Martin P. Lüthi

Mechanisms of fast flow in Jakobshavn Isbræ, West Greenland: Part III. Measurements of ice deformation, temperature and cross-borehole conductivity in boreholes to the bedrock

At a site on the ice sheet adjacent to the Jakobshavn ice stream in West Greenland, ice deformation rates and temperatures have been measured in boreholes to the bedrock at 830 m depth. Enhanced deformation rates were recorded just below the Holocene Wisconsin transition at 680 m depth. A 31 m layer of temperate ice and the temperature minimum of -22°C at 520 m depth were detected. The good agreement of these data with results of a two-dimensional thermomechanically coupled flow model implies that the model input is adequate. Discrepancies between modelled and measured temperature profiles on a flowline at the ice-stream centre have been attributed to effects not accounted for by the model. We have suggested that the convergent three-dimensional flow leads to a vertical extension of the basal ice entering the stream. A thick basal layer of temperate and Wisconsin ice would explain the fast flow of this ice stream. As a test of this hypothesis, the new core-borehole conductivity (CBC) method has been used to compare conductivity sequences from the ice stream to those of the adjacent ice sheet. The correlation thus inferred suggests that the lowest 270 m of the ice sheet correspond to the lowermost 1700 m of the stream, and, consequently, that the lower part of the ice stream has experienced a very large vertical extension.

Modelling heat flow in a cold, high-altitude glacier: interpretation of measurements from Colle Gnifetti, Swiss Alps

Modelling the heat flow in small, cold high-altitude glaciers is important for the interpretation of paleoclimatic data from ice cores. Coupled glacier-flow and heat-flow models are presented that incorporate the densification, heat advection and possible phase transitions at the permafrost boundaries within the bedrock. Marked bends observed in the temperature profiles from two recent boreholes on Colle Gnifetti, Swiss Alps, are interpreted with the help of a transient heat-flow model, driven with a temperature history. The conclusion is that substantial warming of the mean firn temperature at shallow depths has taken place over the last few decades. This has not been observed before in cold-firn regions of the Alps. Modelled heat fluxes in the Monte Rosa massif are strongly influenced by the mountain topography. This leads to a spatial variability of the temperature gradient near the glacier base which has been observed in boreholes to the bedrock. In order to match the measured temperature profiles in the glacier, the vertical heat flux at great depth must be set to an extremely low value. It is shown with the help of the transient heat-flow model that this is a paleoclimatic effect, possibly enhanced by a degrading permafrost base.

Estimating rates of basal motion and internal ice deformation from continuous tilt measurements

Over a two-year period, continuous measurements of temporal changes in tilt, conducted with a string of tilt meters in a borehole on Unteraargletscher, Bernese Alps, Switzerland, have been used to estimate the basal-motion component. This estimation is based on a comparison of the measurements with synthetic tilt curves, computed using a parameterization of a simplified flow field. The best agreement is found for a ratio of basal motion to forward motion due to ice deformation (slip ratio) equal to about 1.2. Measured tilt curves exhibit a number of different transient features. While an overall increase in tilt angle is observed at every tilt-meter location, two of the sensors recorded anomalous tilt behaviour. These anomalies are characterized by sudden and drastic variations in tilt. A particularly intriguing example of such short-term tilt variations was recorded with a tilt meter positioned 40 m above the bed during the 1997 summer melt season.

Thick sediments beneath Greenland’s ablation zone and their potential role in future ice sheet dynamics

The geological nature of glacier beds plays a key role in ice sheet dynamics. Whereas little is known about Greenland’s subglacial geology, the presence of basal sediments is a necessary condition for fast-flowing Antarctic ice streams. Such sediments sustain subglacial till layers, which if water saturated and under high pore-water pressure provide little resistance to ice flow. Using receiver function modeling of teleseismic P-waves, we report a thick (at least tens of meters) sediment layer beneath a site in Greenland’s ablation zone, ∼15 km away from the western ice sheet margin. Although we do not discuss the origin or detailed nature of these subglacial sediments, we suggest that they are capable of sustaining a till layer. Due to the prevalence of an inefficient, pressurized subglacial drainage system, this till layer would typically be under high pore pressures and fail at the shear stresses transmitted from the overlaying ice. Ice flow resistance is thus focused on regions where till is consolidated or absent, or where form drag over obstacles takes place. In contrast to Antarctica, Greenland’s surface melt now affects practically all latitudes, and efficient hydraulic connections between ice sheet surface and bed are pervasive throughout the ablation zone. This implies that rapid, melt-induced changes in subglacial water pressure are possible. The time required for basal till strength to react to these changes depends on the till’s properties. We estimate that formation and destruction of flow resistance can occur on time scales of less than a few years. This could lead to changes in ice flow that are currently difficult to predict.

Caterpillar‐like ice motion in the ablation zone of the Greenland ice sheet

Current understanding of ice dynamics predicts that increasing availability and variability of meltwater will have an impact on basal motion and therefore on the evolution and future behavior of the Greenland ice sheet. We present measurements of ice deformation, subglacial water pressure, and surface velocity that show periodic and episodic variations on several time scales (seasonal, multiday, and diurnal). These variations, observed with GPS and sensors at different depths throughout the ice column, are not synchronous but show delayed responses of ice deformation with increasing depth and basal water pressure in antiphase with surface velocity. With the help of a Full-Stokes ice flow model, these observations are explained as ice motion in a caterpillar-like fashion. Caused by patches of different basal slipperiness, horizontal stress transfer through the stiff central part of the ice body leads to spatially varying surface velocities and ice deformation patterns. Variation of this basal slipperiness induces characteristic patterns of ice deformation variability that explain the observed behavior. Ice flow in the ablation zone of the Greenland ice sheet is therefore controlled by activation of basal patches by varying slipperiness in the course of a melt season, leading to caterpillar-like ice motion superposed on the classical shear deformation.

A description of crevasse formation using continuum damage mechanics

Abstract Continuum damage mechanics describes the progressive deterioration of material subjected to loading. Jointly used with a level-set method, it proves to be a promising approach to computing the interface motion of a damaged material. For polycrystalline ice, a local isotropic damage evolution law (generalized Kachanow’s law) applied to Glen’s flow law allows the description of tertiary creep and facilitates the modeling of crevasse opening using a failure criterion based on damage accumulation. The use of a level-set method permits the description, in a continuum approach, of the motion of a fractured glacier surface. Using these methods, a model is developed. The ability of this model to describe phenomena connected to crevasse opening is presented. The rupture of a large ice block from a hanging glacier is computed and analyzed. The regular acceleration of such an unstable ice block prior to its collapse is calculated and compared to the acceleration function obtained from observations. A good agreement between the two acceleration functions was found.

Transient response of idealized glaciers to climate variations

The transient response of glaciers to climate variations is investigated with a novel low-order model of an idealized glacier resting on uniformly inclined bedrock. The model consists of two volumes representing the accumulation and ablation areas, which are joined by a flux gate controlling mass flow according to the shallow-ice approximation. Under the assumption of a constant vertical mass-balance gradient, a volume-length scaling relation is derived which depends explicitly on mass-balance gradient and bedrock slope. Analytic expressions for the volume and area timescales are given, which are inversely proportional to the mass-balance gradient and a geometric factor which is the ratio between the vertical extent of the ablation area and the ice thickness at the equilibrium line. From the low-order model, a dynamical system in length and volume is obtained. Results from this system are in good agreement with solutions obtained from a transient finite-element model solving the full force-balance and mass-conservation equations. Under periodic forcing there are significant deviations of the length response, which show that the usual relaxation-type parameterization of length change is not well suited for short-term reactions. Switching on and off periodic climate forcing, the model glaciers show surprisingly large initial and final transient responses that have not been investigated before. These results are of significance for the interpretation of length and thickness changes observed on glaciers.

Time-dependent basal stress conditions beneath Black Rapids Glacier, Alaska, USA, inferred from measurements of ice deformation and surface motion

Observations of surface motion and ice deformation from 2002-03 were used to infer mean stress fields in a cross-section of Black Rapids Glacier, Alaska, USA, over seasonal timescales. Basal shear stresses in a well-defined zone north of the center line (orographic left) were approximately 7% and 16% lower in spring and summer, respectively, than in winter. Correspondingly higher stresses were found near the margins. These changes in the basal shear stress distribution were sufficiently large to cause mean surface velocities to be 1.2 and 1.5 times larger in spring and summer than in winter. These results were inferred with a simple inverse finite-element flow model that can successfully reproduce bulk surface velocities and tiltmeter data. Stress redistribution between the well-defined zone and the margins may also occur over much shorter time periods as a result of rapidly changing basal conditions (ice-bed decoupling or enhanced till deformation), thereby causing large variations in surface velocity and strongly influencing the glaciers net motion during summer.

Deducing the thermal structure in the tongue of Gornergletscher, Switzerland, from radar surveys and borehole measurements

Abstract We present the thermal distribution in the confluence area of Gorner- and Grenzgletscher, Valais, Switzerland. The area was mapped by ice-penetrating radar at 1–5 and 40 MHz. The higher-frequency data reveal a thick surface layer of low backscatter in the center of the Grenzgletscher branch. Based on datasets of borehole-temperature measurements and flow velocity, we interpret this as a thick layer of cold ice, advected from the accumulation region of Grenzgletscher. Along seven profiles the base of the low-backscatter zone can be found at a maximum depths between approximately 100 and 200 m. Laterally, the layer extends some 400 m, ∼1/3 of the width of the Grenzgletscher branch. The lower boundary of the low-backscatter zone is systematically higher than the cold–temperate transition surface found in the boreholes. This discrepancy is attributed to the direct sensitivity of radar backscatter to liquid-water inclusions, rather than to the temperature distributions as observed in boreholes. We present the current state of the cold layer and discuss its influence on other glacier characteristics.

Greenland subglacial drainage evolution regulated by weakly connected regions of the bed

Penetration of surface meltwater to the bed of the Greenland Ice Sheet each summer causes an initial increase in ice speed due to elevated basal water pressure, followed by slowdown in late summer that continues into fall and winter. While this seasonal pattern is commonly explained by an evolution of the subglacial drainage system from an inefficient distributed to efficient channelized configuration, mounting evidence indicates that subglacial channels are unable to explain important aspects of hydrodynamic coupling in late summer and fall. Here we use numerical models of subglacial drainage and ice flow to show that limited, gradual leakage of water and lowering of water pressure in weakly connected regions of the bed can explain the dominant features in late and post melt season ice dynamics. These results suggest that a third weakly connected drainage component should be included in the conceptual model of subglacial hydrology.