Clean Glacier Research Articles

Ice sheet growth under dirty conditions: implications of debris cover for early glaciation advances

Debris-covered glaciers respond differently to any given set of climatic conditions than clean glaciers. This difference stems from change in ablation rate caused by a debris cover, approaching zero ablation with sufficiently thick protective cover, a mechanism not yet considered in the context of ice sheet growth. Critical to applying the mechanism to ice sheets is supply of debris. We postulate that periods after major interglacials offer the best conditions for dirty advances. This is because the volume of debris, previously deposited and weathered in interglacial time, the latter a function of exposure length, should be at a maximum. Advances of dirty ice sheets generate landforms and in positions deviating from clean-ice advances under similar climatic conditions. Hence, inferences of both ice sheet properties inferred from such dirty ice advances and the climate conditions related to such advances must be cautious. Furthermore, modeling of past ice sheets must consider the effect of debris cover whenever indications exist for such a cover.

Direct shear tests of materials from a cold glacier: implications for landform development

The mechanical properties of materials at the ice-bed interface exert a major control on glacier behaviour and define the nature and rate of subglacial erosion. This paper presents new data on the strength and behaviour of basal ice and substrate of the Suess Glacier, south Victoria Land, Antarctica which is a small alpine dry-based glacier that has a basal temperature of −17°C. A tunnel excavated in the glacier revealed a substrate composed of frozen sand and gravel and a basal zone that was 3.8-m-thick. From bottom to top, the basal zone was composed of 1.8 m of stratified, complexly deformed layers of ice and debris-laden ice that was overlain by a 0.9-m-thick layer of frozen sediment and a 0.8-m-thick layer of discoloured ice that lies immediately beneath clean glacier ice. Direct shear tests were performed in the field on 36 samples using a strain rate of 0.85 mm/h and on replicate samples in a laboratory at strain rates of 0.08 and 0.01 mm/h. The experiments performed in the field show that the average peak shear strength of substrate samples was 2.53 MPa, which is almost twice as strong as the average value for basal ice (1.28 MPa), and the glacier ice samples (1.39 MPa). The laboratory experiments show that the behaviour and peak strength of samples sheared at the higher strain rates are considerably greater than the peak and residual strengths measured during the lower strain rate tests. The direct shear tests suggest that the glacier substrate is unlikely to deform at the current temperature.

Major end moraines of Younger Dryas age on Wollaston Peninsula, Victoria Island, Canadian Arctic: implications for paleoclimate and for formation of hummocky moraine: Discussion

Discussion In referring to several large moraine belts n Arctic Canada, Dyke and Savelle (2000) state “a consensus is emerging that these moraines are cored by glacier ice” (p. 615). They further state that ice-cored morainal belts in Arctic Canada “would resemble southern areas of hummocky moraine were they to lose their ice cores” (p. 617). More investigation and research is required before these statements can be accepted in lieu of theories proposed by other investigators for the origin of ice within hummocky Arctic topography and for the evolution of this topography. The claimed consensus is that of Dyke et al. (1992), Sharpe (1992), and St-Onge and McMartin (1995). Dyke et al. (1992) stated that many of the moraines present on Prince of Wales Island consist of buried glacier ice. Sharpe (1992) reported exposures of relatively debris-free ice containing a few debris-rich bands underlying up to 2m of diamicton within areas of hummocky moraine. He noted the contact between the diamicton and ice as abrupt. St-Onge and McMartin (1995) reported icy sediments overlain by bouldery diamicton with a sharp and subhorizontal contact; reticulate ice was noted in the diamicton near the contact. All contacts between till and ice appear characteristic of segregation ice developing in the base of and under a finetextured soil (Mackay and Dallimore 1992). In addition, the above sequences are the reverse of the normal continental glacier sequence, where relatively clean glacier ice overlies debris-rich glacier ice. St-Onge and McMartin believed (i) the hydrogeochemistry and isotopic values of reticulate ice, ice-wedge ice, and the observed icy sediments and (ii) the slope of the co-isotope plot from their icy sediment samples support a glacial origin for the icy sediments. The latter is the strongest argument for a glacial origin of the icy sediments as the slope is similar to that for meteoric water (Lorrain and Demeur 1985). However, differentiation of segregation ice and glacial ice by co-isotope relation has since been seriously questioned (French and Harry 1990; Mackay and Dallimore 1992). The hydrogeochemistry and isotope values reported by St-Onge and McMartin (1995) for the different ice types simply confirm different origins of the ice types and do not necessarily indicate a glacial origin for the icy sediments. Dyke and Savelle’s (2000) analysis of ice-cored topography excluded the mainland west of the Amundson Gulf, where a hummocky belt of topography on the Tuktoyaktuk Peninsula and adjacent areas (Fig. 1) was studied in detail by Mackay (cf. 1963, 1971, 1973, 1983) and his colleagues (Mackay and Stager 1966, Rampton and Mackay 1971, Rampton 1974, Rampton and Bouchard 1975, Gell 1976, Rampton 1988a, 1988b, Dallimore and Wolfe 1988, Mackay and Dallimore 1992). They concluded that the ice cores found within hummocky morainal topography along the Tuktoyaktuk Peninsula and Yukon Coastal Plain (Rampton 1982, Pollard and Dallimore 1988, Harry et al. 1988) are intrasedimental ice. The conclusion is based on evidence summarized in Rampton (1988a) and Mackay and Dallimore (1992), namely — similar geochemistry and isotope values for massive ice and ice within the underlying sandy sediments in numerous profiles, indicating a common water source and downward freezing; isotope plots having linear trends through ice and underlying interbedded sand and ice and co-isotope plots showing a linear trend down through till, ice, and sand, both trends suggesting downward freezing; petrographic data from massive ice indicative of a segregation origin; the common presence of reticulate ice within the till immediately over the massive ice (reticulate ice develops in fine-grained sediments as permafrost aggrades); the asso-

Rheology of ice at the bed of Engabreen, Norway

AbstractA three-dimensional finite-element model is used to analyze field data collected as dirty basal ice flowed past an instrumented obstacle at the bed of Engabreen, a temperate glacier in northern Norway The ice is modeled as an incompressible power-law fluid, with viscosity , where ΠD is the second invariant of the stretching tensor, and B and n are two parameters. Using measurements obtained in 1996 and 1997, two values of B are obtained, one using the measured normal stress difference across the obstacle, and the other using the measured bed-parallel force over the instrument. These two values are not equal, probably owing to small frictional forces at the bed unaccounted for in the numerical model. Hence, B ranges between 1.9 × 107 and 3.2 × 107 Pa s1/3 in 1996, and between 2.2 × 107 and 4.1 × 107 Pa s1/3 in 1997. These values are smaller than measured elsewhere for clean glacier or laboratory ice. Field measurements of water content, fabric and texture of the basal ice suggest that unbound water between thin sediment layers and lamellae of clean ice may act as a lubricant and significantly weaken the ice. Near-isotropic fabrics indicate that preferred fabric orientation does not enhance the deformation.

Structure and strength of basal ice and substrate of a dry-based glacier: evidence for substrate deformation at sub-freezing temperatures

AbstractThis paper presents new data and interpretations of the structure, strength and behaviour of basal ice and the substrate of Suess Glacier, south Victoria Land, Antarctica, which is a small alpine dry-based glacier that has a basal temperature of—17°C. A tunnel excavated in the glacier revealed a substrate composed of frozen sand and gravel and a basal zone that was 3.8 m thick. From bottom to top, the basal zone was composed of 1.8 m of stratified, complexly deformed layers of ice and debris overlain by a 0.9 m thick layer of frozen sediment and a 0.8 m thick layer of discoloured ice lying immediately beneath clean glacier ice. Direct shear tests performed in the field on 36 samples show that the average peak shear strength of substrate samples was 2.53 MPa, which is almost twice as strong as the average value for basal ice (1.28 MPa) and the glacier ice samples (1.39 MPa). The direct shear tests suggest that the glacier substrate is unlikely to deform at the current temperature. However, observations of the structure and composition of the basal zone suggest that the substrate contains localized weaknesses that may reduce the peak strength sufficiently to permit bed deformation and entrainment. Two observed forms of weakness are ice lenses and layers in the permafrost and lenses of unsaturated permafrost, which may be inherited from the active zone of a former land surface or produced as pore ice sublimes into cavities which have formed at the glacier bed. Interpretation of data from this study suggests that glacier beds well below the freezing point can be deformed and eroded under certain conditions.

Evidence for an Early Neoglacial Glacier Advance from Rock Glaciers and Lake Sediments in the Sierra Nevada, California, U.S.A.

Lichenometric ages of active glacigenic rock glaciers and Matthes (Little Ice Age) moraines, together with sediment cores from basins below these landforms, provide evidence for a glacier advance -3000 yr ago in the Sierra Nevada, California. Diameters of the largest lichens (70-108 mm) on the Mendel rock glacier indicate that it began forming well before the onset of the Little Ice Age (LIA) (-700 yr BP). Smaller lichens (30-45 mm) on the Matthes moraines of Darwin and Conness glaciers suggest that they formed during the LIA. The apparent lack of pre-LIA Holocene moraines below clean glaciers in the Sierra Nevada demonstrates that the pre-LIA Neoglacial advance must have been less extensive than the Matthes advance. Sediment cores recovered below Matthes glacial deposits provide additional support for a pre-LIA Neoglacial advance. Three cores from a meadow directly below the Powell rock glacier record grus deposition (suggesting nonglacial, summer-dominant mass wasting) between about 5300 and 3600 14C yr BP (--6000-3900 cal yr BP), and alternating gyttja and silt (suggesting cooler, winter-dominated precipitation) after about 3600 14C yr BP (3900 cal yr BP). A lake core from below Conness Glacier shows a transition from higher-organic sediments (10-16% total organic carbon) to lower-organic sediments (2-4% total organic carbon) at approximately 3200 14C yr BP (3400 cal yr BP), suggesting formation and growth of the glacier in the cirque at that time.

Anisotropy of the Reflected Radiation Field Over Melting Glacier Ice: Measurements in Landsat TM Bands 2 and 4

In this article we analyze the anisotropy of the reflected radiation field over melting glacier ice using 118 ground-based radiance measurements in Landsat TM Bands 2 and 4. The measurements were carried out on the tongue of the Morteratschgletscher, Switzerland. On the basis of directional measurements several bidirectional reflectance distribution functions (BRDFs) were derived for dirty and clean glacier ice and for solar zenith angles between 46° and 60°. The anisotropic reflectance factor ranges between 0.70 and 1.60 for TM 2 and between 0.56 and 1.72 for TM 4. The BRDFs exhibit nadir darkening and maximum brightening near the forward limb. Dirty and wet ice reflect more anisotropically than clean ice. We give an example to illustrate the effect of anisotropic reflectance on the TM-derived albedo along a cross-section of the glacier. For solar zenith angles around 48° and in the nadir viewing direction we found differences of 0.05 (in TM 2) and 0.08 (in TM 4) between the albedo (α) and bidirectional reflectance (r). The differences prove to be more or less uniform for both clean ice and dirty ice. It is therefore suggested that one can convert r to α without having to classify the glacier surface.

Characteristics of basal ice at Engabreen, northern Norway

Subglacial intakes in a tunnel system underneath Engabreen, northern Norway, provide access to the underside of a 200 m thick glacier. Detailed observations and measurements were made in several ice tunnels melted out along the glacier bed. A 20–200 cm thick basal sediment layer is overlain by clean glacier ice. Stratigraphy is complex, with alternating sediment-rich and sediment-free layers, and pervasive shearing. Throughout the basal ice are numerous spheroidal water pockets, which increase in both size and degree of elongation with distance from the bed. Ice cores were retrieved from ice-tunnel walls for sediment, cation and isotope analysis. Our observations and measurements provide evidence for both accretion in and water movement through the basal ice. This supports the modification to classical regelation theory proposed recently by Lliboutry in which water flow in the vein network is required to achieve net accretion of regelation layers.

The relative strengths of debris-laden basal ice and clean glacier ice: some evidence from Taylor Glacier, Antarctica

An understanding of the mechanical behaviour of the basal zone of an ice mass is fundamental to understanding the overall dynamics of that ice mass. Despite the fact that debris-laden ice is found in the basal zones of many glaciers and ice sheets, its mechanical behaviour is only poorly understood. This paper attempts to expand our knowledge of the mechanical behaviour of debris-laden ice by examining the uniaxial compressive strength of debris-laden basal ice sampled from the snout of the Taylor Glacier, Antarctica.The mechanical behaviour of debris-laden ice (debris content 5–20% by volume) under uniaxial compression, and the relationship between the behaviours of debris-laden basal ice and ‘clean’ glacier ice, is complex and variable. At the relatively warm temperatures at which uniaxial compressive strength tests were conducted in the field, debris-laden ice was generally weaker than clean glacier ice. At these temperatures, between 0° and −5°C, pressure melting was the dominant deformation mechanism in the debris-laden ice and cracking the dominant deformation mechanism in clean ice. At −25°C, however, debris-laden ice reached higher strengths than lite clean glacier ice and cracking was the dominant deformation mechanism in both ice types. The change in relationship between the strengths of debris-laden and clean ice with temperature is inferred to be attributable to the temperature dependence of the rate of pressure melting.These results suggest that the dynamic effects and significance of the presence of a debris-laden ice layer in the basal zone of an ice mass are likely to be highly variable in space and time.

The relative strengths of debris-laden basal ice and clean glacier ice: some evidence from Taylor Glacier, Antarctica

An understanding of the mechanical behaviour of the basal zone of an ice mass is fundamental to understanding the overall dynamics of that ice mass. Despite the fact that debris-laden ice is found in the basal zones of many glaciers and ice sheets, its mechanical behaviour is only poorly understood. This paper attempts to expand our knowledge of the mechanical behaviour of debris-laden ice by examining the uniaxial compressive strength of debris-laden basal ice sampled from the snout of the Taylor Glacier, Antarctica. The mechanical behaviour of debris-laden ice (debris content 5–20% by volume) under uniaxial compression, and the relationship between the behaviours of debris-laden basal ice and ‘clean’ glacier ice, is complex and variable. At the relatively warm temperatures at which uniaxial compressive strength tests were conducted in the field, debris-laden ice was generally weaker than clean glacier ice. At these temperatures, between 0° and −5°C, pressure melting was the dominant deformation mechanism in the debris-laden ice and cracking the dominant deformation mechanism in clean ice. At −25°C, however, debris-laden ice reached higher strengths than lite clean glacier ice and cracking was the dominant deformation mechanism in both ice types. The change in relationship between the strengths of debris-laden and clean ice with temperature is inferred to be attributable to the temperature dependence of the rate of pressure melting. These results suggest that the dynamic effects and significance of the presence of a debris-laden ice layer in the basal zone of an ice mass are likely to be highly variable in space and time.

Characteristics of basal ice at Engabreen, northern Norway

Subglacial intakes in a tunnel system underneath Engabreen, northern Norway, provide access to the underside of a 200 m thick glacier. Detailed observations and measurements were made in several ice tunnels melted out along the glacier bed. A 20–200 cm thick basal sediment layer is overlain by clean glacier ice. Stratigraphy is complex, with alternating sediment-rich and sediment-free layers, and pervasive shearing. Throughout the basal ice are numerous spheroidal water pockets, which increase in both size and degree of elongation with distance from the bed. Ice cores were retrieved from ice-tunnel walls for sediment, cation and isotope analysis. Our observations and measurements provide evidence for both accretion in and water movement through the basal ice. This supports the modification to classical regelation theory proposed recently by Lliboutry in which water flow in the vein network is required to achieve net accretion of regelation layers.

Subglacial observations from øksfjordjøkelen, North Norway

AbstractStepped bedrock topography at the snout of a small outlet glacier from Øksfjordjøkelen, North Norway, produces an extensive subglacial cavity system which stretches some 70m across and 100m up‐glacier, giving access beneath ice ≤50 m thick. Inside the cavity, regelation ice, clean glacier ice and deforming basal ice have been observed. Samples were taken and basal debris concentrations at the glacier sole were found to vary between 0.005 and 15.38 per cent by volume. The basal ice velocity has been determined using a linear variable differential transformer attached to an analogue recorder, and also by means of measured displacements of ice crews and clasts embedded in the basal ice. Velocities were found to differ both spatially and temporally from a maximum of 2.55 mm h1 to a minimum of 0.3 mm h−1. The measurements and observations, which have been related to present theory, show how spatially averaged values for a number of variables could lead to inaccuracies in predicted erosion values, certainly at a local scale. On the exposed foreland, jointcontrolled lee‐side faces provide evidence for extensive subglacial plucking (here taken to mean the removal of preloosened bed material and/or material resulting from bed failure). Indeed, in the cavity the early stages of removal of joint‐controlled blocks by ice deformation along joints have been observed. The importance of debris‐rich basal ice is shown in the formation of large striations (up to 500cm × 16cm × 2cm) present on the foreland.

The Flow of Ice, Treated as a Newtonian Viscous Liquid, around a Cylindrical Obstacle Near the Bed of a Glacier

AbstractThis paper describes an analytical solution of the equations of motion and heat conduction for ice flowing around a cylindrical solid inclusion and over a solid plane boundary. This is intended to be a simplified representation of the flow of clean glacier ice around a stone and over a rigid rock bed. The ice is treated as a Newtonian viscous liquid and the equations are solved in two dimensions. Regelation boundary conditions are applied at both ice–rock interfaces. It is found that finite solutions for the temperature and stream function only exist for the special cases in which two dimensionless critical wavelengths are zero. That is, unless the stone is very far from the glacial bed, the classical regelation boundary conditions cannot be obeyed over the whole of its surface.

The Flow of Ice, Treated as a Newtonian Viscous Liquid, around a Cylindrical Obstacle Near the Bed of a Glacier

AbstractThis paper describes an analytical solution of the equations of motion and heat conduction for ice flowing around a cylindrical solid inclusion and over a solid plane boundary. This is intended to be a simplified representation of the flow of clean glacier ice around a stone and over a rigid rock bed. The ice is treated as a Newtonian viscous liquid and the equations are solved in two dimensions. Regelation boundary conditions are applied at both ice–rock interfaces. It is found that finite solutions for the temperature and stream function only exist for the special cases in which two dimensionless critical wavelengths are zero. That is, unless the stone is very far from the glacial bed, the classical regelation boundary conditions cannot be obeyed over the whole of its surface.

Ice-Cored Rock Glacier, Galena Creek, Northern Absaroka Mountains, Wyoming

Galena Creek rock glacier (44°38930″ N., 109°47930″ W., elevation 2,680 to 3,110 m, length 1.6 km) originates in a north-facing cirque. Although this rock glacier morphologically resembles others described elsewhere, its upvalley two-thirds is composed of a continuous layer of debris 1 to 1.5 m thick over relatively clean glacier ice and has a maximum measured surface velocity of 80 cm/yr. The downvalley one-third is mantled by 2 to 3 m of debris (measured by seismic refraction) over ice of unknown debris content; it has a maximum measured velocity of 14 cm/yr. The transition zone between these two regions has several large (6-m-high, 90-m-wide) lobes that override one another at a maximum measured velocity of 6 cm/yr. Accumulation occurs primarily as wind-drifted snow in a narrow lens-shaped area against the cirque headwall. Most of the coarse debris is not incorporated in the ice, but is carried past the steep (13° to 33°) snow accumulation area beneath the cirque headwall by snow avalanche and rockfall to form the debris mantle. The debris mantle is sorted, with coarse fragments dominant at the surface and a zone of fines just above the debris-ice contact. The ice beneath the debris mantle contains a maximum of 10 to 12 percent debris by volume, except in probable longitudinal septa downglacier from large debris concentrations in the source area. Intersecting ridges and furrows on the up-valley portion of the rock glacier probably differ in age, according to lichen sizes and ridge sharpness, and are probably formed by compression below steep reaches of the glacier and by collapse into crevasses. Ice-cored rock glaciers uniquely have a very low ratio of accumulation area to ablation area (1:7 in this case). This is mainly the result of an ablation rate beneath the debris mantle that is estimated to be about two orders of magnitude less than that of clean ice. The slow rate of addition of ice makes the glacier thin and thus slow-moving. Because of the debris cover, rock glaciers are not nearly so sensitive to climate as are clean glaciers. The lag effect between retreat of clean glaciers and deactivation of rock glaciers may be several thousand years, and therefore mountain glacier moraines should be correlated with rock glaciers only with extreme care.

An assessment of contemporary glacier fluctuations in Nepal's Khumbu Himal using repeat photography

A preliminary study of glacial fluctuations in Sagarmatha (Mt Everest) National Park, Nepal was undertaken in Oct–Nov 2007 using repeat photography. Photographs from scientific and cartographic expeditions to the upper Imja Khola region ca. 1950 were replicated in order to derive a better, empirically-based understanding of what changes had occurred in the region’s glaciers during the past half century. Over 40 distinct panoramas were replicated which demonstrated the (a) complete loss of certain small (< 0.5 km2), clean glaciers (C-Type) between approximately 5400–5500 masl, (b) the retreat of larger (>0.5 km2) clean glaciers by as much as 50 percent of the ca. 1955 volumes at elevations ranging from approximately 5500–5600 masl, (c) the formation of new and potentially dangerous glacial lakes that had been debris covered glaciers (D-Type) in the 1950s, and (d) the ablation of most of the D-Type glaciers re-photographed. The findings support and complement those of recent investigations based almost entirely on remote sensing and computer modelling. However, detailed, on-the-ground field studies of potential climate change impacts on the people and environments of the Mt. Everest region are disturbingly absent. I suggest that only by systematically combining field and laboratory-based investigations will we acquire the tools to enable us to identify the real threats, non-threats, and ways in which local people can adapt and reduce vulnerabilities to climate change.
 
 Himalayan Journal of Sciences Vol.4(6) 2007 p.21-26

Westland—A New Zealand Province 1

WESTLAND is the province on the western coast of the South Island of New Zealand. Its name brings back a vision of a land covered by forests of tropical luxuriance, rising from a blue sea fringed by a white line of surf to blue mountains capped with fields of snow, of clean glaciers flowing steeply down into glades of tree ferns, and of a succession of pictures so varied and all so perfect in composition that we regard Westland as the most beautiful country it has been our privilege to see.