Recent field studies have confirmed the presence of soluble manganese(III), which along with Mn(II) passes through a 0.2 �m filter, in marine waters. This species accounts for up to 100% of the dissolved Mn pool at the oxic/anoxic interface. Here we show data from two systems where we have applied a spectrophotometric method using a soluble porphyrin as a competitive ligand to calculate the concentrations and kinetics of Mn(II) and Mn(III) recovery, simultaneously. Non-sulfidic porewaters from the hemipelagic sediments of the St. Lawrence estuary show evidence for weak ligands complexing Mn(III), which is formed from Mn(II) oxidation as well as reduction of MnO2 by several processes. Mn(III) accounts for up to 80% of the total dissolved Mn pool in the vicinity of the oxic-suboxic boundary, with concentrations ranging from the detection limit of 50 nM to 80 �M. The anoxic waters of the Chesapeake Bay show evidence for stronger ligands as the porphyrin could not outcompete the natural ligands binding Mn(III). A reductant was required to reduce Mn(III) complexes to measure an increase in Mn(II) for its detection. Soluble Mn(III) comprised up to 52 % of total dissolved Mn. Samples with low H2S (4.9 �M) had high Mn(III) (7.3 �M) whereas those with high H2S (40 �M) had low Mn(III) (1.1 �M) indicating Mn(III) is kinetically stabilized in situ by strong ligands so reduction to Mn(II) is not complete. Assays for MnOx (nano)particles showed these were negligible or not detected except near the oxic-anoxic interface. Mn(III) profiles showed a peak in the suboxic zone. Mn(III) was not observed to flux from the sediments; thus, sediments were not a source of Mn(III) to overlying waters. These profiles likely result from the one electron reductive dissolution of solid MnO2 particles formed at the oxic-anoxic interface, which then fall into the anoxic zone.

Role of soluble Mn(III) in stabilizing suboxic zones in the natural environment

247th National Meeting (2014)

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