Fluxes of iron and manganese across the sediment-water interface under various redox conditions

Fluxes of dissolved forms of iron and manganese across the sediment-water interface were studied it? situ in the Gulf of Finland and the Vistula Lagoon (Baltic Sea), and in the Golubaya Bay (Black Sea) from 2001 to 2005. Fluxes were measured using chamber incubations, and sediment cores were collected and sliced to assess the porewater and solid phase metal distribution at different depths. Measured and calculated benthic fluxes of manganese and iron were directed out of sediment for all sites and were found to vary between 70-4450 and 5-1000 mu mole m(-2) day-(1) for manganese and iron, respectively. The behavior of the studied metals at various redox conditions in the near-bottom water and in the sediment was the main focus in this study. Our results show the importance of bottom water redox conditions for iron fluxes. We measured no fluxes at oxic conditions, intermediate fluxes at anoxic conditions (up to 200 mu mole m(-2) day(-1)) and high fluxes at suboxic conditions (up to 1000 mu mole m(-2) day 1). Total dissolved iron fluxes were generally dominated by iron(II). Contribution of iron(III) to the total iron flux did not exceed 20%. Obtained fluxes of manganese at all studied regions showed a linear correlation (r(2) =0.97) to its concentration in the porewater of the top sediment layer (0-5 mm) and did not depend on dissolved oxygen concentrations of bottom water. Organically complexed iron and manganese were in most cases not involved in the benthic exchange processes. (c) 2007 Elsevier B.V. All rights reserved.