Selective glacial erosion and weathering zones in the coastal mountains of Marie Byrd Land, Antarctica

In the coastal mountains of Marie Byrd Land, Antarctica, there is a juxtaposition of ice moulded landforms at lower altitudes and upstanding summits with weathered surfaces bearing tors. This paper uses geomorphological mapping and exposure dating to test two hypotheses commonly used to explain such a landscape contrast: either the pattern reflects contrasts in glacial erosion related to the basal thermal regime beneath a former ice sheet or it represents a periglacial trimline marking the upper limit of ice cover during the Last Glacial Maximum (LGM). Cosmogenic nuclide measurements on erratics show that the mountains were covered by ice on several occasions, most recently during the LGM. Similar measurements on bedrock show that fragile landforms on weathered surfaces survived many glaciations. Nuclide concentrations in bedrock indicate both that subglacial erosion is insignificant and that ice-free periods of periglacial weathering are longer at higher elevations. We conclude that the pattern of landscape modification reflects both topographically controlled contrasts in the basal thermal regime of overriding ice and the longer exposure to periglacial conditions at higher elevations. In addition, the combination of nuclide data and the wider pattern of striations shows that the LGM ice cover was thinner than predicted by ice sheet models assuming equilibrium conditions, that ice sheet thinning has occurred from 10,400 years ago to the present, and that regional flow by overriding ice has been replaced by radial ice flow from local glaciers centred on individual mountain massifs.