Geochronology of Holocene sediments on the western margin of South Africa

Abstract

The western continental margin of South Africa is a dynamic environment. It is a site of intense upwelling and high productivity, as well as a repository for large quantities of terrigenous sediment delivered by wind and rivers, most notably by the Orange River. Essential to understanding the complex dynamics of sedimentation on this margin is an accurate, robust age model. Here we present 32 radiocarbon ages of mollusc shells and foraminifera tests recovered from Holocene organic-rich mud deposits on the middle-inner shelf (the mudbelt), and from calcareous ooze on the continental slope. Sedimentation rates of between 0.25 and 2.4 mm/year are recorded in the Holocene mudbelt, whereas slope cores have average sedimentation rates of between 0.04 and 0.22 mm/year. Low sedimentation rates in the mudbelt correspond to increases in the coarse-silt fraction, associated with periods of winnowing. In the north, the mudbelt sediments have been deposited since 11 thousand years before present (11 ka), whereas sedimentation in the southern mudbelt was initiated at around 2 ka. From changes in sedimentation rate and the southward younging of the deposit, we infer that the mudbelt depocentre has shifted during the Holocene in response to changes in sea level and accommodation space on the shelf. Holocene carbonate ooze on the slope contains 20 to 40 weight % terrigenous material, indicating significant off-shelf transport. Onshore Orange River palaeoflood deposits provide a terrestrial record spanning 12 ka. Linking high-resolution continental and marine records will allow for the comparison of oceanographic and climatic changes in southern Africa during the Holocene.