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Fig. 7.8. Bottom photograph and nephelometer transect across the northern portion of the South Pacific Ocean (data from Sullivan et al., 1973). Note the bottom nepheloid layer in the eastern portion of basin. Bottom photographs show manganese nodules in the westernmost part of the area. Relatively featureless bottom occurs over most of the western South Pacific seafloor and is separated from seafloor with abundant tracks and trails by the mid-ocean ridge, where basalt outcrops occur. See Fig. 7.7 for discussion of methods used to obtain nephelometer measurements.

WEST OF PACIFtC-ANTAWrTB mpQE

According to Glasby (1976a), high concentrations of nodules (> 75% coverage of the seafloor) occur in five main areas : the Southwestern Pacific Basin, the Tasman manganese pavement, the Bellingshausen Basin (centred on 60°S, 100°W), the central region of the Drake Passage, and the crest of the Pacific-Antarctic Ridge (as manganese-encrusted boulders). High surface densities of nodules on the seafloor are due to a combination of low sedimentation rates and high bottom current velocities. Goodell et al. (1971) argued that the elements comprising the concretions are derived mainly from volcanic sources and transported to the site of deposition by bottom currents, coprecipitating en route. Glasby (1976a), on the other hand, concluded that the high abundance of nodules in the two basins on the flanks of the Pacific-Antarctic Ridge is, in part, due to the high availability of potential nucleating agents derived from the erosion and palagonitization of the volcanic rocks extruded at the ridge crest. The difference in composition between nodules from the Southwestern Pacific Basin enriched in Mn, Ni, Mo, Cu, and Co and those from the Bellingshausen Basin was thought to reflect either differences in sediment type in the two regions (red clay and diato-maceous ooze, respectively), or the characteristics of the nodule nuclei (Glasby, 1976a; Meylan and Goodell, 1976).

Glasby (1976b) concluded that the most extensive manganese deposits in the South Pacific are probably located in the circumpolar siliceous ooze province. Nonetheless, these deposits were considered to have no real economic value because of their relatively low Ni + Cu + Co contents, their dilution by glacial erratics, their great distance from land and the unfavourable weather conditions in the region (cf. Zumberge, 1979,1983).

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