CHEMICAL OCEANOGRAPHY AND MARINE GEOSCIENCE OFF SOUTHERN AFRICA - PAST DISCOVERIES IN THE POST-GILCHRIST ERA, AND FUTURE-PROSPECTS

In 1895/96, John D. F. Gilchrist was appointed marine biologist to the Cape Colony. During voyages extending as far as Walvis Bay and Maputo , he initiated studies of the marine geology and chemical oceanography of the shelf while mapping substrata for new demersal fishing grounds . The shelf sediments off the East Coast are controlled by wave proces ses along the inner shelf and by the poleward-flowing Agulhas Current along the outer shelf. In contrast, South Coast sediments of the easte rn Agulhas Bank consist of wave-dominated, landward-coarsening modern (Holocene) terrigenous muds to sands on the inner shelf and relict wav e-dominated shelly sands on the outer shelf, deposited during Pleistoc ene lowstands within glacial (hypothermal) periods. The Agulhas Curren t also appears to exert a controlling influence over the nutrient chem istry and, hence, primary productivity, on the East and South Coast ma rgins. The surface waters of the Agulhas Current are nutrient-poor and most East Coast areas are consequently considerably less productive t han their West Coast counterparts at the same latitude, but the underl ying South Indian Central Water (SICW) is nutrient-rich. Recent findin gs suggest that the Agulhas Current may induce upwelling of nutrient-r ich bottom water derived from SICW at sites such as the Natal Eight an d off Port Alfred by kinematic upwelling, so enhancing the nutrient co ntent of surface waters and increasing the potential for primary produ ction there. A second physical process, which is also thought to be re lated to interaction of the Agulhas Current and bottom topography, is the dynamic shelf-edge upwelling of SICW onto the shelf along portions of the South Coast where the shelf is wider. It is uncertain whether this is continuous in space or time, but it is possible that the proce ss may prime bottom waters for wind-induced upwelling in the south-wes tern ice of capes along the South Coast. On the West Coast, the outer- shelf sediment consists of Holocene planktonic-foraminiferal ooze, ref lecting the dominating influence of the equatorward-flowing Benguela C urrent. The middle-shelf sediment often consists of glauconitic sand, whereas the sediment of the inner shelf usually has a landward-coarsen ing and -thickening wedge of terrigenous muds to sands. Wind-induced u pwelling is the dominant West Coast physical process of relevance to t he sedimentology and chemistry of the inner shelf and overlying waters . In the southern Benguela, this is seasonal, resulting in seasonal va riability in the abundance of nutrients and the resultant productivity of surface waters and associated biogeochemical processes, such as th e appearance of oxygen-depleted bottom water. There is a northward dec rease in the seasonality of these physical and biogeochemical processe s along the West Coast, which is reflected in an increase in the reduc ing nature of the underlying organic-rich sediments between St Helena Bay and Walvis Bay. In the deep-sea environment of the Cape Basin, the clockwise poleward flow of bath the Antarctic Bottom Water (AABW) and the North Atlantic Deep Water (NADW) is reflected in a major zone of erosion of the sea floor, mantled by abundant ferromanganese nodules, at the foot of the continental rise, which is fed by margin-perpendicu lar slumps, debris flows and canyon-fed turbidity currents. The curren ts, driven by Coriolis Force, both swing left (east) into the Agulhas Passage between the Agulhas Bank and the Agulhas Plateau, before parti ng company in the Transkei Basin, where the AABW is forced eastwards b y the northeast-shallowing contours of the Natal Valley. The E-W-orien tated Agulhas Drift, a contourite drift, is being deposited on the lef t (north) side of the AABW. The NADW then heads into the Natal Valley to deposit margin-parallel contourite drifts at the foot of the contin ental slope as far north as Durban, where the Central Terrace and then the Mozambique Ridge steer the NADW first east and then south hack to the mouth of the Natal Valley.