Neil F. Glasser

Debris entrainment and transfer in polythermal valley glaciers

Hambrey, MJ; Bennett, MR; Dowdeswell, JA, et al. (1999). Debris entrainment and transfer in polythermal valley glaciers. Journal of Glaciology, 45 (149), 69-86. Published: 1999

Glacigenic clast fabrics: genetic fingerprint or wishful thinking?

The interpretation of glacigenic diamictons is a subjective process, for which quantitative support is frequently sought from parameters such as clast shape and fabric. It has been widely suggested that different glacigenic diamicton facies possess distinct clast-fabric signatures. This paper examines this concept using a data set of 111 clast fabrics, and a synthesis of published results. Eigenvalues are calculated and compared for a variety sedimentary facies. It is concluded that clast fabric alone is not able to discriminate between different glacigenic facies, and it is argued that clast fabric offers little quantitative support in the interpretation of glacigenic sediments. It is suggested, therefore, that although clast fabric may continue to have a role as an indicator of relative strain at specific sites, its use in the discrimination of glacigenic facies is limited. Consequently, we should be much more selective in undertaking such analyses in the future. Copyright

Styles of sedimentation beneath Svalbard valley glaciers under changing dynamic and thermal regimes

Glacier beds provide crucial information concerning past and present ice dynamics and thermal regime. In this paper we present structural and sedimentological data from four valley glaciers (Austre Lovénbreen, Midtre Lovénbreen, Vestre Lovénbreen and Austre Brøggerbreen) on Brøggerhalvøya, NW Spitsbergen. The main focus of this paper is Midtre and Austre Lovénbreen, two typical High-Arctic land-based polythermal valley glaciers; the former has a long record of documentation regarding its response to twentieth century climatic warming. Structural mapping on the glacier surface and analysis of sediments in the proglacial area of Midtre Lovénbreen indicate that the dynamic regime and thermal structure of the glacier have changed through time. Dynamically, Midtre Lovénbreen was once heavily crevassed and relatively fast moving, but now is virtually crevasse-free and slow moving. The sedimentary record indicates that extensive areas of this glacier were wet-based when it was in a more advanced state, probably at its Neoglacial maximum (late nineteenth/early twentieth centuries). During this advance, a thin deforming layer of diamicton, commonly fluted, was draped over the existing morphology in the presently exposed proglacial area. This morphology consisted of large streamlined ridges aligned parallel to ice flow. Erosion of the underlying bedrock appears to have been limited. Radio-echo soundings of the glacier show that, at present, it is characterized by a polythermal basal thermal regime, with wet-based subglacial conditions only beneath its thicker parts. Modification of the bed is limited under this thermal regime and, as a result, the supraglacial environment dominates modern sedimentation. Comparative studies on Austre Lovénbreen, which also is probably polythermal, indicate similar sedimentary characteristics and facies associations, although here there are much more extensive areas of striated bedrock indicative of former basal sliding. In contrast both Austre Brøggerbreen and, by analogy, Vestre Lovénbreen are known to be predominantly cold-based. Collectively, these four glaciers suggest a trend of glacier recession and thinning accompanied by a change in thermal regime from predominantly wet-based, through partly frozen, to completely frozen. This study suggests that Svalbard valley glaciers have several dynamic modes and that glaciers switch between these modes largely as a reaction to changes in mass balance.

Characteristics of tide-water calving at Glaciar San Rafael, Chile

Glacial calving is a poorly understood process. This study tests the influence of local environmental variables on the magnitude and frequency distributions of calving behaviour at Glaciar San Rafael, Chile. Near the terminus of the glacier, surface speeds average 17 m d −1 in summer and calving is profuse and continual. The size, location and characteristics of over 7000 calving events were recorded during 32 d in 1991 and 1992, together with meteorological, bathymetric and oceanographic data. Mean daily calving exceeds 400 events per day and the mean calving flux is more than 2 Mm 3 d −1 . Mean annual calving speed and calving flux are about 4500 m a −1 and 2.0 km 3 a −1 , respectively. This calving speed is higher than that predicted by the established empirical relationship between tide-water calving speed and water depth. This is surprising, given the low salinity of Laguna San Rafael and that fresh-water calving speeds are commonly much lower than those in tide water. Daily patterns of calving frequency and flux correlate poorly or not at all with meteorological variables, but tidal stage may have some control over the timing of large submarine calving events. Submarine calving produced the largest bergs. However, the relatively small total flux recorded from the submerged part of the ice cliff may imply unusually rapid melt rates.

Karakoram glacier surge dynamics

Quincey, D. J., Braun, M. Glasser, N. F., Bishop, M. P., Hewitt, K., Luckman, A. (2011). Geophysical Research Letters, 38, Article Number: L18504.

The landform and sediment assemblage produced by a tidewater glacier surge in Kongsfjorden, Svalbard

Abstract This paper describes the landform and sediment assemblage produced by a surge (in 1948) of the Kongsvegen/Kronebreen tidewater glacier complex in northwest Spitsbergen. The main geomorphological products of this advance are two large thrustmoraine complexes on opposite sides of the fjord, and a system of geometrical ridges revealed on glacier decay. The thrust-moraines are composed largely of diamicton, sandy and muddy gravel, gravelly sand, sand and mud, with minor laminites. All of these appear to be derived from the fjord floor and represent both fine fjord basin sediments and coarse grounding-line fan deposits. Thrusting was the principal mode of emplacement of the sediment onto the adjacent land areas during the 1948 advance. However, the geomorphology of the thrust-moraine complexes on either side of the fjord is quite different, reflecting a transpressive regime on the southwest side (mainly long ridges) and a normal compressive regime on the northeast side (short ridges and pinnacles of a ‘hummocky’ nature). The advance which produced the moraine complex has previously been attributed to a surge of Kongsvegen, but the glaciological and geomorphological evidence suggests that the advance involved both Kongsvegen and Kronebreen. Comparison of the landform assemblage produced by this event with that produced by other tidewater glacier surges demonstrates the diverse range of landform assemblages associated with glacier surges, or other episodes of rapid flow, within glaciomarine environments.

Optical remote sensing techniques in high-mountain environments: application to glacial hazards:

Remote sensing studies have shown that glaciers and their proximal environments exhibit unique temporal, spatial and spectral characteristics that can be analysed to better quantify glacial hazard potential. In this review, the optical remote sensing data sources available to glacial hazard assessors are considered and the range of information on glacial environments that can be derived is analysed. The review shows that the integration of a variety of data sources can provide geoscientists with information regarding glacial lakes and lake development, glacier dynamics, avalanche sources and ice-marginal fluctuations. Such data can be used to complement and, in many cases, improve field-based glacial hazard assessments. The review concludes that aerial photography still remains the main source of data for measuring a number of glacier characteristics, but that fine to moderate spatial resolution satellite sensors (e.g., ASTER, SPOT 5 HRVIR, Landsat ETM) also provide useful information that can be used to support the assessment of hazards in high-mountain glacierized terrain.

The structural glaciology of Kongsvegen, Svalbard, and its role in landform genesis

Mapping of the structural glaciology of Kongsvegen, Svalbard, reveals evidence for four main deformational structures. These are stratification, longitudinal foliation, thrusts and crevasse traces. These structures are considered in terms of their contribution to debris entrainment, transport and subsequent landform development. Stratification is associated with small amounts of supraglacial debris that has been folded with flow-parallel axes; longitudinal foliation in places incorporates basal glacial sediments along folds with flow-parallel axes; and thrusts transport basal debris to the glacier surface. Crevasse traces are not significant in terms of debris entrainment. The entrainment of basal debris along longitudinal foliation is not a universally recognised process. At Kongsvegen this process is attributed to the development of a transposition foliation, in combination with incorporation of debris-rich basal ice or soft basal sediment in the fold complex. Mapping of the landforms in the proglacial area shows that debris incorporated along longitudinal foliation is released as “foliation-parallel ridges” and that transverse ridges mark debris-bearing thrusts. The role of longitudinal foliation in landform development has never been documented in this manner. Although the preservation potential of such ridges may be limited, recognition of foliation-parallel ridges in the Pleistocene landform record has important implications for the interpretation of the dynamics of former ire masses.

A modelling reconstruction of the last glacial maximum ice sheet and its deglaciation in the vicinity of the Northern Patagonian Icefield, South America

ABSTRACT. A time‐dependent model is used to investigate the interaction between climate, extent and fluctuations of Patagonian ice sheet between 45° and 48°S during the last glacial maximum (LGM) and its subsequent deglaciation. The model is applied at 2 km resolution and enables ice thickness, lithospheric response and ice deformation and sliding to interact freely and is perturbed from present day by relative changes in sea level and equilibrium line altitude (ELA). Experiments implemented to identify an LGM configuration compatible with the available empirical record, indicate that a stepped ELA lowering of 750 to 950 m is required over 15000 years to bracket the Fenix I‐V suite of moraines at Lago Buenos Aires. However, 900 m of ELA lowering yields an ice sheet which best matches the Fenix V moraine (c. 23000 a BP) and Caldenius’ reconstructed LGM limit for the entire modelled area. This optimum LGM experiment yields a highly dynamic, low aspect ice sheet, with a mean ice thickness of c. 1130 m drained by numerous large ice streams to the western, seaward margin and two large, fast‐flowing outlet lobes to the east. Forcing this scenario into deglaciation using a re‐scaled Vostok ice core record results in an ice sheet that slowly shrinks by 25% to c. 14500 a bp, after which it experiences a rapid collapse, loosing some 85% of its volume in c. 800 years. Its margins stabilize during the Antarctic Cold Reversal after which it shrinks to near present‐day limits by 11 000 a bp.

Evolution of large roches moutonnees.

There is something of a mismatch between our understanding of small scale glaciological processes and the way they combine to form large scale landforms of glacial erosion. This paper develops an empirically-based model of the evolution of large roches moutonn6es, based on features in eastern Scotland with lee side cliffs up to 160 m high. Block removal by ice begins some way down the lee slope of a hill or spur at a point where cavities start to form at the ice/ rock interface. Under these circumstances stress gradients are high within the rock and, given fluctuating water pressures within the cavities, rock fracture and entrainment may proceed. With time, block removal produces a cliff which retreats upglacier, producing a characteristic staircase morphology to the cliff in response to limiting crack length and the presence of pre-existing joints in the rock. The link between plucking and subglacial meltwater helps to explain the distribution of roches moutonnees along the axis of the Dee valley, the location of a major ice stream within the former Scottish ice sheet. Surface meltwater may also have been instrumental in plucking, since it is likely to have reached the glacier bed in lee side locations via crevasse fields associated with ice flow over convexities. Whereas this link with meltwater helps to explain the importance of glacial erosion during full ice sheet conditions, a phase of plucking also occurred at the very end of the last glaciation beneath thin ice. The implication of the latter discovery is that a narrow ice-marginal zone of plucking may migrate across the landscape as ice sheets wax and wane.