Dating Quaternary Terraces in Northern Switzerland with Cosmogenic Nuclides

pdf NAB 19-25 Dating Quaternary Terraces in Northern Switzerland with Cosmogenic Nuclides(14.62 MB)

This report consists of three main parts. The first part introduces the topic of dating the Deckenschotter in Northern Switzerland, particularly using cosmogenic nuclides, and addresses the related motivation and research questions. In short, the aim is to better constrain the existing, poorly developed chronologies of the Deckenschotter and to improve the understanding of landscape evolution during the Quaternary.

The second part contains an assessment of depth profile dating of the Deckenschotter using Terrestrial Cosmogenic Nuclides (TCN). An in-depth literature review provides the required background to the depth profile dating method. A recently introduced modelling algorithm, the Hidy code, was used for systematic tests of ¹⁰Be depth profiles, tracking their evolution over time. For this purpose, crucial parameters such as profile erosion rates were taken from a recently published paper which investigated the Deckenschotter from the Stadlerberg site in the Swiss foreland. Additionally, the Stadlerberg data from the paper itself were thoroughly tested using the algorithm and other tools. Altogether, these tests show that ¹⁰Be depth profiles quickly achieve steady state within 0.2 – 0.3 Myr for erosion rates of 5 cm/kyr. Without using a constraint such as total erosion or independent age, absolute ages cannot be modelled, and only minimum depositional ages can be obtained. Consequently, it is not possible to determine a depositional age with any degree of confidence using the published ¹⁰Be data for the Stadlerberg site.

The third part consists of a study of dating the Deckenschotter using the ²⁶Al/¹⁰Be TCN burial method approach. Two methods were used for this report: true/simple and isochron burial dating. Methodological as well as technical aspects are described, explaining the challenges of measuring low nuclide concentrations, particularly ²⁶Al, in Alpine samples. Available codes for age modelling of ²⁶Al/¹⁰Be data are then described and discussed.

The sampling sites were selected on the basis of an outcrop catalogue map and applied selection criteria. Two sites belonging to the Höhere Deckenschotter, Tromsberg (Dürn-Gländ) and Feusi (Egg) and one site related to the Tiefere Deckenschotter, Iberig, were sampled. Two sites related to the Hochterrasse, namely Beringen and the Glatt Valley, were additionally sampled. In total, 41 ²⁶Al/¹⁰Be data are presented. Modelled ages yield the most likely reliable ages ranging from 0.95 – 1.3 Myr for the Höhere Deckenschotter at the Feusi site. The dataset for Tromsberg (HDS) indicates a presumed age range of ~ 1.25 – 1.55 Myr, whereas the TDS site at Iberig yielded an age of ~ 0.65 – 0.95 Myr. However, both datasets require more data to consolidate the regression analysis. The datasets for the Hochterrasse sites Beringen and Glatt Valley cannot be used to determine an age due to data scatter. Relevant challenges in terms of methodology, measuring techniques and parameters used for age calculation are discussed. Finally, a landscape evolution model based on the cosmogenic data is presented. In this, the measured ²⁶Al/¹⁰Be concentrations at the Feusi site (HDS) that are relatively and significantly higher than at the Tromsberg and Iberig sites are explained as having originated from one of the first advances of Alpine glaciers into the Swiss foreland at the beginning of the Mid-Pleistocene Transition around 1 – 1.3 Myr. Several glaciations that followed caused a drop in the ²⁶Al/¹⁰Be concentrations as observed at the Tromsberg and Iberig sites. However, the dataset needs to be expanded to further constrain this model.

Besides the established chronologies, the working principles of the method as well the weaknesses and challenges are widely discussed. Applying ²⁶Al/¹⁰Be isochron burial to samples originating from a glacial landscape will require more systematic tests and improved measuring techniques. Nonetheless, geomorphic processes such as glacial erosion representing the source of the Deckenschotter sediments are the cause of the low nuclide concentrations measured. Thus, achieving a solid database with a representative spread in the data and a well-defined regression analysis can only be obtained through the analysis of additional samples.