Does the Antarctic Ice Sheet transport glacial erratics?

Dear Malcolm,

Thank you for your question on glacial erratics. Below is a summary of the main debris transport pathways in glacial systems.

Debris can be supplied to a glacier system from supraglacial or subglacial sources. Supraglacial sources include rock or snow avalanches from valley sides or mountain peaks that rise above an ice sheet/cap surface (these are known as nunataks). Debris of a smaller grain-size (e.g. volcanic ash, dust) can also be added to glacier surfaces from windfall. Subglacial debris can either be derived from erosion at the glacier bed, or where supraglacial debris is lowered through a glacier (e.g. via crevasses or moulins).

Whether debris is supplied at the glacier surface (supraglacial) or at the bed (subglacial) goes some way in determining its transport pathway. Debris transported on a glacier surface is referred to as high-level debris transport. This material is also commonly termed passively transported debris because there is little modification of rock debris transported on the glacier surface. However, debris that falls on the glacier surface does not always remain there. It may descend to an englacial or subglacial position due to burial within primary ice stratification in the accumulation zone, or it may descend through crevasses or moulins.

By contrast, debris transported at the glacier bed is known as low-level debris transport. This type of debris is often termed actively transported because it is modified (e.g. rock edges are rounded off) during the process of transport. At the bed, debris can be transported via traction, within subglacial meltwater streams, or within the basal ice itself. Once entrained in basal ice debris may be transferred to different levels in a glacier. The folding and thrusting of ice as it deforms (e.g. where squeezed between narrow valley walls) can be enough to elevate basal debris into englacial, or even supraglacial, positions. Similarly, basal debris often emerges at the glacier surface close to the snout, due to compressive ice flow (put simply, slower flowing ice at the margin may be overridden by faster flowing ice immediately up-glacier, bringing with it basal debris).

To return to your question about Antarctic erratics, like all glaciers, debris is transported at the surface, within, and at the bed of the Antarctic Ice Sheet. An interesting point to note is that there is very little terrain above the ice sheet surface in Antarctica, which almost completely submerges the underlying topography (including mountain chains). This provides a natural limit on the amount of debris supplied to the ice sheet surface. However, surface debris does occur, and erratic rocks have been identified from past advances of the Antarctic Ice Sheet, such as those on James Ross Island, and they provide a valuable means of reconstructing ice sheet history through cosmogenic isotope dating techniques.

I hope this answer has been useful in outlining the main transport pathways in Antarctic glaciers, and glacial systems more generally. Do not hesitate to get in touch with more questions!

Jacob