研究業績

Deep-sea mud enriched in rare-earth elements and yttrium (REY-rich mud) has been recognized as a new resource for REY and scandium (Sc). Recently, highly REY-rich mud with more than 2,000 ppm of REY was found in the western North Pacific Ocean. The high REY content was attributed to enhanced deposition of fish debris, which acts as a host for REY, resulting from fish proliferation caused by topographically induced upwelling of nutrients. To investigate the spatial extent of the highly REY-rich mud, we conducted mineralogical and chemical analyses on selected sediment cores collected near topographic highs in the Central Pacific Ocean. The maximum REY content of the study cores was 4,489 ppm. The correlation between REY contents and abundance of fish debris indicated that the presence of large amounts of fish debris was responsible for the REY enrichment. The results of the mineralogical analysis suggested that there are three layers of REY-rich mud on the basis of the presence/absence of zeolite group minerals in the study cores. The first layer with phillipsite formed as a result of fish proliferation due to interactions between water currents and topography in the latest Eocene to early Oligocene. The layers with clinoptilolite probably formed as the result of an increased depositional flux of fish debris in the equatorial high-surface-productivity area in the middle Eocene. The layer without zeolite group minerals likely formed in relation to increased fish debris production caused by increased biological activity and high ecosystem efficiency together with slow sedimentation of other components in the South Pacific Gyre. Calculation of the resource amount in a 1-km2 area around the study sites, based on mining to the depth where the average grade is maximum when mined from the surface, revealed that the highly REY-rich mud could provide 1.4% of the global annual mine production of REY and 0.8?1.3 times the global annual supply of Sc. This finding suggests that REY-rich mud in the Central Pacific Ocean is a prospective REE resource.