Vertically mixed and unmixed: Do surface features tell the whole story? An investigation of glacial regolith profiles using in-situ produced cosmogenic radionuclides

Whether a regolith is unmixed or mixed is critical to determining its erosion rate or age from in situ-produced cosmogenic nuclides. We use in situ-produced 10Be and 26Al in quartzite clasts extracted from depth profiles to investigate mixing of a periglacially-sorted till blanketing a plateau in the northern Swedish mountains. Our data indicate significant intra-site variations from a completely unmixed to a fully mixed regolith. We conclude that caution must be exercised in assuming that an entire regolith is either unmixed or mixed from interspersed depth profiles and that the degree of mixing may differ significantly from that indicated by observation of surface features. From the difference between the surface isotope concentration of an unmixed profile and the average isotope concentration of a fully mixed profile, we confirm that the regolith is a glacial till and that it could have been emplaced in a single event. Incorporating isotope concentrations, 26Al/10Be ratios, and an isostasy and ice sheet burial model we date the till emplacement to the Saalian glaciation (~ 200 to 130 ka).