Geochemistry of rare earth elements in two-color core sediments from the Korea Deep Ocean Study (KODOS)-90 site, NE equatorial Pacific

Abstract

Major factors controlling the variation of rare earth element (REE) contents in deep-sea sediments were identified using box-core sediments from the Korea Deep Ocean Study (KODOS)-90 site in the western part between the Clarion-Clipperton fracture zones, northeast equatorial Pacific. Four of ten core columns can be divided into two layers by a sharp color boundary; an upper pale brown layer (Unit A) and a lower dark brown layer (Unit B). Trivalent REEs (3 + REEs) as well as micronodules, smectite, smectite aggregate, and fish remains (spicules and spines) are highly enriched in Unit B compared to Unit A. 3 + REEs in micronodule, smectite, and smectite aggregate are not significantly concentrated compared to those in bulk Unit B sediments, 3 + REE contents in coarse-grained fish remains (>25 μm), which is likely biogenic apatite, are about seven and two times higher, respectively, than the contents in Units A and B. Contents of fish remains in bulk Unit B sediments, however, are too low to explain the enrichment of 3 + REEs in Unit B. Based on the results of phosphorus speciation and linear relationships in two pair diagrams (La contents vs. total inorganic phosphorus and 1/(Ce/Ce*) vs. total inorganic phosphorous), it can be inferred that 3 + REEs are dominantly enriched in apatite in fine sediments (<25 μm) with strong negative Ce anomaly and convex-type distribution pattern of REEs normalized with NASC. It is suggested that apatite grains, supplied abundantly from surface water during the time of Unit B formation with high paleo-productivity, had incorporated and concentrated 3 + REEs.