Seasonal dynamics of marly badlands illustrated by field records of hillslope regolith properties, Draix–Bléone Critical Zone Observatory, South‐East France

Abstract

Sparsely vegetated badlands are loci of intense erosion that is sufficiently rapid to have observable effects on human timescales. Characterizing and understanding the physical weathering processes in these settings are key to predicting the temporal variability of regolith production and sediment flux, as well as their evolution under changing climate conditions. Here, we study intra‐annual changes of hillslope properties and explore the relationship between sediment production and transport in steep marly badland catchments of the Draix–Bléone Critical Zone Observatory (SE France), where decades‐long monitoring records show rapid morphological changes. There is evidence for seasonal dynamics of these badlands, but characterization and quantification of physical weathering processes have been lacking up to now. We explore this gap by monitoring key regolith parameters including grain‐size distribution (characterized by D50), surface resistance and water content in the regolith layer (surface to ∼10 cm depth) at different locations, through repeated field surveys over a 2.5‐year period. While water content appears to be directly controlled by the last previous rainfall event, the cyclic annual pattern in observed D50 suggests that loose and finely fragmented regolith is mainly produced and accumulates during the winter season, whereas sediment transport is dominant during spring–summer. This dynamic reduces regolith thickness and induces coarsening of hillslope surface material between early spring and autumn. Principal component analysis (PCA) highlights the strong correlation between resistance and D50. We therefore suggest that D50 provides the best proxy of regolith weathering in these marls. The spatial variability of the regolith was analysed through a geophysical profile, highlighting distinct behaviour depending on slope aspect. However, the distribution of slope angles is independent of aspect at the catchment scale. These results corroborate the strong annual dynamics of these catchments, where hillslopes and gullies are drained during spring and early summer high‐intensity precipitation events, inducing high sediment yields.

Probing the exchange of CO2 and O2 in the shallow critical zone during weathering of marl and black shale

“…During drier conditions, cracks may significantly increase gas exchange between the rocks and the atmosphere (Maier & Schack-Kirchner, 2014;Weisbrod et al, 2009). In the study area, desiccation cracks typically appear at steep slopes during summer, when erosion by runoff is less prevailing than in spring and autumn, whereas a thick layer of loose detrital cover can be accumulated during winter due to frost weathering (Ariagno et al, 2022(Ariagno et al, , 2023, when movement of surface materials is limited to solifluction (Bechet et al, 2016). Thus, the diffusivity of the rock surface presumably changes over time, with greater values during dry summer conditions (Fig.…”

“…The sediment export fluxes of the Brusquet catchment are on average ~ 70 t km -2 yr -1 , and ~ 5,700 t km -2 yr -1 and ~ 14,300 t km -2 yr -1 for the Moulin and Laval catchments, respectively (Carriere et al, 2020;Mathys et al, 2003). Taking a regolith bulk density of ~ 1.3 t m -3 -1.8 t m -3 into account (Ariagno et al, 2023;Bechet et al, 2016;Mallet et al, 2020;Mathys & Klotz, 2008;Oostwoud Wijdenes & Ergenzinger, 1998), a physical erosion rate of ~ 0.04 mm yr -1 -0.05 mm yr -1 , ~ 3.2 mm yr -1 -4.4 mm yr -1 and ~ 8 mm yr -1 -11 mm yr -1 can be estimated for the Brusquet, Moulin and Laval catchments, respectively. However, these values are catchment-scale averages, and the physical erosion can significantly vary spatially.…”