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Wire regelation is a common tabletop demonstration of the pressure-dependence of the ice melting temperature where a loaded wire moves from top to bottom through a block of ice, yet leaves the block intact. With the background temperature fixed at the bulk melting point ∼0°C, the elevated ice and liquid pressures beneath the wire cause melting because of the negative Clapeyron slope, while refreezing takes place above the wire where the pressures are reduced. Regelation is a model for temperate glacier ice moving through small bedrock obstacles. Laboratory experiments demonstrate that regelation continues to occur, albeit at much slower velocities, when the fixed background ice temperature is cold enough that the wire load is insufficient to produce bulk melting, suggesting that premelting plays a central role. Here, we compile available data for wire regelation at all temperatures. We then develop a model for the subtemperate data points, where the film thickness depends on the temperature below the melting point. We find agreement between the power-law model and the laboratory data for slow regelation velocities, allowing us to characterize the dominant premelting mechanisms for different wire compositions. These results advance our understanding of the role of premelting in subtemperate glacier sliding. [ABSTRACT FROM AUTHOR]

Small quantities of liquid water lining triple junctions in polycrystalline glacier ice form connected vein networks that enable material exchange with underlying basal environments. Diffuse debris concentrations commonly observed in ice marginal regions might be attributed to this mechanism. Following recent cryogenic ring-shear experiments, we observed emplacement along grain boundaries of loess particles several tens of microns in size. Here, we describe an idealized model of vein liquid flow to elucidate conditions favoring such particle transport. Gradients in liquid potential drive flow toward colder temperatures and lower solute concentrations, while deviations of the ice stress state from hydrostatic balance produce additional suction toward anomalously low ice pressures. Our model predicts particle entrainment following both modest warming along the basal interface resulting from anticipated natural changes in effective stress, and the interior relaxation of temperature and solute concentration imposed by our experimental protocols. Comparisons with experimental observations are encouraging, but suggest that liquid flow rates are somewhat higher and/or more effective at dragging larger particles than predicted by our idealized model with nominal parameter choices. Diffuse debris entrainment extending several meters above the glacier bed likely requires a more sophisticated treatment that incorporates effects of ice deformation or other processes. [ABSTRACT FROM AUTHOR]

The importance of glacier sliding has motivated a rich literature describing the thermomechanical interactions between ice, liquid water and bed materials. Early recognition of the gradient in melting temperature across small bed obstacles led to focused studies of regelation. An appreciation for the limits on ice deformation rates downstream of larger obstacles highlighted a role for cavitation, which has subsequently gained prominence in descriptions of subglacial drainage. Here, we show that the changes in melting temperature that accompany changes in normal stress along a sliding ice interface near cavities and other macroscopic drainage elements cause appreciable supercooling and basal mass exchange. This provides the basis of a novel formation mechanism for widely observed laminated debris-rich basal ice layers. [ABSTRACT FROM AUTHOR]

Freezing in porous media is associated with a host of dynamic phenomena that stem from the presence and mobility of premelted liquid at subzero temperatures. Accurate assessments of the progressive liquid‐ice phase transition is required for predictive models of frost damage, glacier‐till coupling, and many other cold regions processes, as well as for evaluating the capacity for water storage in near‐surface extraterrestrial environments. We use a Monte Carlo approach to sample the pore space in a synthetic 3D packing of poly‐dispersed spherical particles and evaluate local geometrical constraints that allow us to assess changes in the relative proportions of pore fluid and ice. By approximating the phase boundary geometry in fine‐grained pores while considering both the curvature of the liquid‐ice interface and wetting interactions with matrix particles, our model predicts changes in phase equilibrium in granular media over a broad temperature range, where present accounting for the colligative effects of chloride and perchlorate solutes. In addition to formulating the constitutive behavior needed to better understand properties and processes in frozen soils, our results also provide insight into other aspects of phase equilibria in porous media, including the formation of methane hydrates in permafrost and marine sediments, and the partitioning between liquid water and vapor in the vadose zone. Plain Language Summary: When saturated soils become frozen at subzero temperatures, the water between the soil grains turns into ice. The soil freezing curves (SFCs) describe how the amount of water remaining liquid in pore spaces changes with temperature, which is also determined by the geometry of pores. Previous researches tend to idealize the pores as spheres or cylinders, but the actual pores have irregular geometry, and the interface between water and ice can be so curved that liquid water may exist at temperatures much lower than bulk freezing point. To quantitatively assess the relation between the undercooling and residual liquid water in frozen soils, we construct synthetic soil models with packed spherical particles and use a Monte Carlo approach to track the ice‐liquid interface over a large temperature range. We examine cases with soil particles of uniform size and log‐normal size distributions and compare our results with ice‐infiltration experiments, field observations, and mercury intrusion measurements. Additional factors, including changes in wetting behavior and various dissolved salts, are also taken into account. The model can shed light on the formation of ice and gas hydrate in permafrost and extraterrestrial systems. Key Points: We develop numerical models to evaluate pore geometric constraints on soil freezing with temperatureOur method is validated by comparing modeled results against empirical data and field observationsSoil freezing curves are predicted for log‐normal particle‐packings in pure and salt‐doped systems [ABSTRACT FROM AUTHOR]

Glacier sliding over small obstacles relies on melting on their upstream sides and refreezing downstream. Previous treatments have appealed to 'pressure melting' as the cause of the spatial variations in melting temperature that drive this regelation process. However, we show that typical liquid pressure variations across small obstacles are negligible and therefore variations in ice pressure closely approximate variations in effective stress. For a given change in effective stress, the equilibrium melting temperature changes by an order of magnitude more than when the pressure of ice and liquid both change by an equal amount. In consequence, the temperature gradients that drive heat flow across small obstacles are larger than previously recognized and the rate of regelation is faster. Under typical conditions, the transition wavelength at which ice deformation and regelation contribute equally is of m-scale, several times longer than previous predictions, which have been reported to underestimate field inferences. [ABSTRACT FROM AUTHOR]

The article discusses a study on effective stress profiles and seepage flows beneath soft-bedded glaciers and ice sheets. The study examined the distribution of water pressure at the glacier bed that controls the resistance to sliding and till deformation under the soft-bedded glaciers. The study presented a description of the conditions for thermodynamic and mechanical equilibrium at the glacier base, heat transfer and fluid mass. The study found that seepage transport allows water exchange between major subglacial conduits during basal heat flow that leads to net melting or freezing.

Discharge from sliding outlet glaciers controls uncertainty in projections for future sea level.Remarkably, over 90% of glacial area is subject to gravitational driving stresses below 150kPa (median ∼70 kPa). Longstanding explanations that appeal to the shear-thinning rheologyof ice tend to overpredict driving stresses and are restricted to areas where ice sheets onlydeform (roughly 50%). Over the more dynamic portions that slide, driving stresses must bebalanced by thermo-mechanical interactions that control basal strength. Here we show thatmedian bed strength is comparable to a threshold effective stress set by ice–liquid surfaceenergy and till pore size. Above this threshold, ice infiltrates sediment to produce basallayers of debris-rich ice, even where net melting takes place. We demonstrate thatthe narrow range of inferred bed strengths can be explained by the mechanicalresistance to sliding where roughness is enhanced by heterogeneous freeze-on. [ABSTRACT FROM AUTHOR]