Erosion of the eastern United States observed with 10Be

The problem of identifying areas of accelerated erosion in a dynamic landscape is complicated. The limited history of sediment yield measurements makes this task difficult even if geomorphic evidence is available. Beryllium-10, a cosmogenic isotope produced by cosmic rays interacting with the earth's atmosphere and surface, has chemical and physical properties that make it useful as a tracer for erosion and sediment transport processes. The rarity of the stable isotope, ⁹Be, allows ¹⁰Be to be detected with accelerator mass spectrometry in natural materials at extremely low levels. Backgrounds for rocks and sediments below 10⁵ atom per g are now attainable, a value to be compared with an average deposition rate of 1.3 × 10⁶ atom cm^?2 yr^?1. The affinity of Be for the components of soil and sediment is sufficiently high that it is effectively immobilized on contact, thereby allowing ¹⁰Be to function as a tracer of sediment transport. To a good approximation all the ¹⁰Be transport out of a drainage basin is on the sediment leaving it. The number of ¹⁰Be atoms passing the gauging station can be determined by measuring the concentration of the isotope in the sediment, if the annual sediment load is known. The ratio of the ¹⁰Be carried from the basin by the sediment to that incident upon it, called the erosion index, has been determined for 48 drainage basins within the same physiographic province, which allows them to be reasonably compared, all of which have sediment yield data. Basins located in the Atlantic coastal plain have an average index of 0.3 with the maximum observed being 0.9. Basins located between the fall line and the mountains, a region called the Piedmont, have an average value of 2.2 with individual values ranging from 0.6 to 6.7; this marked difference is thought to result from two centuries of farming on land of moderate gradient. Basins in the highland regions reflect local conditions with low indices for those in grass and timber and high indices associated with destructive land use. The data allow an estimate of the erosion index for the pre-colonial Piedmont, which then allows the pre-colonial sediment yield to be calculated. A number of basins have also been examined world wide with similar conclusions derived. An important deviation from the rule is noted for rivers that erode large regions of loess, such as the Mississippi, Hwang Ho, and Yangtze. Large aeolian deposits were laid down during the ice age in these basins, deposits that brought inherited ¹⁰Be with them and that are easily eroded.