TRACING THE ANTHROPOGENIC COSMOGENIC NUCLIDE SIGNAL IN RIVER SEDIMENT

Human land use has left both visible and isotopic marks on the South Fork Eel River basin, northern California. Anthropogenically disturbed catchments in this area are scarred by logging roads, drag trails and deforestation-induced slumps and landslides. We have developed a methodology to constrain the erosional effect of human land use, by using terrestrial in situ cosmogenic nuclide (TCN) concentrations (¹⁰Be) in river sand from active stream channels, flood bars, and infilled saw-mill ponds.

¹⁰Be concentrations from active channel deposits are significantly lower in disturbed areas compared with adjacent, lithologically similar, undisturbed areas within the UC – Heath and Marjorie Angelo Reserve. Higher concentrations of ¹⁰Be in river sediment from these undisturbed watersheds within the Reserve match ¹⁰Be concentrations in the bars deposited prior to the extreme land use change. In disturbed areas, lowered ¹⁰Be concentrations can reflect an order-of-magnitude increase in soil erosion and/or deeper (sub-soil) erosion. In the case of soil erosion, anthropogenic disturbances frequently result in disequilibrium TCN concentrations in soil, precluding steady-state erosion rate estimation in modern river sediments. In the case of sub-soil erosion, erosion from the cut sides of deeply inset logging roads may be contributing a large portion of ‘¹⁰Be dead' material derived from deep in the exponential ¹⁰Be production profile. These end-member cases can be modeled and provide constraints on logging road sediment contribution and century-timescale soil erosion due to logging.

Our data suggests that erosion rates determined from flood bar deposits can be used to assess the natural ‘baseline' erosion rate prior to human disturbance. Moreover, this methodology could be exported to other logged areas and to agricultural watersheds and has implications for TCN in river sediment sampling methodology.