研究業績

Deep-sea sediments containing high concentrations of rare-earth elements and yttrium (REY), termed REY-rich mud, are widely distributed in the Pacific and Indian oceans. Mud layers with very high total REY (ΣREY) concentrations (>5000 ppm of ΣREY with ~1000 ppm of heavy rare-earth elements) have been discovered within the Japanese exclusive economic zone surrounding Minamitorishima Island, western North Pacific. The number of highly REY-enriched layers in the sediment column, the depths at which the layers occur, and the maximum ΣREY values of the layers are, however, quite variable on a scale of several tens of kilometers. The factors controlling the formation and distribution of the REY-enriched layers are still poorly understood. Here, we produce a chemostratigraphic scheme for 1240 sediment samples from 49 piston cores collected within the Minamitorishima exclusive economic zone. Detailed investigation of the bulk sediment geochemistry, which exhibits distinctive compositional data structures in multi-elemental subspaces, revealed that the samples can be categorized into five units (Units I to V) in addition to REY peaks (>2000 ppm). These units appear in a consistent stratigraphic sequence in the studied cores. This chemostratigraphy reveals that highly REY-enriched layers formed at least three times, and that almost all of these layers were accompanied by erosion during deposition. Our results suggest that erosion events of deep-sea sediment play an important role in the formation of highly REY-rich mud layers.