Open-incubation, diffusion methods for measuring solute reaction rates in sediments
Sedimentary solute distributions and fluxes are determined in part by the kinetics of production/consumption reactions in a deposit. It is possible to estimate rates and investigate kinetic relations in undisturbed or manipulated sediments by documenting build-up or depletion patterns of solutes allowed to diffuse either between relatively thin sections of sediment and a well-stirred water reservoir (plug incubation); or through a large section of sediment without an overlying reservoir (whole-core incubation). The time-dependent concentrations in the sediment in both cases depend on reaction rates, kinetics, diffusion coefficients, and geometric scaling of the sediment and contacting reservoir. Major advantages of the plug incubation method are that interactions between classes of sedimentary reactions can be examined by manipulating the composition of the stirred water reservoir, and kinetic relations, such as reaction order, can be inferred from comparisons of reaction rate with steady-state concentrations of pore water solutes. The water reservoir size and sediment thickness can be altered to allow rapid estimates of reaction rates at near steady-state or to examine nonsteady-state behavior. Nonsteady-state models are always required for the whole-core incubation method. This latter method has the advantages that it is less labor-intensive than many other rate measurement methods and the incubations can be performed in situ. Experimental comparisons between open-incubation and more traditional closed-incubation estimates of reaction rates show good agreement for solutes such as NH4+, SO4–, HPO4– and I–. In some cases, such as Mn++, Fe++, and HPO4– production, where major back-reactions with sediment occur, open-incubations without substantial build-up of solutes may provide the most accurate method for estimating production rates. In principle, the open incubation methods described in this paper can be used for any diffusable species.
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Document Type: Research Article
Publication date: May 1, 1989
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