Controlling Factors on REY Enrichments in Basins From the Pacific Ocean: Early Diagenesis and Local Constraints

Increasing interest in high-tech metals (e.g., rare earth elements and yttrium, REY) has triggered extensive research on the enrichment of these metals in pelagic sediments. In the sediments collected worldwide, Ca-phosphate and Fe-Mn (oxyhydr)oxides are targeted as the two most important REY carriers based on multiple geochemical and mineralogical analyses. However, the formation of the REY-rich stratum remains enigmatic as the influences of diagenesis and the local restrictions of different oceanic basins are complex, which require further investigation. In our study, we analyzed four sediment cores from the Western, Central North, and Eastern South Pacific (WP, CNP, and ESP), which are three of the most promising REY-rich regions in global oceans. Sequential leaching quantifies the dominance of Ca-phosphate by controlling ∼73.9%–93.3% of ΣREY and the secondary role of Fe-Mn (oxyhydr)oxides by holding up to ∼97.8% of Ce and ∼4.82%–13.9% of ΣREY. The shallowly and deeply buried sediments show similar features regarding the REY proportions hosted by different phases, which indicates that the REY accumulation by Ca-phosphate might majorly occur on the seawater-sediment interface. The geochemistry of the studied sediment cores reveals that (a) REY enrichment in ESP is affected by hydrothermal Fe-Mn (oxyhydr)oxides which can promote adsorptions of P and REY. (b) The Fe and Mn contents in WP sediments show an inverse relationship against REY enrichment which might be explained by the dust input from Central Asia. (c) The slow sedimentation rate of CNP enables the extraordinary REY-uptake by Ca-phosphate, but the low flux of phosphorus constrains the formation of high-grade REY deposits. Key Points The rare earth elements and yttrium (REY) held by Ca-phosphate and Fe-Mn (oxyhydr)oxides in pelagic sediments are quantified by sequential extraction Ca-phosphate mainly scavenges REY at the seawater-sediment interface on the seafloor Local constraints on REY-rich sediments from different pelagic basins relate to their material sources (e.g., eolian dust and hydrothermal vent) Plain Language Summary Rare earth elements and yttrium (REY) are useful for geological studies and valuable materials for cutting-edge technologies. Recently, scientists found high REY contents in deep-sea sediments, which could be future resources. However, its enriching mechanism is not fully understood. Here, we analyze the geochemistry of four sediment cores across the Pacific. The samples were sequentially leached with chemical solutions to separate different mineral phases. The results reveal that Ca-phosphate (e.g., fish fossils) is the main carrier holding ∼73.9%–93.3% of REY, and Fe-Mn (oxyhydr)oxides (e.g., small Fe-Mn nodules) hold ∼4.82%–13.9%. The proportions of REY hosted by the carriers are similar among the layers ranging from the surface to 8-m depth. This indicates that the REY accumulation mostly happens on the seafloor surface. Additionally, this study reveals that the REY-rich sediments in the Eastern South Pacific are likely related to underwater hot vents that release more Fe and Mn to accumulate P and REY. In the Western Pacific, the levels of REY seem to be affected by dust coming from Central Asia. In the Central North Pacific with slow sedimentation rates, fish fossils can take up high REY, but their low amount limits the formation of high-quality REY deposits.