The transformation and accumulation mechanism of rare earth elements in deep-sea sediments from the Wharton Basin, Indian Ocean

Recently, rare-earth elements and yttrium (REYs, including La-Lu and Y) have been found to be remarkably enriched in Pacific and Indian Ocean deep-sea sediments, as the extreme enrichment of critical metals have received unprecedented attention. However, the origins, transformation and accumulation mechanism of REYs in deep-sea sediments remain controversial. In this study, the mineralogical and geochemical characteristics of REY-rich sediments from the Wharton Basin, Indian Ocean (core GC34) were systematically analyzed. Our results indicate that bioapatites are the most significant host phase of REYs, ambient solutions (seawater and pore water) are the main sources of REYs in deep sea sediments. Besides being the other important host phase of REYs, Fe-Mn micronodules also act as the links between sources and sinks of REYs in deep-sea sediments. Based on the variations in the characteristic profiles of ΣREY, δCe, and MREE/MREE* with depth, combined with the fractionation characteristics of LREE, MREE and HREY in bulk sediments, bioapatites, and Fe-Mn micronodules, we present a model of REYs transformation and accumulation processes in the deep-sea sediments, which mainly occurs in two intervals. The first and dominant accumulation process is in the sediment-water interface (SWI) and shallow sediments, where Fe-Mn micronodules and bioapatites adsorb and/or substitute REYs directly from ambient solutions, resulting in the initial accumulation of REYs in bioapatites and micronodules. The secondary accumulation process in the deep subsurface sediments, due to desorption and resorption processes in bioapatites and release processes in micronodules. The significant connection between P/Th and ΣREY suggests that the physical enrichment of REY-rich materials (bioapatites and micronodules) contributed to the REYs enrichment in deep-sea sediments.