C34A-01 16:00h
Ice Sheet Dynamics Along the Western Margin of the Scandinavian and Barents Sea-Svalbard ice Sheets on the Basis of Submarine Landforms
Morphological interpretation of regional and detailed bathymetric data sets (multibeam bathymetry and 3D seismics) on the 2,500 km-long Norwegian shelf from the North Sea (57N) to Svalbard (80N) has revealed a dynamic ice-flow pattern along the western margin of the north European ice sheets. About 20 cross-shelf troughs with megascale glacial lineations (MSGL) are interpreted as former pathways for fast-flowing ice streams. On intermediate shallower banks less dynamic ice probably existed. The two largest paleo-ice streams were the Norwegian Channel Ice Stream and Bear Island Trough Ice Stream, each 150-200 km wide at the mouth. The lengths of individual MSGL vary from hundreds of meters to several tens of kilometers, and the distance between ridges varies from 0.1-3 km. MSGL amplitudes reach 15 m, but are commonly less than 10 m. The onset of MSGL and, hence, fast ice flow is generally close to the outer coast, at the border zone between crystalline rocks and softer sedimentary rocks. Transverse submarine ridges on various scales, commonly parallel to the shelf edge, reflect either the maximum ice-sheet position or the recessional pattern of the ice sheet. Lateral ice stream moraines several tens of kilometers long have been mapped along the sides of several cross-shelf troughs, identifying the border zone between fast ice flow and stagnant or slow-flowing ice on intervening banks. An ice sheet model for the whole area during the last glacial maximum will be presented
C34A-02 16:15h
Landform Formation Under Ice Sheets
We present a new mathematical model for the formation of subglacial landforms such as drumlins under a warm-based, soft-bedded ice sheet. At the heart of the model is a channelized drainage system in which smaller channels grow at the expense of larger ones, leading to the continuous creation and extinction of drainage paths, and to a spatially distributed imprint on the landscape. We demonstrate how interactions between such a drainage system, bed topography and ice flow can lead to the spontaneous formation of subglacial landforms, and discuss the effect of different sediment transport characteristics in the drainage system on the shape and migration of these landforms. This mathematical model is the first component of a study of landscape/ice-sheet self-organization, which is inspired and guided, in part, by new digital topographic data (LIDAR) that are revealing with unprecedented detail the striking grain of glacially scoured topography on length scales ranging from 0.5 to 20 km.
C34A-03 16:30h
Debris-Bed Friction of Hard-Bedded Glaciers
Field measurements of debris-bed friction on a smooth rock tablet at the bed of Engabreen, a hard-bedded, temperate glacier in northern Norway, indicated that basal ice containing 10% debris by volume exerted local shear traction of up to 500~kPa. The corresponding bulk friction coefficient between the dirty basal ice and the tablet was between 0.05 and 0.08. A model of friction in which non-rotating spherical rock particles are held in frictional contact with the bed by bed-normal ice flow can account for these measurements if ice is Newtonian. Numerical calculations of the bed-normal drag force on a sphere in contact with a flat bed show that this force can reach values several hundred times that on a sphere isolated from the bed, thus drastically increasing frictional resistance. Various estimates of basal friction are obtained from this model. For example, the shear traction at the bed of a 200~m thick glacier sliding at 20~m$\,$a$^{-1}$ with a geothermally induced melt rate of 0.006~m$\,$a$^{-1}$ can exceed 100~kPa. Debris-bed friction can, therefore, be a major component of sliding resistance, contradicting the common assumption that debris-bed friction is negligible.
C34A-04 16:45h
Glaciology and Geomorphology at the Mountain Crest: Cirque Development
The geomorphic signatures of glacial erosion are ubiquitous in alpine landscapes. Cirques are particularly intriguing features in that they are often used as proxies for past climate, e.g., cirque elevations are assumed to represent elevation of the zero-degree isotherm during the LGM. The detailed physical processes acting to form cirques, including cirque glacier ice dynamics, subglacial erosion, and headwall backwearing, are not well understood. We initiated a field study at Grinnell Glacier in Montana to examine the relevant glacial and geomorphic processes driving cirque development. In July 2004, we installed several temperature sensors and snow stakes to monitor snow and ice melt across Grinnell Glacier, as well as a GPS unit to measure ice motion. During July and early August, air temperature and snowmelt correlate well, with typical melt rates between 3 and 6 cm d$^{-1}$ (water equivalent). Average velocity near the center of the glacier, where ice thickness was $\sim$44 meters, was 4.5 to 5.5 cm d$^{-1}$ during this time. Diurnal variability in ice surface velocity, in addition to calculations of expected velocity due to internal deformation only, suggesting sliding is an important component of measured surface motion. Significant debris cover on the glacier, as well as active rockfalls during our field excursions, imply rapid headwall backwearing, perhaps on par with estimated rates of subglacial erosion.
C34A-05 17:00h
Glacial Erosion Patterns in Valley Systems in Northern Sweden Investigated Using Cosmogenic Nuclides
Erosion patterns associated with glaciation of the V\'{a}vl\'{a}j$\aa$hk$\aa$, Dievssaj\'{a}vri, and R\'{a}vtasj\'{a}vri valley systems in northern Sweden were investigated using cosmogenic nuclide ($^{10}$Be and $^{26}$Al) apparent exposure ages and inferred inheritance signals. Sequences of samples taken across valleys known to have been covered repeatedly by the Fennoscandian ice sheet revealed two primary patterns of erosion. In V\'{a}vl\'{a}j$\aa$hk$\aa$ the exposure age pattern has consistent deglaciation ages (8-10 kyr) along the entire profile, indicating erosion of $>$$\sim$2 m at all sites during the last glacial cycle. At R\'{a}vtasj\'{a}vri and Dievssaj\'{a}vri, deglaciation ages in the valley bottom contrast with apparent exposure ages two to four times older than deglaciation on the valley sides. Older ages on the valley sides are interpreted as reflecting cosmogenic nuclide inheritance due to limited ($<$2 m) erosion of the valley sides during the last glacial cycle. The pattern of erosion at R\'{a}vtasj\'{a}vri and Dievssaj\'{a}vri indicates that in some locations glacial valley formation is a result of multiple glacial cycles rather than the result of topographic modification during a single glacial cycle. Initial data comparing hanging valley and trunk valley sites do not show distinct differences in apparent exposure ages. Slightly older ages on samples from hanging valley bottoms may suggest inheritance indicating lower erosion than trunk valleys, as would be expected given the marked topographic step between hanging and trunk valleys. Overall, initial data from this pilot study suggest that patterns of cosmogenic apparent exposure age data can in some cases provide constraints on spatial patterns of erosion and help refine understanding of the timing and scope of landform modification by glaciation.
C34A-06 17:15h
Sub-glacial Water Pressures Over Long Time Scales and Extended Spatial Scales
Data on the sub-glacial hydrologic conditions under the Bench Glacier, coastal Alaska, have been obtained over the full length of the glacier and have been recorded continuously spanning 3 summer melt seasons and 2 winters. Data includes basal water system measurements of ionic conductivity, turbidity, water speed, and water pressure. This talk focuses on approximately 2.5 years of pressure data from up to 43 boreholes, spanning the full length of the glacier and taken at time intervals ranging from 15min intervals down to months of 1 second data. This unique data set reveals the spatial and temporal variability of the sub-glacial hydraulic system. Despite gross similarities between years, there are major differences in the yearly behavior. In particular, both the long term average and short term detailed pressure behavior varies from year to year, both in individual boreholes and in regions of the glacier. Strong differences are noted between the ablation region, which shows a complex record, and the accumulation zone, with a subdued pressure history. The difference appears to be stepwise in space and not a gradual change. Although there is correlation between individual glacial movement or speed events and basal water pressure events, there is no evidence of persistent correlations between ice sliding velocity and basal water system pressures averaged over any spatial or temporal scale. We also report on several anomalous events seen in the record, in particular, we report on sort lived, very high pressure spikes that are rare but apparently not unique aspects of the basal hydrologic system.
C34A-07 17:30h
Warming-induced glacial advance in Taylor Valley?
Changes in the extent of the polar alpine glaciers in Taylor Valley, Antarctica, are important for understanding past climates and changes in ice-dammed lakes. Glacier length changes are slow compared to surface energy balance changes (which govern lake levels) due to the relatively long response time of glaciers. Lyons and others (1998) infer a draw-down 1200 years ago of the lakes in Taylor Valley, when conditions were colder by about 3$^{\circ}$ C compared with today (Clow, pers. comm.). Presumably, colder temperatures reduced melt water production from the glaciers and lake levels dropped. Air temperatures warmed following the drawdown, and increased melt refilled the lakes. Ground-based photography taken over a 20-year period, combined with field survey measurements, show thinning and a 2 to 100m advance for three glaciers in Taylor Valley. While these advances seem to indicate cooler conditions in this part of Antarctica compared to other areas of Antarctica, we propose that long-term climate warming can explain the observed advances through ice softening. We test this hypothesis by using a flow-band model that includes a temperature-dependent softness term and apply the model to Commonwealth Glacier. Results show that a 2$^{\circ}$ C warming 2000 years ago can cause a 25m advance and thinning in the ablation zone, consistent with observations. Increasing the accumulation rate in the accumulation zone by 10% also explains the advance, but predicts 7m of thickening in the ablation zone. Our observations of thinning in the ablation zone thus support a temperature-driven glacial advance rather than an accumulation-driven advance. Our findings indicate that the dynamic behavior of temperate glaciers in response to temperature changes cannot be directly applied to polar glaciers. The sluggish behavior of polar glaciers is a consequence of the ice temperature and small magnitude of mass exchange. These conditions mask an intriguing and complex response to temperature change.
C34A-08 17:45h
Response of debris-covered glaciers to climate change
The presence of supraglacial debris strongly influences glacier ablation, and the mass balance of debris-covered glaciers differs significantly from that of clean glaciers in similar climatic settings. Predicting the response of debris-covered glaciers to climate change is important for hazard mitigation strategies in many high mountain environments, especially where temporary lakes are likely to form on stagnating glacier tongues. Accurate prediction of glacier evolution requires a robust mass balance function which incorporates the effect of debris cover. We present a new model for calculating ablation beneath supraglacial debris layer from meteorological data, based on coupling the surface energy balance and conductive heat flux through the debris layer. The model performs well in a wide range of climatic settings, and results correlate well with measured melt rates in the European Alps and Svalbard. The ablation model is used to construct theoretical mass balance curves for debris covered glaciers, providing surface boundary conditions for glacier flow models. Modelled mass balance curves display reverse gradients on glacier termini where the effect of thickening debris cover with decreasing altitude outweighs that of higher air temperatures. This explains the widely-noted tendency for debris-covered glaciers to stagnate under warming climates. When the mass balance of the glacier as a whole is negative, increasing ablation with altitude causes the lower tongue to decrease in gradient. As gradients and ice thicknesses decline, the process is reinforced by a positive feedback with velocity, so less ice is delivered to the terminal zone. Low surface gradients encourage the formation of supraglacial ponds which can grow rapidly, significantly increasing mass loss from the glacier and potentially posing flood hazards.