Moraine formation in a polar arid environment, McMurdo dry valleys, Antarctica

The prevailing view of cold-based glaciers in geomorphology and glacial geology is that they are ineffective agents of erosion. This view stems from studies that have concluded that the adhesive bond between ice and the glacier substrate is greater than the maximum average shear stress normally generated at the base of a glacier. However, several researchers have suggested that erosion by cold ice is possible because theoretical and laboratory studies have demonstrated that sliding is possible at subfreezing temperatures. More recently studies of moraines and erosional landforms have documented entrainment and erosion by cold glaciers and observations beneath Meserve Glacier concluded that slow sliding and limited debris entrainment occurred at -17°C. Although the work described above has challenged the previously held assumptions that cold-based glaciers do not slide or abrade their beds, reviews of glacial processes and paleoglaciological studies continue to associate cold-based glaciers with no or minimal erosion.

Several approaches can be taken in the study of geomorphological processes associated with glaciers. Typically these approaches are based on the study of the morphology, structure and sedimentology of deposits that formed during the Pleistocene Period when glaciers were much more extensive on the Earth. Although it is possible to study contemporary processes of erosion, transportation and deposition in modern glaciers such an approach is constrained by limited access to the beds of glaciers. In the late 1990s and early 2000s several tunnels were excavated beneath glaciers in the McMurdo Dry Valleys in order to understand how the glaciers are coupled to their beds and the circumstances that lead to erosion. The data and observations provide new insights into erosion processes beneath cold glaciers and highlight the importance of material properties in controlling deformation processes.

Cold-based glaciers can be defined as glaciers in which the basal temperature lies below the pressure melting point throughout the glacier (Figure 1). The low basal temperatures mean that there is no liquid water at the bed although thin films of liquid may exist and facilitate very slow sliding. In the McMurdo Dry Valleys These glaciers are typically thin (<300m) and their basal temperatures are close to the mean annual air temperatures, which vary between -17.7°C for relatively warm locations such as the Taylor Valley and -27.4°C for colder valley-floor locations such as the upper Victoria Valley. The glaciers typically have very low concentrations of supraglacial debris, surfaces characterized by well-developed cryoconite and distinctive 15-30m-high cliffs at the glacier margins.

Moraines can be observed at the margin of many glaciers, particularly those that rest on frozen unconsolidated sediments on the valley floors (Figures 1 and 2). Exposures of these moraines show that they consist of blocks of sediment that have been eroded from the permafrost, entrained by the glaciers and stacked at the ice margins where they form low relief mounds. The internal structure of the moraines is characterized by angular unconformities between blocks of frozen sand and gravel that are approximately 0.5 m thick. These moraines appear to be the product of deformation, erosion and deposition of subglacial permafrost that rests beneath the glaciers.
In order to examine the processes of erosion at the glacier bed several tunnels were excavated beneath the glaciers (Figure 3). These tunnels were used to examine the entrained debris, and to monitor the movement of basal ice and the underlying permafrost for periods up to 4 years. The tunnels show that the basal zone of the glaciers consists layers of sand and gravel interspersed with segregated ice from the underlying permafrost (Figure 4). Analysis of the stable isotopes of the ice suggests that the debris and permafrost ice has been mixed with the overlying glacier ice. Strain measurements in the tunnels show that the ice deforms relatively rapidly whereas the strain rates within the frozen blocks of sediment are negligible.

This pattern suggests that the ice content of the permafrost on which the glaciers rest determines whether the permafrost will be deformed and entrained by glaciers. The permafrost is very sensitive to deformation by glacier ice flowing above it when the permafrost is ice-rich. The result of permafrost deformation beneath a glacier is that both the ice and sediment in the permafrost can be entrained by the glacier and incorporated in the glacier basal zone. The velocity structure of Suess Glacier shows that the overridden permafrost has become part of the base of the glacier and suggests that the basal décollement is within the permafrost rather than at the glacier ice-permafrost boundary. When the entrained material reaches the glacier margin this material accumulates and has very similar physical characteristics and may be mistaken for proglacial glaciotectonic landforms.

The observations and measurements around the margins and beneath cold glaciers resting on frozen sediment demonstrate that these glaciers can achieve significant erosion. Furthermore, this study together with previous work on cold-based glaciers furnish a portrait of the behaviour of cold-based glaciers that is increasingly difficult to resolve with ice sheet reconstructions that associate zones of erosion with wet-based ice and zones of no erosion or landscape modification with cold ice. Such a simplistic "binary" approach to paleoglaciological reconstructions seems to represent a precarious separation between glacial geology and glaciology.

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