Quantifying Rates of Landscape Unzipping

Measuring rates of valley head migration and determining the timing of canyon-opening are insightful for the evolution of planetary surfaces. Spatial gradients of in situ-produced cosmogenic nuclide concentrations along horizontal transects provide a framework for assessing the migration of valley networks and similar topographic features. We developed a new derivation for valley head retreat rates from the concentrations of in situ-produced cosmogenic radionuclides in valley walls. The retreat rate is inversely proportional to the magnitude of the spatial concentration gradient and proportional to local nuclide production rates. By solving for a spatial gradient in concentration along a valley parallel transect, we created an expression for the explicit determination of valley head retreat, which we refer to herein as unzipping. We applied this expression to a seepage-derived drainage network developing along the Apalachicola River, Florida, USA. Sample concentrations along a valley margin transect varied systematically from 2.9 x 10(5) to 3.5 x 10(5) atoms/g resulting in a gradient of 160 atoms/g/m, and from this value a valley head retreat rate of 0.025 m/y was found. The discrepancy between overall network age and current rates of valley head migration suggests intermittent network growth which is consistent with glacial-interglacial precipitation variations during the Pleistocene. This method can be applied to a wide range of Earth-surface environments. For the Be-10 system, this method should be sensitive to unzipping rates bounded between 10(-6) and 10(0) m/y.

Automated summary

Measuring rates of valley head migration and determining the timing of canyon-opening are insightful for the evolution of planetary surfaces. Spatial gradients of in situ-produced cosmogenic nuclide concentrations along horizontal transects provide a framework for assessing the migration of valley networks and similar topographic features. We developed a new method for determining the retreat rates of valley heads based on the concentrations of in situ-produced cosmogenic radionuclides in valley walls. This method was applied to a seepage-derived drainage network in Florida, USA, and revealed intermittent network growth.