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
Authors: Jautzy Timothée , Rixhon Gilles , Braucher Régis , Schmitt Laurent , Team* Aster 
Published: March 2022

&lt;p&gt;&lt;span&gt;*Georges Auma&amp;#238;tre, Didier L. Bourl&amp;#232;s, Karim Keddadouche&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;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 (&gt;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.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;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&lt;/span&gt;&lt;span&gt;, lithological control, morphometry and climatic forcing.&lt;/span&gt;&lt;span&gt; Modern stream sediments from 21 river catchments draining the whole massif were sampled for &lt;/span&gt;&lt;span&gt;&lt;em&gt;in situ&lt;/em&gt;&lt;/span&gt; &lt;sup&gt;&lt;span&gt;10&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;Be and &lt;/span&gt;&lt;sup&gt;&lt;span&gt;26&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;Al concentration measurements at the outlet of their mountainous reach. The mean Channel Steepness Index (k&lt;/span&gt;&lt;sub&gt;&lt;span&gt;sn&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;) was computed as a morphometric &amp;#8220;predictor&amp;#8221; of denudation rates. Groups of lithologically uniform catchments were statistically identified based on their lithological surficial proportions.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Catchment-wide denudation rates inferred from cosmogenic &lt;/span&gt;&lt;sup&gt;&lt;span&gt;10&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;Be and &lt;/span&gt;&lt;sup&gt;&lt;span&gt;26&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;Al concentrations range from 33 to 83 mm/ka and 38 to 337 mm/ka, respectively. The [&lt;/span&gt;&lt;sup&gt;&lt;span&gt;26&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;Al]/[&lt;/span&gt;&lt;sup&gt;&lt;span&gt;10&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;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 [&lt;/span&gt;&lt;sup&gt;&lt;span&gt;26&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;Al]/[&lt;/span&gt;&lt;sup&gt;&lt;span&gt;10&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;Be] ratio.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;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 &lt;/span&gt;&lt;sup&gt;&lt;span&gt;10&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;Be concentrations only, this study highlights the need of using a pair of cosmogenic nuclides (i.e. &lt;/span&gt;&lt;sup&gt;&lt;span&gt;26&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;Al/&lt;/span&gt;&lt;sup&gt;&lt;span&gt;10&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;Be) to check whether stream sediments in formerly glaciated catchments have experienced complex exposure history.&lt;/span&gt;&lt;/p&gt;