Holocene climatic optimum in southwest Africa - Evidence from the marine clay mineral record

Sediment composition, grain size and clay mineral record of a high-resolution sediment core from the continental slope off Namibia was investigated to gain information on the deposition of terrigenous matter in this part of the Southwest African continental margin during the last 18 k.y. The depositional processes involved are fluvial input by the Kunene River and eolian input from the Namib and Kalahari deserts, each supplying characteristic mineral suites. During low sea level, erosion of the exposed shelf yields additional material. The amount of eolian or fluvial matter depends on the strength of the transport process, which is a function of aridity or humidity of the source area, thus allowing paleoclimatic interpretations. Arid conditions prevailed during a low sea level from 18 to 15 ka and unconsolidated shelf sediment was mobilized and supplied to the slope by short-distance transport by southerly winds. A dramatic increase in the accumulation of terrigenous sediment is recorded from 15 to 10 ka without major changes in sediment composition, which is attributed to increased runoff of the Kunene River and fluvial erosion of shelf sediments. This period coincides with a strengthening of the monsoonal system during a precessional minimum, which is observed in numerous sites in Central Africa and indicates an intensified influence of the monsoon at the Kunene headwaters. A distinct shift in day mineralogy towards river-derived material marks a second period of increased river runoff-during high sea level-from 9 to 5 ka, indicating maximum humidity in the source area from 6 to 5 ka. This corresponds to the Holocene climatic optimum observed in the arid belts of Northern Africa. The present balance between fluvial and eolian input was reached approximately at 4 ka.