The ferromanganese crusts in the central Pacific seamounts have been believed as most abundant and enriched in cobalt according to the earlier studies by the USGS, KORDI, MMAJ and GSJ since 1980's. We investigated the ferromanganese crust areas using a TV camera-monitored drill machine (BMS) after full multi-narrow beam mapping over three flat-top seamounts in the Marshall Islands area. The cruise data revealed characteristics of small-scale variation in relation to water depth, substrate geology, and often topography. The mapping and BMS sampling have shown that the thickness (up to 120 mm) and growth patterns of the crusts are basically related to substrate geology and topography, which in turn indicates possibility of geology and topography is significant parameters in estimating development and continuity of the sea-bed crusts over seamounts. On the other hand, fine-scale (in millimeter) compositional and structural variations were also described for several nice drill cores, which is another important factor in estimating ore reserves. Our preliminary data of depth- compositional profiles demonstrated significant enrichment of cobalt in younger layers a crust, as high as those of regional variations. More importantly, some of the chemical profiles are correspondent to each other even between the studied seamounts far apart over the western Pacific.


The hydrogenetic ferromanganese crusts in the Marshall Island area have been documented as most abundant in thickness and enriched in cobalt and nickel as well as in Line Islands, Magellan seamounts, and French Polynesia Islands (Hein et al, 1988, 1990). The earlier reconnaissance surveys show that the crusts commonly cover the tops and flanks of seamounts and guyots over the central to south Pacific oceans. On the other hand, small-scale variability has not been well characterized in each seamount, which is most probably closely related to past geological and oceanographic environment.

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