Linking paleoproductivity to REE-rich mud formation: geochemical evidence from the Clarion-Clipperton Fracture Zone in the pacific

Deep-sea sediments enriched in rare earth elements, termed “REE-rich mud,” represent a promising resource with substantial economic potential. Nevertheless, the role of paleoproductivity in their formation remains poorly understood. In this study, we focus on the geochemical characteristics of two adjacent sediment cores (GC02 and GC03) from the Clarion-Clipperton Fracture Zone (CCFZ) in the Pacific. Compared to core GC03 with an average concentration of lanthanoids and yttrium (ΣREY) of 477 ppm, the REE-rich muds in GC02 show a significantly higher average ΣREY of 843 ppm, coupled with lower δ¹³C_TOC and δ¹⁵N values. Additionally, the REE-rich muds contain elevated concentrations of bio-SiO₂ and excess Ba. These geochemical features suggest markedly enhanced paleoproductivity during the deposition of the REE-rich muds. The significantly lower δ¹⁵N values in GC02 (averaging 5.9 ‰ vs. 9.9 ‰ in GC03) indicate that rapid expansion of oxygen minimum zones (OMZs) was triggered by high productivity, which intensified water-column denitrification and depleted the regional nitrogen inventory during the Oligocene. To compensate for nitrogen loss, significantly enhanced nitrogen fixation occurred, sustaining persistently high productivity levels. Crucially, the expansion of OMZs reduced the degradation of organic matter in the mesopelagic zone, thereby increasing its export flux to the seafloor. During intense early diagenesis, decomposition of this organic matter at the sediment–water interface released REE, which were subsequently incorporated into abundant biogenic apatite formed under high productivity.