Freshwater lenses on coral islands as sources of cobalt and nickel in the Entrecasteaux lagoon system (New Caledonia)

On tropical coral and atoll islands, rainfall infiltrates highly permeable soils to form freshwater lenses floating atop seawater, creating stratified aquifer systems. These lenses interact with underlying lithology as well as tidal forcing, mediating the transport of trace metals and nutrients to adjacent reef ecosystems. Here, we investigated the Entrecasteaux system in New Caledonia, Huon and Surprise Islands, to quantify the potential trace metal contribution of submarine groundwater discharge (SGD). We showed that water preferentially discharges along beachrock fringes, where density-driven flow and tidal pumping release metal- and radium-enriched waters compared to offshore water. Although dissolved Co (dCo) and Ni (dNi) concentrations are exceptionally high within the lenses, effective SGD dCo fluxes range only from 25 to 184 μmol d −1 at Huon and 5–15 μmol d −1 at Surprise, reflecting >99% removal during subsurface transport under these seasonal conditions. Dissolved Pb, puCu, and Mn exhibited intermediate attenuation (by factors of 2–30), reflecting the highly reactive nature of such subterranean estuaries, where oxidative precipitation, adsorption onto Fe-Mn oxides, and co-precipitation with carbonates occur. Deep submarine pathways extend SGD beyond the nearshore reef, highlighting aquifer heterogeneity and karstic conduits as key controls on metal transport. Geochemical data suggest that, during our sampling window, ultramafic weathering of subsurface peridotite and lateritic saprolite constitutes the main source of Co- and Ni-enriched, U-rich/Th-poor water in the lens, while seabird-derived inputs ( i.e. , guano) supply only minor amounts of labile metals such as Cu.