Maganese geochemistry in Galveston Bay sediment

Abstract

The sediment geochemistry of Mn at a site in Galveston Bay, Texas, was investigated through a three-fold approach. A field study demonstrated that despite the coarse, quartz-rich nature of the bottom sediments, Mn was extensively remobilized within the sediment and that a large fraction of the incoming Mn flux was returned to the water column ([approximately]80-90%, relative to overlying suspended matter). Laboratory experiments showed that Mn(II) adsorption onto natural sediment follows a nonlinear Freundlich isotherm, and is dependent upon the relative proportions of quartz sand and fine-grained aluminosilicate material. Manganese(II) oxidation in the presence of Galveston Bay sediment is also dependent upon grain size distribution, follows Michaelis-Menten-type kinetics with respect to dissolved O₂, and is independent of dissolved Mn(II) within the concentration range 20-300 μM. Below 20 μM, nonlinear adsorption effects prevent unambiguous interpretation of oxidation behavior. Sediment incubation experiments showed that production of dissolved Mn(II) through reduction of Mn oxides occurs shortly after isolation of sediment from the atmosphere, and that it is independent of Mn oxide concentration over a wide concentration range. These experiments also resulted in the first laboratory determination of the Mn(II) removal rate constant for anoxic sediments. Numerical modelling of dissolved and solid phase Mn, using reaction rate constants determined in the laboratory investigation, failed to predict profiles and fluxes observed in the field when the only mechanism for loss was molecular diffusion. Manipulation of physical and reaction parameters in the model demonstrated that the high Mn(II) oxidation rate was responsible for almost quantitative Mn retention, even though the O₂-containing layer was only 2-3 millimeters in thickness. This observation and the absence of worm tubes at the collection site indicate that export of Mn is accomplished through some process other than molecular diffusion or sediment irrigation by biota. Additional modelling indicated that the Mn depletion observed in sediment cores may result from either episodic erosive or anoxic events, or from continuous resuspension of surficial sediment. Finally, modelling of the effect of an impermeable object, located at the sediment:water interface, indicates that in nearshore areas where the oxidized surface layer is only millimeters thick, distributions and fluxes of both O₂ and dissolved Mn(II) may be affected locally by objects with diameters on the order of one millimeter.