Andreas Dehnert

Isochron-burial dating of glaciofluvial deposits: First results from the Swiss Alps

In this study, we use isochron-burial dating to date the Swiss Deckenschotter, the oldest Quaternary deposits of the northern Alpine Foreland. Concentrations of cosmogenic 10Be and 26Al in individual clasts from a single stratigraphic horizon can be used to calculate an isochron-burial age based on an assumed initial ratio and the measured 26Al/10Be ratio. We suggest that, due to deep and repeated glacial erosion, the initial isochron ratio of glacial landscapes at the time of burial varies between 6.75 and 8.4. Analysis of 22 clasts of different lithology, shape, and size from one 0.5 m thick gravel bed at Siglistorf (Canton Aargau) indicates low nuclide concentrations: <20000 10Be atoms/g and <150000 26Al atoms/g. Using an 26Al/10Be ratio of 7.6 (arithmetical mean of 6.75 and 8.4), we calculate a mean isochron-burial age of 1.5 ± 0.2 Ma. This age points to an average bedrock incision rate between 0.13 and 0.17 mm/a. Age data from the Irchel, Stadlerberg, and Siglistorf sites show that the Higher Swiss Deckenschotter was deposited between 2.5 – 1.3 Ma. Our results indicate that isochron-burial dating can be successfully applied to glaciofluvial sediments despite very low cosmogenic nuclide concentrations.

Isochron-burial dating of glaciofluvial deposits: First results from the Swiss Alps: ISOCHRON-BURIAL DATING OF GLACIOFLUVIAL DEPOSITS

In this study, we use isochron-burial dating to date the Swiss Deckenschotter, the oldest Quaternary deposits of the northern Alpine Foreland. Concentrations of cosmogenic 10Be and 26Al in individual clasts from a single stratigraphic horizon can be used to calculate an isochron-burial age based on an assumed initial ratio and the measured 26Al/10Be ratio. We suggest that, due to deep and repeated glacial erosion, the initial isochron ratio of glacial landscapes at the time of burial varies between 6.75 and 8.4. Analysis of 22 clasts of different lithology, shape, and size from one 0.5 m thick gravel bed at Siglistorf (Canton Aargau) indicates low nuclide concentrations: <20000 10Be atoms/g and <150000 26Al atoms/g. Using an 26Al/10Be ratio of 7.6 (arithmetical mean of 6.75 and 8.4), we calculate a mean isochron-burial age of 1.5 ± 0.2 Ma. This age points to an average bedrock incision rate between 0.13 and 0.17 mm/a. Age data from the Irchel, Stadlerberg, and Siglistorf sites show that the Higher Swiss Deckenschotter was deposited between 2.5 – 1.3 Ma. Our results indicate that isochron-burial dating can be successfully applied to glaciofluvial sediments despite very low cosmogenic nuclide concentrations.