Erosion Rates and Debris Flow History Reconstruction: a Comparison of Carbon and Cosmogenic Nuclide Dating Techniques

Hillslope scale erosion rates in landslide-dominated landscapes are difficult to measure. These small spatial scales often lack representative alluvium which is typically used to infer catchment-averaged erosion rates. Furthermore, bedrock sampling tends to underestimate true erosion rates while areas with complete preservation of eroded material and datable markers occur only under special circumstances. We seek to use cosmogenic nuclide concentrations accumulated in debris flow material to determine hillslope scale erosion rates. Two small, completely preserved fans deposited on a strath terrace of the South Fork Eel River in northern California provide an opportunity to test this technique. The debris flows, likely induced by deep-seated landsliding, are preserved in the fans as episodic mass flow events with interlaced transient, fluvial activity. We collected material for cosmogenic nuclide analysis from prescribed depth intervals in profile at the upper and lower limits of the fan. These cosmogenic erosion rates are compared with volumetrically determined rates based on carbon dating. Two charcoal samples from the upper and lower limits of the fan act to constrain deposition to a period of 5 kyr during the Holocene. The fan volumes, divided by development time and upslope drainage areas, give volumetric lowering rates of approximately 0.1 and 0.5 mm/yr. These results straddle other researchers' erosion rates obtained from modern stream sediment and strath terraces in the area using cosmogenic techniques. Similar results between our two techniques will support the use of cosmogenic nuclides in determining erosion rates in debris flow material. Such uses, not currently demonstrated, will allow local erosion rates to be derived in a variety of geomorphically active areas, ultimately helping to quantify landscape formation and evolution.