Improved Schmidt-hammer exposure ages for active and relict pronival ramparts in southern Norway, and their palaeoenvironmental implications

Abstract

Eight pronival (protalus) ramparts from alpine southern Norway were dated using Schmidt-hammer exposure-age dating (SHD) based on local high-precision linear calibration equations. SHD ages were −1355±1240 to 815±1000years for three active ramparts and 8730±1050 to 14,635±1060years for five relict ramparts. Different parts of the same rampart yielded ages that were in generally good agreement. R-value frequency distributions enabled the identification single-age and mixed-age boulder populations, and the differentiation of active and relict rampart surfaces. The relict ramparts are located outside Younger Dryas glacier limits and probably formed over a period of ~6000years between deglaciation (following the Last Glacial Maximum of the late Weichselian, ~18,000years ago) and the end of the Younger Dryas (~11,700years ago). At that time, glacial debuttressing and permafrost degradation apparently enhanced the release of rock debris from rock faces to the ramparts. The active ramparts, which are located in areas deglaciated after the Younger Dryas, are relatively uncommon in the study area due to diminished paraglacial effects and limited debris supply associated with seasonally frozen ground during the Holocene. Thus, the legacy of glaciers, glacier–permafrost interaction, and the specific type of periglacial environment must all be considered when assessing the palaeoenvironmental and palaeoclimatic significance of pronival ramparts.