Erosion-weathering partitioning from paired-mineral and weathering-corrected cosmogenic nuclide approaches

Abstract

Chemical weathering can bias the interpretation of cosmogenic nuclide measurements for denudation rates, because soluble minerals have shorter regolith residence times than insoluble minerals. Paired mineral measurements in target minerals with different solubility and corrections based on stream-water-derived weathering rates theoretically offer ways to mitigate these biases. Here we test these approaches in the carbonate landscapes of the Tabular Jura and Black Forest in Switzerland and Germany. We measured 26 36Cl and 10Be concentrations for catchment-average denudation rates in calcite and quartz grains, respectively, and derive weathering rates from stream water chemistry along a denudation rate gradient. In catchments with homogeneous lithology, the 10Be-36Cl paired nuclide approach predicts weathering rates that agree with stream-water derived measurements. In catchments with heterogeneous lithology, stream-water derived weathering rates were used to correct 10Be and 36Cl denudation rates. Weathering-corrected denudation rates increase by 100% for 10Be and decrease by 5% for 36Cl compared to uncorrected ones, illustrating that the magnitude of weathering correction depends on the abundance of target minerals.We find denudation rates of 50 – 100 mm/ka on the low relief carbonate-dominated Bözberg plateau and intermediate relief incised carbonate valleys of the Randen, where weathering accounts for about 75% and 50% of the denudation, respectively. Along the Bözberg plateau flanks and the mid-relief Wutach tributaries denudation rates range between 100 – 200mm/ka, with erosion and weathering each contributing about 50%. In the rapidly incising Wutach gorge, denudation rates are 300 – 500 mm/ka with less than 20% of weathering. These findings show that, similarly to crystalline rocks, the fraction of weathering in mixed sedimentary rocks inversely scales with denudation rate. Our study demonstrates that when sampling sites are chosen to ensure that target minerals are sourced from the same area, paired mineral measurements and stream-water weathering corrections can be effectively used to determine denudation and weathering rates.