Manganese complexation at hydrothermal vents

Veronique Oldham, University of Rhode Island, Graduate School of Oceanography, Narragansett, United States, Timothy J Shaw, University of South Carolina Columbia, Chemistry and Biochemistry, Columbia, SC, United States, Aubin Thibault de Chanvalon, University of Delaware, College of Earth, Ocean, and Environment, Lewes, DE, United States, Bradley M Tebo, Oregon Health & Science University, Division of Environmental and Biomolecular Systems, Portland, OR, United States and George W Luther III, University of Delaware, School of Marine Science and Policy, Lewes, United States
Hydrothermal vents are a significant source of trace elements to the global ocean, particularly elements with high crustal abundance like iron (Fe) and manganese (Mn). At the interface of hot, sulfidic water and cool oxygenated water, a number of redox reactions affecting the speciation of such metals can occur. For Mn, this should include the oxidation of soluble Mn(II) emitted from vents to solid Mn(III/IV) oxides, which is catalyzed by abiotic reactions and/or Mn-oxidizing organisms. These oxidation reactions are balanced by reduction of sinking Mn oxide particles by vent emitted hydrogen sulfide, back to soluble Mn, as Mn does not form sulfides. Along both the oxidative and reductive pathways from the soluble to solid phase and back again, Mn(III) as a soluble species can be stabilized as an intermediate via complexation to organic ligand (L). This type of complexation has been shown in a number of marine systems, but until now, not at hydrothermal vent systems. Here we examine the speciation of Mn in the rising plume of three vent sites at the East Pacific Rise hydrothermal vent system. We find that Mn(III)-L makes up to 50 % of the total Mn in waters just a few meters above the plume source, and that Mn(III)-L complexes are responsible for non-conservative mixing behavior from the vent to overlying water. Water column data collected using a CTD above the rising plume indicates that these Mn(III)-L complexes can be transported tens of meters up into the water column. Further, the mixing profiles of our three sites were all different from one another, suggesting that there are many factors at play during mixing of vent waters with ambient waters. We find that Mn(III)-L at these sites may be formed when Mn(II) reacts with superoxide, which forms when Fe(II) rich vent waters react with O2 from ambient bottom waters.These results suggest that soluble Mn may not be as good a mixing tracer for hydrothermal vents and that complexation of Mn is important in transporting soluble Mn from vent sites to the global ocean.