DOI: 10.5061/dryad.bvq83bk8p
title: 10Be concentrations constraining surface age and valley growth rate in a seepage-derived drainage network in the Apalachicola River basin, Florida

description: Measuring rates of valley head migration and determining the timing of
 canyon-opening are insightful quantifications for the history and
 evolution of planetary surfaces.  Horizontal spatial gradients of
 in situ-produced cosmogenic nuclide concentrations provide a framework for
 assessing the migration of these and similar topographic features. We
 developed a theoretical model for the concentration of in situ produced
 cosmogenic radionuclides in valley walls during retreat of a valley head.
 The retreat rate is inversely proportional to the magnitude of the spatial
 concentration gradient and proportional to local nuclide accumulation
 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, termed unzipping.  We applied this theory
 to a developing seepage-derived drainage network along the Apalachicola
 River, Florida, USA.  Sample concentrations along a valley margin
 transect vary systematically from 2.9 x 105 atoms/g to 3.5 x 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 is 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 m/y and 100 m/y.
description: Beryllium extraction was
 performed at the University of Pennsylvania cosmogenic lab between April
 and May of 2011. The chemical blank ratio
 (<sup>10</sup>Be/<sup>9</sup>Be) run with this
 sample was 4x10<sup>-15</sup>; 3.4x10<sup>3</sup>
 atoms of <sup>10</sup>Be were added to the samples via the
 chemical blank. The quartz samples were spiked with 250 ml of a
 10<sup>-3</sup> g/g <sup>9</sup>Be solution from
 CEREGE, Aix-en-Provence, France before dissolution. An adaption of the
 Kohl and Nishiizumi (1992) chemical procedure was used for quartz
 purification and Be extraction. The ratio of cosmogenic to native
 (carrier) Beryllium was measured via AMS at PRIME Lab at Purdue University
 during January of 2012.
description: Six samples were collected from a vertical profile at the channel
 head to a depth of 2 m. An additional sample was collected at a depth of 5
 m below the upland surface by excavating horizontally ~1 m. This sample
 was considered shielded from spallation production and was used to
 determine the inherited concentration for the site. Using the CRONUS-Earth
 calculator (Balco et al., 2008), a production rate for the surface in
 these locations was found to be 4.1 atoms/g/y including modern topographic
 shielding. The sediment bulk density is 1.6 g/cm3.  A series of 5 samples were
 collected from a 460 m long transect of the southwestern slope of a valley
 axis. Samples were collected on valley slopes approximately 5 m below the
 upland surface and from 0.3 m beneath the ground surface. The average
 production rate in this interval was 3.2 atoms/g/y. Most samples were approximately
 1-2 kg of material. Sample elevations are all 60±10 m.