A general theory of glacier surges

Abstract

We present the first general theory of glacier surging that includes both temperate and polythermal glacier surges, based on coupled mass and enthalpy budgets. Enthalpy (in the form of thermal energy and water) is gained at the glacier bed from geothermal heating plus frictional heating (expenditure of potential energy) as a consequence of ice flow. Enthalpy losses occur by conduction and loss of meltwater from the system. Because enthalpy directly impacts flow speeds, mass and enthalpy budgets must simultaneously balance if a glacier is to maintain a steady flow. If not, glaciers undergo out-of-phase mass and enthalpy cycles, manifest as quiescent and surge phases. We illustrate the theory using a lumped element model, which parameterizes key thermodynamic and hydrological processes, including surface-to-bed drainage and distributed and channelized drainage systems. Model output exhibits many of the observed characteristics of polythermal and temperate glacier surges, including the association of surging behaviour with particular combinations of climate (precipitation, temperature), geometry (length, slope) and bed properties (hydraulic conductivity). Enthalpy balance theory explains a broad spectrum of observed surging behaviour in a single framework, and offers an answer to the wider question of why the majority of glaciers do not surge.

Dilation of subglacial sediment governs incipient surge motion in glaciers with deformable beds

“…Differentiating Eq. 2.9 in time yieldsφ 10) an expression that indicates that shearing of the till layer causes it to compact (φp < 0) when θ is below steady state (θ < dc/u b ) and to dilate when θ is above steady state. Such behavior is consistent with observations of the response of over-and under-consolidated soils to shear [50].…”

“…Flow speeds during a surge can reach 5-100 times typical quiescent-phase velocities because of commensurate increases in the rate of slip at the ice-bed interface, hereafter called basal slip rate. Accelerated basal slip rates are facilitated by changes in the mechanical, thermal, and hydrological properties of the bed, which may work independently or in concert to initiate, sustain, and arrest glacier surges [2][3][4][5][6][7][8][9][10].…”

“…Many existing models of glacier surges rely on an evolving subglacial hydrological system, which can influence water pressure and thereby drag at the bed [3,10,21]. One such model posits that incipient surge motion arises from a switch in the subglacial hydrological system from a relatively efficient channelized system to an inefficient distributed, or linked-cavity, system [3,21], though recent work suggests that a distributed hydrological system primes but does not trigger surges [10]. Throughout the surge phase, the basal hydrological system likely remains relatively inefficient, facilitating rapid basal slip due to lubrication from high basal water pressures, until reestablishment of an efficient channelized system reduces basal water pressure and terminates the surge [10,[21][22][23][24].…”

“…One such model posits that incipient surge motion arises from a switch in the subglacial hydrological system from a relatively efficient channelized system to an inefficient distributed, or linked-cavity, system [3,21], though recent work suggests that a distributed hydrological system primes but does not trigger surges [10]. Throughout the surge phase, the basal hydrological system likely remains relatively inefficient, facilitating rapid basal slip due to lubrication from high basal water pressures, until reestablishment of an efficient channelized system reduces basal water pressure and terminates the surge [10,[21][22][23][24]. Given a supply of water to the bed, this theory has the potential to explain rapid surge motion and coincident increases in basal water pressure, at least in glaciers with rigid beds [21].…”

“…The prevalence of till layers beneath surge-type glaciers suggests that changes in the mechanical properties of till caused by dilation and variable pore water pressure are a promising complement to existing models of incipient surge mechanisms [10,38]. It would be difficult to overstate the complexity of granular mechanics in subglacial till [39], which is especially pronounced where the till contains coarse clasts, where ice at the ice-bed interface is laden with debris [40][41][42], where the ice slides over the ice-till interface [41,43,44], where clasts frozen into the ice can plow through the till [45], and where the till is mobilized during surging [46].…”