We demonstrate that cosmogenic radionuclides can be employed to trace sediment when the sources have sufficiently distinct concentrations, and develop a theoretical mixing model for a rocky coastline littoral system. We combine the resulting mixing model with existing cosmogenic radionuclide methods that quantify river inputs and terrace ages to determine the major components of the long-term littoral sediment budget of the Santa Cruz, California, coastline. Sediment derived from coastal basins eroding at 0.2 mm/yr has a much lower concentration than sediment derived from 60–84 ka terrace sands atop backwearing seacliffs. The complex pattern of cosmogenic radionuclide concentrations in littoral sands along >100 km of coastline can be explained by mixing sediment derived from seacliffs backwearing at 10 cm/yr with that delivered by rivers having widely different amounts of sediment discharge.

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