ABSTRACT
Extreme aridity in the Namib Desert is the result of several interacting atmospheric and oceanic phenomena. The presence immediately offshore of the Benguela Current is one of the major controlling factors. These cold, upwelled waters cool moistureladen sea breezes, and combined with the atmospheric factors, prevent rain from falling in the Namib. If we can establish the time of the initiation of major cooling and upwelling in the Benguela Current System, we can approximate the time when aridification of the Namib was initiated, or at least greatly intensified.
Recent deep drilling on the Walvis Ridge Abutment has recovered a complete sequence of sediments ranging from Middle Eocene to Late Pleistocene. These open-ocean biogenic sediments provide a wealth of information on the history of the overlying waters in which they were formed: the Benguela Current.
Studies of sediment accumulation rates, diatom frustule abundance, planktonic Foraminifera and calcareous nannoplankton temperature preferences, primary productivity (expressed in Corg) and phosphorus incorporation in calcareous skeletons all suggest changes in the characteristics of the Benguela Current. These sedimentological, palaeontological and geochemical data suggest weak, spasmodic introduction of cool, upwelled waters along this coast from Middle or Late Oligocene until Middle Miocene times. In the early Late Miocene conditions changed markedly, strongly suggesting intensification of upwelling which brought cold, nutrient-rich waters to the surface along this coast.
Onshore faunal remains indicate that the Namib was mostly wooded-grasslands until Middle Miocene times, suggesting that the early spasmodic conditions of the Benguela did not cause significant aridification. It is suggested that the major coolingupwelling of the Benguela in early Late Miocene times initiated aridification of the Namib Desert.
