Organic matter controls rare earth element hyper-enrichment in Pacific deep-sea bioapatite

Bioapatite is a predominant sink for rare earth elements (REE) in global deep-sea sediments, where the REE accumulate either via organic matter remineralization or Fe–Mn (oxyhydr)oxide dissolution. While the sequestration of REE into bioapatite has been extensively documented, the mechanistic pathways governing REE transfer from these carrier phases to bioapatite during diagenesis remain poorly constrained. This limits understanding of REE hyper-enrichment in deep-sea sediments, and impacts the reliability of REE signatures in bioapatite as palaeoceanographic tracers. In this study, we elucidate the mechanisms of REE hyper-enrichment from marine porewaters into bioapatite in the presence of Fe–Mn (oxyhydr)oxides and organic matter, using a combination of adsorption experiments integrated with micro-X-ray absorption fine structure (μ-XAFS), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) analyses. Results demonstrate that bioapatite can rapidly adsorb REE from a simulated porewater environment (equilibrium time <1 h) via the formation of stable inner-sphere complexes. XANES and EXAFS evidence further reveals that these complexes are strongly coordinated by PO₄³⁻ groups, as exemplified by Y³⁺ with well-defined coordination shells: Y–O (2.37 Å), Y-P₁ (3.50 Å), and Y-P₂ (3.71 Å), which maintains long-term REE retention in bioapatite. Furthermore, we show that organic matter regulates REE cycling through coupled direct and indirect pathways: (1) enhancing short-term REE adsorption onto bioapatite through complexation with Cdouble bondO and O–H bonds, accounting for 79–94% of initial REE uptake; and (2) indirectly promoting long-term REE sequestration by driving the reductive dissolution of Fe–Mn (oxyhydr)oxides, which enhances the REE supply to porewaters for subsequent adsorption by bioapatite. We therefore propose that organic matter plays a central role in controlling REE hyper-enrichment by coupling REE sources (organic matter and Fe–Mn (oxyhydr)oxides) with long-term retention in bioapatite during early diagenesis. This explains the spatial variability in REE enrichment observed across Pacific sediments, and establishes a refined framework for evaluating the early diagenetic alteration of REE signatures.