Crozet Basin Research Articles

Geochemistry of manganese deposits in relation to environment on the sea floor around southern Africa

Manganese deposits from the Madagascar Basin, the Crozet Basin and the basins west of the Madagascar Ridge around southern Africa differ chemically from one another. This appears to be largely because of differences in the degree of oxygenation of the environment; most highly oxidizing is the Madagascar Basin, where the Mn Fe ratio is very low and where the manganese deposits probably consist largely of “birnessite”. Continental-margin samples differ from basin samples in having higher Mn Fe ratios, probably because of diagenetic remobilization processes within the sediment column. The ratios of Mn and Fe to certain metals ( Mn Ni , Mn Zn , Fe Co and Fe As ) are the opposites of those found in other continental-margin environments, suggesting either a difference in the diagenetic process, or an additional metal source, off southern Africa. Both seawater and bottom sediments of the southern African margin are metal-enriched, and could act as sources of metals for growing manganese nodules and encrustations.

Deep flow in the Madagascar and Mascarene basins

Confirming a prediction of deep circulation theory, hydrographic sections along 3ats. 12°S and 23°S between Madagascara nd the Central Indian Ridge exhibit a western boundary current flowing northward close against Madagascar at depths greater than 3000–3500 m. It is 300–400 km wide at Lat. 12°S, and its volum transport is etiamted geostrophically to be 4–5 × 10 6 m 3s −1. The current dervies from deep water int he Circumpolar Current, which moves northward into the western Crozet Basin and appears to pass into the Madagascar Basin near 27°S, 65°E. It is uncertain what fraction of the current continues northward into the Somali Basin. In the layer 2000–3000 m, property distributions in the Madagascar and Mascarenee basis are ambiguous concerning the sense of flow, and there are no boundary currents; geostrophic considerations suggest a net flow northwards of perhaps 1 × 10 6 m 3 s −1 in this layer. Evidently very little of the high-salinity deep water in the Mozambique Basin passes northward into other parts of the Indian Ocean. Some general problematical aspects of these observations for deep circulation theory are noted briefly.

Seismic refraction measurements in the southern Indian Ocean

Twenty-seven seismic refraction profiles in the southern Indian Ocean are described. They straddle four major physiographic features: Two branches of the mid-ocean ridge system, Broken ridge, and the Ninetyeast ridge. The Réunion and Crozet basins are structurally very similar to the flanks and adjacent basins of the northern mid-Atlantic ridge. North of Amsterdam Island the mantle beneath the crest of mid-ocean ridge has a velocity of 7.61 km/sec; away from the crest the mantle has normal velocity but is exceptionally shallow. Broken ridge has a quasi-continental structure, and Ninetyeast ridge appears to be upfaulted oceanic crust rather than volcanic accumulation. A station in the West Australian basin revealed a shallow, low-velocity mantle.