ALBERT

Description: Fallout radionuclides 137 Cs and 210 Pb are well established as tracers of surface and sub-surface soil erosion contributing sediment to river systems. However without additional information it has not been possible to distinguish sub-surface soil erosion sources. Here we use the fallout radionuclide 7 Be (half-life 53 days) in combination with 137 Cs and excess 210 Pb to trace the form of erosion contributing sediment in large river catchments in eastern Australia; the Logan River (area 3,700 km 2 ), Bowen River (9,400 km 2 ) and Mitchell River (4,700 km 2 ). We show that the combination of 137 Cs, excess 210 Pb and 7 Be can discriminate horizontally-aligned sub-surface erosion sources (rilled and scalded hillslopes and the floors of incised drainage lines and gully ‘badland’ areas) from vertical erosion sources (channel banks and gully walls). Specifically, sub-surface sources of sediment eroded during high rainfall and high river flow events have been distinguished by the ability of rainfall-derived 7 Be to label horizontal soil surfaces, but not vertical. Our results indicate that in the two northern catchments erosion of horizontal sub-surface soil sources contributed almost as much fine river sediment as vertical channel banks, and several times the contribution of hillslope topsoils. This result improves on source discrimination provided previously and indicates that in some areas erosion of hillslope soils may contribute significantly to sediment yield, but not as topsoil loss. We find that in north-eastern Australia scalded areas on hillslopes and incising drainage lines may be sediment sources of comparable importance to vertical channel banks. Previous studies have used the combination of 137 Cs, excess 210 Pb and 7 Be to estimate soils losses at the hillslope scale. Here we show that with timely and judicious sampling of soil and sediment during and immediately after high flow events 7 Be measurements can augment fallout 137 Cs and 210 Pb to provide important erosion source information over large catchments. This article is protected by copyright. All rights reserved.