ALBERT

Description: Nine glaciers in Alaska and Washington, U.S.A., originally mapped as part of the International Geophysical Year (IGY) in 1957—58, were re-mapped between 1993 and 1996, eight using airborne surface elevation profiling and the ninth using ground-based kinematic global positioning system methods. Elevation, volume and terminus changes were determined for the approximately 38 year period between the IGY mapping and the profiling, All nine glaciers showed substantial thinning at lower elevations; seven of the nine thickened at higher elevations. None of the glaciers had a significant net volume increase; two had close to zero change, and the others had a decrease. For the eight glaciers for which we could obtain quantitative information, the mean thickness change was -10 m with a large scatter, 8 m standard deviation. The volume and terminus changes had no clear geographic pattern, and no simple relationship between volume change and terminus advance or retreat was identified. The largest error in the estimated volume changes is due to map errors.

Description: Ice temperature was measured in and around the chaotically crevassed south margin of Ice Stream B, Antarctica, from 1992 to 1994. The temperatures at 30 m depth in the chaotic zone are about 12 K lower than in the adjacent uncrevassed ice, due to the ponding of cold winter air. At depths greater than 150 m, there is clear evidence of internal heating of the ice due to the large shear déformation rate in the marginal zone. Analysis of the depth of cooling below the crevasses and of the internal heating gives two pieces of information. First, over the last half century the lateral shear stress averaged 2.0 x 105 Pa in the top third of the margin and, second, the margin moved outward at an average rate of 7.3 m a−1. These values do not involve any assumptions about the How law of ice. The uncertainties are roughly 20%. The value of lateral shear stress indicates that the most of the drag on the ice stream is along its sides.

Description: We have analyzed the flow of polythermal McCall Glacier in Arctic Alaska. Using measurements of surface velocity from the 1970s and 1990s, together with measurements of ice thickness and surface slope, we have investigated both the present flow and seasonal and long-term flow variations. Our analysis of the present flow reveals that (i) longitudinal stress coupling is important along the entire length of the glacier, and (ii) there is significant basal sliding beneath a 2 km long section of the lower glacier. This sliding exists year-round and it accounts for more than 70% of the total motion there. We have developed a numerical model which shows that such a sliding anomaly causes an asymmetric decrease in ice thickness. Accompanying this decrease in thickness is a decrease in surface slope at the center of the anomaly and an increase in slope up-glacier from it. Both effects are reflected in the observed surface profile of McCall Glacier.The longitudinal stress-coupling length of McCall Glacier is three times the ice thickness, almost twice that typical of temperate glaciers. This is a direct effect of lower strain rates, which themselves are associated with the smaller mass-balance gradients of Arctic and continental glaciers. Long-term variations in surface velocity between the 1970s and 1990s are explained solely by the effects of changes in glacier geometry on the deformational flow contribution. This means that long-term variations in the spatial patterns of longitudinal stresses and basal sliding must have been small. Seasonally, Velocities reach their annual minimum in spring and increase during the short summer nick season by up to 75% above mean winter values. However, the extra motion associated with the period of elevated velocities is only about 5% of the total annual motion. The speed-up is due to an increase in basal sliding. This implies that most of the glacier bed is at the melting point. The zone a affected by the melt-season speed-up extends well up-glacier of any moulins or other obvious sources for melt water at the bed.

Description: A surge of West Fork Glacier, a temperate glacier in the Susitna Basin of the Alaska Range, began soon after the end of the 1987 melt season and terminated on 6 July 1988. Reconnaissance measurements of balance, elevation and speed had been made from 1981 to 1983. Daily measurements of surface speed at two points 9 km apart and of the characteristics of the stream draining the glacier were begun during the surge and continued through the following year. The maximum displacement of the ice during the surge was about 4 km; the maximum change in surface elevation was about 120 m. Between the time of the start of detailed observations on 12 February 1988 and the onset of a complex termination phase during the last month of the surge, the speed was almost constant, and the water discharge was totally free of turbidity, indicating that no basal water was escaping from the glacier. During the termination phase, sharp changes in speed occurred, almost simultaneously at the two observation sites; each deceleration event was accompanied by high sediment concentration and high water discharge. This behavior is similar to that observed on Variegated Glacier during its 1982-83 surge. The mechanism of triggering (related to surface water input and the disruption of the internal drainage system) and the cause of the fast motion were probably the same for both surges, even though there are substantial differences in size and mass-balance characteristics.

Description: The speed of Ice Stream B, Antarctica, was measured twice a day-over a 1 month study period, and found to be steady at about the ±3½% level, the sensitivity of the measurements. The vertical strain was measured at three sites over a 1 year period at 1 h intervals with sensitivities of 2 or 0.2 ppm. The strain rate varied on all time-scales. Events of high strain rate were observed, but never at more than one site at a time. They can probably be understood in terms of local modification of the strain field associated with crevassing. Diurnal variation in strain rate was observed at one and possibly two sites during two summers. The seismicity was measured at all three sites, and diurnal and seasonal variations were prominent at all, the seismicity being much more intense in winter. Several possible causes of the diurnal variations in strain and seismicity are considered: thermal and atmospheric effects, and the effects of tides in the Ross Sea.

Description: We have analyzed the flow of polythermal McCall Glacier in Arctic Alaska. Using measurements of surface velocity from the 1970s and 1990s, together with measurements of ice thickness and surface slope, we have investigated both the present flow and seasonal and long-term flow variations. Our analysis of the present flow reveals that (i) longitudinal stress coupling is important along the entire length of the glacier, and (ii) there is significant basal sliding beneath a 2 km long section of the lower glacier. This sliding exists year-round and it accounts for more than 70% of the total motion there. We have developed a numerical model which shows that such a sliding anomaly causes an asymmetric decrease in ice thickness. Accompanying this decrease in thickness is a decrease in surface slope at the center of the anomaly and an increase in slope up-glacier from it. Both effects are reflected in the observed surface profile of McCall Glacier.The longitudinal stress-coupling length of McCall Glacier is three times the ice thickness, almost twice that typical of temperate glaciers. This is a direct effect of lower strain rates, which themselves are associated with the smaller mass-balance gradients of Arctic and continental glaciers. Long-term variations in surface velocity between the 1970s and 1990s are explained solely by the effects of changes in glacier geometry on the deformational flow contribution. This means that long-term variations in the spatial patterns of longitudinal stresses and basal sliding must have been small. Seasonally, Velocities reach their annual minimum in spring and increase during the short summer nick season by up to 75% above mean winter values. However, the extra motion associated with the period of elevated velocities is only about 5% of the total annual motion. The speed-up is due to an increase in basal sliding. This implies that most of the glacier bed is at the melting point. The zone a affected by the melt-season speed-up extends well up-glacier of any moulins or other obvious sources for melt water at the bed.

Description: We employed a commercial wireline drill rig to investigate the subglacial conditions of Black Rapids Glacier, a well-studied surge-type glacier in the central Alaska Range. The four main goals were: to assess the capabilities of the commercial drilling industry for sampling subglacial material, to investigate the basal morphology, to determine the subglacial geology and to emplace borehole instruments. The drilling was done in an area where seasonal and secular variations in speed are large, and where seismic studies suggested the presence of a till layer. Four holes were drilled at three locations to a maximum depth of 620 m. Three holes yielded samples of basal ice and till, although recovery of the latter was generally poor. Bedrock was sampled in one or possibly two of the holes. In the area sampled, t he glacier is underlain by a till layer some 4–7 m thick, confirming the seismic interpretation. It consists of a sandy matrix at least 20–30% of which comprises larger clasts. Limited samples of the matrix indicate that near the top of the till the porosity is 40%, and t hat some of the pore water is frozen. Geologic studies suggest that the drilling area lies to the north of the Denali Fault, a major tectonic boundary followed by the glacier, and that most of the till is locally derived with transport distances of

Description: McCall Glacier has the only long-term mass-balance record in Arctic-Alaska. Average annual balances over the periods 1958–72 and 1972–93 were –15 and –33cm, respectively; recent annual balances (1993–96) are about –60 cm, and the mass-balance gradient has increased. For an Arctic glacier, with its low mass-exchange rate, this marks a significant negative trend.Recently acquired elevation profiles of McCall Glacier and ten other glaciers within a 30 km radius were compared with topographic maps made in 1956 or 1973. Most of these glaciers had average annual mass balances between –25 and –33 cm, while McCall Glacier averaged –28 cm for 1956–93, indicating that it is representative of the region. In contrast, changes in terminus position for the different glaciers vary markedly. Thus, mass-balance trends in this region cannot be estimated from fractional length changes at time-scales of a few decades.We developed a simple degree-day/accumulation mass-balance model for McCall Glacier. The model was tested using precipitation and radiosonde temperatures from weather stations at Inuvik, Canada, and Barrow, Kaktovik and Fairbanks, Alaska, and was calibrated with the measured balances. The Inuvik data reproduce all measured mass balances of McCall Glacier well and also reproduce the long-term trend towards more negative balances. Data from the other stations do not produce satisfactory model results. We speculate that the Arctic Front, oriented east–west in this region, causes the differences in model results.

Description: The rate of margin migration of an ice stream can be determined using repeat measurements of the surface velocity profile within the marginal shear zone. The method relies on the assumption that the velocity profile is a characteristic feature of the margin, and that this profile is shifted laterally as the margin migrates. Application of the method to Ice Stream B, Antarctica, indicates that the southern margin is moving outward into the inland ice at a rate of at least 9.7 ± 1.1 m a−1.

Description: The rate of margin migration of an ice stream can be determined using repeat measurements of the surface velocity profile within the marginal shear zone. The method relies on the assumption that the velocity profile is a characteristic feature of the margin, and that this profile is shifted laterally as the margin migrates. Application of the method to Ice Stream B, Antarctica, indicates that the southern margin is moving outward into the inland ice at a rate of at least 9.7 ± 1.1 m a−1.