Importance of the different manganese species in the formation of water column redox zones: Observations and modeling
The joint analysis of manganese species distributions (dissolved Mn, particulate Mn, and bound Mn) obtained in the Black Sea, the Baltic Sea (Landsort Deep) and Oslo Fjord (Baerumsbassenget), enabled common features to be revealed that demonstrate the similarity in the mechanisms of pelagic redox-layer formation. Our investigations demonstrate that Mn bound in stable complexes with hypothetical P-containing compounds or organic matter (OM) is observed in the redox zones at significant (average: 0.5-1.0 μM) concentrations and is likely present as Mn (III), an intermediate product of Mn (II) oxidation. This bound Mn (III) can explain phosphate distribution in redox interfaces and the formation of the so-called "phosphate dipole" with a minimum level above the sulfidic boundary, a maximum just below and a rapid increase in concentration within this range. This dipole structure serves as a geochemical barrier that decreases the upward flux of phosphate from the anoxic layer. Modeling results show that the manganese cycle (formation of settling Mn (IV) particles and presence of dissolved Mn (III)) is the main cause of lack of oxygen and hydrogen sulfide contact. According to the model, the formation of the "phosphate dipole" may also be connected with manganese cycling and different factors (i.e. mixing, particle sinking rate, presence of Mn in the water column) should affect the peculiarities of the observed distributions at the different geographical locations. © 2009 Elsevier B.V. All rights reserved.