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 × 105 to 3.5 × 105 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 10Be system, this method should be sensitive to unzipping rates bounded between 10−6 and 100 m/y. Plain Language Summary: The pace at which landforms develop is an important control on many biological, chemical, and physical processes operating at Earth's surface. Rates of landscape change are often quantified by measuring the accumulation of cosmogenic radionuclides in near‐surface Earth materials, as an indicator of the erosion rate and age of landforms. In this study, we advance a method for querying the rate of horizontal topographic changes, such as valley growth or ice margin retreat, by sampling material in a horizontal transect and observing patterns in the nuclide concentrations in a spatial gradient. We present the results of numerical modeling that describe the limits of this approach due to the rate and consistency of ongoing landscape evolution processes. We present the first empirical data on the growth rate of a well‐studied seepage channel network in Florida, which suggests that the time‐averaged channel advance rate is 0.025 m/y. Furthermore, our measurements indicate that the age of the incising plateau surface is between 2 and 2.5 My and that the regional uplift rate is between 27 and 38 m/My. Key Points: Propagation rates of morphologic features determined by nuclide concentration gradients along a horizontal transectUplift rate of the Florida panhandle estimated between 0.027 and 0.038 mm/y from a nuclide depth profileChanges in climate over the Quaternary likely drove variable growth rates of seepage valleys along the Apalachicola River [ABSTRACT FROM AUTHOR]