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  1. Abstract

    The lateral migration of a river meander is driven by erosion on the outer bank and deposition on the inner bank, both of which are affected by shear stress (and therefore channel slope) through complex morphodynamic feedbacks. To test the sensitivity of lateral migration to channel slope, we quantify slope change induced by glacial isostatic adjustment along the Red River (North Dakota, USA and Manitoba, Canada) and two of its tributaries over the past 8.5 ka. We demonstrate a statistically significant, positive relationship between normalized cutoff count, which we interpret as a proxy for channel lateral migration rate, and slope change. We interpret this relationship as the signature of slope change modulating the magnitude of shear stress on riverbanks, suggesting that slope changes that occur over thousands of years are recorded in river floodplain morphology.

     
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  2. null (Ed.)
  3. Abstract

    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 × 105to 3.5 × 105atoms/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 the10Be system, this method should be sensitive to unzipping rates bounded between 10−6and 100 m/y.

     
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  4. null (Ed.)
    Abstract. Meteoric 10Be (10Bemet) concentrations insoil profiles have great potential as a geochronometer and a tracer of Earthsurface processes, particularly in fine-grained soils lacking quartz thatwould preclude the use of in situ produced 10Be (10Bein situ). Oneprerequisite for using this technique for accurately calculating rates anddates is constraining the delivery, or flux, of 10Bemet to a site.However, few studies to date have quantified long-term (i.e., millennial)delivery rates, and none have determined a delivery rate for an erodingsoil. In this study, we compared existing concentrations of 10Bein situ with new measurements of 10Bemet in eroding soils sampledfrom the same depth profiles to calibrate a long-term 10Bemetdelivery rate. We did so on the Pinedale (∼ 21–25 kyr) and BullLake (∼ 140 kyr) glacial moraines at Fremont Lake, Wyoming(USA), where age, grain sizes, weathering indices, and soil properties areknown, as are erosion and denudation rates calculated from 10Bein situ. After ensuring sufficient beryllium retention in each profile,solving for the delivery rate of 10Bemet, and normalizing forpaleomagnetic and solar intensity variations over the Holocene, we calculate10Bemet fluxes of 1.46 (±0.20) × 106 atoms cm−2 yr−1 and 1.30 (±0.48) × 106 atoms cm−2 yr−1 tothe Pinedale and Bull Lake moraines, respectively, and compare these valuesto two widely used 10Bemet delivery rate estimation methods thatsubstantially differ for this site. Accurately estimating the 10Bemetflux using these methods requires a consideration of spatial scale andtemporally varying parameters (i.e., paleomagnetic field intensity, solarmodulation) to ensure the most realistic estimates of10Bemet-derived erosion rates in future studies. 
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