Polythermal glaciers, Svalbard

Glacier Dynamics, Structure and Sedimentation of Polythermal Glaciers, Svalbard

Hambrey, Michael, N. F. Glasser and B. Hubbard

Studying Arctic glaciers is of value for several reasons:

• Most Arctic glaciers have receded since the post-glacial maximum positions of around AD 1900, and these changes can be linked to historical and climatic records, so, indirectly, the effects of industrialization may be observed.
• Climatic models predict that global warming will be most pronounced in the Polar Regions. As Arctic glaciers are particularly sensitive to climatic changes, they are useful monitors of such changes, and assist in the assessment of global sea-level change.
• More specifically, ice-volume and ice-marginal changes can be used to determine how much mass has been lost since the postglacial maximum positions of around AD 1900, and these changes can be linked to historical and climatic records. Thus, indirectly, we can observe the effects of industrialization through time on glaciers.
• Many glacial landforms and sediments exist in currently non-glaciated areas, such as North America, the British Isles, northern and central Europe. To understand the origin and climatic significance of these landforms we need to study modern glaciers. For example, those in Svalbard are useful analogues for late-glacial features in Britain.
• Glacial sediments in formerly glacier-covered regions are characterised by extensive sheets of unconsolidated deposits.

The importance of Arctic glaciers has been recognised by the Natural Environment Research Council. Since 1998, a special programme called ARCICE (Arctic Ice and Climate Variability) has been operating, with a view to better understanding glacier dynamics on all time-scales, glacial processes, and their links with climatic change.

Aberystwyth scientists have worked on the glaciers of Svalbard since 1993, focusing initially on the development of sediment/landform assemblages, and more recently on long-term changes in glacier dynamics. The most recent project, "Multi-annual dynamics of Svalbard glaciers", funded under the ARCICE programme, involves interdisciplinary co-operation with Dr Tavi Murray (University of Leeds), Dr Jemma Wadham (University of Bristol), Dr Adrian Luckman (University of Wales, Swansea), Prof Julian Dowdeswell (University of Cambridge), Prof. Jon-Ove Hagen (University of Oslo), and Dr Jack Kohler (Norwegian Polar Institute). Our research has been undertaken on Svalbard, a cluster of Norwegian islands in latitude 76o to 80oN. The archipelago is 60% ice-covered, but most glaciers have suffered major recession since their 1900 AD maxima. However, about a third of the glaciers "surge" (undergo short-lived, rapid advances), making links with climate complicated.

Svalbard archipelago with the locations of the glaciers studied.

There are few detailed studies of the velocity distribution in the glaciers of Svalbard, but much can be gleaned about their dynamic behaviour from a variety of geophysical techniques and by examining their surface structures. One of the most useful geophysical techniques is "ground-penetrating radar" which "sees" through the glacier enabling the observer to distinguish areas of warm or temperate ice (at the pressure melting point) from cold ice (below the pressure melting point). Glaciologists believe that many glaciers in Svalbard are ‘polythermal', i.e. they have a frozen rim and a wet, sliding core. A glacier named midre Lovénbreen in NW Spitsbergen is a typical polythermal glacier, and here Norwegian glaciologists have obtained a mass-balance record that spans some 35 years. The focus of the Aberystwyth team, in association with the universities of Leeds, Bristol, and Swansea has been to record ice-structures with a view to establishing how the dynamics of the glacier have changed through time. Glacier structures are like those in the rocks of many mountain belts, and result from deformation of ice at, or close to, the melting point. Structures visible at the ice surface include crevasses and associated fractures, foliation, folds and thrusts. Field measurements of structures in 3-d and radar surveys help define the internal structure of the glacier. Many of these structures in Svalbard glaciers are inherited from a time when the glacier was much more dynamic.

Neil Glasser fixing sampling locations on Midtre Lovénbreen using a GPS. Photo: M. J. Hambrey.

We also see ways in which debris can be incorporated in relation to structures. Debris of angular character, which is embedded in the snow pack in the accumulation area is subject to folding, ultimately producing medial moraines. Debris also freezes onto the base of the glacier, giving rise to a layer of dirty ice, several metres thick. Some of this debris, along with slabs of sediment from below, is raised towards the ice surface via thrusts All these processes have potential for the creation of depositional landforms.

The processes alluded to above have a strong bearing on the distribution of sediments and landforms in the proglacial areas of Svalbard glaciers. Key features are:

• An outer Neoglacial moraine-round complex, formed by thrusting when the glacier was at its most dynamic. These are similar to "hummocky moraines" described from North America, Scandinavia and the UK. Materials consist of diamicton, sand, gravel and poorly sorted sediment interpreted as a basal till, deposited when wet-based conditions prevailed, although following recession this material is emerging from beneath frozen ice.
• Small linear ridges formed parallel to foliation at depth in the glacier, and also composed of diamicton. We call these foliation-parallel ridges to distinguish them from the better known flutes which are also present
• Flow-parallel trains of angular debris (debris stripes) derived from medial moraines (which in turn are the product of folding in the ice). These features drape all the other forms described above
• Geometrical ridge networks where foliation-parallel ridges are cut by thrust moraines

Comparative studies on the Late Glacial landform/sediment assemblages in the mountain areas of Britain indicate a close similarity with features around modern glaciers in Svalbard. Around 10,000 years ago the Western Highlands of Scotland had an extensive icefield, while smaller valley and cirque glaciers existed elsewhere in the Highlands, the Lake District and Wales. The landforms arising from this period, known locally as the Loch Lomond Stadial (or internationally as the Younger Dryas), are "fresh" in appearance, and appear to retain their original morphology and sedimentology. Our work has shown that the similarity of form and composition between the two areas suggests that the British landforms are also the product of ice deformational processes. For example, we believe that the moraine-mound complexes of Coire a' Cheud Cnoic in Torridon, NW Scotland; of Ennerdale in the Lake District; and of Cwm Idwal in Snowdonia, are the product of thrusting.

Fresh-looking hummock moraines in Coire a' Cheud Cnoic (Corrie of a Hundred Hills) in Torridon, NW Scotland - arguably the finest example in the British Isles. The origin of these 10,000 year-old features has been the subject of much debate. Our interpretation is that they were generated by englacial thrusting.