Sediment residence time variations in an Alpine river system inferred by uranium activity ratio.

The sediment residence time represents the time elapsed since the formation of the sediment in soils until its deposition. In order to better constrain timescales of sedimentary processes (erosion, transport, and deposition), it is important to understand to what extent sediment residence time is controlled by geomorphological parameters (e.g. elevation, curvature, slope). Uranium isotopes have been used to infer the time elapsed since the formation of fine detrital grains (<63 µm) by physical and chemical weathering (i.e. comminution age).

In this study, uranium isotopes were measured in fluvial sediments (<63 µm) sampled at different locations in a catchment (Var, France) to determine the variation of uranium activity ratio (²³⁴U/²³⁸U) along the river profile. The absence of fluvial plain implies that the sediment residence time mainly represents the storage time on hillslopes, as sediment transport is expected to be very rapid in this mountainous sedimentary system. 

The catchment was divided into 27 sub-catchments to investigate the variability of the geomorphological parameters that have been extracted from spatial analysis. Additionally, sediment residence time was estimated based on soil thickness prediction data combined with denudation rate information to compare this predicted residence time to the one calculated with (²³⁴U/²³⁸U).

The correlation between (²³⁴U/²³⁸U) and the estimated sediment residence time confirms that (²³⁴U/²³⁸U) can be modelled to infer sediment residence time. Furthermore, the correlations between the slope, the elevation and (²³⁴U/²³⁸U) highlight the geomorphological controls on the sediment residence time. The use of (²³⁴U/²³⁸U) in sedimentary archives will help to determine past geomorphological variations and re-construct past links between catchment erosion and climate change.