The Rate of Unzipping a Landscape
Abstract
Rates of landscape evolution tell us much about the history and processes of planetary surfaces. Particularly insightful are the quantifications of rate and timing of opening of canyons and valleys. Horizontal spatial gradients of in situ produced cosmogenic nuclide concentrations provide a framework by which the migration rates of these and similar topographic features can be quantified. We develop a theoretical model for the in situ production of cosmogenic radionuclides in valley walls during retreat of a valley head. The rate of retreat is inversely proportional to the magnitude of the spatial gradient and proportional to local production rates corrected for nuclide decay By applying the chain rule to the differential equation that describes cosmogenic radionuclide concentration and solving for a spatial gradient in concentration along a valley parallel transect, we create an expression for the explicit determination of valley head retreat. In cases where the time since opening of the valley is small and accumulated concentrations are low, radionuclide decay can be neglected with only small errors, and only production needs to be accounted for. In contrast, when a valley has been open for many millions of years, the entire surface should be at equilibrium, and no spatial gradient should exist. We apply this model to the development of a seepage-derived drainage network near the Alum Bluff of the Apalachicola River, Florida, USA. The substrate in this area is a sequence of unconsolidated Plio-Pleistocene sands. A series of five samples were collected along a 400 m transect of a valley wall starting just below the valley head. A vertical profile through the upland surface was also collected. Sample concentrations vary systematically between 2.9 x 10^5 atoms/g and 3.5 x 10^5 atoms/g. With a calculated age of 570 kyr of the upland surface, nuclide decay can be neglected. This nuclide accumulation over the time span of the valley creation results in a gradient of 160 atoms/g/m. Using the locally scaled production rate of 4.1 atoms/g/yr and a measured bulk density of 1600 kg/m^3, we calculate a valley head retreat rate of 0.02 m/yr.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMEP13C0850M
- Keywords:
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- 1150 GEOCHRONOLOGY / Cosmogenic-nuclide exposure dating;
- 1815 HYDROLOGY / Erosion;
- 1824 HYDROLOGY / Geomorphology: general