Cosmogenic signal of the stream boulder cover effect

Boulders are ubiquitous in bedrock river channels, moving only when the discharge and slope are large enough to transport them; as a result, large boulders in river channels may retard stream incision by blocking the erodible surface of the channel bed. This cover effect of boulders in stream channels has been demonstrated in models, but we lack field data to test it and understand how it varies with rock type and climate. We collected amalgamated stream boulder samples for in situ ¹⁰Be and ¹⁴C cosmogenic radionuclide analysis from catchments in humid-temperate, mediterranean, and semi-arid zones in the Chilean Coastal Cordillera. We present preliminary results, including new cosmogenic nuclide data from small (1-5 km²) main catchments in each climate zone and three very small (~0.05 km²) catchments in the semi-arid zone along with grain size analysis (Wolman pebble counts; >100 measurements per locality) for the catchments and adjacent hillslopes, and compare with published ¹⁰Be concentrations of stream sediment from van Dongen et al. (2019) and hillslope boulders from Lodes et al. (2023). The humid-temperate catchment has the lowest mean stream boulder ¹⁰Be concentrations (normalized to sea level high latitude production rates) at 0.52×10⁵ a g^-1, followed by the mediterranean (1.82×10⁵ a g^-1) and the semi-arid catchment (4.56×10⁵ a g^-1). We obtained a ¹⁴C concentration for one boulder sample in the humid-temperate zone so far, at 2.31×10⁵ a g^-1. In the main catchments of all three climate zones, stream sediment samples show a negative trend where larger sample grain sizes have lower ¹⁰Be concentrations, possibly due to increased erosion of the boulder surfaces or shielding due to overturning. Deposition by landslides can be ruled out in the humid and semi-arid catchments, but not in the mediterranean. In contrast, the three very small catchments in the semi-arid zone show a positive relationship between grain size and ¹⁰Be concentrations, suggesting that in these catchments, discharge and slope are too low to frequently overturn or erode the largest grains. In addition, in the semi-arid catchments boulders on hillslopes are larger and have higher ¹⁰Be concentrations than boulders in streams, whereas in the mediterranean catchment we observe the opposite pattern, suggesting that in the semi-arid catchments, the largest boulders may stay on the hillslopes until they have weathered to a size small enough to be transported to the stream channels. Next steps include analyzing ¹⁴C data for the remaining catchments and further analysis of boulder nuclide concentrations using a modelling approach.