Quantifying the flux and size of sediment in landscapes is central to understanding the interplay of climate and tectonics in erosion and weathering. Cosmogenic nuclides in stream sediment are widely used to measure spatially averaged erosion rates. Though this use of streams as integrators of erosion has proven powerful, it does not readily capture erosional heterogeneities that, if measured, could yield fresh perspectives on sediment production and transport. Here we show how such heterogeneities can be quantified using cosmogenic nuclides, apatite (U-Th)/He dating, and numerical modeling. As evidence, we present results from Inyo Creek, which drains a small, high-relief catchment in the eastern Sierra Nevada, California. We find that detrital apatite (U-Th)/He ages are markedly older on average in very coarse gravel than in sand. This signature of erosional heterogeneity validates the need to sample and study coarse sediment, particularly when it accounts for a large fraction of sediment flux, as may often be the case in steep catchments. The measured difference in ages at Inyo Creek demonstrates that the coarser sediment is eroded from higher elevations, implying it has a 20% higher cosmogenic nuclide production rate, on average, relative to the sand. This illustrates a potentially significant source of bias in cosmogenic nuclide studies of erosion; if the grain size of eroded sediment from slopes varies with elevation, then altitudinal variations in the nuclide production rates can impart a grain-size dependence on cosmogenic nuclide concentrations in sediment. Our analysis calls for caution in interpreting spatially averaged erosion rates from cosmogenic nuclides measured in a single grain size. It also demonstrates a framework for quantifying heterogeneities in sediment production, flux and size from cosmogenic nuclides and apatite (U-Th)/He ages measured in multiple grain sizes.
Lukens, C., Riebe, C.S., Sklar, L.S., and Shuster, D.L. (2012): Moving beyond the average in cosmogenic nuclide studies of erosion and weathering. Fall Meeting, American Geophysical Union, December 2012. Abstract EP41D-0822. .