To reconstruct the timing of Alpine glacier advances onto the Jura Mountains, we sampled 17 Alpine erratic boulders within and beyond the LGM (Last Glacial Maximum) extent of the Valais Glacier along three transect for the analysis of cosmogenic 10 Be. Our results indicate at least two advances into the study area in Northwestern Switzerland; one during the LGM and one beforehand. Reaching of the maximum extent of by the Valais Glacier into the Jura mountains during the LGM occurred at 21.9 ± 1.8 ka based on erratic boulders exposure ages. Ages of boulders located beyond the LGM extent of the Valais Glacier vary from 19.9 ± 1.6 to 144.0 ± 9.6 ka. These ages indicate that many of the boulders were displaced and re-deposited by local ice in the Jura Mountains both during and perhaps even prior to the LGM. Therefore, the chronology of the pre-LGM advance(s) of Alpine glaciers into the Jura Mountains remains still to be elaborated.
Our new dataset from the Rivoli‐Avigliana end‐moraine system, the westernmost amphitheatre of the Italian Alps, provides an important step towards understanding foreland‐reaching glaciations before and during the Last Glacial Maximum (LGM) in the Western Alps. 10Be data from six boulders in pre‐LGM deposits gave ages between 26.8 ± 2.1 and 41.2 ± 1.9 ka. Based on morphological and pedological data, we interpret the oldest age as a minimum age for the glacier advance(s). 10Be results suggest that the LGM occurred in two major steps. During the first at 24.0 ± 1.5 ka, several ridges were constructed demonstrating oscillation of the Dora Riparia glacier snout at the maximum position. Our data demonstrate a significantly larger LGM extent in the Rivoli‐Avigliana amphitheatre than shown on previous maps. The maximum advance was followed by a short re‐advance of the glacier at 19.6 ± 0.9 ka, as recorded by 10Be ages from boulders in lateral positions along the lower Susa Valley. The maximum ice surface during the LGM was at 1000–920 m a.s.l. in the final reach of the valley (560–620 m of elevation above the alluvial plain) and at 620–340 m a.s.l. at the continuous moraines in the amphitheatre.
The Jura Mountains are a perialpine chain bending from the SW to the NW of Switzerland. Alpine ice reached only marginal areas of this chain during the Last Glacial Maximum (LGM) but remaining erratic boulders of alpine origin indicate older glaciations of unknown age, tentatively correlated either with the classical Rissian or with the Most Extensive Glaciation. Presented here are the first results of dating these pre-LGM boulders by using cosmogenic radionuclides ( 10 Be) and noble gases ( 21 Ne). Both data sets are in good agreement within 1s error, with radionuclide apparent exposure ages ranging from 60 to 107 ka, and noble gas apparent ages from 73 to 123 ka. Considering the importance of erosion on older boulders, the age of the erratics corresponds most probably to Marine Isotope Stage 6. These results are encouraging for further studies on pre-LGM erratic boulders in the mid-latitudes.
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