ABSTRACT

Seven piston cores were collected from the seafloor ~250 km south of Minamitorishima Island in the western North Pacific Ocean during the cruise KR13-02 of R/V Kairei; in some portions of cores PC04 and PC05, the total contents of rare-earth elements and yttrium (∑REY) exceeded 4,000 ppm. Microscopic observations showed that the highly REY-enriched layers (∑REY > ~2,000 ppm) contained significant amounts of calcium phosphate and phillipsite. We conducted microscopic observations and grain size distribution analyses of bulk sediments and distinctive components (calcium phosphate and phillipsite) from cores PC04 and PC05 to elucidate the mechanism of the anomalous REY enrichment. The shapes of the calcium phosphate grains suggest that they were mostly biogenic in origin. The grain size distributions of bulk sediments from the REY-enriched layers of cores PC04 (7.8–8.6 mbsf) and PC05 (2.6–3.6 mbsf) were bimodal, with peaks at fine (~4 μm) and coarse (~40–80 μm) sizes. Calcium phosphate and phillipsite were the major components of the coarse-grained portions of these REY-enriched layers. The bulk ∑REY content was mainly controlled by the amount of biogenic calcium phosphate, which is well known to concentrate REY. Thus, increased accumulation of biogenic calcium phosphate was responsible for the REY enrichment. The volume-based cumulative median diameters of calcium phosphate and phillipsite grains appeared to be proportional to the contents of both calcium phosphate and ∑REY. Increased phillipsite grain size suggests a low sedimentation rate, which may have allowed biogenic calcium phosphate to accumulate without dilution by low-∑REY components. In addition, increased grain sizes of calcium phosphate and phillipsite suggest that sorting by a bottom current allowed coarse-grained biogenic calcium phosphate to become concentrated in sediments by removing fine-grained particles, including low-∑REY components. Multiple factors should be considered to explain the overall features of the highly REY-enriched layers.