RECORDS OF PAST EROSION RATES FROM COSMOGENIC-NUCLIDE CONCENTRATIONS IN OLD ALLUVIUM

The dynamic relationship between uplift of a landscape by tectonic processes, and denudation of that landscape by erosional ones, cannot be fully understood unless we can compare records of past erosion rate changes with corresponding records of paleoclimate or tectonics. Thus, our goal is to use cosmogenic-nuclide measurements on alluvial sediment sections preserved in depositional basins to obtain a record of past erosion rates in the source area of the sediment. Just as the nuclide concentration in sediment leaving a drainage basin now reflects the present basin-averaged erosion rate, the nuclide concentration in sediment that left the basin at some time in the past, and has been stored in a sedimentary basin since then, reflects the past erosion rate. The challenge in interpreting such a record, however, is to account for the additional nuclide concentration which accumulated during burial and storage of the sediment. We give an example from Fisher Valley, Utah, where middle Pleistocene alluvial sediment shed from an eroding basin has been trapped in a salt-cored anticline. The present basin-averaged erosion rate in Fisher Valley, inferred from modern stream sediment, is near 125 m/Myr. Middle Pleistocene erosion rates inferred from four samples of old stream sediment were between 90 and 250 m/Myr and suggest a large decrease in erosion rate near 0.65 Ma. Our uncertainty analysis shows that, in cases where erosion rates were low (leading to high nuclide concentrations at deposition) and sediment deposition rates were high (that is, postdepositional nuclide production is small), we can accurately infer the past erosion rate. Where erosion and deposition rates have the opposite relationship, we have little confidence in past erosion-rate estimates. Future applications of this method will be most successful in situations where postdepositional nuclide production is low, for example in lakes or marine basins.