Luminescence dating of the last glaciation maximum in the southern Black Forest, Germany: Preliminary results

Abstract

<p>During the last glaciation maximum, an ice cap and its more than 20 kilometres-long outlet glaciers covered the highest summit of the Black Forest, the Feldberg (1493 m above sea-level), and the surrounding region (Hofmann et al., 2020). This event is tentatively correlated with the global last glacial maximum (at ca. 22-19 ka). However, this hypothesis has never been tested by applying up-to-date geochronological techniques to glacial deposits or landforms. Due to the lack of significant topographic control, the climate probably mainly controlled the mass balance of the ice cap. Dating its last maximum extent may thus have important implications for the reconstruction of atmospheric circulations patterns during the Pleistocene. A last glaciation maximum out of phase with the Alps would strengthen the hypothesis that a meridional atmospheric circulation prevailed over Europe during the last glaciation maximum in the Alps (at ca. 25 ka).</p><p>We aim at filling this gap by re-investigating a well-preserved multi-ridged terminal moraine complex several kilometres north-west of the Feldberg. Since the beginning of the twentieth century, this landform is undisputedly assigned to the last glaciation maximum. As units of sorted sediments occur in two sections on one ridge of the terminal moraine complex, luminescence dating was deemed a suitable technique to infer the age of the landform.</p><p>Luminescence dating was applied to both feldspar and quartz for suitable comparison, as these dosimeters have different drawbacks. Since quartz from crystalline source areas often shows no or only a relatively weak luminescence signal, we anticipated that this also applies to the sampled sediments from the crystalline part of the Black Forest. Optically-stimulated luminescence (OSL) measurements revealed a bright signal in quartz as well as equivalent doses in the order of 200 Gy considerably below the saturation level. We did not observe any obvious signs for partial bleaching, but this possibility has to be confirmed via further OSL measurements, since the sampled sediments were not transported over large distances. Component analysis will also be performed.</p><p>Infrared-stimulated luminescence (IRSL) measurements on feldspar revealed higher preliminary luminescence ages. It is expected that anomalous fading and partial bleaching will be two major future challenges. To overcome the first problem, post-infrared IRSL dating will be applied. As resetting of this signal is more difficult when compared to IRSL and OSL signals, problems associated with incomplete bleaching may arise in future work.</p><p>Our preliminary quartz OSL ages tentatively suggest that the last glaciation maximum in the southern Black Forest preceded the last glaciation maxima in other formerly glaciated uplands in Central Europe and the global last glacial maximum. This result needs to be evaluated by luminescence dating of other ice-marginal landforms correlative with the last glaciation maximum. Other geochronological techniques, such as <sup>10</sup>Be cosmic-ray exposure dating, will also be applied to reveal their age.</p><p> </p><p><strong>Reference</strong></p><p>Hofmann, F. M., Rauscher, F., McCreary, W., Bischoff, J.-P., and Preusser, F.: Revisiting Late Pleistocene glacier dynamics north-west of the Feldberg, southern Black Forest, Germany, E&G Quaternary Science Journal, 69, 61–87, https://doi.org/10.5194/egqsj-69-61-2020, 2020.</p><p> </p><p> </p><p> </p>