Evaluation of luminescence techniques for investigating the transport and abrasion of pebbles in fluvial systems: the Ardèche River (France)

Luminescence is now used to date the exposure of rocks at the Earth’s surface or to measure their erosion rates. These methods are based on the observation that the luminescence of a rock exposed to sunlight is not only reset (bleached) at the surface but also at depth below the surface. For a stable surface, this bleaching depth (PBl) increases over time, forming a “bleaching front” that propagates into the rock interior. For an eroding surface, models show that a balance establishes between erosion and bleaching, and that PBl then depends on the erosion rate. Here, we present luminescence measurements performed on pebble surfaces sampled along a river in order to assess whether longitudinal variations in PBl could be used to quantify pebble transport durations or their abrasion (attrition).

Our study focuses on the Ardèche River (France), a ~120 km long river which is a tributary of the Rhône River. In its headwaters, in the Cévennes (french Massif Central), the eroded bedrock consists of Paleozoic metamorphic and plutonic rocks (including granites) and Quaternary volcanics. Then, along most of its course (~ 90 km) the river flows and cuts into Mesozoic carbonates. In its downstream end, it forms a  ~200 m deep and 30 km long gorge (“Gorges de l’Ardèche”), before connecting to the Rhône.

We consider here the luminescence of pebbles and cobbles sampled on twelve site along the Ardèche modern floodplain. For this purpose, we first characterized the granulometric distribution of alluvial bars on the 12 sites and then sampled granitic cobbles considering the D50 and D90 of the distributions.

Luminescence profiles (IRSL) of granite pebbles show PB1 depths ranging between 2 and 10 mm. For the D50 fraction, the luminescence signal reflects a progressive downward deepening of PB1, from the upstream area to the entrance of the gorges. Within the gorges, a slight reduction in PB1 depth is observed that we attribute to enhanced pebble abrasion within the gorges. Our results suggest that luminescence could form the basis of a new method for investigating the transport and abrasion of pebbles in fluvial systems.