The biogeochemistry of manganese and iron in the Black Sea

Abstract

The solution speciation and solid-phase suspended particulate fractionation of Mn and Fe were investigated in the Black Sea in an effort to understand the biogeochemical cycling of Mn and Fe across redox boundaries and to study the scavenging/precipitation reactions affecting their distributions. The redox cycling of Mn (in a distinct “suboxic” zone from 15 to 50 m thick) and the redox cycling of Fe (coincident with total sulfide concentrations exceeding 0.4 μM) occur along isopycnal surfaces which deepen sharply towards the Turkish coast. Dissolved Mn behaves primarily as the free hydrated Mn 2+ species and approaches saturation with respect to MnS 2 (haurite) in the deep anoxic waters. In the oxic zone, colloidal and organically-complexed Fe species account for 10–30% of the total dissolved Fe, while colloidal Fe-sulfides account for 30–60% of the total in the mid-depth dissolved Fe(II) maximum. The deep waters are close to saturation with respect to FeS (mackinawite) or Fe 3S 4 (greigite). A weak-acid soluble Mn phase dominates in the broad particulate Mn maximum in the suboxic zone and appears to be associated with Mn-oxidizing bacteria. More resistant Mn and Fe phases, presumed to be sulfide precipitates, were found in the deep anoxic waters. Particulate Al showed a broad maximum below the sulfide interface, presumably due to offshore transport of resuspended sediment. A vertical mixing model for dissolved Mn in the central basin of the Black Sea yields removal rates consistent with measured bacterial Mn oxidation rates. The redox cycling of Fe occurs somewhat deeper in the water column. The vertical supply of oxygen cannot account for the Mn oxidation rate. However, horizontal advection and/or seasonal vertical mixing could provide enough oxidizing equivalents.