ALBERT

Description: An electromagnetic stress-wave generator which was developed to study shock waves in snow and ice is described. This system works on the principle of generating large electrical currents to produce highly transient loads on the test specimen. In its present configuration, the generator can produce pressures ranging from a few kilopascals to as large as 104 kPa and load frequencies as, high as 150 kHz. The system has been found to have high repeatability and has good turn-round time.

Description: Glaciomarine sediments (GMS) comprise detrital, biogenic, and authigenic materials of two principal facies: laminated deposits and massive aqueous till. The processes governing sedimentation of the ice-rafted debris (IRD) component of GMS are investigated in the marine zone around Antarctica. Four controlling factors are identified: nature and disposition of sediments at the grounding line, transition from grounded to floating ice (ice shelves, outlet glaciers, and ice cliffs), processes of under-side melting and freezing of these ice masses, and, finally, mechanisms of iceberg calving, fragmentation, and melt-release of debris in the open ocean. Modelling studies of Brunt and Ross ice shelves suggest two main conclusions. (1) Ice shelves are of major importance for sedimentation on the continental shelf. Bulk •debris release occurs within the grounding-line zone which may frequently oscillate, producing pronounced diachronism. Bottom melting removes all debris prior to calving at the ice front so that ice shelves do not play a part in deposition in the open ocean. (2) Outlet glaciers, in contrast, have high sediment content, calve rapidly, and produce debris-rich icebergs which contribute the major portion of IRD in the ocean.

Description: The morphology, sediments, and processes associated with the construction of a moraine along the western margin of the ice shelf in George VI Sound, Antarctica, are discussed. The moraine occurs as a double ridge where the ice sheet grounds against promontories on Alexander Island and is approximately horizontal over a distance of 120 km. It consists of exotic rock debris carried into the ice shelf by Antarctic Peninsula glaciers and local rock debris derived from the grounding line on Alexander Island. As the coast steepens, so the proportion of exotic rocks increases. The transport of basal material from the peninsula implies that there can be little bottom melting beneath this part of the ice shelf. The moraine is modified by streams and marginal lakes which periodically drain into and through the ice shelf. Tidal lakes are impounded against the ice shelf in shallower embayments and consist of fresh water overlying sea-water. A conceptual model of the moraine is developed and may help to explain some features of puzzling horizontal moraines found in formerly glaciated areas.

Description: Radiocarbon dates of 13 000 a BP at the Cheboygan bryophyte bed in Michigan and 11 800 a BP at the Two Creeks site in Wisconsin bracket three ice advance/retreat cycles and one advance to the Two Creeks site of the Lake Michigan lobe. These advances are documented by individual till sheets separated by lacustrine fine sand and silt-clay units. The tills are distinguishable only by small differences in grain-size distribution and clay-mineral content. They probably reflect closely the composition of the sediment being deposited in the Lake Michigan basin between advances. Because of lack of exposure and probable erosion of pre-existing material by each successive ice advance, the maximum extent of retreat between the deposition of the tills cannot be documented. We can demonstrate, however, that a total of at least 850 and perhaps over 1 000 km of combined retreat and advance took place during this period of 1 200 a. This implies that the change in ice-margin position averaged 0.7 to 0.9 km a-1, a rate higher than most recorded on modern glaciers. Since this is an average rate over 1 200 a and encompasses several advances and retreats, the actual rate of change in ice-margin position must at times have been considerably more rapid.There is very little evidence in the pollen record of climatic changes that would explain rapid advances and retreats of this magnitude. This observation has led to suggestions that late Wisconsin age advances in various parts of the Great Lakes were surges unrelated to climate change.We suggest instead that the shape of the Lake Michigan basin, and the substantial changes in water level that might have occurred in it, could have greatly amplified the smaller fluctuations of the ice margin that have been documented across the eastern United States (presumably resulting from changes in mass balance), and so could have produced the rapid advances and retreats seen in the stratigraphic record.The Lake Michigan basin consists of two deep areas with a high area in between. Present water depths are over 280 m in the northern basin, less than 60 m on the mid-lake high, and more than 160 m in the southern basin. All of the ice advances discussed above seem to have stopped either on the northern (up-stream) side or on the crest of the mid-lake high. Even conservative estimates of the amount of isostatic crustal depression at that time suggest that if water could drain into the eastern Great Lakes during retreat of the ice to the north end of the basin, lake level could have dropped as much as 220 m. Although there is no stratigraphic evidence that drainage out of the north end of the basin took place between these ice advances, there are valleys cut in drift and bedrock extending from the north basin eastward toward the Lake Huron basin. It is possible that these valleys formed during the rapid draining of the lake between ice advances. Whenever ice advanced, blocking the northern outlet, lake level rose back to the Glenwood level (which has well-developed beaches), and the lake drained through the Chicago outlet to the south.Our model is as follows. Consider a grounded ice sheet filling the northern basin and terminating at an equilibrium position on the crest or up-stream side of the mid-lake high. The ice sheet would be unstable in that any initial retreat of the grounding line (i.e. the boundary between the grounded ice and either floating ice or open water) would accelerate as the grounding line moved northward into deeper water. Now let a general marginal retreat occur. Rapid retreat of the grounding line follows until the grounding line passes the northern lake outlet. The consequent large drop in lake level leads then to a readvance of the grounding line, re-blocking the outlet and causing the basin to refill with water. A new equilibrium position of the grounding line is established on the northern side of the basin. As the mass balance increases again, associated with a general marginal advance of the ice sheet, an ice shelf forms (if it was not already there) and grows southward until it grounds on the mid-lake high. That then causes the grounding line to advance rapidly to the position of the margin, whereupon the process is ready to repeat. Each advance would be smaller than the previous because rebound would be tilting the north end of the lake upward, shoaling the water and causing ice shelves to ground further north on the mid-lake high. In addition, the mass of ice to the north was probably shrinking.Grain-size distribution and mineralogic characteristics of the tills along the Lake Michigan shoreline have been analyzed extensively. However, the existence of a floating ice shelf in the basin at various times during this period cannot, at present, be deduced from the sediment.

Description: Recent work by Clapperton (1975) proposes that the rapid rates of advance experienced by glaciers which surge may lead to enhanced debris incorporation, increased compressive flow near the glacier snout at the point of maximum extension, and to the upward translation and vertical stacking of debris near the glacier snout and margin. Five glaciers in Spitsbergen (Battyebreen, Holmströmbreen, Lisbetbreen, Vonbreen, Elnabreen) display morphological features which are widely accepted as being diagnostic of surging glaciers.Results of detailed observations regarding the nature, distribution, melt-out, and reworking of englacial debris at Battyebreen are presented. Basally derived till is brought to the surface of the glacier in narrow lateral and terminal belts, no more than 100 m wide. Within this zone, (i.e. up-valley from the snout and towards the valley centre) the ice is debris-free with the exception of small amounts of en-glacial debris which form the core of lobate medial moraines. Differential ablation of debris-free and debris-rich ice leads to the production of a topographic basin within which melt-out and reworking processes occupy restricted locations, as follows. Immediately inside the encircling melt-out till, a zone of flow tills is found. Melt streams are located at the foot of, the flow till-mantled slope, producing narrow (150 m wide) outwash trains, which merge into deltas. The central area of the topographic basin is occupied by a supraglacial lake.Observations of the remaining four locations confirm that other glaciers in the vicinity, which display similar characteristics associated with surging, are developing a stagnant-ice zone of identical appearance. The pattern of processes observed at Battyebreen is thus repeated at each site.A simple model of depositional landscape development is proposed for surging valley glaciers in a sub-polar environment.

Description: Sampling of sediment from the fjord floor in front of tidewater glaciers in Glacier Bay, Alaska, has provided information about processes in this restricted glacimarine (Dreimanis 1979) setting. Sediment sampling, in conjunction with oceanographic and glacial dynamics data, has also enabled the discrimination of sediment types and their facies associations. Deposits are strongly controlled by: sea-water characteristics, position and sediment discharge of melt-water streams, iceberg calving, and rate of glacierfront retreat.Five distinct facies associations have been found to reflect glacier-fjord regimes. The facies associations and ice-fjord conditions responsible for them form the basis for constructing a preliminary model for glacimarine sedimentation by tidewater glaciers. The model can be used to predict (i) rapid retreat of an actively calving ice front, (ii) slow retreat or stabilization of a calving ice front at a channel constriction, (iii) stabilization of a melting (very rarely calving) ice front when the glacier base is near tidewater elevation, and (iv) large outwash delta progradation into a fjord when the ice front retreats onto land. This model can be used to interpret facies associations found in a stratigraphic record.