IN SITU TERRESTRIAL COSMOGENIC NUCLIDES IN ALLUVIAL SEDIMENT: GRAIN SIZE MATTERS

The method for estimating long-term erosion rates from in situ terrestrial cosmogenic nuclide (TCN) inventories in alluvial sediment has developed quickly over the last decade. Despite widespread use of the method, extrapolation of nuclide concentrations to understand how landscapes are eroding and evolving is a complicated task and several model parameters are still in need of further study. Foremost among the analytical problems is the grain size dependency observed in some studies conducted in different climatic and tectonic settings, such that larger grains contain lower nuclide abundance, inferring faster erosion rates. Here we present a study designed specifically to investigate the processes contributing to the grain size dependency. The Clearwater River basin, western WA, features a unique tectonic and geomorphic setting that is conducive to rigorously testing hypotheses related to the grain size dependency of TCN inventories. Our experimental design isolates several tributary basins experiencing uniform uplift from headwaters to mouth, along a tectonic gradient. Uniform uplift within each basin allows us to isolate the problem of downstream fining, which mixes larger grains containing lower TCN concentrations with finer grains, which contain higher concentrations of TCNs. Among-basin differences in uplift permits us to investigate the grain size dependency in the context of a range of hillslope processes, ranging from creep that delivers finer grain sizes and landslides, which tend to deliver coarser grains. Preliminary grain size data indicate that downstream fining occurs in the Clearwater, but it is unclear as to how this fining should be apportioned to the spatial distribution of hillslope processes or physical breakdown of grains during transport. Preliminary TCN data, from a single grain size (0.25-0.35 mm), indicate basin wide erosion rates of 0.3 ±.14 mm/yr consistent with erosion rates estimated from extensive apatite fission track thermochronometry in the basin, and suggesting that mixing between grain sizes is sufficient. Nevertheless, further study is imperative to better understand the grain size phenomenon and link it to the spatially segregated hillslope processes.