Highly positive Ce anomalies of hydrogenetic ferromanganese micronodules from abyssal basins in the NW and NE Pacific: Implications for REY migration and enrichment in deep-sea sediments

Abstract

Ferromanganese micronodule, ubiquitous in aquatic environments, is a key authigenic phase in the rare earth elements and yttrium (REY) enriched deep-sea sediments. This is due to the high porosity and strong scavenging REY capacity of Fe-Mn (oxyhydr)oxides. In this study, micronodules were selected from surface sediments of the NW Pacific inter-seamount basins and the NE Pacific Clarion-Clipperton Zone (CCZ), both distal from oceanic ridges and largely impervious to hydrothermal activities. Hydrogenetic micronodules of this study have commonly consistent and highly positive Ce anomalies. Based on big data statistics, amounts of geochemical data of ferromanganese deposits (n > 1350) were compiled, and those of hydrogenetic ones (including micronodules, (macro)nodules and crusts) were selected. Comparative analyses reveal that both the hydrogenetic crusts (n = 289) and nodules (n = 159) have weaker positive Ce anomalies (avg. 2.23 and 2.97, respectively) than hydrogenetic micronodules (n = 204, avg. 6.27). Combined with the similar Ce content in all three hydrogenetic ferromanganese deposit types, and the lower concentrations of other REYs in hydrogenetic micronodules, the migration of REYs (except for Ce) from micronodules to porewater is indicated. While Ce is likely retained via oxidizing to insoluble Ce(IV), thus forming the highly positive Ce anomalies. Therefore, our study suggests that ferromanganese micronodules could serve as a buffer in the REY enrichment of pelagic sediments.