Antarctic blue-ice moraines: Analogue for Northern Hemisphere ice sheets?

Abstract

This paper reviews the distribution, character and age of blue-ice moraines in Antarctica and asks whether there are implications for the study of former Pleistocene ice sheets in the Northern Hemisphere. Blue-ice forms where acceleration of downslope katabatic winds removes snow and causes ice to ablate. Upward ice flow compensates for the surface ablation and in some places brings rock debris to the surface to form a blue-ice moraine. Blue-ice moraines occur where topography focuses katabatic winds, notably outlet glaciers cutting through mountains and in the lee of nunataks and escarpments nearer the coast. Many Antarctic blue-ice moraines have been accumulating for millions of years. The cyclic growth and decay of Pleistocene ice sheets and the dominance of surface ablation near the ice-sheet margins are clearly different, yet there are aspects that apply to Pleistocene ice sheets. Based on Antarctic blue-ice deposits, equivalent deposits associated with former Pleistocene ice sheets are likely to: (1) lie in topographic sediment traps such as in side valleys or embayments next to outlet glaciers, (2) occur in the lee of mountains, (3) display a morphology indicating ice flow into the embayment or towards the mountain front, (4) include a wide range of lithologies derived from the inland ice sheet, yet consist wholly of local debris in places, (5) accumulate a thick deposit perhaps over successive glaciations. Further, (6) the location and intensity of moraine formation will change spatially and vertically in response to changes in the relative elevation of the surrounding mountains and its effect on ice flow. The extent to which these criteria will help in interpreting the behaviour of Pleistocene ice sheets is uncertain. But we use examples from the Greenland, Laurentide and Eurasian ice sheets to suggest that the concept of enhanced ablation by katabatic winds encouraging surface moraine formation helps resolve several puzzles.