Acoustic characterization of pelagic sediments using sub-bottom profiler data: Implications for the distribution of REY-rich mud in the Minamitorishima EEZ, western Pacific

Sub-bottom profiling was conducted in the Japanese Exclusive Economic Zone (EEZ) around Minamitorishima Island, western Pacific Ocean, to investigate the features and distribution of mud rich in rare earth elements and yttrium (REY-rich mud). Based on the echogram records, we distinguished three acoustic facies: opaque (O), transparent (T), and layered (L). The O-type facies is acoustically opaque and highly reflective, without visible structures beneath the top surface. The T-type facies is acoustically transparent, with a basal reflector from the acoustic basement. This facies is subdivided into irregular (TI) and smooth (TS) types according to the topography of its upper surface. The irregular surface morphology of the TI-type is generally parallel to the topography of the acoustic basement, whereas the smooth morphology of the TS-type is independent from the basement topography. The L-type facies is characterized by a layered sequence of multiple reflectors. It always overlies the T-type facies. Correlation of the acoustic facies of T-and L-types with lithological and geochemical characteristics of sediment core samples shows that T-type facies correspond to REY-rich mud and L-type facies correspond to non-REY-rich sediment covering REY-rich mud. Distribution of the O-type facies is restricted to seamounts or their immediate vicinity, suggesting that it corresponds to rocky outcrops. The T-type facies (REY-rich mud) mainly occurs in the southern and southeastern part of the Minamitorishima EEZ, whereas the L-type facies (non-REY-rich sediment) is widespread in the northern and western part of the EEZ. Our results reveal that, in the southern and southeastern part of the Minamitorishima EEZ, REY-rich mud lies at a shallow depth below the seafloor without a cover of non-REY-rich sediment. These areas, therefore, should be a primary target for future mining of REY-rich mud.