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

Abstract

SUMMARY 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 processes along the inner shelf and by the poleward-flowing Agulhas Current along the outer shelf. In contrast. South Coast sediments of the eastern Agulhas Bank consist of wave-dominated, landward-coarsening modem (Holocene) terrigenous muds to sands on the inner shelf and relict wave-dominated shelly sands on the outer shelf, deposited during Pleistocene lowstands within glacial (hypotbermal) periods. The Agulhas Current also appears to exert a controlling influence over the nutrient chemistry and, hence, primary productivity, on the East and South Coast margins. The surface waters of the Agulhas Current are nutrient-poor and most East Coast areas are consequently considerably less productive than their West Coast counterparts at the same latitude, but the underlying South Indian Central Water (SICW) is nutrient-rich. Recent findings suggest that the Agulhas Current may induce upwelling of nutrient-rich bottom water derived from SICW at sites such as the Natal Bight and off Port Alfred by kinematic upwelling so enhancing the nutrient content of surface waters and increasing the potential for primary production there. A second physical process, which is also thought to be related 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 process may, prime bottom waters for wind-induced upwelling in the south-western lee of capes along the South Coast. On the West Coast, the outer-shelf sediment consists of Holocene planktonic-foraminiferal ooze, reflecting the dominating influence of the equatorward-flowing Benguela Current. The middle-shelf sediment often consists of glauconitic sand, whereas the sediment of the inner shelf usually has a landward-coarsening and -thickening wedge of terrigenous muds to sands. Wind-induced upwelling is the dominant West Coast physical process of relevance to the sedimentology and chemistry of the inner shelf and overlying waters. In the southern Benguela this is seasonal, resulting in seasonal variability in the abundance of nutrients and the resultant productivity of surface waters and associated biogeochemical processes, such as the appearance of oxygen-depleted bottom water. There is a northward decrease in the seasonality of these physical and biogeochemical processes along the West Coast, which is reflected in an increase in the reducing 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 pole-ward flow of both 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-perpendicular slumps, debris flows and canyon-fed turbidity currents. The currents, driven by Coriolis Force, both swing left (east) into the Agulhas Passage between the Agulhas Bank and the Agulhas Plateau, before parting company in the Transkei Basin, where the AABW is forced eastwards by the northeast-shallowing contours of the Natal Valley. The E-W-orientated Agulhas Drift, a contourite drift, is being deposited on the left (north) side of the AABW. The NADW then heads into the Natal Valley to deposit margin-parallel contourite drifts at the foot of the continental slope as far north as Durban, where the Central Terrace and then the Mozambique Ridge steer the NADW first east and then south back to the mouth of the Natal Valley.