Geochemical and mineralogical characteristics of deep-sea sediments from the western North Pacific Ocean: Constraints on the enrichment processes of rare earth elements

Deep-sea sediments containing high concentrations of rare earth elements (REE) and yttrium (Y), termed REYrich sediments, have attracted considerable attention and are widely distributed in the Pacific Ocean. However, the processes and mechanisms of REY enrichment in REY-rich sediments remain controversial. Here, we present our investigation of the mineralogical and geochemical characteristics of deep-sea sediments in three sediment cores from the western North Pacific Ocean. REY-rich sediments with very high total REY (TREY) concentrations (>3000 ppm) were discovered within the study area, which is characterized by middle REE (MREE) and heavy REE (HREE) enrichment. REY-rich sediments have unique geochemical characteristics and are different from known onshore REE ore deposits. Our results suggest that the pelagic clay sediments have higher TREY concentrations than the carbonate ooze sediments. Bioapatite fossil and micronodule are the main host minerals of REY in REY-rich sediments, whereas terrigenous, volcaniclastic, and calcareous materials can dilute TREY concentrations. The contribution of micronodule to REY enrichment in REY-rich sediments is relatively moderate compared to that of bioapatite fossil. Seawater is the main source of REY in REY-rich sediments. During diagenesis, bioapatite fossils absorb REY from seawater/porewater by substitution, whereas micronodules release REY (especially Ce) into the ambient solution. Thus, a migration of REY from micronodules to bioapatite fossils occurs. The bioapatite fossil is likely to act as the ultimate host of REY, while the micronodule acts as a temporary sink for REY. The enrichment and transformation of REY in the bioapatite fossils and micronodules mainly occur in the seawater column and the sea-sediment interface. The REY enrichment in deep-sea sediments was caused by the physical enrichment of REY-rich materials (bioapatite fossil and micronodule). The sedimentation rate, therefore, is an important controlling factor for REY enrichment. The formation of REY-rich sediments with TREY concentrations > 2000 ppm was likely to occur due to the additional contributions of enriched bottom currents.