First in situ studies of nodule distribution and geotechnical measurements of associated deep-sea clay (Northeastern Pacific Ocean)

Abstract

Deep-sea pelagic clay, particularly that associated with manganese nodules, has long been studied by acoustic survey methods, seabed photographs and sampling. The first manned submersible study of the geological environment of three manganese nodule mining areas using Le Nautile was performed in late 1988 between the Clarion and Clipperton Fracture Zones (Northeastern Pacific Ocean). This operation, based on detailed in situ observations and specific measurements, provides a new outlook on the distribution of nodule facies according to the nature and the physical properties of the underlying sediments. Nodules appear to be located in fields whose existence and boundaries are totally controlled by the bottom morphology and the thickness of the Plio-Quaternary sequence consisting of siliceous pelagic clay (corresponding to the upper acoustic transparent unit). The in situ undrained shear strength ( S u) measured in surficial clay (up to 60 cm below the seafloor) by a vane shear testing device shows systematic and significant variations not detected previous laboratory measurements. These variations are considered to be characteristic of the overall Plio-Quaternary sequence and follow the thickness variations. They reflect the local change of sedimentation rate within the overall pelagic clay sequency, which also appears to control nodule abundance and distribution.