human impacts. Anthropogenic increases in river fluxes can be estimated by quantifying terrigenous mud accumulation offshore adjacent to rivers, and in the case of South Africa indicate a 12-fold increase since the Pliocene for the east coast (Martin, 1987) and a 13-fold increase since the Neogene for the west coast (Dingle and Hendey, 1984). However, deriving natural back- ground rates of erosion over long, million-year timescales is com- plicated by tectonic and climatic variations, which may have significantly altered river drainage, sediment discharge and dis- persal (Partridge and Maud, 2000). Therefore, it is desirable to compare modern river discharge with that of the Holocene, a period during which tectonism and climate were largely similar to today. Here, we estimate the mass of terrigenous mud on the western continental margin to derive the mean Holocene mud flux of the Orange River and reconcile the large increase in the modern flux with soil erosion within the Orange River catchment. Introduction Soil erosion is a major environmental threat globally and in South Africa is highly variable and linked to past land-use change (Garland et al., 1999) and likely to be influenced by future cli- mate change (eg, Lobell et al., 2008). A pre-dam (1930–1969) sediment discharge of 60 million metric tons/year (Mt/yr), of which 50 Mt/yr is mud, makes the Orange River the most turbid in Africa and the fourth most turbid river in the world (Bremner et al., 1990). Most rainfall and erosion occur in the eastern por- tion of the 0.9 million km 2 catchment (Figure 1). The suspended mud load of rivers can provide a useful measure of accelerated soil erosion, but requires an estimate of river fluxes prior to Abstract: Soil erosion poses a major threat to sustainable agriculture in southern Africa but is difficult to quantify. One measure of soil erosion is the sediment flux of rivers. The Orange River is the principal source of sediment to the western margin of South Africa with an estimated mean mud flux over the last 11 500 years (the Holocene epoch) of 5.1 (3.2–7.4) million metric tons/year (Mt/yr). A total of 43 gigatons (Gt; 10 15 g) representing 72% of the Holocene mud flux has accumulated on the shelf in the Orange River prodelta and mudbelt, a clayey fine- silt deposit focused on the inner to middle shelf. Only 8% (5 Gt) of the mud flux occurs in Holocene calcare- ous ooze on the slope. Comparison of the clay to mud ratio of offshore deposits with Orange River suspended sediment and catchment soils indicates that 20% (11 Gt) of the Holocene mud flux has been lost as clay beyond the margin. The Orange River mud flux prior to the building of large dams (1930–1969) is ten times greater than the mean Holocene mud flux and is reconciled with estimates of soil erosion within the catchment. A ten- fold increase in the Orange River mud flux implies up to a hundredfold increase in total soil erosion depending on the extent of mud storage over periods of decades to centuries within the catchment. Erosion has shifted from areas of high relief and rainfall of the Drakensberg escarpment during the Holocene to intensely cultivated lands of low relief having moderate to high rainfall in the eastern catchment and to a lesser extent, grazing areas of the southern Orange River catchment. Key words: Soil, erosion, Orange River, South Africa, continental margin, Holocene. The Holocene (2010) pp. 115–122 © The Author(s), 2010. Reprints and permissions: http://www.sagepub. co.uk/journalsPermissions.nav 10.1177/0959683609348860 *Author for correspondence (e-mail: john.compton@uct.ac.za) A tenfold increase in the Orange River mean Holocene mud flux: implications for soil erosion in South Africa John S. Compton, 1 * Caren T. Herbert, 1 M. Timm Hoffman, 2 Ralph R. Schneider 3 and Jan-Berend Stuut 4 ( 1 Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa; 2 Plant Conservation Unit, Botany Department, University of Cape Town, Rondebosch 7700, South Africa; 3 Institute of Geosciences, Christian Albrechts University, D-24118 Kiel, Germany; 4 Research Center Ocean Margins, University of Bremen, 28334 Bremen, Germany) Received 9 January 2009; revised manuscript accepted 3 July 2009 at Staats-Und Universitaets Bibliothek Bremen on March 8, 2010 http://hol.sagepub.com Downloaded from