Cosmogenic 3HE Measurements Provide Insight into Lithologic controls on Bedrock Channel Incision:Examples from the South African Interior

Resistant bedrock outcrops can exert control on river long profile adjustment, upstream transmission of base level fall, and valley development, particularly in post-orogenic settings. To examine how variation in lithologic resistance impacts landscape development in the post-orogenic eastern South African interior, cosmogenic 3He in pyroxene from Karoo dolerite was measured in samples from valleys of the Klip and Mooi Rivers and the Schoonspruit. The denudation rates measured from cosmogenic 3He in the Klip and Mooi Rivers and Schoonspruit are widely variable, with channel bed denudation rates ranging from 14-255 m/Myr and valley side and top denudation rates ranging from 11-50 m/Myr. Various processes of channel bed erosion occurring at grain to block scales (abrasion, plucking, subaerial weathering) result in the widely ranging channel bed incision rates. In this setting, river incision rates are restricted by moderate unit stream powers (~20 to >50 W/m2) and by limited sediment supply, resulting in a lack of abrasive tools. In many dolerite valleys, channel bed incision is commonly slow enough for local base levels to remain essentially stable for extended periods of time (>10 kyr). These results suggest that in the post-orogenic eastern South African interior, resistant dolerite in channel long profiles can result in spatially variable rates of channel bed incision, with implications for the patterns and tempo of wider landscape dynamics

The rarefied (non-continuum) conditions of tracer particle transport in soils, with implications for assessing the intensity and depth dependence of mixing from geochronology

We formulate tracer particle transport and mixing in soils due to disturbance-driven particle motions in terms of the Fokker–Planck equation. The probabilistic basis of the formulation is suitable for rarefied particle conditions, and for parsing the mixing behavior of extensive and intensive properties belonging to the particles rather than to the bulk soil. The significance of the formulation is illustrated with the examples of vertical profiles of expected beryllium-10 (10Be) concentrations and optically stimulated luminescence (OSL) particle ages for the benchmark situation involving a one-dimensional mean upward soil motion with nominally steady surface erosion in the presence of either uniform or depth-dependent particle mixing, and varying mixing intensity. The analysis, together with Eulerian–Lagrangian numerical simulations of tracer particle motions, highlights the significance of calculating ensemble-expected values of extensive and intensive particle properties, including higher moments of particle OSL ages, rather than assuming de facto a continuum-like mixing behavior. The analysis and results offer guidance for field sampling and for describing the mixing behavior of other particle and soil properties. Profiles of expected 10Be concentrations and OSL ages systematically vary with mixing intensity as measured by a Péclet number involving the speed at which particles enter the soil, the soil thickness, and the particle diffusivity. Profiles associated with uniform mixing versus a linear decrease in mixing with depth are distinct for moderate mixing, but they become similar with either weak mixing or strong mixing; uniform profiles do not necessarily imply uniform mixing.</p

Soil particle transport and mixing near a hillslope crest: 2. Cosmogenic nuclide and optically stimulated luminescence tracers

We examine probabilistic elements of how cosmogenic nuclide (e.g., ¹⁰Be) concentrations and optically stimulated luminescence (OSL) particle ages are distributed within a soil mantle near a hillslope crest as a consequence of disturbance-driven transport and particle mixing. We use an Eulerian-Lagrangian algorithm in which fluctuating particle motions, representing depth-dependent mixing, are superimposed on a two-dimensional mean motion. The intensity of mixing is characterized by a Péclet number involving the vertical speed of particles entering the soil mantle at the soil-bedrock interface, the mechanically active soil thickness, and a particle diffusivity at the soil surface. With weak mixing, the vertical profile of ¹⁰Be concentration reflects the strong influence of the mean motion in which particles spend much of their lives in the higher part of the soil column with higher ¹⁰Be production rates. With increasing mixing intensity, the profile becomes linear, then uniform, and the vertically averaged concentration is larger than that expected with one-dimensional motion. With weak mixing, particles possessing a finite OSL age tend to remain near the soil surface; with increasing mixing they become more uniformly distributed with depth. Depth-interval-averaged OSL ages increase linearly with depth and then become uniform with strong mixing. With moderate to strong mixing, the probability distribution of OSL ages is approximately exponential with an average much less than the mean residence time of particles. The formulation is consistent with profiles of 10Be concentrations and interval-averaged OSL particle ages compiled from published data, suggesting moderate to strong mixing in the cases examined

Implications of the loess record for Holocene climate and human settlement in Heye Catchment, Jiuzhaigou, eastern Tibetan Plateau, Sichuan, China

We examine the Holocene loess record in Heye Catchment on the margins of the Tibetan Plateau and China Loess Plateau to determine to which of those regions the climate is more similar; temporal change to wind strength; and modification of the loess record by mass wasting and human activity. Luminescence and radiocarbon dating demonstrate loess deposited in two periods: >11–8.6 ka and <5.1 ka. The 8.6–5.1 ka depositional hiatus, which coincides with the Mid-Holocene Climatic Optimum, is more similar to the Tibetan Plateau, where loess deposition stops, than to the China Loess Plateau, where deposition slows. Grain-size analysis suggests the Heye loess is a mixture of at least three different grain size distributions and that it may derive from multiple sources. A greater proportion of coarse sediments in the older loess may indicate stronger winds compared to the more recent depositional period. Gravel incorporated into younger loess most likely comes from bedrock exposed in slump scarps. Human occupation of the catchment, for which the earliest evidence is 3.4 ka, post-dates the onset of slumping and thus the slumps may have created an environment that people could live in