Geochemical characteristics of polymetallic nodules and adjacent sediments in the western Pacific Ocean: effects of sedimentary environments on nodules

Polymetallic nodules and rare earth-rich sediments are developing simultaneously in the western Pacific Deep Sea Basin, but there are few studies on the relationship between polymetallic nodules and their surface sediments. In this study, the geochemical analysis of polymetallic nodules and surface sediments was used to reveal the enrichment and fractionation processes of critical metal elements in nodules-sediments and to explore the influence of deposition processes and environment on the growth of polymetallic nodules. The polymetallic nodules in the study area have relatively high Co and REY contents and low Mn/Fe ratios, indicating a typical hydrogenetic precipitation. The results of principal component analysis and correlation analysis indicate the nodule formation process is a selective enrichment of Fe-Mn oxides for various metal elements in seawater and pore water under the influence of hydrogenesis, diagenesis and input from terrestrial debris and bioclastic. The surface sediments in the study area are mainly deep-sea clays, which are more enriched in Si, Al, Na, K, and other elements than the polymetallic nodules, which are enriched in most metal elements. The enrichment of metallic elements such as Co, Ni, and Cu in the sediments is related to the content of Fe-Mn micronodules, while REY is more closely related to the phosphate fraction. the Fe-Mn oxide fraction selectively adsorbs metallic elements in seawater to form multi-element enrichment and significant positive Ce anomalies and negative Y anomalies, while the phosphate fraction mainly inherits the rare earth characteristics of seawater, and their content determines the content and pattern of metallic elements and rare earth elements in the sediments. The nodule and sediments have similar processes in the enrichment of critical elements, and Fe-Mn oxide components are the carriers of elemental enrichment processes in both. The low biological productivity and low sedimentation rate in the study area result in reduced sedimentation fluxes and slower sedimentation rates of the relevant elements in seawater, providing favorable conditions for hydroformed nodule growth. Affected by the Antarctic Bottom Water, the bottom seawater in the study area is characterized by oxygen enrichment, and under the effect of its strong bottom current scouring causes frequent deposition interruptions, which promotes the formation of Fe-Mn oxides and the enrichment of critical metal.