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Establishing background (geologic) rates of erosion is prerequisite to quantifying the impact of human activities on Earth's surface. Here, we present 10Be estimates of background erosion rates for ten large (10,000-100,000 km²) river basins in the southeastern United States, an area that was cleared of native forest and used intensively for agriculture. These 10Be-based rates are indicative of the pace at which the North American passive-margin landscape eroded before European settlement (~8 m/m.y.). Comparing these background rates to both rates of post-settlement hillslope erosion and to river sediment yields for the same basins, we find that following peak disturbance (late 1800s and early 1900s), rates of hillslope erosion (~950 m/m.y.) exceeded 10Be-determined background rates more than one-hundred fold. Although large-basin sediment yields during peak disturbance increased 5-10x above pre-settlement norms, rivers at the time were transporting only ~6% of the eroded material; work by others suggests that the bulk of historically eroded material remained and still remains as legacy sediment stored at the base of hillslopes and along valley bottoms. Because background erosion rates, such as we present here, reflect the rate at which soil is generated over millennial time scales, they can inform and enhance landscape-management strategies. [ABSTRACT FROM AUTHOR]

Soil mixing plays a significant role in contaminant transport, carbon sequestration, and landscape evolution, yet the rates and driving mechanisms are poorly constrained. Here we use depth profiles and advection-diffusion modeling of fallout nuclides to quantify differences in short-term (<100 yr) physical soil mixing across contrasting landscapes. We constrain advection in soils using the distribution of cosmogenic 7Be and weapons-derived isotopes, and quantify mixing with a steady-state model of vertical 210Pb transport. On a forested landscape in the Bega Valley in southeastern Australia and on grasslands in Mann County, California, where bioturbation is documented as the dominant sediment transport mechanism, we calculate diffusion-like mixing coefficients of 1-2 cm² yr-1. In montane forest soils of northern New England, we observe little field evidence of short-term mixing, and find that the traditional advection-diffusion model fails to describe 210Pb profiles. Because nuclide profiles here can be described with a simple model of litterfall, organic matter decay, and radioactive decay, we argue that diffusion-like processes are barely active on short time scales, and that the advection-diffusion model overestimates diffusion-like transport. While animal bioturbation and soil freezing cycles have little effect on the fate of elements in New England, physical soil mixing drives transport at Bega Valley and Mann County. We suggest that the absence of soil stirring that we quantify in New England forests may explain the slow physical erosion here (~0.2 cm/k.y.) relative to the actively bioturbated soils of Bega Valley and Mann County (5-10 cm/k.y.). [ABSTRACT FROM AUTHOR]