Ultraselective enrichment of trace elements in seawater by Co-rich ferromanganese nodules

Ferromanganese nodules are huge metal resources and windows into Earth processes, being widely distributed in vast deep-sea basins covered by sediments. Co-rich ferromanganese nodules are typical hydrogenetic deposits that can effectively scavenge and enrich multiple trace elements in seawater. However, uncertainty still exists regarding the enrichment process of hydrogenetic nodules and their interactions with the distribution of trace elements in seawater. Herein, we analyzed up to 73 elements in Co-rich ferromanganese nodules from the western Pacific and found that high-field strength and redox-sensitive elements are selectively distributed between the Fe and Mn hydroxide phases. These elements are highly enriched in ferromanganese nodules over seawater and upper continental crustal values. The enormous amounts of ferromanganese deposits make them the major, even exclusive, budget for Te, Mn, Co, Ce, Pb, Bi, Pt, Ru, Rh, Ni, and Mo. The distributions of trace elements in seawater are both the cause and result of scavenging by ferromanganese deposits and of biogeochemical cycling. In particular, ferromanganese deposition is responsible for the distributions of scavenged-type elements such as Mn, Co, Ce, Pb, Bi, and Te in seawater. Based on the distributions of elements in seawater and at the water–sediment interface, we propose a new two-stage model for nodule metallogenesis. Stage I is the initial enrichment of trace elements by the sinking of ferromanganese hydroxide colloids, which regulate the distributions of scavenged-type elements in the water column. Stage II is the top-down migration of trace elements dominated by bioparticle cycling, which promotes the re-enrichment of trace elements by ferromanganese hydroxides at the water–sediment interface.