Cosmogenic nuclides and the dating of Lateglacial and Early Holocene glacier variations:: The Alpine perspective

Based on cosmogenic Be-10 data from four sites in the Alps we discuss geological uncertainties associated with the dating of former cirque and small valley glacier margins. At the Early Lateglacial Gschnitz site (Trins, Austria), a 3000 yr spread in Be-10 exposure ages points to prolonged boulder instability. Three out of seven ages are not included in the mean age calculation, which yielded 15,400 +/- 1000 yr (indistinguishable from the oldest boulder age of 16,130 +/- 1040 yr). As a result of the distinctive morphology at Julier Pass (Switzerland) site we are able to exposure date the early (12,300 +/- 1300 yr) and the late (11,300 +/- 600 yr) Egesen stadial glacier advances (Younger Dryas equivalent), not just final retreat. At the Kromer site (Austria), Be-10 exposure ages from five clast-supported boulders are indistinguishable within the analytical uncertainties (mean age: 8400 +/- 500 yr). In addition to moraine age, key factors that may lead to "too young" ages include degree of matrix- vs. clast-support of the boulders, post-depositional periglacial activity and tree coverage. At the Nagelisgratli bedrock site near Grimsel Pass (Switzerland) exposure ages of 10,760-11,720 yr are consistent with Early Holocene cirque glacier retreat, and underline the marked lack of nuclide inheritance in bedrock exposures in the Alps. (C) 2007 Elsevier Ltd and INQUA. All rights reserved.