Last Glacial Maximum glacier fluctuations on the northern Alpine foreland: Geomorphological and chronological reconstructions from the Rhine and Reuss glacier systems

Abstract

Fresh glacial landforms of the Alpine forelands evidence the presence and extent of large piedmont glaciers during the Last Glacial Maximum (LGM) and yield valuable insights into LGM glacier dynamics. This study assesses widespread ice marginal landforms preserved within the limit of the former LGM Rhine glacier and the eastern lobes of the LGM Reuss glacier system by means of geomorphological mapping. Timing of formation of the studied ice margins in Rhine and Reuss systems are chronologically framed by new 10Be and 36Cl surface exposure ages, as well as new radiocarbon dates. This includes redating of radiocarbon samples first determined in the 1980s. Results of this and an earlier study focussing on outwash deposits downstream of the outer LGM ice margin, suggest that the Rhine glacier advanced to and reached its LGM maximum between ca. 26–22 ka, thereby forming a broad >100 km wide foreland piedmont lobe and constructing prominent and largely continuous chains of frontal moraines (outer Schaffhausen moraines). The eastern lobes of the Reuss glacier likely advanced to their LGM maximum position by 25/24 ± 2 ka as indicated by published luminescence dates. Stabilization of the corresponding Untertannwald ice margin and the slightly internal yet more prominent Mellingen moraines (Reuss glacier system) occurred no later than 22 ± 1 ka and 21 ± 1 ka, respectively. Glacier oscillations following the LGM maximum position are evidenced in both Rhine and Reuss systems but show varying degrees of preservation. Largely contemporaneous, late LGM readvances of Rhine (Stein am Rhein stadial) and Reuss (Bremgarten stadial) glaciers occurred after 20.6 ± 1.7 ka and 20.8 ± 1.3 ka, respectively. Absence of upstream moraines suggests rapid ice decay without marked stabilization, thereafter. Despite clear differences in the size and nature of their foreland lobes, with Rhein glacier as a broad piedmont lobe and narrow valley glacier like lobes characterising the eastern Reuss system, a remarkable similarity in timing is shown between the two.