Microbial iron cycling illuminates the biological contribution and potential climate drivers of deep-sea rare earth element enrichment

Deep-sea sediments enriched in rare earth elements represent important potential resources and paleoceanographic archives. The transfer of these elements from marine particles (e.g., Fe-Mn (oxyhydr)oxides or organic matter) to phosphate phases is essential for their enrichment during early diagenesis. However, the biogeochemical processes driving this transfer remain unresolved. Here, we investigate how microbial reductive diagenesis influences rare earth element enrichment in deep-sea sediments of the northwestern Pacific. By integrating geochemical, magnetic, and microscopic analyses of iron biogeochemical cycling and its signature mineral, biogenic magnetite, we demonstrate that microbial reduction drives the post-depositional mineralization of sinking marine particles. This process releases rare earth elements into porewater, thereby promoting their uptake into bioapatite or authigenic apatite. This mechanism was likely enhanced during past global cooling events that increased ocean productivity. Our findings provide valuable insights for tracing microbial contributions to deep-sea rare earth element enrichment and for investigating their underlying drivers. Microbial iron cycling can enhance the accumulation of rare earth elements and yttrium in host minerals by facilitating their release from iron oxides back into the pore water, according to an integrated geochemical, magnetic, and microscopic analysis.