The nature of soluble Mn(III) in oxygenated surface waters and in future oceanic O2 regimes

Veronique Oldham, University of Delaware, Oceonography, Lewes, DE, United States, Alfonso Mucci, McGill University, Department of Earth and Planetary Sciences, Montreal, QC, Canada, George W Luther III, University of Delaware, College of Earth, Ocean and Environment, Lewes, DE, United States, Kristen E Burk, University of Miami, Laramie Jensen, Texas A&M and Megan T Miller, Georgia Institute of Technology
Abstract:
The speciation of soluble Mn (dMn) was previously assumed to be dominated by Mn(II) in surface waters, and recent work has shown that the redox reactive species, Mn(III)L, can dominate suboxic and anoxic systems. The stabilization of dMn is favorable in low O2 regimes, and the predicted increase in oceanic oxygen minimum zones (OMZs) could lead to greater stabilization, cycling and transport of Mn(III)L. We find that Mn(III)L makes up to 99% of dMn in the surface waters of the Broadkill River, DE and up to 86% in the St. Lawrence Estuary (SLE), indicating, for the first time, that Mn(III)L complexes are also stable in oxygenated waters. In the SLE, dMn fluxed out of the sediments both in the Saguenay Fjord and the lower estuary. The vertical profile of dMn was over 20-80% higher in the lower SLE than in 1974, corresponding to lower dO2 in the system. Here, dMn was much higher in bottom waters, with Mn(III)L stabilized above the sediment water interface. However, in the Saguenay Fjord, the profile revealed that Mn(III)L was not only entering via sediment flux and subsequent stabilization by ambient ligands, but also by surface waters via wetland runoff, indicating that terrestrial ligands may also stabilize Mn(III). This behavior is more clearly shown in the horizontal surface profiles of dMn along a salinity gradient in the Broadkill River, a coastal waterway bordered by salt marshes. Here, high Mn(III)L corresponded to high humic material, as indicated by characteristic UV absorption peaks. Additionally, an assay of ligand character was made after precipitating humic matter, and confirmed that Mn(III)L complexes had humic character. These findings indicate that humic Mn(III)L complexes are likely prevalent in coastal terrestrial systems. Thus, humic material serves to bind Mn(III), which transports both Mn(III) and organic matter to the coastal ocean. Finally, processes like these will become even more important as oceanic OMZs increase and Mn(III) plays a greater role in the coupled cycles of Fe, S, O, N and C.
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