LUMINESCENCE CHRONOLOGY HIGHLIGHTS CLIMATE CONTROLS ON SEDIMENTATION AND EROSION ACROSS THE SOUTH AFRICAN INTERIOR

Soil erosion is one of South Africa’s most pressing environmental problems. Across the interior, many river channels are currently incising and numerous dongas (gullies) have formed in colluvial and alluvial sediments. The chief driver(s) of this incisional phase is contested. Donga formation has traditionally been attributed to anthropogenic drivers (eg. overgrazing), but stratigraphic evidence indicates that dongas also have naturally formed and infilled in response to late Quaternary environmental changes.

To constrain the timing of sedimentation and erosion and examine the temporal correlation with late Quaternary environmental and anthropogenic land use changes, optically stimulated luminescence (OSL) ages have been determined for sediments associated with dongas at three sites across interior South Africa: i) Modder River, Free State; ii) Steelport region, Limpopo Province; iii) upper Blood River region, KwaZulu-Natal. OSL ages for three key sediment types are presented: i) alluvial and colluvial sediment units, which indicate the timing of major late Quaternary sedimentation episodes; ii) ‘recent’ channel fills in older dongas, which provide maximum ages for the onset of the present channel incision and donga formation phase; and iii) alluvial fan sediments, which indicate the timing of ‘recent’ upslope donga formation.

OSL chronologies indicate that overbank and hillslope sedimentation, interspersed by soil development, have been dominant for most of the last ~40 ka, ~82 ka and >~100 ka at the Modder River, Steelport and Blood River sites respectively. In contrast, within the last ~2 ka, but no later than 0.29 ka, river channels deeply incised, and numerous dongas formed. Onset of incision predates European colonisation and agricultural intensification (late 18^th century onwards), and instead coincides with periods of frequent, abrupt climate changes associated with the Medieval Warm Period and the Little Ice Age. We propose that these changes destabilised landsurfaces, initiating a shift from net sedimentation to erosion. Additional controls on the patterns, magnitude and timing of channel incision and donga formation include soil type and local baselevel falls that are related to the breaching of resistant rock barriers in channel beds.