Geotechnical and Sedimentological Study of Deep-sea Sediments from a Manganese Nodule Field of the Northeastern Pacific

Abstract

In order to assess deep-sea bed mining impacts on the undisturbed benthic environments, reliable sedimentological and soil mechanical baseline data are essential. Such data also are required by the engineer to support the design and construction of stable and environmentally acceptable sub-sea structures and bottom vehicles. We sampled deep-sea surface sediments from the Clarion-Clipperton fracture zone in the northeast equatorial Pacific, focusing on the Korea contract areas (under the International Seabed Authority) for manganese nodule exploration. We classify the sea floor sediments in this study area into three different types: 1. Biogenic Siliceous Sediment: The seabed of the middle part (8-12°N) of this study area is mainly covered with biogenic siliceous sediment. 2. Pelagic Red Clays: In the northern part of the study area (16-17°N), the seabed sediments consist primarily of argillaceous muds, with minor biogenic components. 3. Calcareous Sediments: The southern part of the study area (5-6°N) is dominated by calcareous. This area is generally shallower than the carbonate compensation depth (CCD). Based on their physcal index properties (i.e., grain size distribution, mean grain size, water content, specific grain density, wet bulk density, void ratio, and porosity) and geotechnical properties (i.e., shear strength, consistency limits), the sediments were classified into 4 types, and each type was evaluated in terms of miner maneuverablility and potential environmental impacts arising from mining activities. The index properties of sediments distinctly show the latitudinal differences in this study area. The results of this study not only provide useful information for the formulation of the mining code of the International Seabed Authority but also enhance our understanding of sedimentation processes in this area. This study is supported by the Korea Deep-Ocean Environmental Study, a component of the KODOS.