Description: The determination of basin-averaged denudation rates from cosmogenic nuclide concentrations in stream sediments depends on the surface production rate, scaling methods of cosmic ray intensities, and the correction algorithms for skyline, snow and vegetation shielding. While much work has been devoted to the calculation of skyline shielding factors (Dunne et al. 1999, Codilean 2006), the pitfalls of and potential solutions to the derivation of skyline shielding factors for large areas has never been addressed. Specifically, the resolution of common topographic datasets, 30 to 90 m, are coarse enough that significant underestimations, up to nearly 20 percent, of the shielding factor can occur. This effect is greatest in mountainous regions with high relief, i.e. exactly those landscapes which are most often studied with cosmogenic methods. By combining measurements of surface roughness from high resolution topographic data with cosmogenic ray shielding laws, we determined an empirical model for the calculation of accurate skyline shielding factors. Codilean A. 2006. Calculation of the cosmogenic nuclide production topographic shielding scaling factor for large areas using DEMs. Earth Surface Process and Landforms 31: pp. 785-794. Dunne J, Elmore D & Muzikar P. 1999. Scaling factors for the rates of production of cosmogenic nuclides for geometric shielding and attenuation at depth on sloped surfaces. Geomorphology 27: pp. 3-11.

Description: Landscape evolution models can be used to assess the impact of rainfall variability on bedrock river incision over millennial timescales. However, isolating the role of rainfall variability remains difficult in natural environments, in part because environmental controls on river incision such as lithological heterogeneity are poorly constrained. In this study, we explore spatial differences in the rate of bedrock river incision in the Ecuadorian Andes using three different stream power models. A pronounced rainfall gradient due to orographic precipitation and high lithological heterogeneity enable us to explore the relative roles of these controls. First, we use an area-based stream power model to scrutinize the role of lithological heterogeneity in river incision rates. We show that lithological heterogeneity is key to predicting the spatial patterns of incision rates. Accounting for lithological heterogeneity reveals a nonlinear relationship between river steepness, a proxy for river incision, and denudation rates derived from cosmogenic radionuclide (CRNs). Second, we explore this nonlinearity using runoff-based and stochastic-threshold stream power models, combined with a hydrological dataset, to calculate spatial and temporal runoff variability. Statistical modeling suggests that the nonlinear relationship between river steepness and denudation rates can be attributed to a spatial runoff gradient and incision thresholds. Our findings have two main implications for the overall interpretation of CRN-derived denudation rates and the use of river incision models: (i) applying sophisticated stream power models to explain denudation rates at the landscape scale is only relevant when accounting for the confounding role of environmental factors such as lithology, and (ii) spatial patterns in runoff due to orographic precipitation in combination with incision thresholds explain part of the nonlinearity between river steepness and CRN-derived denudation rates. Our methodology can be used as a framework to study the coupling between river incision, lithological heterogeneity and climate at regional to continental scales.

Description: Tropical mountain areas may undergo rapid land degradation as demographic growth and intensified agriculture cause more people to migrate to fragile ecosystems. To assess the extent of the resulting damage, an erosion rate benchmark against which changes in erosion can be evaluated is required. Benchmarks reflecting natural erosion rates are usually not provided by conventional sediment fluxes, which are often biased due to modern land use change, and also miss large, episodic events within the measuring period. To overcome this, we combined three independent assessment tools in the southern Ecuadorian Andes, an area that is severely affected by soil erosion. First, denudation rates from cosmogenic nuclides in river sediment average over time periods of 1–100 k.y. and establish a natural benchmark of only 150 ± 100 t km-2 yr-1. Second, we find that land use practices have increased modern sediment yields as derived from reservoir sedimentation rates, which average over periods of 10–100 yr to as much as 15 × 103 t km-2 yr-1. Third, our land cover analysis has shown us that vegetation cover exerts first-order control over present-day erosion rates at the catchment scale. Areas with high vegetation density erode at rates that are characteristically similar to those of the natural benchmark, regardless of whether the type of vegetation is native or anthropogenic. Therefore, our data suggest that even in steep mountain environments sediment fl uxes can slow to near their natural benchmark levels with suitable revegetation programs. A set of techniques is now in place to evaluate the effectiveness of erosion mitigation strategies.

Description: In-situ produced cosmogenic nuclides are increasingly used to quantify geomorphological process rates, and the laboratory and computational techniques required for cosmogenic nuclide studies are becoming increasingly available to the earth science community. As analytical and theoretical errors continue to be reduced, the relative importance of the common correction factors, such as topographic and snow shielding, increases. The derivation of an additional correction factor for topographic or skyline shielding for large areas is still problematic. One important issue that has yet to be addressed is the effect of the accuracy and resolution of terrain representation by a digital elevation model (DEM) on topographic shielding correction factors. Most topographic metrics derived from DEMs such as slope and roughness scale with the resolution of the input elevation data. This effect is primarily due to terrain smoothing, resulting in a reduction in the relief (local and global) across an area. Therefore, DEM-based estimates of topographic shielding of cosmic rays are expected to be dependent on the pixel size of the input topographic data as well. Here, we undertake a systematic study of the effect of scaling caused by terrain smoothing on estimates of topographic shielding. We selected a wide range of natural terrains in order to capture maximum variability of surface roughness : (1) extremely flat terrain, Flanders, Belgium; (2) mature fluvial valleys, Appalachians, USA; (3) incised canyons, Arizona, USA; (4) steep mountain valleys, Andes, Ecuador; (5) glacial mountain valleys, Wyoming, USA. The effect of terrain smoothing on the resulting basin-wide topographic shielding factors was quantified by calculating the bias of shielding factors for the range of grid resolutions that are commonly used in geomorphological applications. Our data indicate that basin-average topographic shielding factors increase as the grid size of the input elevation data increases. This effect is accentuated in high-relief mountain basins, the rough landscapes which are exactly those that are most often studied with cosmogenic nuclide derived basin-averaged denudation rates. Our results indicate that errors in excess of common analytical errors are possible in steep mountainous settings when using medium resolution (30-100 m) DEMs input data for estimation of topographic shielding. Based on our calculations for a wide range of topographic settings, we derived an empirical model for estimating topographic shielding factors based on simple landscape metrics, which allows a determination of the errors associated with terrain smoothing for various topographic settings.

Description: Soil thickness and residence time are regulated by a dynamic interplay between soil formation and lateral transport of soil particles and solutes. To unravel this interplay and infer patterns and rates of chemical weathering, soil physical and chemical properties can be used. Here, we present an integrated approach combining numerical modeling with field measurements to assess the impact of slope gradient on soil thickness and chemical weathering at a regional scale. We first perform a number of synthetic model runs simulating soil formation, weathering, erosion, and deposition, which show that soil thickness and weathering degree decline with increasing slope gradient. We then evaluate how those functional relationships compare to soil‐landscape data observed in the field. Soils are sampled at 100 midslope positions under varying slope gradient. The weathering degree is determined using three chemical weathering indices: ratio of iron oxides to total iron (Fed/Fet), chemical index of alteration (CIA), and total reserve in bases (TRB). Finally, we calibrate the Be2D model to our field data to constrain soil residence times and chemical weathering rates. The modeled weathering rates decrease with increasing soil residence time and decreasing slope gradient. The application of the soil‐landscape evolution model in Southern Brazil shows that weathering rates can vary up to 2 orders of magnitude and depend on hillslope gradient. Notwithstanding model limitations and data uncertainties, we demonstrate the potential of an integrated approach, where field data and numerical modeling are integrated to unravel the timescale of soil weathering along transport over hillslopes.

Description: Escarpments are prominentmorphological features along high-elevation passive margins. Recent studies integrating geomorphology, thermochronology, and cosmogenic nuclide-based denudation rate estimates suggest a rapid phase of denudation immediately after the earliest stages of seafloor spreading, and subsequent slow denudation rates since. To constrain the geomorphic evolution of passive margins, we have examined the development of the Sri Lankan escarpment. Cosmogenic nuclide data on river sediment along a north-south transect across the southern escarpment reveal that the landscape is eroding ten times more rapidly in the escarpment zone (26 to 71 mm kyr-1) than in the high-elevation plateau above it and in the lowland plain beneath it (2.6 to 6.2 mm kyr-1). Unlike these low denudation rate areas, the escarpment denudation is strongly and linearly hill slope-dependent. This shows that denudation and retreat are tightly interlinked within the escarpment, which suggests that the escarpment is evolving by rift-parallel retreat, rather than by escarpment downwearing. Supporting evidence is provided by themorphology of rivers draining the escarpment zone. These have steep bedrock channels which show sharp and prominent knickpoints along their longitudinal profiles. It appears that fluvial processes are driving escarpment retreat, as rivers migrate headwards were they incise into the high-elevation plateau. However, the average catchment-wide denudation rates of the escarpment zone are low compared to the denudation rates that are estimated for constant escarpment retreat since rifting. In common with other escarpments worldwide, causes for this slow down can be tectonic change related to flexural bending of the lithosphere, climate change that would vary the degree of precipitation focused into the escarpment, or the decrease in the contributing catchment area, which would reduce the stream power available for fluvial erosion.