An artificial seafloor sediment plume was created to obtain its dispersion data in 1994 in the Clarion-Clipperton Fracture Zones in the Pacific. The result showed that most of the resuspended sediments settled down immediately, though the deep-sea sediments consisted of fine silt and clay particles. In order to evaluate the quick settlement, size distribution of the sediments aRer the resuspension was examined in laboratory. Settling velocity analyses without dispersant in distilled water and without dispersant in seawater were tried in the examination to clarify an effect of dispersanL Larger size distributions of the sediments than the ones measured by a geotechnical standard settling velocity analysis were detected from the examination. An even larger size distribution of the sediments was obtained with a preliminary dispersion test of sediment-water mixture conducted in a small seawater tank.
Deep-sea manganese nodules, which have been found in large quantities at the Clarion-Clipperton Zones (CCZ) in the Pacific, have been considered as potential resources for Cu, Ni, Co, and Mn in the next generation (Herrouin et al., 1987). Three major sources ofin-situ environmental impacts are expected during manganese nodule mining operation. They are direct tracking of a collector on seafloor, a seafioor plume created with recovered and discharged deep-sea sediments by the collector, and a surface plume created with lifted bottom water, deep-sea sediments, and nodule fragments and discharged from a mining vessel as shown in Fig. 1. A large amount of deep-sea sediments is recovered with the nodules during manganese nodule excavation by a hydraulic type pickup device on sea.floor. Most of the recovered sediments are separated from the nodules and discharged from the collector immediately. They make sediment-water mixture, suspend near the seafloor as a seafloor plume, and seule on the seafloor again fmally.
