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Increase of human over natural erosion rates in tropical highlands constrained by cosmogenic nuclides

¹ Isotope Geology, University of Berne, Erlachstrasse 9a, 3012 Berne, Switzerl
² Paul Scherrer Institute, Institute of Particle Physics, ETH Hönggerberg, 8093 Zürich, Switzerland

We quantify the difference between the human-caused sediment yield and the natural rates of soil production and bedrock erosion in a now largely deforested tropical highland. The present-day rate of soil loss in the Upper Mahaweli catchment, Sri Lanka, is calculated by using suspended river-load fluxes. These data provide spatially averaged sediment yields of 130–2100 t·km^–2·yr^–1. Local rates of soil loss from agricultural plots on hillslopes are as high as 7000 t·km^–2·yr^–1. By comparison, natural rates of sediment generation, as determined by measuring cosmogenic ¹⁰Be in quartz from sediments and soils, are only 13–30 t·km^–2·yr^–1. The natural rates presented here provide a benchmark against which recent erosion rates, determined by various sediment gauging techniques, can be referenced. In the Sri Lankan highlands, these results suggest that soil is now being lost 10–100 times faster from agriculturally utilized areas than it is being produced.

Key Words: cosmogenic nuclides • erosion • sediment yield • tropical highlands • Sri Lanka

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