Hillslope sediment size distributions revealed by granulometric cosmogenic nuclides, detrital thermochronology, and experimentally calibrated particle wear relationships

The rate of bedrock river incision both regulates and depends on the size distribution of sediment produced on hillslopes. Quantifying how hillslope sediment size varies across catchment scales is therefore fundamental to understanding feedbacks between weathering, erosion, and tectonic uplift in mountain landscapes. Here, we quantify spatial variations in hillslope sediment size distributions within a steep mountain catchment using a numerical model that combines a granulometric analysis of detrital cosmogenic nuclide and apatite (U–Th)/He age measurements from each of twelve sediment size classes ranging from medium sand to boulders. The model accounts for sediment production, mixing, and particle size evolution during transport using particle-wear relationships calibrated from tumbling experiments conducted in rotating wheels of varying size. Because these experiments span five orders of magnitude in calculated sediment energy, they enable upscaling of measured abrasion and fragmentation relationships from laboratory to field conditions. 

Measured age distributions by size class show excesses and deficits relative to spatially uniform erosion that we detect using a Monte Carlo-based departure analysis. For example, cobbles at the outlet are relatively old and thus preferentially derived from higher elevations while boulders are relatively young and thus preferentially derived from lower elevations. When these granulometric elevation distributions are combined with measured granulometric variations in cosmogenic nuclides, our model predictions are consistent with independent field-based measurements of hillslope sediment size distributions and their spatial variability across the catchment. Hence, the measured size-dependent variations in cosmogenic nuclides at our study site need not be attributed solely to depth-dependent shielding of relatively coarse material on steep hillslopes. Instead, the granulometric variability in isotopic tracers can be explained by the linkage between erosion rate and particle size production. Together, these results demonstrate that coupling granulometric cosmogenic nuclides and tracer thermochronology with empirically calibrated particle-wear relationships provides a powerful framework for predicting spatial variations in sediment production and erosion in mountain landscapes.