Manganese and iron at the redox interfaces in the black sea, the baltic sea, and the oslo fjord
Abstract
© Springer-Verlag Berlin Heidelberg 2011. The joint analysis of the data of manganese and iron species distributions (dissolved Mn, dissolved bound Mn, dissolved Fe(II) and Fe(III), particulate Fe and Mn) obtained in the Black Sea, the Baltic Sea, and the Oslo Fjord allowed to reveal the common features that testify the similarity of the mechanism of the redox layer biogeochemical structure formation in these regions. Our investigations demonstrated that Mn bound in stable complexes with hypothetically organic matter or pyrophosphate is observed in the redox zones in significant concentrations (up to 2 μM), and is likely presented by Mn(III), an intermediate product of Mn(II) oxidation and Mn(IV) reduction. This bound Mn(III) can explain phosphate distribution in redox interfaces – formation of so-called phosphate dipole with a minimum above the sulfidic boundary and a maximum just below, and with a steep increase in the concentrations between these two. This dipole structure serves as a geochemical barrier that decreases the upward flux of phosphate from the anoxic layer. On the base of the recent data obtained in the 100th cruise of RV “Professor Shtokman” (March to April, 2009), it was found that the bound Mn could exist in two forms – colloidal (0.02–0.40 μm) and truly dissolved (<0.02 μm) that perhaps result from complexing with different types of ligands. The flushing events, river input, sporadically increased mixing, and anoxygenic photosynthesis affect the distributions of the redox zone parameters. Response time for changes in the microbial processes involved in reduction and/or reoxidation of Mn and Fe lags behind that for oxygen injection into water. Concentrations of redox-sensitive species of Mn and Fe should thus be useful as a tracer to inter prior hypoxic/anoxic conditions not apparent from oxygen levels at the time of sampling. Modeling results showed that the manganese cycle [formation of sinking down Mn(IV) and presence of dissolved Mn(III)] is the main reason of oxygen and hydrogen sulfide direct contact absence. Modeling allowed to study the role of affecting factors in the formation of the observed distributions.