Evidence for long-lasting landform surface instability on hummocky moraines in the Pamir Mountains (Tajikistan) from 10Be surface exposure dating

Abstract

Surface exposure dating is a new, but already widely used tool in geomorphological studies, aiming, e.g. at the establishment of glacial chronologies. Sometimes, however, results from samples taken from one and the same moraine scatter widely and thus interpretation turns out to be difficult. Here we assess sample-specific effects (topographic shielding, sample geometry and erosion) on 17 surface exposure ages obtained by analysis of in situ produced cosmogenic 10Be in boulders from moraines in the Pamir Mountains (Tajikistan). A wide scatter of exposure ages is observed only for hummocky moraines (> 45 to 10 ka BP), but not for distinct lateral moraines (∼ 20 and 60 ka BP, respectively). It is hypothesized that inheritance (prior exposure) and moraine degradation may be responsible for the wide scatter of exposure ages on the hummocky moraines. To proof this hypothesis, a data set of 72 boulders from the Pamir Mountains is used. We conclude that most likely hummocky moraines experienced long-lasting landform surface instability, probably due to buried glacier ice that is protected from melting several ten thousand years by a thick debris cover.