Differences in Be-10 concentrations between river sand, gravel and pebbles along the western side of the central Andes

Abstract

Cosmogenic nuclides in river sediment have been used to quantify catchment-mean erosion rates. Nevertheless, variable differences in Be-10 concentrations according to grain size have been reported. We analyzed these differences in eleven catchments on the western side of the Andes, covering contrasting climates and slopes. The data include eight sand (0.5-1 mm) and gravel (1-3 cm) pairs and twelve sand (0.5-1 mm) and pebble (5-10 cm) pairs. The difference observed in three pairs can be explained by a difference in the provenance of the sand and coarser sediment. The other sand pebble pairs show a lower Be-10 concentration in the pebbles, except for one pair that shows similar concentrations. Two sand-gravel pairs show a lower Be-10 concentration in the gravel and the other five pairs show a higher Be-10 concentration in the gravel. Differences in climate do not reveal a particular influence on the Be-10 concentration between pairs. The analysis supports a model where pebbles and gravel are mainly derived from catchment areas that are eroding at a faster rate. The five gravel samples with high Be-10 concentrations probably contain gravel that were derived from the abrasion of cobbles exhumed at high elevations. In order to validate this model, further work should test if pebbles are preferentially exhumed from high erosion rate areas, and if the difference between pebbles with high Be-10 concentrations and sand decreases when the erosion rate tends to be homogeneous within a catchment.