It would be nice to be able to read the paper. Since 10Be tends to deposit in soil/sediment/dust, perhaps it has just built up over the years from melt water at the bottom of the glacier. There is also the confounding factor from 10B -- if not separated, AMS has trouble distinguishing 10Be from 10B.
Edit: I see that their methods/etc doc is available to view. Methods seem ok -- I guess that they are basing their soil claim on the observed Carbon and Nitrogen ratios in the samples.