Glacial thermal regime

Introduction | Thermal regimes | Warm-based glacial processes | Polythermal glacial processes | Cold-based glacial processes | References | Comments |

Introduction

Glacial thermal regime is one of the most important factors in determining subglacial processes. The amount of meltwater at the base of a glacier controls entrainment, transfer and deposition of debris, as well as being an important factor in controlling glacier velocities and ice deformation. Glacial sediments and landforms vary widely between different landsystems[1, 2]. The breadth of temperatures and environments found in Antarctica, from the northern Antarctic Peninsula to the cold Dry Valleys, means that the whole spectrum of glacier thermal properties exist, with a wide variety in glacial processes across the Antarctic continent. Together, thermal regime, topographic setting and tectonic regime control sedimentary outputs[3]. In this section, we describe, firstly, different kinds of thermal regimes, before looking at their processes and products in Antarctica.

Thermal regimes

The thermal regime of a glacier is a function of ice temperature (which again is a function of air and ground temperatures, with some glaciers being heated from below by geothermal heating) and the pressure of the ice. In temperate regions, such as the Alps[e.g., 4], many glaciers reach pressure melting point, where ice at the base of a glacier melts[1, 5]. Debris may be an obstacle to the ice, which causes melting and refreezing in the lee of the object. This process of melting and refreezing readily entrains debris into the basal layers of glacier ice. Ice at warmer temperatures is more plastic and deforms more easily, encouraging movement[5]. Finally, the presence of meltwater at the base of a glacier encourages basal sliding, and rapid ice velocities.  These temperate, wet-based glaciers can erode and transport large volumes of sediment, resulting in large landforms such as moraines, drumlins, scoured bedrock or mega-scale glacial lineations. Temperate glaciers can also grind down rocks to fine silts and clays, mixing them with rocks and boulders to form subglacial tills[6]. In Britain, there are fine examples of strongly deformed subglacial tills well exposed in coastal cliff sections in Durham[7-9] and Norfolk[10-13]. The photographs below illustrate some examples of glaciotectonic structures; see Davies et al. 2009; 2012a; 2012b for more information on Warren House Gill and Whitburn Bay.

In Antarctica, thermal regimes pass through the end members of cold, polythermal and warm (wet-based). This means that under some glaciers in cold environments, such as the Dry Valleys in Antarctica, pressure melting point is not reached and the glacier remains frozen to its bed. Typically, under cold-based glaciers, there is little movement, debris entrainment or deposition, and landforms are typically subdued[14-17]. Delicate features, such as tors, may be preserved under cold ice caps and glaciers, and the plateaux of Scotland were thought to bear cold-based ice domes during the Last Glacial Maximum[18]. This is because a frozen glacier bed inhibits the rapid-flow mechanisms of sediment deformation, ice deformation and basal sliding[14, 19].

Between these two end members, polythermal glaciers have beds that are frozen and unfrozen. Many small glaciers in Svalbard are polythermal; cold temperatures mean that higher basal pressures must be reached to attain pressure melting point, and thin valley glaciers are typically frozen around their margins but wet-based in their upper reaches[20-22]. In reality, most valley glaciers probably are polythermal, but range from mostly warm-based to mostly cold-based. Many outlet glaciers in Svalbard are polythermal, and can be viewed on the Glaciers Online website.

Within ice sheets, Kleman and Glasser 2007[14] identify four major ice-dynamical components found within ice sheets, including the Antarctic Ice Sheet. They are: frozen-bed patches, ice streams, ice stream tributaries, and lateral shear zones.

Warm-based glacial processes

When glaciers recede, the generally expose glacial deposits (including “till”; see photographs above). This till deforms by shear under certain conditions, and deformation of till has been observed in situ under ice streams and outlet glaciers in Antarctica and Iceland[5, 19]. When saturated sediments at the base of Whillans Ice Stream (Ice Stream B) were sampled via boreholes, they had a shear strength of only a few kPa. The ice stream had a driving force of only 20 kPa, but was able to deform the sediment and slide along its surface [see Cuffey and Paterson 2010 for a summary]. These saturated sediments are often referred to as a “deforming bed”.

The majority of the work of glacial geologists over the last 20 years has focussed on rapid ice motion through subglacial deformation[23], with the majority of glacier motion occurring through pervasive deformation of saturated subglacial sediments[24]. More recent work has focussed on basal sliding beneath soft-bedded glaciers[25] through the intensive shear of a thin subglacial layer.

The Antarctic Ice Sheet comprises large areas of slow ice flow, drained by a number of rapidly-flowing ice streams fed by dendritic tributaries extending far into the ice sheet[26] (see Ice Streams). The rapid velocities of ice streams is sustained by basal sliding and subglacial sediment deformation[27], with a high pore-water pressure at the ice-bed interface. For example, the Siple Coast ice streams slide over deformable subglacial sediments. However, the beds of modern ice streams are directly inaccessible to scientists, and so we must look to the palaeo-record to understand wet-based glacial processes in Antarctica.

On the continental shelf around Antarctica, there is ample evidence of wet-based glacial processes beneath the palaeo-ice streams that crossed the continental shelf during the Last Glacial Maximum. The soft sediments beneath these ice streams facilitated high ice discharge[28, 29].

Polythermal glacial processes

This section is mostly from Hambrey and Glasser 2012.

Polythermal glaciers are an intermediate type, with a complex thermal structure. Typically, the snout, margins, sides, and surface ice are below the pressure-melting point, while thicker ice higher up in the accumulation area is warm-based[3]. These glaciers typically move via basal sliding or subglacial deformation under wet (warm)-based ice in the accumulation area, but only by internal ice deformation in the colder parts. These glaciers are generally drained by supraglacial and englacial (within the glacier) meltwater channels, and meltwater channels at the base of the glacier are rare[3, 30]. Debris entrainment and transportation is controlled by the structure of the glacier, and deformation of permafrost may be important in the formation of push- and ice-cored moraines. Stress is transmitted to proglacial permafrost, resulting in deformation[3, 31, 32]. These proglacial sediments can be folded, thrust-faulted or overridden[3].

The Antarctic Ice Sheet is polythermal. Up to 55% of the grounded ice sheet may be underlain by ice at the pressure melting point[33]. Wet-based areas include ice streams, outlet glaciers, and regions underlain by subglacial lakes[3, 34]. On James Ross Island, NE Antarctic Peninsula, most of the small outlet glaciers are polythermal, with smaller niche glaciers being cold-based.

Cold-based glacial processes

This section is mostly from Hambrey and Fitzsimons 2010.

Despite a long history of papers arguing that cold glaciers do not erode or deposit glacial sediments, this paradigm is now being challenged, with a growing number of papers describing processes of debris entrainment, transportation and deposition at the margins of cold-based glaciers, where the ice at the ice-bed interface is not at pressure melting point[16, 24, 35]. Numerical ice sheet models have in the past assumed no movement where the glacier is cold-based[24], with geologists assuming little debris entrainment or movement, preserving delicate landforms and preglacial land surfaces[36].

However, there are a few studies challenging these views. The Dry Valleys, Antarctica, are in Southern Victoria Land near McMurdo station. They are the largest ice-free region in Antarctica[17], and are thought to be the closest place on Earth to Mars. In this polar desert, rainfall is unknown, and there is only 10 mm snow fall (water equivalent) per year. Mean annual air temperature is around -19.8°C, and the majority of the local glaciers are cold throughout[17]. These glaciers have basal temperatures of around -17°C[17, 37], and no free running water[38].

Wright Lower Glacier has a 3.5 km broad tongue that terminates as a degraded ice front in the frozen Lake Brownworth, which has an ice thickness of 9 m[17]. Next to the glacier is a sediment apron and there is a braid plain around the lake. The northern margin of the glacier has a 5-10 m high ice cliff, from which large blocks fall (dry calving)[17]. There are moraines within and beyond the lake, which have a similar plan view as the dry margin of Wright Lower Glacier.

The ice margin comprises pinnacles and gullies, with windblown sand-covered and clean ice parts melting at different rates. The ice margin is not very well defined and merges with the lake ice via debris-covered, ice-cored moraines parallel to the ice front with intervening ponds.

Hambrey and Fitzsimons (2010) found that the ice-contact debris apron was mostly made up of sand, and extended for several hundred meters towards the lake. It is dissected by several gullies, cut into the unconsolidated sand by streams (melting from the glacier surface is encouraged by the accumulation of dark wind-blown sand, which absorbs solar radiation)[17].

Hambrey and Fitzsimons (2010) argued that debris was entrained in Lower Wright Glacier by two mechanisms:

1. Supraglacially, from windblown sand;
2. Subglacially, where the basal ice layer is several meters thick.

The ice-proximal debris apron is similar to modern fluvial systems, with inclined bedding related to uplift of the region following rebound of the earth’s crust following the removal of glacier mass (isostatic uplift). All these glacigenic sediments have been reworked by flowing water and wind[17]. The debris apron has also been modified by glaciotectonic deformation. Deformation structures include angular bubbly ice blocks, boudin and thrust blocks in the northern margin of the debris apron. This range of structures indicates a heterogeneous deformation regime within the basal ice of Wright Lower Glacier in the Dry Valleys of Antarctica. Strain rates measured within the basal debris-laden ice indicate that simple shear is occurring, resulting in foliation and boudin formation. The clean and debris-rich ice is has brittle failure, resulting in landforms similar to thrust-block moraines.

The work of these authors[17] and others[16] indicates that cold-based glaciers can generate landforms, and erode, transport and deposit sediment. Bedrock erosion occurs through fracture and abrasion[16] as well as deposition. However, in comparison to glaciers in warmer climates, there is less abrasion at the ice-bed interface, resulting in coarser sediments and less clays and silts being produced. Sand is the dominant product[17]. Pre-existing sediments have been reworked without much modification. The lack of free-flowing water has resulted in this lack of modification.

In summary, the glaciers in the Dry Valleys of Antarctica represent the end-members in the glacier thermal spectrum, being the coldest glaciers on earth. However, these glaciers are capable of erosion and deposition. Debris entrainment encompasses the detachment of frozen blocks of sediment from the subglacial substrate, which is then folded and thrusted[17]. The geomorphological features that are created include sedimentary ridges and aprons with glaciotectonised sand and glacier ice, draped with a veneer of wind-blown sand. Supraglacial streams, which melted following increased albedos as a result of accumulations of wind-blown sand on the glacier surface, rework proglacial sediments, including the debris apron.  All the glacial sediments bear little resemblance to their counterparts from warmer climates, and the preservation potential of these sediments is high[17].

Summary

The thermal regime is very important for how glaciers move, flow and operate, and is dependent on basal ice temperature, ice thickness, and the substrate. The chart below illustrates the different processes operatiing beneath temperate ice, cold-based glaciers, surging glaciers and polythermal glaciers.

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