From Australia to Nepal: Detrital cosmo from the ends of the Earth

For over a decade cosmogenic nuclide (10-Be and 26-Al) concentrations measured in stream and river sediments have been used to yield an extraordinary wealth of information on how the Earth's surface is changing. We focus here on using detrital concentrations to infer erosion rates, a methodology that is widely used across diverse landscapes. There have, however, been relatively few studies systematically testing the three principal assumptions that underlie the methodology: 1) The mobile soil or regolith layer contributing sediment to the channel is well mixed; 2) Subcatchments contribute sediment in proportion to their long-term erosion rate; and 3) The mean residence time of the sediment in storage and transport is relatively short. Recent studies have shown effective interpretation of detrital signals where one or more of these assumptions did not hold using more complicated modeling. Here we present data from field sites in Australia and Nepal that span about three orders of magnitude in effective exposure age to reassess the veracity of the above assumptions. In Australia we have detrital nuclide concentrations from "source to sink" and from a well-studied soil-mantled landscape that test the first and third assumption, while in Nepal we have concentrations that may test the second and third for a landscape that is eroding up to three orders of magnitude more rapidly. These studies emphasize the remarkable applications that this methodology enables and caution against assuming that the simplest assumptions always hold.