Relating Suspended Sediment to its Original Soil Depth Using Fallout Radionuclides

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Abstract
We present and test a new method for determining the original soil depth of suspended sediment through analysis of fallout radionuclide properties. We propose that combining the different depth distributions of fallout 7Be, 210Pb, and 137Cs in soils with one another provides reference curves that can be used as a framework for interpreting the tracer concentrations of sediments derived from them. If it is assumed that the soil depth sources of the sediment are from the surface and the subsurface, the tracer concentrations from these two sources can be used to calculate the actual soil depth(s) from which the sediment originated. The proportion of sediment from each depth can also be calculated. We tested this method by generating sediment from a hillslope with treatments simulating: (T1) surface erosion, (T2) shallow rills to 10 mm, (T3) deep rills to 100 mm, and (T4) small gullies to 250 mm. The tracer‐based calculations of sediment source matched the known incision depths of these features. Material was predicted to be from 1 ± 2 mm and 4 ± 4 mm soil depth for T1 and T2 respectively. In T3, the material was predicted to be from 1.25 ± 0.5 mm at the surface and >78 ± 6 mm depth from the deep rills. In T4, the sediment was predicted to be from 86 ± 6 mm from the small gullies. The subsoil contribution to sediments increased from ≈50% (T3) to 75% (T4) as rill and gully depths and widths increased.

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