Trace metal enrichments in the Mediterranean Sea

Abstract

Copper, nickel, and cadmium occur at higher concentrations in the surface waters of the Mediterranean than in nutrient-depleted waters of the open ocean. Trace metal profiles have been taken on both sides of the Strait of Gibraltar to constrain a mass balance. Deep Mediterranean waters have trace metal levels comparable to surface waters in regions of Mediterranean deep water formation, implying a minimal role for vertical transport by biological activity. Excess trace metals are not present in open-Atlantic source waters, but a detailed map of trace metal distributions in the Alboran Sea (immediately east of the Strait) reveals a metal-enriched plume with concentrations as high as copper, 3.6 nmol/kg; nickel, 2.8 nmol/kg; and cadmium, 120 pmol/kg. A substantial enrichment in trace metal concentrations therefore can occur in transit from the open Atlantic through the coastal zone into the Strait. This trace metal enrichment may derive either from river inflow (masked by evaporation), diffusion out of continental shelf sediments, eolian particulate fallout, or anthropogenic sources. Metal sources (fluvial, sedimentary, or atmospheric; natural or anthropogenic) are difficult to discern from the large-scale oceanographic distribution. Metal retention in nutrient-depleted surface waters is not due to unusual metal fluxes relative to nutrients, but instead is a consequence of less frequent recycling of nutrients in the Mediterranean compared to the open ocean. Metal enrichments would be expected before human intervention, with anthropogenic input reflected in proportion to the relative augmentation of inputs over natural levels. Mediterranean water produces small trace metal anomalies in the outflow core (positive for Ni, negative for Cd). Present observations are consistent with this prediction, but high precision data would be necessary to observe these at distance from Gibraltar.