Alan W. Rempel

Water pressures in conduit networks (e.g. linked cavities, R-channels, subglacial water sheets) support a fraction of glacier weight; the remainder is carried by elevated normal stresses where basal ice is separated from the bed by extremely thin water films. We show how the requirements for melt flow toward conduits constrain film water pressure regimes. Where permeable bed materials facilitate efficient drainage, film pressures approach conduit water pressures, and deficits below the ice normal stress require the direct ice–bed stress transfer, termed thermomolecular pressure, that accompanies interfacial premelting. Beneath warm-based regions of the Greenland Ice Sheet, premelting is likely wherever meter-scale till depths are present; bulk melting conditions, with film water pressures balancing ice normal stresses, are anticipated where the ice and bed are both effectively impermeable. Englacial transport through vein networks can facilitate premelting over small film areas on impermeable beds; absent significant vein transport, such locations are prone to the formation of basal crevasses, which can buffer water pressures near the minimum compressive stress. Interfacial premelting is important for the preservation of unconsolidated bedforms, the strengthening of debris–bed frictional contacts and the enhancement of basal temperature fluctuations; anticorrelations between nearby borehole pressure records are expected when bulk melting prevails.

Frost heave occurs when the ground swells during freezing conditions due to the growth of ice lenses in the subsurface. The mechanics of ice-infiltrated sediment, or frozen fringe, influences the formation and evolution of ice lenses. As the frozen fringe thickens during freezing, progressive unloading can result in dilation of the pore space and the formation of new ice lenses. Compaction can also occur as water is expelled from the pore space and freezes onto the ice lenses. We introduce a mathematical model for compaction within frozen fringe to explore how internal variability influences the fundamental characteristics of frost heave cycles. At faster freezing rates, compaction below ice lenses can generate complex dynamics and chaos when the frozen fringe follows a consolidation law based solely on the sediment yield stress. The complex oscillations arise because a downward water flux below the compacting zone generates a distributed zone of weakening. We introduce viscous effects into the compaction law through a bulk viscosity to determine how the cycles could be influenced by the creep of ice through the pore space. Localised zones of decompaction in the viscoplastic model can prevent the feedback mechanisms that cause complex oscillations in the perfectly plastic model.

Glaciers and ice streams flowing over sediment beds commonly have a layer of ice‐rich debris adhered to their base, known as a “frozen fringe,” but its impact on basal friction is unknown. We simulated basal slip over granular beds with a cryogenic ring shear device while ice infiltrated the bed to grow a fringe, and measured the frictional response under different effective stresses and slip speeds. Frictional resistance increased with increasing slip speed until it plateaued at the frictional strength of the till, closely resembling the regularized Coulomb slip law associated with clean ice over deformable beds. We hypothesize that this arises from deformation in a previously unidentified zone of weakly frozen sediments at the fringe's base, which is highly sensitive to temperature and stress gradients. We show how a rheologic model for ice‐rich debris coupled with the thermomechanics of fringe growth can account for the regularized Coulomb behavior.

Wire regelation is a common tabletop demonstration of the pressure-dependence of the ice melting temperature in which 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 $\sim0\,^{\circ}$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 the fundamental 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 in this regime. 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 excellent model-data agreement 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.

Ice-infiltrated sediment, or frozen fringe, is responsible for phenomena such as frost heave, ice lenses and metres of debris-rich ice under glaciers. Understanding frozen fringes is important as frost heave is responsible for damaging infrastructure at high latitudes and sediment freeze-on at the base of glaciers can modulate subglacial friction, influencing the rate of global sea level rise. Here we describe the thermomechanics of liquid water flow and freezing in ice-saturated sediments, focusing on the conditions relevant for subglacial environments. The force balance that governs the frozen fringe thickness depends on the weight of the overlying material, the thermomolecular force between ice and sediments across liquid premelted films and the water pressure required by Darcy flow. We combine this mechanical model with an enthalpy method that conserves energy across phase change interfaces on a fixed computational grid. The force balance and enthalpy model together determine the evolution of the frozen fringe thickness and our simulations predict frost heave rates and ice lens spacing. Our model accounts for premelting at ice–sediment contacts, partial ice saturation of the pore space, water flow through the fringe, the thermodynamics of the ice–water–sediment interface and vertical force balance. We explicitly account for the formation of ice lenses, regions of pure ice that cleave the fringe at the depth where the interparticle force vanishes. Our model results allow us to predict the thickness of a frozen fringe and the spacing of ice lenses in subaerial and subglacial sediments.

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.

Currently, biotechnologies that aim to optimize the residual lignocellulosicbiomass are receiving widespread attention, mainly when it comes to developingintegrated systems that allow the generation of multi-products in industrial plants,especially for ethanol production. One of the main bottlenecks for efficient conversionof lignocellulosic biomass into ethanol is the limitation of Saccharomyces cerevisiae,the most widely used yeast in bioethanol production, in metabolizing xylose. Thispentose is the main constituent of the hemicellulose fractions in plant cell walls and thesecond most abundant monosaccharide in lignocellulosic biomass. This challenge isbeing overcome by the isolation and intense molecular evaluation of new yeast species,mainly members of the genera Spathaspora and Scheffersomyces, since they haveshown high capacities for xylose assimilation, which has been corroborated throughstudies aimed at improving ethanol production and other products via the association ofthese yeasts with improved fermentation capacity. In this sense, this chapter addressesthe recent advances in the identification of novel isolates of the genera Spathasporaand Scheffersomyces, particularly emphasizing the applications of these genera inethanol and xylitol production.

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 focussed 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.

Flow through partially frozen pores in granular media containing ice or gas hydrate plays an essential role in diverse phenomena including methane migration and frost heave. As freezing progresses, the frozen phase grows in the pore space and constricts flow paths so that the permeability decreases. Previous works have measured the relationship between permeability and volumetric fraction of the frozen phase, and various correlations have been proposed to predict permeability change in hydrology and the oil industry. However, predictions from different formulae can differ by orders of magnitude, causing great uncertainty in modeling results. We present a floating random walk method to approximate the porous flow field and estimate the effective permeability in isotropic granular media with specified particle size distributions, without solving for the entire flow field in the pore space. In packed spherical particles, the method compares favorably with the Kozeny‐Carman formula. We further extend this method with a probabilistic interpretation of the volumetric fraction of the frozen phase, simulate the effect of freezing in irregular pores, and predict the evolution of permeability. Employing no adjustable parameters, our results can provide insight into the coupling between phase transitions and permeability change, which plays important roles in hydrate formation and dissociation, as well as in the thawing and freezing of permafrost and ice‐bed coupling beneath glaciers.