Rare earth elements and yttrium (REY) exhibit distinct distributions in deep-sea sediments and are widely used as tracers of marine geochemical processes. However, the mechanisms governing REY cycling during early diagenesis remain incompletely understood. Here, we present integrated geochemical analyses of sediments, porewaters, and overlying seawater from two short sediment cores (24–36 cm) recovered from the central Indian Ocean basins. Despite predominantly oxic sedimentary conditions, porewaters display elevated dissolved REY concentrations (18–103 nM) and pronounced middle-REY enrichment, indicating active remobilization of REY during early diagenesis. These dissolved REY are inferred to originate primarily from reductive and non-reductive dissolution of Fe–Mn (oxyhydr)oxides. Variations in shale-normalized porewater REY patterns reflect dynamic partitioning between porewaters and reactive sediment phases, highlighting intense sediment–porewater exchange during early diagenesis. The high dissolved REY concentrations are likely sustained by relatively low phosphate contents in sediments together with colloidal stabilization of dissolved REY, as indicated by the near-flat patterns obtained after normalizing reactive sediment fractions to porewater compositions. Sequential extraction reveals contrasting REY-hosting phases between the two cores, further demonstrating that porewaters mediate REY transfer among Fe–Mn (oxyhydr)oxides, authigenic phosphates, and dissolved phases. Together with previous studies, our results indicate that efficient REY enrichment in deep-sea sediments is favored by oxic bottom waters, low sedimentation rates, limited carbonate dilution, sufficient reactive REY-bearing phases, and moderate hydrothermal inputs. Overall, porewaters function as a dynamic reservoir and transport medium during early diagenesis, regulating REY mobilization, redistribution, and eventual fixation into sedimentary mineral phases. These findings provide new insights into early diagenetic REY cycling in deep-sea sediments and improve the interpretation of REY signatures preserved in marine sedimentary archives.

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