Measuring 10Be and 26Al concentrations in stream sediments from the Vosges Mountains (NE France) to explore the respective role of lithologic, topographic and climatic control on massif-wide denudation

<p><span>*Georges Auma&#238;tre, Didier L. Bourl&#232;s, Karim Keddadouche</span></p><p><span>Located in northeastern France, the Vosges Mountains (VM) belongs to these Hercynian ranges strewn across the European alpine foreland. Peaking at ~1425 m of elevation, it presents four contrasting primary characteristics. Firstly, the geological basement allows a bipartite N-S subdivision: the Palaeozoic southern part (crystalline Vosges) composed of various igneous, metamorphic and sedimentary rocks contrasts with the much more homogeneous Triassic cover in the northern part (sandstone Vosges). Secondly, a clear E-W topographic gradient is reflected by steep hillslopes on the eastern side (Alsace) and gently-sloping hillslopes on the western side (Lorraine). Thirdly, a sharp W-E precipitation gradient (>1000mm/yr) is recorded between the windward and the leeward side. Finally, the imprint left by Quaternary climatic fluctuations yields a N-S gradient: whereas the crystalline Vosges hosted abundant valley glaciers, the sandstone Vosges were void of ice cover.</span></p><p><span>Owing to these advantageous characteristics, this contribution aims to present the first data of catchment-wide denudation at the massif scale and to explore the long-term interactions between denudation</span><span>, lithological control, morphometry and climatic forcing.</span><span> Modern stream sediments from 21 river catchments draining the whole massif were sampled for </span><span><em>in situ</em></span> ^{<span>10</span>}<span>Be and </span>^{<span>26</span>}<span>Al concentration measurements at the outlet of their mountainous reach. The mean Channel Steepness Index (k</span>_{<span>sn</span>}<span>) was computed as a morphometric &#8220;predictor&#8221; of denudation rates. Groups of lithologically uniform catchments were statistically identified based on their lithological surficial proportions.</span></p><p><span>Catchment-wide denudation rates inferred from cosmogenic </span>^{<span>10</span>}<span>Be and </span>^{<span>26</span>}<span>Al concentrations range from 33 to 83 mm/ka and 38 to 337 mm/ka, respectively. The [</span>^{<span>26</span>}<span>Al]/[</span>^{<span>10</span>}<span>Be] ratio range from 1.43 to 7.96, highlighting a complex exposure history for the glaciated catchments. At the massif scale, results show (i) no relation between denudation and steepness, (ii) a strong positive relation between denudation and precipitations when lithological groups are considered and (iii) a negative relation between the surficial proportion of fluvio-glacial deposits in the catchment and the [</span>^{<span>26</span>}<span>Al]/[</span>^{<span>10</span>}<span>Be] ratio.</span></p><p><span>To our knowledge, this contribution is the first massif-scale attempt to quantify denudation in an European low- to medium-altitude mountain range. This is especially relevant as long-term landscape evolution in the Variscan belt, by contrast to the numerous works focusing on denudation in high-mountains ranges (e.g. the Alps), has been regularly disregarded in recent geomorphological studies. Importantly, whereas a vast majority of studies measuring denudation rates rely on </span>^{<span>10</span>}<span>Be concentrations only, this study highlights the need of using a pair of cosmogenic nuclides (i.e. </span>^{<span>26</span>}<span>Al/</span>^{<span>10</span>}<span>Be) to check whether stream sediments in formerly glaciated catchments have experienced complex exposure history.</span></p>