ABSTRACT:

An artificial impact experiment named DIETS, whose objective was an analysis of the direct destruction of the benthos occurring in the running tracks of a nodule collector, was conducted in a terrace on a seamount in the southeast region of Marcus Island in 1999. The depth was 2,200 m, and the nodule population was 15 kg/m2 in the terrace. In the experiment, towing a system called a "scraper," the nodules and sediments on the seafloor surface were partially removed and piled. Afterwards, the sediment cores were sampled with a multiplecorer, and results of the impact on physical properties of the sediment cores and benthos were analyzed. Moreover, the seafloor features were observed with a towed camera system. In this paper, an outline of the experiment and some results of the post-experiment monitoring are introduced The conceptual system design and the performance are discussed

INTRODUCTION

Manganese nodules and cobalt-rich manganese deposits, found on the deep-sea floor in the Pacific Ocean, are expected to be important supply sources for copper, nickel, cobalt, and the like in the next generation (Mero. 1965: Cronan, 1980; Halbach, 1982; Manheim, 1985). Separating these sediments in the mining system as quickly as possible improves the transportation efficiency later. This means most of the sediments are e:(dlausted from the nodule collector immediately after collection, at which time they create a plume, are diffused, subside, and are re-deposited on the seafloor (Yamazaki.et al., 1991). Because this re-deposition occurs from tens of thousands to millions of times faster than the natural particle sedimentation rate, it likely has a significant influence on the benthos ecosystem of the seafloor. Wherever the nodule collector is used, sediment layers on the tracks are destroyed to a considerable depth, and, moreover, the ecosystem is directly destroyed immediately.

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