|
PAGES Africa
Pan Africa Workshop, July 2004
Oceanographic Change and Submarine Earthquakes Recorded Off South Africa Over the Past Million Years
Amanda Rau1, John Rogers1 and Min-Te Chen2
University of Cape Town1 and National Taiwan Ocean University2
The Quaternary record in marine sediments off southern Africa has been studied in recent years via long cores from the continental slope and rise. The longest cores were recovered from the French research vessel, the MARION DUFRESNE, using the CALYPSO corer, that recovered cores up to 45m in length during two major research cruises, one in 1996 and a second in 2002. Three of the cores, recovered in 1996, have been studied at the University of Cape Town, MD962084 NW of Cape Town, MD962080, SSW of Cape Town and MD962077 south of Durban.. MD962084 (35.28m) contains a detailed record of the Quaternary variability of the Southern Benguela Upwelling System off the west coast of South Africa, whereas MD962080 (22.23m) is well positioned to monitor latitudinal variations in the position of the Subtropical Convergence. This is a major oceanic boundary between the cold waters of the Southern Ocean and the temperate waters of the Atlantic, Indian and Pacific Oceans.
Sand-size microfossils of both bottom-dwelling and planktonic foraminifera and grains of quartz sand were studied in the cores. Sediments of glacial periods are characterised by increased proportions of colder-water assemblages of planktonic foraminifera, dominated by Neogloboquadrina pachyderma (right-coiling) and, in MD962080, by higher proportions of very fine quartz sand, washed off the continental shelf. In contrast, sediments of interglacial periods are characterised by warmer-water assemblages of planktonic foraminifera, dominated by Globorotalia inflata but including the tropical foraminifer Globorotalia menardii. The latter persists in trace amounts in the glacial periods, showing that leakage of warm Indian Ocean water into the South Atlantic never ceased entirely, during equatorward excursions of the Subtropical Convergence.
Five thin layers of quartz sand, three major and two minor, are a distinctive feature of core MD962077 (35.54m) off Durban. The sandy layers are interpreted as the deposits of very fast, sediment-laden bottom currents, turbidity currents, probably triggered by submarine earthquakes. Therefore they are termed turbidites. The ages of the major turbidites, range in age from 510 000 years to 250 000 years, with an average interval of 140 000 years. If they were triggered by major earthquakes, it is argued that an even bigger earthquake is overdue, because the last one was 250 000 years ago.
Figure 1: Schematic of the Agulhas Current and Southern Benguela Region. Agulhas rings (1) and filaments (2) are shed at the Agulhas Retroflection (3) and are carried equatorward by the Benguela Current. The rest of the water of the Agulhas Current flows eastward along the Subtropical Convergence (STC) (4). Cool waters of the Benguela coastal upwelling system (BUS) are shown along the west coast of South Africa. The different surfaces water masses are characterised by planktonic foraminiferal assemblages, typified by (a) Globigerina bulloides (offshore of upwelling zones), (b) Globorotalia inflata (transitional waters), (c) Neogloboquadrina pachyderma (dextral) (northern subpolar waters), (d) Globorotalia menardii (tropical Agulhas waters). AFR= Agulhas Fracture Ridge (After Rau et al., 2002). Core locations are indicated by numbered triangles.
|
