Basin scale distributions of dissolved manganese, nickel, zinc and cadmium in the Mediterranean Sea

Samples for dissolved trace metal concentrations were collected during GEOTRACES expedition GA04-N in summer/spring in the Mediterranean Sea, starting in the Atlantic Ocean and sampling both deep basins of the Mediterranean Sea. Outflow of Mediterranean Outflow Water leads to elevated concentrations of Mn, Ni and Zn in the Atlantic Ocean, but a concentration minimum in the Atlantic distribution of Cd. Nevertheless, when comparing the in- and outflow, the Mediterranean is a net source of Cd to the Atlantic Ocean. Surface concentrations of Mn, Ni, Zn and Cd are elevated in the Mediterranean relative to the Atlantic Ocean where Ni and Cd gradually increased along the eastward surface water flow path, Zn reached a homogenous concentration in the order of 1 to 1.5 nM, and Mn displayed a patchy surface distribution. The observed differences are attributable to the different dynamics of their biogeochemical cycling, notably the partitioning between the dissolved and particulate phases due to biological uptake, scavenging and possibly organic complexation. The elevated surface concentrations of Mn, Ni, Zn and Cd in the Mediterranean are derived from atmospheric deposition, where most likely Zn and Cd are mainly sourced from anthropogenic origin, Mn mostly from lithogenic origin and Ni from both anthropogenic and lithogenic origin. Dissolved Zn and Cd, as well as phosphate and nitrate, display striking inter-basin fractionations with elevated concentrations in the deep water of the western basin compared to the deep eastern basin, without a coinciding increase in the apparent oxygen utilization. Given that physical circulation or contribution from biogenic particulate metals cannot explain the elevated dissolved concentrations, an external non-biological source is required. This source is most likely a vertical flux of metal laden particles dissolving through the water column of the western Mediterranean where these particles, most likely from anthropogenic origin, are derived from either atmospheric deposition or particulate material deposited on the continental shelves that makes its way into the deep basin. To confirm or detect trends in dissolved metal concentrations in the deep basin, regular basin wide assessments of the trace metal distributions in the Mediterranean are needed. The distributions of Mn, Ni, Zn and Cd in the Sea of Marmara illustrate that all of these metals can be affected by anthropogenic surface sources and highlight the different susceptibilities of the dissolved metal distributions to supply from remineralization, and to removal through scavenging. This study provides a first baseline to assess future changes and underlines that the Mediterranean marine environment is susceptible to anthropogenic disturbances, with varying effects for different metals due to differing source strengths and biogeochemical cycles.