Oxidative and reductive processes contributing to manganese cycling at oxic-anoxic interfaces

Manganese (Mn) is important in seawater as a micronutrient, scavenger (as MnO₂) and as a reactant in the redox cycles of many other biologically important elements. In seawater, Mn cycles between oxidized insoluble Mn(III/IV) oxides (MnO_x) and soluble Mn (II/III) species (dMn_T), which includes Mn(III) complexed to organic ligands (Mn(III)-L) and reduced Mn(II). Mn(III)-L complexes have been shown to be stable in separate oxic, suboxic and anoxic systems, but not in an entire water column with a transition from an oxic to anoxic zone. Additionally, the formation pathways for these complexes have not been well described at one location over a short timescale. In order to better understand these pathways, dissolved and particulate Mn speciation was determined in the water column of the seasonally stratified Chesapeake Bay basin over a 2-day period, using pump profiling for better spatial resolution (10 cm) of redox active species and a modified, low-level spectrophotometric method for soluble Mn speciation (detection limit = 3 nM). Our data suggest that Mn(II) fluxes out of the anoxic sediments (bottom water total Mn: 2.23–3.80 μM; Mn(II) = 61–84% of total Mn) and is oxidized to MnO_x at the top of suboxic zone of the water column at non-detectable dO₂ levels (≤ 3 μM). These biogenic Mn oxides are reduced at the suboxic-anoxic interface via a combination of strong ambient Mn(III)-binding ligands, H₂S and/or microbial activity, resulting in the disappearance of MnO_x and formation of Mn(III)-L complexes and Mn(II). The oxic water column has lower concentrations of dMn_T (0.05–0.18 μM), with Mn(III)-L complexes present in all oxic samples (33–80% of dMn_T). The complexes in the oxic water column arise from processes distinct from those occurring in the suboxic and anoxic zones.