Using the surface profiles of modern ice masses to inform palaeo-glacier reconstructions

Morphometric study of modern ice masses is useful because many reconstructions of glaciers traditionally draw on their shape for guidance. Here we analyse data derived from the surface profiles of 200 modern ice masses—valley glaciers, icefields, ice caps, and ice sheets with length scales from 100 to 103 km—from different parts of the world. Four profile-attributes are investigated: relief, span, and two parameters C∗ and C̃ that result from using Nye’s (1952) theoretical parabola as a profile descriptor. C∗ and C̃ respectively measure each profile’s aspect ratio and steepness, and are found to decrease in size and variability with span. This dependence quantifies the competing influences of unconstrained spreading behaviour of ice flow and bed topography on the profile shape of ice masses, which becomes more parabolic as span increases (with C∗ and C̃ tending to low values of 2.5–3.3 m1/2). The same data reveal coherent minimum bounds in C∗ and C̃ for modern ice masses that we develop into two new methods of palaeo-glacier reconstruction. In the first method, glacial limits are known from moraines, and the bounds are used to constrain the lowest palaeo ice surface consistent with modern profiles. We give an example of applying this method over a three-dimensional glacial landscape in Kamchatka. In the second method, we test the plausibility of existing reconstructions by comparing their C∗ and C̃ against the modern minimum bounds. Of the 86 published palaeo ice masses that we put to this test, 88% are found to be plausible. The search for other morphometric constraints will help us formalise glacier reconstructions and reduce their uncertainty and subjectiveness.