Trace metal composition of suspended particulate matter in the water column of the Black Sea

We analyzed the chemical composition of suspended matter from the Black Sea for a complete suite of elements used as tracers for various sources and processes. Samples of suspended matter were collected at four stations from the Bosporus to the center of the western gyre during a R/V Bilim cruise in September 2000. Sample depths were from the oxic surface layer, the suboxic zone and the upper part of the anoxic (sulfidic) layer. All samples were analyzed for Al, Ti, V, Cr, Mn, Fe, Co, Cu, Zn, Mo, Ag, Cd, Ba, Pb, U and P using ICP-MS, AAS, and spectrophotometer. The main objective of this study was to determine the processes that influence the distributions of particulate trace metals in the water column under various redox environments. The composition of the particulate matter was influenced by lithogenic input from surrounding rivers, biological production in the surface layer, biogeochemical (e.g. redox) processes and anthropogenic inputs. Almost all elemental concentrations decreased from the coastal to the offshore stations to the center of the western gyre. Metal/Al ratios were compared with the ratios of average crust and Danube and Turkish rivers. Deviations from these lithogenic sources were used to examine the anomalies due to geochemical, biological and anthropogenic processes. In general, the largest metal enrichments relative to crust were observed inthe surface samples away from coastal sources. Metal/P ratios were used to compare data for some of the elements with previous data from the open ocean. The Me/P ratios for Cu, Zn, Mo and Ni were much higher than open ocean studies, while the ratios for Cd and Co were comparable. The high Me/P (and Me/Al) for several elements suggest anthropogenic input may contribute to these high surface concentrations. The composition of particulate matter from the suboxic zone showed that some elements were strongly enriched. Enrichments of Mn and Fe in the suboxic zone were observed, as expected. Mn and Fe oxides play an important role in scavenging processes. The redox dependent processes in the suboxic - anoxic interface influence the vertical distribution of U, V, Ni, Co, Cu, Zn, Ba and possibly Mo, Cr. Elements influenced by sulfide formation in the anoxic layer were Fe, Cr, Ni, Co, Mo, and Ag.

Research highlights
► SPM compositions showed large spatial variations. Lithogenic detrital material decreased from the Bosporus to the W. Gyre.
► Enrichments due to biological processes and possibly anthropogenic inputs were observed in the surface samples.
► Elements that appeared to be influenced by Fe/Mn oxides scavenging in the suboxic zone were U, V, Ni, Co, Cu, Zn and Ba.
► Redox dependent processes in the suboxic -anoxic interface influence the distribution of U, V, Ni, Co, Cu, Zn, Ba, Mo, Cr.
► Elements influenced by sulfide formation in the anoxic layer were Fe, Cr, Ni, Co, Mo, and possibly Ag.