Can cold-based glaciers pick up erratic blocks?

Wright Lower Glacier, Antarctica -- one of the coldest glaciers on earth.

From Bethan Davies’s Antarctic Glaciers web site:

http://www.antarcticglaciers.org/glacier-processes/glacier-flow-2/glacial-processes/

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:
• Supraglacially, from windblown sand;
• 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 author s[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].

Three more photos of Wright Lower Glacier