Amanda Rau’s research while affiliated with University of Cape Town and other places

Three giant piston cores were recovered in 1996 from the slope and rise off South Africa by the MARION DUFRESNE. The cores are MD962084, NW of Cape Town, MD962080, SSW of Cape Town and MD962077 off East London in the Natal Valley. MD962084 (35.28 m) contains a detailed record of the Quaternary variability of the Southern Benguela, whereas MD962080 (22.23 m) records latitudinal variations in the position of the Subtropical Convergence. Sediments of hypothermal periods are dominated by Neogloboquadrina pachyderma (right-coiling) and, in MD962080, higher proportions of very fine quartz sand, washed off the continental shelf during lowstands. In contrast, sediments of hyperthermal periods are dominated by Globorotalia inflata and increased amounts of the tropical Globorotalia menardii, which is also found in trace amounts in the hypothermal periods. This shows that Agulhas water transfer never completely ceased, even during equatorward excursions of the Subtropical Convergence during hypothermals. Three major turbidites of quartz sand characterise the Natal Valley core, MD962077 (35.54 m) and they have ages of 510, 340 and 250 ka. If they were triggered by major earthquakes, it is argued that an even bigger earthquake is overdue, 250 ka after the last one.

The Agulhas Bank region, south of Africa, is an oceanographically important and complex area. The leakage of warm saline Indian Ocean water into the South Atlantic around the southern tip of Africa is a crucial factor in the global thermohaline circulation. Foraminiferal assemblage, stable isotope and sedimentological data from the top 10 m of core MD962080, recovered from the western Agulhas Bank Slope, are used to indicate changes in water mass circulation in the southeastern South Atlantic for the last 450 kyr. Sedimentological and planktonic foraminiferal data give clear signals of cold water intrusions. The benthic stable isotope record provides the stratigraphic framework and indicates that the last four climatic cycles are represented (i.e. down to marine isotope stage (MIS) 12). The planktonic foraminiferal assemblages bear a clear transitional to subantarctic character with Globorotalia inflata and Neogloboquadrina pachyderma (dextral) being the dominant taxa. Input of cold, subantarctic waters into the region by means of leakage through the Subtropical Convergence, as part of Agulhas ring shedding, and a general cooling of surface waters is suggested by increased occurrence of the subantarctic assemblage during glacial periods. Variable input of Indian Ocean waters via the Agulhas Current is indicated by the presence of tropical/subtropical planktonic foraminiferal species Globoquadrina dutertrei, Globigerinoides ruber (alba) and Globorotalia menardii with maximum leakage occurring at glacial terminations. The continuous presence of G. menardii throughout the core suggests that the exchange of water from the South Indian Ocean to the South Atlantic Ocean was never entirely obstructed in the last 450 kyr. The benthic carbon isotope record and sediment textural data reflect a change in bottom water masses over the core location from North Atlantic Deep Water to Upper Southern Component Water. Planktonic foraminiferal assemblages and sediment composition indicate a profound change in surface water conditions over the core site approximately 200-250 kyr BP, during MIS 7, from mixed subantarctic and transitional water masses to overall warmer surface water conditions.