Sediment budgeting using cosmogenic nuclides
For a more detaileed accout please see the publshied manuscript in Geology, 2005, v. 33, p. 133-136.
The piedmont extending from the Chemehuevi Mountain piedmont, Mojave Desert, California, has an ephemeral channel network that transports sediment derived from the mountain source basins, from a bedrock pediment (A), from the interpiedmont Sawtooth Range, and from incised alluvium (B). At ~10 km from the rangefront, the incised alluvium merges (C) with a uniformly active wash surface (D).
The cosmogenic nuclide signature in the ephemeral channels increases in a regular fashion down piedmont.
Pediment (open squares and open triangles), the Sawtooth Range (filled square), and incised alluvium (open circles) have distinct nuclide signatures and sediment generation rates.
Using a simple mixing models we calculated the long-term sediment and nuclide budgets for the Chemehuevi Mountain piedmont. A. Schematic diagram of Chemehuevi Mountain piedmont from the source basins to the last transect 12 km from mountain front. Black represents bedrock surface, gray represents alluvial surface, and white represents ephemeral channels. Top numbers represent average addition of mass (kg y-1 km-2) from each geomorphic unit; italicized numbers represent nuclide activity (atoms g-1). Total mass flux increases down piedmont as shown by graph adjacent to piedmont. B. Mass flux of sediment moving down the Chemehuevi Mountain piedmont. Arrow thickness represents cumulative flux of sediment. Numbers represent mass fluxes of sediment from transect locations, on geomorphic units, to ephemeral channels. CM = Chemehuevi Mountains and SR = Sawtooth Range. C. Flux of nuclides moving down the Chemehuevi Mountain piedmont. Horizontal arrow thickness represents cumulative nuclide flux resulting from addition of nuclides from geomorphic surfaces represented by vertical numbers. Small diagonal arrows and associated numbers represent additional nuclide activity from dosing as sediment moves down piedmont. D. Nuclide activity of ephemeral channel sediment (solid squares) and nuclide balance model (open squares) without dosing during transport. Nuclide balance model predicts well the channel measurements in pediment region where sediment speeds are fast. Nuclide balance model and channel data diverge on distal piedmont where sediment speeds are slower. Difference is dosing by cosmogenic radiation during transport (represented by diagonal arrows in C).
In order to reproduce the nuclide signature of sediment moving down the ephemeral channel network, we calculated that sediment speeds decrease from 10s of meters per year in the incised bedrock to meters per year in the incised alluvium to decimeters per year across the wash surface.