Radar Layering and the Internal Flow of

Some ice-sheet models are capable of calculating the flow of ice in three dimensions. Ice penetrating radar information can be used to verify calculated flow paths because it reveals internal layering which is assumed to be isochronous. Internal layering is detected by changes in the electrical properties of ice. However, there are three different ways by which these changes can occur, yielding three main types of internal layering. The first type of internal layer is when there are changes in the density of ice. This is a dominant process at ice depths less than 700 m. However, below this level, the density of ice does not change very much and so internal reflections must be caused by other processes. The second form of layering is caused by the acidity of ice. Layers with an acidity in excess of the normal background level of glacier ice are formed when the aerosol products of ancient volcanic activity are incorporated in the snow chemistry on the former ice surface. This acid snow is subsequently buried by later snow fall to its present-day position, several tens of thousand years later. These acid layers are therefore isochronous surfaces. The third type of layering is where there are changes in the crystallography of ice. Such layering is thought to develop from acidic layers in the presence of enhanced stress across the stoss face of subglacial hills (Fujita et al., 1999).

Internal layers are often continuous across large sections of the Antarctic Ice Sheet (Siegert, 1999). Because they are isochronous, their patterns can be used to match the 3D flow of ice calculated in numerical models. However, as yet, very few ice-sheet modelling investigations have utilized this potentially powerful measurement. Leysinger Vieli et al. (2007) have recently identified a means by which internal layering can assist large-scale ice-sheet modelling, and this marks an important new area of model validation.

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