Stochastic processes of soil production and transport: Erosion rates, topographic variation and cosmogenic nuclides in the Oregon Coast Range

Landscapes in areas of active uplift and erosion can only remain soil-mantl ed if the local production of soil equals or exceeds the local erosion rate . The soil production rate varies with soil depth, hence local variation in soil depth may provide clues about spatial variation in erosion rates. If uplift and the consequent erosion rates are sufficiently uniform in space a nd time, then there will be tendency toward equilibrium landforms shaped by the erosional processes. Soil mantle thickness would adjust such that soil production matched the erosion. Previous work in the Oregon Coast Range su ggested that there may be a tendency locally toward equilibrium between hil lslope erosion and sediment yield. Here results from a new methodology base d on cosmogenic radionuclide accumulation in bedrock minerals at the base o f the soil column are reported. We quantify how soil production varies with soil thickness in the southern Oregon Coast Range and explore further the issue of landscape equilibrium. Apparent soil production is determined to b e an inverse exponential function of soil depth, with a maximum inferred pr oduction rate of 268 m Ma(-1) occurring under zero soil depth. This rate de pends, however, on the degree of weathering of the underlying bedrock. The stochastic and large-scale nature of soil production by biogenic processes leads to large temporal and spatial variations in soil depth; the spatial v ariation of soil depth neither supports nor rejects equilibrium morphology. Our observed catchment-averaged erosion rate of 117 m Ma(-1) is, however, similar to that estimated for the region by others, and to soil production rates under thin and intermediate soils typical for the steep ridges. We su ggest that portions of the Oregon Coast Range may be eroding at roughly the same rate, but that local competition between drainage networks and episod ic erosional events leads to landforms that are out of equilibrium locally and have a spatially varying soil mantle. Copyright (C) 2001 John Wiley & S ons, Ltd.