Impact of iron biogeochemical cycling in deep-sea environments on rare earth element enrichment

Abstract

The enrichment mechanisms of deep-sea rare earth elements and yttrium (REY) are critical for both strategic resource exploration and paleoceanographic reconstruction. Iron biogeochemical cycling plays a central role in the dynamics of REY, mediating their pre-enrichment in seawater and their remobilization at sediment-porewater interfaces. However, the spatial heterogeneity of iron (oxyhydr)oxides distributions and their associated microbial processes in REY enrichment remain poorly understood. Here, we employ a multidisciplinary approach, integrating geochemical, magnetic, microscopic, and metagenomic analyses, to compare iron cycling regimes in two contrasting Pacific REY-rich provinces: the ferruginous southeastern Pacific and the oligotrophic northwestern Pacific basins. Our results reveal that quantitative scavenging by iron (oxyhydr)oxides dominates REY pre-enrichment in the southeastern Pacific, whereas selective adsorption prevails in the iron-depleted northwestern Pacific. Additionally, microbial iron reduction coupled with secondary mineralization enhances the remobilization of REY to porewater and their ultimate accumulation in both regions. This study advances our understanding of the biogeochemical coupling between marine iron and REY, providing new insights into their co-evolution in deep-sea environments.