A 190 ky record of lithogenic grain-size on the Namibian slope: Forging a tight link between past wind-strength and coastal upwelling dynamics

We inferred wind-strength variations at the Namibian coast and its effect on upwelling intensity over the last 190 ky, by comparing grain-size data and alkenone-based sea surface temperatures (SSTs) from one sediment core located on the upper slope off Lüderitz (25°6′S, 13°38′E). The wind-strength proxy was defined by partitioning terrigenous grain-size distributions (GSDs) into two sub-populations owing to a simple statistical model. The fine, poorly sorted component is assumed to include both fine aeolian dust and hemipelagic mud carried by rivers or winnowed from the shelf. Those two fractions are assumed to have been identically sorted during settling in the water column and are not distinguished by our approach. The mode of the coarse, well-sorted sub-population displays rapid, high amplitude variations through time. We ascribe a strictly aeolian origin to this component, and consider that low-altitude winds are able to carry coarse-grained particles to the ocean, as shown by grain-size measurements of present dust material from the adjacent continent [Stuut, J.B.W., Prins, M.A., Schneider, R.R., Weltje, G.J., Jansen, J.H.F., Postma, G., 2002. A 300-kyr record of aridity and wind-strength in southwestern Africa: inferences from grain-size distributions of sediments on Walvis Ridge, SE Atlantic. Mar. Geol. 180, 221–233]. ions in the coarse-grained dust diameter are closely correlated with SST changes. Past SSTs records from the Namibian upper slope have been previously hypothesized as reflecting upwelling intensity combined with occasional injection of warm-water from the Angola Dome. We challenge this view in considering that past long-term SSTs variations are primarily induced by the wind-strength and witness coastal upwelling intensity. Early MIS 6 is the only period with evidences of warm, poleward water intrusion at the core site. The windiest conditions occurred during isotopic stages 3 and 4. The wind-strength proxy defined in this study can be used confidently as an independent proxy for past upwelling intensity over the last two climatic cycles.