Douglas I. Benn

Mass balance and equilibrium-line altitudes of glaciers in high-mountain environments

Abstract The mass-balance characteristics of glaciers in high-mountain environments complicate the relationship between glacier equilibrium-line altitudes (ELAs) and climatic variables such as precipitation and air temperature. Therefore, methods of ELA reconstruction employed in low-relief environments are commonly not applicable in high mountains, or require some modification. We review the concept of the ELA, with reference to the mass balance of a range of glacier types found in high-mountain regions. We examine the applicability of several commonly used methods of ELA reconstruction for different glacier types, and propose some general principles to guide the choice of appropriate methods.

Calculating ice melt beneath a debris layer using meteorological data

Generalized numerical models of sub-debris ice ablation are preferable to empirical approaches for predicting runoff and glacier response to climate change, as empirical methods are site-specific and strongly dependent upon the conditions prevailing during the measurement period. We present a modified surface energy-balance model to calculate melt beneath a surface debris layer from daily mean meteorological variables. Despite numerous simplifications, the model performs well and modelled melt rates give a good match to observed melt rates, suggesting that this model can produce reliable estimates of ablation rate beneath debris layers several decimetres thick. This is a useful improvement on previous models which are inappropriate for thick debris cover.

A physically based calving model applied to marine outlet glaciers and implications for the glacier dynamics

We present results from numerical ice-flow models that include calving criteria based on penetration of surface and basal crevasses, which in turn is a function of longitudinal strain rates near the glacier front. The position of the calving front is defined as the point where either (1) surface crevasses reach the waterline (model CDw), or (2) surface and basal crevasses penetrate the full thickness of the glacier (model CD). For comparison with previous studies, results are also presented for a height-above-buoyancy calving model. Qualitatively, both models CDw and CD produce similar behaviour. Unlike previous models for calving, the new calving criteria are applicable to both grounded termini and floating ice shelves and tongues. The numerical ice-flow model is applied to an idealized geometry characteristic of marine outlet glaciers. Results indicate that grounding-line dynamics are less sensitive to basal topography than previously suggested. Stable grounding-line positions can be obtained even on a reverse bed slope with or without floating termini. The proposed calving criteria also allow calving losses to be linked to surface melt and therefore climate. In contrast to previous studies in which calving rate or position of the terminus is linked to local water depth, the new calving criterion is able to produce seasonal cycles of retreat and advance as observed for Greenland marine outlet glaciers. The contrasting dynamical behaviour and stability found for different calving models suggests that a realistic parameterization for the process of calving is crucial for any predictions of marine outlet glacier change.

The genesis and significance of ‘hummocky moraine’: Evidence from the Isle of Skye, Scotland

Abstract The range of genetic and climatic interpretations of Scottish ‘hummocky moraine’ is reviewed, and new data are presented from the Isle of Skye, western Scotland, which are used as the basis of a genetic classification. ‘Hummocky moraine’ on Skye is shown to consist of three principal sediment-landform associations: (1) recessional moraines; (2) chaotic ice-stagnation moraines; and (3) drumlins and fluted moraines. The recessional moraines consist of transverse moraine ridges and chains of mounds, and were formed by a combination of glaciotectonics and debris accumulation at active ice margins. Second, chaotic moraines consist of randomly-distributed hummocks, mounds and rim-ridges and record deposition in contact with inactive ice. Finally, drumlins and fluted moraines are longitudinally-oriented subglacial bedforms formed by a combination of lodgement and sediment deformation. Individual occurrences of ‘hummocky moraine’ may comprise one, two or all of these associations. The detailed study and differentiation of Scottish ‘hummocky moraine’ provides a valuable source of information on former glacier dynamics and landscape change.

Growth and drainage of supraglacial lakes on debris-mantled Ngozumpa Glacier, Khumbu Himal, Nepal

Ablation of debris-mantled glaciers in Nepal has resulted in the formation of several potentially unstable moraine-dammed lakes, some of which constitute serious hazards. Ngozumpa Glacier, Khumbu Himal, has undergone significant downwasting in recent decades, and is believed to lie close to the threshold for moraine-dammed lake formation. The debris-mantled ablation area of the glacier is studded with numerous supraglacial lakes, the majority of which occupy closed basins with no perennial connections to the englacial drainage system (perched lakes). Perched lakes can undergo rapid growth by subaerial and water-line melting of exposed ice faces, and calving. Subaerial and subaqueous melting beneath thick ( > 1 m) debris mantles is comparatively insignificant. Although lake expansion can contribute substantially to ablation of the glacier, perched lakes cannot continue to grow indefinitely, but are subject to rapid drainage once a connection is made to englacial conduits. The level of one of the lakes on the Ngozumpa, however, is controlled by the altitude of a spillway through the lateral moraine of the glacier. This lake underwent only limited growth in the period 1998-2000, but is likely to experience monotonic growth if glacier mass balance continues to be negative.

'Calving laws', 'sliding laws' and the stability of tidewater glaciers

Abstract A new calving criterion is introduced, which predicts calving where the depth of surface crevasses equals ice height above sea level. Crevasse depth is calculated from strain rates, and terminus position and calving rate are therefore functions of ice velocity, strain rate, ice thickness and water depth. We couple the calving criterion with three ‘sliding laws’, in which velocity is controlled by (1) basal drag, (2) lateral drag and (3) a combination of the two. In model 1, velocities and strain rates are dependent on effective pressure, and hence ice thickness relative to water depth. Imposed thinning can lead to acceleration and terminus retreat, and ice shelves cannot form. In model 2, ice velocity is independent of changes in ice thickness unless accompanied by changes in surface gradient. Velocities are strongly dependent on channel width, and calving margins tend to stabilize at flow-unit widenings. Model 3 exhibits the combined characteristics of the other two models, and suggests that calving glaciers are sensitive to imposed thickness changes if basal drag provides most resistance to flow, but stable if most resistance is from lateral drag. Ice shelves can form if reduction of basal drag occurs over a sufficiently long spatial scale. In combination, the new calving criterion and the basal–lateral drag sliding function (model 3) can be used to simulate much of the observed spectrum of behaviour of calving glaciers, and present new opportunities to model ice-sheet response to climate change.

Paraglacial Slope Adjustment and Resedimentation Following Recent Glacier Retreat, Fabergstolsdalen, Norway

Withdrawal of glacier ice from upper Fabergstolsdalen after A.D. 1930 has exposed steep drift-covered slopes on the north side of the valley. Since 1943 these have experienced radical transformatio...

Himalayan glacial sedimentary environments: a framework for reconstructing and dating the former extent of glaciers in high mountains

Abstract Reconstructing paleoenvironmental change from glacial geologic evidence in the Himalayas has been difficult because of the lack of organic material for radiocarbon dating and the problems of correctly identifying the origin of highly dissected landforms. Studies of the contemporary glacial depositional environments, and ancient landforms and sediments in the Hunza valley (Karakoram Mountains), the Lahul and Garhwal Himalaya, and the Khumbu Himalaya illustrate the variability in processes, landforms and sediment types. These studies can be used to interpret ancient landforms and sediments for paleoenvironmental reconstructions, and aid in forming strategies for sampling sediments and rocks for the developing techniques of cosmogenic radionuclide (CRN) surface exposure and optically stimulated luminescence (OSL) dating. Many Himalayan glaciers have thick covers of supraglacial debris derived from valley sides, and such debris-mantled glaciers exhibit important differences from ‘clean’ glaciers, both in terms of debris transport processes, and the depositional landforms that they produce. Analysis of sediment-landform associations can be used to reconstruct processes of sediment transport and deposition, and the relationship between moraines and other landforms and climatic forcing cycles. Such analysis is of fundamental importance in guiding sampling and interpretation in CRN and OSL dating work.

Timing of late Quaternary glaciations south of Mount Everest in the Khumbu Himal, Nepal

Moraines south of Mount Everest in the Khumbu Himal were dated using optically stimulated luminescence. Clustering of ages and morphostratigraphy allowed three advances to be dated: (1) the Periche Glacial Stage (ca. 18‐25 ka), (2) the Chhukung Glacial Stage (ca. 10 ka), and (3) the Lobuche Stage (ca. 1‐2 ka). The Periche Stage is coincident with Oxygen Isotope Stage 2; the Chhukung Stage represents a late glacial or early Holocene glacial advance; and the Lobuche Stage is a late Holocene glacial advance that predates the Little Ice Age.

Quantification of Everest region glacier velocities between 1992 and 2002, using satellite radar interferometry and feature tracking

Quincey, D. J., Luckman, A., Benn, D. (2009). Quantification of Everest-region glacier velocities between 1992 and 2002, using satellite radar interferometry and feature tracking. Journal of Glaciology, 55(192): 596-606. Sponsorship: Knowledge Transfer Project No. 3742