution vertically-polarized L-band brightness temperature imagery (2015-2019) generated using observations collected over the GrIS by NASA’s Soil Moisture Active Passive (SMAP) satellite to map perennial firn aquifer and ice slab areas together as a continuous englacial hydrological system. We use an empirical algorithm previously developed to map the extent of Greenland’s perennial firn aquifers via fitting exponentially decreasing temporal L-band signatures to a set of sigmoidal curves. This algorithm is recalibrated to also map the extent of ice slab areas using airborne ice-penetrating radar surveys collected by NASA’s Operation Ice Bridge (OIB) campaigns (2010-2017). Our SMAP-derived maps show that between 2015 and 2019, perennial firn aquifer areas extended over 64,000 km2, and ice slab areas extended over 76,000 km2. Combined together, these sub-facies are the equivalent of 24% of the percolation facies of the GrIS. As Greenland’s climate continues to warm, seasonal surface melting will increase in extent, intensity, and duration. Quantifying the possible rapid expansion of these sub-facies using satellite L-band microwave radiometry has significant implications for understanding ice sheet-wide variability in englacial firn hydrology that may drive meltwater-induced hydrofracturing and accelerated ice flow as well as high-elevation meltwater runoff that can impact the mass balance and stability of the GrIS.
Surface crevasses on the Greenland Ice Sheet deliver significant volumes of meltwater to the englacial and subglacial environment, but the topic has received little attention compared to supraglacial lake and moulin drainage. Here, we explore relationships between crevasse hydrology and the surface stress regime at a fast-flowing, marine-terminating sector of the Greenland ice sheet. Regional-scale observations of surface water, crevasses, and stress were made across a 3,000 km2 region using satellite data. Contemporaneous high spatio-temporal resolution observations were obtained from uncrewed aerial vehicle surveys on Store Glacier using a supervised classifier and feature-tracked velocities. While previous studies have identified crevasses using von Mises stress thresholds, we find these are insufficient for predicting crevasse hydrology. We found that dry crevasse fields, where no ponded meltwater was observed through the entire melt season, were more likely to exist in tensile mean stress regimes, which we interpret to be due to meltwater draining continuously into the englacial system. Conversely, wet crevasse fields, hosting ponded meltwater, were more likely to exist in compressive mean stress regimes, which we interpret to be a result of closed englacial conduits. We show that these ponded crevasses drain through episodic rapid drainage events (i.e. hydrofracture). Mean stress regime can therefore inform spatially heterogeneous styles of meltwater delivery through crevasses to the bed of ice sheets, with distinct consequences for basal processes such as subglacial drainage efficiency and cryo-hydrologic warming. Thus, we recommend simple guidelines for improving the representation of crevasse hydrology in regional hydrological models.
Hydrofracturing can enhance the depth to which crevasses propagate and, in some cases, allow full depth crevasse penetration and iceberg detachment. However, many existing crevasse models either do not fully account for the stress field driving the hydrofracture process and/or treat glacier ice as elastic, neglecting the non-linear viscous rheology. Here, we present a non-local continuum poro-damage mechanics (CPDM) model for hydrofracturing and implement it within a full Stokes finite element formulation. We use the CPDM model to simulate the propagation of water-filled crevasses in idealized grounded glaciers, and compare crevasse depths predicted by this model with those from linear elastic fracture mechanics (LEFM) and zero stress models. We find that the CPDM model is in good agreement with the LEFM model for isolated crevasses and with the zero stress model for closely-spaced crevasses, until the glacier approaches buoyancy. When the glacier approaches buoyancy, we find that the CPDM model does not allow the propagation of water-filled crevasses due to the much smaller size of the tensile stress region concentrated near the crevasse tip. Our study suggests that the combination of non-linear viscous and damage processes in ice near the tip of a water-filled crevasse can alter calving outcomes.
Ground-penetrating radar data acquired in the 2016/17 austral summer on Sørsdal Glacier, East Antarctica, provide evidence for meltwater lenses within porous surface ice that are conceptually similar to firn aquifers observed on the Greenland Ice Sheet and the Arctic and Alpine glaciers. These englacial water bodies are associated with a dry relict surface basin and consistent with perennial drainage into an interconnected englacial drainage system, which may explain a large englacial outburst flood observed in satellite imagery in the early 2016/17 melt season. Our observations indicate the rarely-documented presence of an englacial hydrological system in Antarctica, with implications for the storage and routing of surface meltwater. Future work should ascertain the spatial prevalence of such systems around the Antarctic coastline, and identify the degree of surface runoff redistribution and storage in the near surface, to quantify their impact on surface mass balance.
Glacier-erosion rates range across orders of magnitude, and much of this variation cannot be attributed to basal sliding rates. Subglacial till acts as lubricating ‘fault gouge’ or ‘sawdust’, and must be removed for rapid subglacial bedrock erosion. Such erosion occurs especially where and when moulin-fed streams access the bed and are unconstrained by supercooling or other processes. Streams also may directly erode bedrock, likely with strong time-evolution. Erosion is primarily by quarrying, aided by strong fluctuations in the water system driven by variable surface melt and by subglacial earthquakes. Debris-bed friction significantly affects abrasion, quarrying and general glacier flow. Frost heave drives cirque headwall erosion as winter cold air enters bergschrunds, creating temperature gradients to drive water flow along premelted films to growing ice lenses that fracture rock, and the glacier removes the resulting blocks. Recent subglacial bedrock erosion and sediment flux are in many cases much higher than long-term averages. Over glacial cycles, evolution of glacial-valley form feeds back strongly on erosion and deposition. Most of this is poorly quantified, with parts open to argument. Glacial erosion and interactions are important to tectonic and volcanic processes as well as climate and biogeochemical fluxes, motivating vigorous research.
We analyze Landsat-7 imagery spanning a 13-year period (2000–2012) for the Jakobshavn Ablation Region (JAR) along the west coast of Greenland. In addition, we introduce a new semi-automated technique for the mapping of melt-lakes using FoveaPro image-processing software (plug-in to Adobe Photoshop™), greatly simplifying the process, and resulting in more-precise spatial melt-lake statistics over existing manual methods. We found a total mean melt-lake area of 0.30 ± 0.12 km² (±1σ), with maximum melt-lake area increasing at an average rate of 0.032 km² d⁻¹ across the study periods. Additionally, we note a yearly seasonal increase (∼1.8 m d⁻¹) in the overall mean lake elevation (∼200 m per season) as well as an optimal elevation of the largest-area melt-lakes of ∼1320 ± 20 m (±1σ). We also found an increase in the maximum average melt-lake elevation (MAME) of ∼3.8 m a⁻¹ (∼50 m). Based on data recorded at nearby automated weather stations, the mean seasonal temperature increased ∼1.6°C over the 13-year period at an average rate of 0.125°C a⁻¹. Although temperature is a driver for meltwater production, we conclude that mechanisms related to the surface topography are more likely modulating the spatial pattern and characteristics of melt lakes in the ablation zone.
Subglacial water drainage plays a significant role in glacier flow dynamics. Various forms of subglacial drainage have been observed beneath modern ice sheets, and the same forms of drainage likely occurred beneath paleo-ice sheets, such as the Laurentide Ice Sheet. Records of paleo drainage are occasionally preserved in the geomorphic record and can serve as a widespread and easily accessible means to investigate aspects of subglacial drainage that are difficult to directly study on modern-day glaciers. Linear surface depressions that extend tens of kilometers inward from the margin of Laurentide lobes are found throughout the Midwest of North America and are hypothesized to form from incision by channelized subglacial water flow. Here we estimate the subglacial hydropotential and its gradient along the western margin of the Marine Isotope Stage 2 Laurentide Ice Sheet's Green Bay Lobe to identify potential locations for subglacial water pooling. We find that linear surface depressions correlate well with areas where subglacial water likely pooled. We use a combination of active and passive seismic analyses to estimate the actual size of an incised subglacial channel in Waushara County, Wisconsin, that was subsequently infilled with sediment after the formation. We find that the channel incised ca. 65 m into the surrounding unlithified material, which is 6 times greater than the modern surface expression, and has a width of 450 m, which is nearly equal to the width of the surface expression. We estimate the channel would have taken ca. 60 days of water flow to form and that the flow of pore water from the surrounding till into the low-pressure channel could have increased the strength of the till by as much as 140 kPa following a single drainage by as much as 175 kPa in a smaller region following a series of 15-day-long drainage events. Till strengthening could have been a factor of 7 to 25× greater than predrainage strength values.
The interior dynamics of Helheim Glacier were monitored using an autonomous phase-sensitive radio-echo sounder (ApRES) during two consecutive summers. The return signals from all observational sites exhibited strong non-tidal, depth-dependent diurnal variations. We show that these variations in the glacier interior can be explained by an englacial diurnal meltwater cycle: a data interpretation that assumes constant ice-column composition through time leads to dynamical inconsistencies with concurrent observations from GPS and terrestrial radar. The observed diurnal meltwater cycle is spatially variable, both between different sites and in the vertical, consistent with the existence of a dense and complex englacial hydrologic network. Future applications of this observational technique could reveal long-term meltwater behavior inside glaciers and ice sheets, leading to an improved understanding of the spatiotemporal evolution of the basal boundary conditions needed to simulate them realistically.
Distant storms, tsunamis, and earthquakes generate waves on floating ice shelves. Previous studies, however, have disagreed about whether the resulting wave-induced stresses may cause ice shelf rift propagation. Most ice shelf rifts show long periods of dormancy suggesting that they have low background stress concentrations and may therefore be susceptible to wave-induced stresses. Here, I quantify wave-induced stresses on the Ross Ice Shelf Nascent Rift and the Amery Ice Shelf Loose Tooth T2 Rift using passive seismology. I then relate these stresses to a fracture mechanical model of rift propagation that accounts for rift cohesive strength due to refrozen melange, ice inertia, and spatial heterogeneity in fracture toughness due to the presence of high toughness suture zones. I infer wave-induced stresses using the wave impedance tensor, a rank three tensor that relates seismically observable particle velocities to components of the stress tensor. I find that wave-induced stresses are an order of magnitude larger on the Ross Ice Shelf as compared to the Amery Ice Shelf. In the absence of additional rift strength, my model predicts that the Nascent Rift should have experienced extensive rift propagation. The observation that no such propagation occurred during this time therefore suggests that the Nascent Rift experiences cohesive strengthening from either refrozen melange or rift tip processes zone dynamics. This study illustrates one way in which passive seismology may illuminate glacier calving physics.
Distant storms, tsunamis, and earthquakes generate waves on floating ice shelves. Previous studies, however, have disagreed about whether the resulting wave-induced stresses may cause ice shelf rift propagation. Most ice shelf rifts show long periods of dormancy suggesting that they have low background stress concentrations and may therefore be susceptible to wave-induced stresses. Here I quantify wave-induced stresses on the Ross Ice Shelf Nascent Rift and the Amery Ice Shelf Loose Tooth T2 Rift using passive seismology. I then relate these stresses to a fracture mechanical model of rift propagation that accounts for rift cohesive strength due to refrozen melange, ice inertia, and spatial heterogeneity in fracture toughness due to the presence of high toughness suture zones. I infer wave-induced stresses using the wave impedance tensor, a rank three tensor that relates seismically observable particle velocities to components of the stress tensor. I find that wave-induced stresses are an order of magnitude larger on the Ross Ice Shelf as compared to the Amery Ice Shelf. In the absence of additional rift strength, my model predicts that the Nascent Rift should have experienced extensive rift propagation. The observation that no such propagation occurred during this time therefore suggests that the Nascent Rift experiences strengthening from either refrozen melange or rift tip processes zone dynamics. This study illustrates one way in which passive seismology may illuminate glacier calving physics.
Supraglacial lakes are known to trigger Antarctic ice-shelf instability and break-up. However, to date, no study has focussed on lakes on Greenland’s floating termini. Here we apply lake boundary/area and depth algorithms to Landsat 8 imagery to analyse the inter- and intra-seasonal evolution of supraglacial lakes across Petermann Glacier’s (81°N) floating tongue from 2014-2016, while also comparing these lakes to those on the grounded ice. Lakes start to fill in June and quickly peak in total number, volume and area in late June/early July in response to increases in air temperatures. However, through July and August, total lake number, volume, and area all decline, despite sustained high temperatures. These observations may be explained by the transportation of meltwater into the ocean by a river, and by lake drainage events on the floating tongue. Further, as mean lake depth remains relatively constant during this time, we suggest that a large proportion of the lakes that drain, do so completely, likely by rapid hydrofracture. The mean areas of lakes on the tongue are only ~20% of those on the grounded ice and exhibit lower variability in maximum and mean depth, differences likely attributable to the contrasting formation processes of lakes in each environment.
The modified ring test is used to determine the fracture toughness of synthetic, granular, fresh-water ice average density 0.891 Mg m ⁻³ and firn (average density 0.605 Mg m ³ ) from depths between 26 and 27.2 m in the E core of the Greenland Ice Sheet Project II. Average fracture toughness is 145.7kPa m ² for the manufactured ice and 108.6kPam ½ for the firn. Comparison between the ice and firn suggests that ice-fracture toughness decreases with decreasing density (i.e. increasing porosity), suggesting lateral and vertical variations in the near-surface fracture resistance of glaciers and ice sheets may be related to firn densification. The modified ring test has many advantages over conventional, notch-based specimens in that complications which arise in notched specimens due to crack-length, loading-rate, notch-acuity and specimen-size effects are irrelevant for a modified ring-specimen geometry.
A mathematical model is presented for the stability of sheet flow of water beneath a temperate glacier. Enhanced viscous heat dissipation in thick parts of the sheet tends to make sheet flow unstable, the instability increasing as sheet thickness and pressure gradient increase. However, incipient channels may be destroyed as the glacier slides over protuberances on its bed. Quasi-stable sheet flow may be possible for sheets up to several mm in thickness, especially beneath glaciers that have relatively gentle surface slopes and slide at moderate to high speeds. The presence of numerous water-filled cavities at the glacier bed will tend to reduce the sheet thickness and lessen the degree of 'lubrication' of the glacier bed by the water sheet.-Author Dept Geology, Stanford Univ, Stanford, California 94305, U.S.A.
Two disintegration events leading to the loss of Larsen A and B ice shelves, Antarctic Peninsula, in 1995 and 2002, respectively,proceeded with extreme rapidity (order of several days) and reduced an extensive, seemingly integrated ice shelf to a jumble of small fragments. These events strongly correlate with warming regional climate and accumulation of surface meltwater, supporting a hypothesis that meltwater-induced propagation of pre-existing surface crevasses may have initiated ice-shelf fragmentation.Weaddress here an additional, subsequent mechanism that may sustain and accelerate the ice-shelf break-up once it begins.The proposed mechanism involves the coherent capsize of narrow (less than thickness) ice-shelf fragments by rolling 90° in a direction toward, or away from, the ice front. Fragment capsize liberates gravitational potential energy, forces open ice-shelf rifts and contributes to further fragmentation of the surrounding ice shelf.
The numerous debris bands in the terminus region of Matanuska Glacier, Alaska, U.S.A., were formed by injection of turbid meltwaters into basal crevasses. The debris bands are millimeter(s)-thick layers of silt-rich ice cross-cutting older, debris-poor englacial ice. The sediment grain-size distribution of the debris bands closely resembles the suspended load of basal waters, and of basal and proglacial ice grown from basal waters, but does not resemble supraglacial debris, till or the bedload of subglacial streams. Most debris bands contain anthropogenic tritium (3H) in concentrations similar to those of basal meltwater and ice formed from that meltwater, but cross-cut englacial ice lacking tritium. Stable-isotopic ratios (δ18O and δD) of debris-band ice are consistent with freezing from basal waters, but are distinct from those in englacial ice. Ice petrofabric data along one debris band lack evidence of active shearing. High basal water pressures and locally extensional ice flow associated with overdeepened subglacial basins favor basal crevasse formation.
Ice flow at a location in the equilibrium zone of the west-central Greenland Ice Sheet accelerates above the midwinter average
rate during periods of summer melting. The near coincidence of the ice acceleration with the duration of surface melting,
followed by deceleration after the melting ceases, indicates that glacial sliding is enhanced by rapid migration of surface
meltwater to the ice-bedrock interface. Interannual variations in the ice acceleration are correlated with variations in the
intensity of the surface melting, with larger increases accompanying higher amounts of summer melting. The indicated coupling
between surface melting and ice-sheet flow provides a mechanism for rapid, large-scale, dynamic responses of ice sheets to
climate warming.
Cambridge Core - Hydrology, Hydrogeology and Water Resources - Principles of Glacier Mechanics - by Roger LeB. Hooke
Nye has estimated that the depth L of crevasses is equal to Tjog, where T is the tensile stress causing extending flow, Q is the density of ice, and g is the gravitational acceleration. This expression for L is derived on the assumption that the crevasses are closely spaced and free of water. It is shown in this paper that the depth of an isolated crevasse is a factor sr/2 greater than the depth calculated by Nye for closely spaced crevasses. It is shown further that the presence of water in a crevasse can increase its depth. A crevasse filled with water up to at least 97.4% of its depth can penetrate to the bottom of a glacier. Water-filled cavities can exist directly beneath water filled crevasses. RÉSUMÉ. Une crevasse emplie d'eau peut-elle atteindre la surface basale d'un glacier ? Nye a estimé que la profondeur L des cre-vasses est égale à Tjog, où Test la contrainte d'élongation causant un écoulement d'extension, est la densité de la glace et g est l'accélération de la pesanteur. Cette expression pour L est dérivée en supposant que les crevasses sont proches et vides d'eau. On montre ici que la profondeur d'une crevasse isolée est plus grande d'un facteur de JT/2 que la profondeur calculée par Nye pour les crevasses proches l'une de l'autre. Il est montré de plus que la présence d'eau dans une crevasse peut augmenter sa profondeur. Une crevasse emplie d'eau d'au moins 97,4% de sa profondeur peut pénétrer jusqu'au fond d'un glacier. Des cavités pleines d'eau peuvent exister directement sur des crevasses emplies d'eau.
The stress intensity factor for a semicircular edge crack is derived. Numerical values for axial, bending, and thermal loads in half spaces and plates are presented. The results show that a magnification of the stress intensity factor of about 20 percent occurs at the free surface.
An expression is developed for the stress intensity factor of a penny-shaped crack in an infinite elastic solid subjected to nonaxisymmetric normal loading. The stress intensity factor can then be determined for penny-shaped cracks in infinite or finite solids subjected to symmetric loading about the plane containing the crack. The singular state associated with the embedded crack with finite, nonaxisymmetric normal loading is that of plane strain. Results are also presented for two problems: A penny-shaped crack subjected to two symmetrically located concentrated forces and a penny-shaped crack in a large beam subjected to pure bending.
Observations from Ellesmere Island suggest that the connection between surface and subglacial drainage on a predominantly cold glacier is made abruptly by hydrologically-driven propagation of fractures from the surface to the bed. Where ice is 150 m thick, water ponded to a depth of 6.9 m within a supraglacial stream system before establishing a permanent bed connection. Multiple premonitory drainage events preceded the final drainage of ponded water, implying that fracturing is necessary, but insufficient, to establish a permanent link between surface and subglacial drainage. Refreezing of water that penetrates the first fractures to form may reseal the connection, while flow resistance within the subglacial system may delay the onset of continuous through-flow. A large volume of ponded water is required to enlarge fractures sufficiently by melting to maintain continuous drainage, while feedbacks between subglacial hydrology and ice dynamics may assist in maintaining the connection and initiating subglacial outflow.
A simple model, based on linear elastic fracture mechanics, is used to investigate conditions allowing the existence of bottom crevasses. On grounded glaciers, these crevasses can only occur if the basal water pressure is close to the ice overburden pressure. If the piezometric head drops more than ∼10 m below the flotation level, very large stretching rates are required for appreciable bottom crevasses to form. This combination of special conditions may explain the lack of observations of basal crevassing on grounded glaciers.
The theory of the flow of water at the base of a glacier is reviewed and extended. A detailed analysis is made of flow through Röthlisberger channels. (Röthlisberger channels are channels at the base of a glacier that are incised upward into the ice mass.) It is shown that, in general, a pressure gradient drives water melted from the bottom surface of a glacier away from Röthlisberger channels. Thus these channels are not good collectors of the water produced at the bottom surface of a glacier or ice sheet. If Nye channels exist (Nye channels are incised downward into the bedrock of a glacier bed), they should be important in the discharge of water at the base of a glacier. Nye tributary channels are expected to be spaced over several hundred meters apart. The analysis indicates that sheet flow is the main mechanism by which water melted from the bottom ice surface flows out of a glacier or ice sheet. The sheet flow described in this paper is a modification of that considered in our earlier papers. The thickness of the present sheets is not roughly uniform, but rather squeezes to negligible values at high-pressure zones, where ice moves over the upstream side of obstacles and irregularities in the bed. The pressure of the water in the sheets is lower than the ice overburden pressure. The difference between the pressure in the sheets and the ice overburden pressure is estimated with the aid of the recent major refinements to glacier sliding theory made by J. F. Nye and by B. Kamb. (L. Lliboutry made the original suggestion that the pressure of the water at the base of a glacier is smaller than the ice overburden pressure.) Analysis indicates that a single Röthlisberger channel runs down the center of a glacier over a major fraction of its length. Pressure gradients, produced by the concave shape of the transverse bed profile, exist that drive water in the sheet toward this channel. However, in the upper parts of a glacier, this channel may take the form of a thick water layer of finite breadth.
This review begins by highlighting the observations of ancient and modern dikes that lay the groundwork for current modeling. The emphasis then shifts to the dominant physical processes involved - host rock fracture and deformation (elastic and inelastic), magma flow, and heat transfer. This review attempts to provide a framework for thinking about important but poorly understood processes such as dike initiation, the role of dike propagation in the ascent of granitic magmas, and earthquakes accompanying magma transport. -from Author
Vigorous flow of central regions of Ice Stream C, West Antarctica, near the UpC camp ended about the year 1830, based on analysis of a firn and ice core taken at the camp. Ice-stream flow was characterized by repeated fracturing and healing, probably subsurface, especially near the onset of streaming flow. High longitudinal stresses caused fracturing, recrystallization of the ice and elongation of bubbles, and enhanced densification rates of high-density firn indicating power-law-creep behavior.
To test models of the hydraulics and geometry of englacial conduits, 48 holes (3900 m of ice) were drilled into Storglaciaren, Sweden, in search of conduits. About 79% of the holes intersected a hydraulically connected englacial feature. A video camera was used to examine the features and measure local water-flow rates. Because of the extremely clear ice that surrounded most features, their geometry could not be discerned. Of the remaining features, 80% (36) were fracture-like, 16% (6) were of complex geometry, and 4% (2) exhibited a conduit-like geometry. The fracture-like features exhibited steep plunges (∼70°), narrow openings (∼40 mm) and slow water-flow speeds (∼10 mm s−1). We argue that these fracture-like features are indeed englacial fractures of unknown origin. The depth to fractures intersection varied from near the glacier surface to 96% of local ice depth, with a maximum depth of 131 m. Few hydraulically connected fractures exhibited water motion, indicating some preferential flow pathways exist. We found one 'traditional' englacial conduit after an intentional search in a field of moulins. These results suggest that englacial water flow is conveyed through a ubiquitous network of fractures and that conduits are relatively rare.
Reversals in the ice/air surface slope are important in geomorphic and glaciological contexts, thus motivating consideration of the conditions under which they form. Surface slope reversals are seen in numerous places, such as ice rumples on ice shelves, as surficial lakes, and at the down-glacier end of Vostok lake, Antarctica. Such slope reversals can reduce or reverse the subglacial hydrological gradient, thereby rerouting subglacial water transport and possibly leading to the creation of subglacial lakes. Supraglacial lakes produced by slope reversals in ablation zones may aid in driving water-filled cracks that allow surface water access to the bed. Surface slope reversals, in the absence of a concomitant reversal in ice flow, indicate a local violation of the so-called 'shallow-ice' approximation, and in this circumstance the longitudinal deviatoric stress becomes critical in the stress equilibrium. Using a simple numerical model, we have explored the conditions under which surface slope reversals form for certain simple scenarios. The results indicate that ice which initially possesses a normal slope will tend toward a reversed slope if the ice is thinned, the bed is strengthened or the downstream buttressing is increased.
A review of in situ and remote-sensing data covering the ice shelves of the Antarctic Peninsula provides a series of characteristics closely associated with rapid shelf retreat: deeply embayed ice fronts; calving of myriad small elongate bergs in punctuated events; increasing flow speed; and the presence of melt ponds on the ice-shelf surface in the vicinity of the break-ups. As climate has warmed in the Antarctic Peninsula region, melt-season duration and the extent of ponding have increased. Most break-up events have occurred during longer melt seasons, suggesting that meltwater itself, not just warming, is responsible. Regions that show melting without pond formation are relatively unchanged. Melt ponds thus appear to be a robust harbinger of ice-shelf retreat. We use these observations to guide a model of ice-shelf flow and the effects of meltwater. Crevasses present in a region of surface ponding will likely fill to the brim with water. We hypothesize (building on Weertman (1973), Hughes (1983) and Van der Veen (1998)) that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat. A thermodynamic finite-element model is used to evaluate ice flow and the strain field, and simple extensions of this model are used to investigate crack propagation by meltwater. The model results support the hypothesis.
Buoyancy-driven flows resulting from the introduction of fluid of one density into a crack embedded in an elastic solid of different density are analysed. Scaling arguments are used to determine the regimes in which different combinations of the buoyancy force, elastic stress, viscous pressure drop and material toughness provide the dominant pressure balance in the flow. The nonlinear equations governing the shape and rate of spread of the propagating crack are formulated for the cases of vertical propagation of buoyant fluid released into a solid of greater density and of lateral propagation of fluid released at an interface between an upper layer of lesser density and a lower layer of greater density. Similarity solutions of these equations are derived under the assumption that the volume of fluid is given by Qt α , where Q and α are constants. Both laminar and turbulent flows are considered.
Fluid fracture is an important mechanism for the transport of molten rock from the region of production in the Earth's mantle to surface eruptions or near-surface emplacement. The theoretical solutions provide simple models which describe the relation between the elastic and fluid-mechanical phenomena involved in the vertical transport of melt through the Earth's lithosphere and in the lateral intrusion of melt at a neutral-buoyancy level close to the Earth's surface.
Theoretical studies of glacial outburst floods (jökulhlaups) assume that: (i) intraglacial floodwater is transported efficiently in isolated conduits; (ii) intraglacial conduit enlargement operates proportionally to increasing discharge; (iii) floodwater exits glaciers through pre-existing ice-marginal outlets; and (iv) the morphology and positioning of outlets remains fixed during flooding. Direct field observations, together with historical jökulhlaup accounts, confirm that these theoretical assumptions are not always correct. This paper presents new evidence for spatial and temporal changes in intraglacial floodwater routing during jökulhlaups; secondly, it identifies and explains the mechanisms controlling the position and morphology of supraglacial jökulhlaup outlets; and finally, it presents a conceptual model of the controls on supraglacial outbursts. Field observations are presented from two Icelandic glaciers, Skeiðarárjökull and Sólheimajökull. Video footage and aerial photographs, taken before, during and after the Skeiðarárjökull jökulhlaup and immediately after the Sólheimajökull jökulhlaup, reveal changes in floodwater routing and the positioning and morphology of outlets. Field observations confirm that glaciers cannot transmit floodwater as efficiently as previously assumed. Rapid increases in jökulhlaup discharge generate basal hydraulic pressures in excess of ice overburden. Under these circumstances, floodwater can be forced through the surface of glaciers, leading to the development of a range of supraglacial outlets. The rate of increase in hydraulic pressure strongly influences the type of supraglacial outlet that can develop. Steady increases in basal hydraulic pressure can retro-feed pre-existing englacial drainage, whereas transient increases in pressure can generate hydraulic fracturing. The position and morphology of supraglacial outlets provide important controls on the spatial and temporal impact of flooding. The development of supraglacial jökulhlaup outlets provides a new mechanism for rapid englacial debris entrainment. Copyright © 2000 John Wiley & Sons, Ltd.
Outburst flooding from subglacial reservoirs and associated surging of ice-streams are caused by processes following climatic cooling that produce ice-shelf grounding on proglacial sills trapping subglacial water, according to an hypothesis presented here. Glaciers often advance into proglacial water bodies. Cooling may allow ice-shelf formation and advance causing grounding on a proglacial sill, trapping subglacial water. Ice-shelf freeze-on to the sill and development of a local reversal in ice–air surface slope over the sill then are likely, allowing ice thickening and water overpressurization. If basal thawing then occurs, as is likely, an outburst flood and a surge may be triggered. These processes may have been involved in Heinrich events, generation of Antarctic subglacial lakes, and meltwater scouring of some regions.
In linear elastic fracture mechanics, the stress intensity factor is used to describe elastic stresses near the tip of a crack. Crack growth occurs when the stress intensity factor is larger than a critical value, the fracture toughness, which is a material parameter that applies to cracks of any size. For surface crevasses on glaciers, the net stress intensity factor can be calculated by superimposing the effects of a tensile stress, the weight of the ice, and water pressure if the crevasse is filled with water. The analysis is applied to individual and multiple crevasses to investigate the important factors determining crevasse depth. The model indicates that a single crevasse can only exist if the tensile stress is larger than 30-80 kPa, depending on the fracture toughness of glacier ice. Multiple crevasses result in a decrease in stress intensity factor for any crevasse, thus reducing their depth (all other factors being equal). Consequently, in a field of crevasses, a larger tensile stress is needed compared to an individual crevasse, to allow crevasse formation. Further, a water-filled crevasse or field of crevasses can reach the bottom of a glacier provided that the water level is about 15 m below the surface, or higher, and the tensile stress is larger than ~150 kPa. Compared to earlier studies it is shown that accounting for the finite thickness is larger than ~0.3. However, such deep crevasses can only exist if filled with water, in which case the crevasse may penetrate the ice completely and the small error introduced by approximating the glacier as a semi-infinite plane is unimportant. It is more important to account for the lower density of the upper firn layers: this effect increases the maximum depth of crevasses by almost a factor of two compared to the solution of Weertman (1973) [Weertman, J., 1973. Can a water-filled crevasse reach the bottom surface of a glacier? IASH Publ. 95, 139-145].
The Greenland ice sheet is likely to make a faster contribution to sea-level rise in a warming world than previously believed, based on numerical modelling using a parameterization of recent results showing surface-meltwater lubrication of ice flow. Zwally et al. (Science 297(557) (2002) 218) documented correlation between increased ice velocity and increased surface melt (as parameterized by positive degree days). They argued that surface water is piped directly to the bed with little delay, causing increased basal-water pressures and basal-sliding velocities, an effect not included in recent Greenland ice-sheet models known to the authors.Using the Pennsylvania State University/University of Chicago thermomechanical flowline model, numerous simulations were conducted to test a wide range of parameter space linking surface melt with a new sliding law based on the Zwally et al. data under three different global warming scenarios (2×CO2, 4×CO2, and 8×CO2). Comparisons to reconstructions generated with a traditional sliding parameterization illustrate an enhanced sensitivity of the ice sheet to surface warming resulting in higher ablation rates, additional thinning and retreat of the margin, and a reduction in ice volume leading to an increased contribution to global sea-level rise.