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Mn cycling; water column; Gotland Basin; Baltic Sea
Abstract:
Manganese plays an important role as both an electron donor and acceptor in redox processes of stratified marine environments. Here we present results on Mn cycling in the water column of the Gotland Deep, Baltic Sea—a basin with periodically observed anoxic conditions in bottom waters. In the period 1999–2001, the deeper part of the Gotland Basin was permanently anoxic, and no inflows of significant amounts of oxygenated North Sea water were recorded below the halocline. The upward Mn(II) flux at the chemocline varied between 28 and 77 μmol m−2 day−1 (avg. 53 μmol m−2 day−1) and it was balanced by the downward flux of oxidized Mn over the entire year. The vertical flux of Mn(II) in the water column was mostly regulated by the flux of settling Mn oxides with a minor contribution of Mn(II) diffusing from bottom sediments (7.1–8.2 μmol m−2 day−1). The potential Mn(II) oxidation contributed no more than 2% to the total transferable electron flux (potential chemosynthesis) from the anoxic into the oxic zone, whereas the flux of settling Mn oxides presumably accounted for 3–30% of sulfide oxidation. Two different morphotypes of Mn-rich particles, formed during Mn(II) oxidation in the Gotland Basin chemocline, were identified using SEM-EDX: amorphous particles (92%) and Metallogenium-like particles. About 68% of all Mn-rich particles were associated with large aggregates containing an organic matrix. Manganese represented more than 60% of the total elemental composition of the particles; calcium, iron and silica were also detected.
