Regolith evolution on the millennial timescale from combined U–Th–Ra isotopes and in situ cosmogenic 10Be analysis in a weathering profile (Strengbach catchment, France)

• Published in: Earth and planetary science letters Vol. 453; pp. 33 - 43
• Main Authors: Ackerer, J., Chabaux, F., Van der Woerd, J., Viville, D., Pelt, E., Kali, E., Lerouge, C., Ackerer, P., di Chiara Roupert, R., Négrel, P.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.11.2016
• Subjects: Catchments; denudation; Erosion; Erosion rate; in situ 10Be; Landscapes; Monitoring; Regolith; U-series nuclides; Watersheds; Weathering; ANE, France; regolith; weathering; in situ 10Be; denudation; U-series nuclides
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2016.08.005

U–Th–Ra disequilibria, cosmogenic in situ 10Be concentrations and major and trace element concentrations have been analyzed in a 2 m-deep weathering profile sampled at the summit of the granitic Strengbach catchment (France). The data have been used to independently estimate both the long-term regolith production and denudation rates and the weathering and erosion rates. Modeling of the 238U–234U–230Th–226Ra disequilibrium variations in the lower part of the profile yields a regolith production rate of 12±4 mm/kyr (30 ± 10 T/km2/yr), while modeling of the high-resolution 10Be concentration profile leads to an exposure age of 19.7±2.2 kyr, an inherited concentration of 15,000 ± 1,000 at/g in quartz and a mean denudation rate of 22 ± 10 mm/kyr (37 ± 15 T/km2/yr). The consistency between production and denudation rates suggests that, on a millennial timescale, the regolith mass balance at the summit of the catchment is close to a steady state, even if the watershed may have been impacted by Quaternary climatic changes and by recent anthropogenic perturbations (e.g., 20th century acid rain and recent afforestation efforts). The results also indicate that physical erosion is likely the dominant long-term process of regolith denudation in the catchment. Furthermore, the comparison of the long-term production and denudation rates and of weathering and erosion rates determined from the depth profile analyses with the current weathering and erosion rates estimated at the outlet of the watershed based on monitoring of the water chemistry and sediment fluxes suggests that physical erosion may have varied more than the chemical weathering flux during the last 150 kyr. Although very few other sites with U-series, in situ 10Be and stream monitoring data are available for comparison, the current data suggest that (1) the mass balance steady state of regolith might be commonly achieved in soil mantled landscapes, and (2) physical erosion has varied much more than chemical weathering in mid-mountain catchments over the last 10–150 kyr. These results highlight the importance of the combined analysis of U-series nuclides and in situ 10Be in the same weathering profile for the determination of key geomorphic parameters, which are important to constraining landscape stability and the responses of landscapes to natural or anthropogenic forcing. •Analysis of U-series isotopes and in situ 10Be in a single regolith profile.•Independent determination of long term regolith denudation and production rates.•Lower regolith is the most suitable place to quantify regolith production rate.•A high spatial resolution profile is required for an extensive modeling of in situ 10Be.•Higher temporal variability of physical erosion than chemical weathering.