Some areas within ice sheet boundaries retain
pre-existing landforms and thus either remained as ice free islands (nunataks) during glaciation, or were
preserved under ice. Differentiating between these alternatives has significant im
plications for
paleoenvironment, ice sheet surface elevation, and ice volume reconstructions. In the northern Swedish mountains, in situ cosmogenic
10Be and
26Al concentrations from glacial erratics on relict surfaces as well as glacially eroded bedrock adjacent to these surfaces,
provide consistent last deglaciation ex
posure ages (
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phs/BQ1.GIF>8–13 kyr), confirming ice sheet overriding as o
pposed to ice free conditions. However, these ages contrast with ex
posure ages of 34–61 kyr on bedrock surfaces in these same relict areas, demonstrating that relict areas were
preserved with little erosion through multi
ple glacial cycles. Based on the difference in radioactive decay between
26Al and
10Be, the measured nuclide concentration in one of these bedrock surfaces suggests that it remained largely unmodified for a minimum
period of 845
−418+461 kyr. These results indicate that relict areas need to be accounted for as frozen bed
patches in basal boundary conditions for ice sheet models, and in landsca
pe develo
pment models. Subglacial
preservation also im
plies that source areas for glacial sediments in ocean cores are considerably smaller than the total area covered by ice sheets. These relict areas also have significance as
potential long-term subglacial biologic refugia.