Conventional wisdom holds that sediment yields in Madagascar are uncommonly high, and that human activities drive the erosion. The country has a "world-record erosion rate" of 20,000-40,000 tons per square km per year according to the World Bank, and the widespread hillslope gullies called lavaka are featured in every guidebook as examples of the catastrophic denudation of the country's landscape. This viewpoint is based, however, on measurements that are few and local, and in many respects represents "received wisdom" rather than rigorously determined interpretation. Systematic analysis is therefore necessary to measure the erosion rate of Madagascar. Cosmogenic 10Be, a short-lived isotope created at the Earth's surface by cosmic-ray bombardment, can determine landscape erosion rates. Undisturbed surface materials accumulate substantial amounts of 10Be, but where erosion is high then surface materials are low in this isotope. Preliminary 10Be analyses of sediment from Madagascar rivers indicate erosion rates of only about 32 tons per square km per year, suggesting that the Madagascar landscape has been quite stable on timescales of thousands to hundreds of thousands of years. On the other hand, direct measurements of sediment yield in rivers can provide erosion rates on shorter timescales (years to centuries). Because humans arrived in Madagascar less than 2000 years ago, collecting and comparing 10Be and sediment gauge data will document erosion rates in the pre-human to modern time span. This project will sample drainage networks from broad swaths of Madagascar, omparing 10Be data with recent sediment data from stream-sediment gauging and reservoir sedimentation records.in order to quantify both background and modern erosion rates. GIS analysis will investigate the relationship between erosion rate and the intensity of lavaka gullying in the watersheds.
This project contributes to ongoing efforts to quantify the effects of humans as geomorphic agents world-wide. Madagascar's status as a biodiversity hotspot and a threatened environment lend added significance to the work, as these questions loom large in discussions of conservation and international aid policy. The project will involve both Malagasy and U.S. students and personnel. The scientific results of this project will have direct relevance for environmental planning and erosion mitigation strategies for Madagascar. Understanding rates of erosion, the role of lavakas in sediment generation, and the role of humans in the process will help determine whether Madagascar is naturally characterized by high erosion rates or whether they are of recent and anthropogenic origin. This award is co-funded by NSF's Office of International Science and Engineering.
