Quantifying solute distributions in the bioturbated zone of marine sediments by defining an average microenvironment

The bioturbated zone of marine sediments is a region having a complex, time-dependent geometry of diffusion and chemical reactions. It is possible to simplify this geometry by postulating an average sediment microenvironment and modelling it as representative of the sediment body as a whole. The microenvironment is assumed to correspond to a single, tube-dwelling animal together with its surrounding sediment and can be represented by a finite hollow cylinder. A transport-reaction model derived from this postulate produces good agreement between observed and predicted pore water profiles using realistic physical constants. The average vertical distributions of pore water solutes and their sediment-water fluxes are influenced by the presence of irrigated burrows to varying degrees depending on the kind of reactions governing their behavior. Pore water profiles of solutes, such as NH⁺₄, subject to zero order reaction rates are highly sensitive to the abundance and sizes of burrows while the net flux of the constituent across the sediment-water interface is not. In contrast, profiles of solutes such as Si that are subject to first order reaction rates are less sensitive to the presence of irrigated burrows but net fluxes are greatly affected. Average pore water concentrations, fluxes of solutes like Si and the apparent one-dimensional diffusion coefficients required to match vertical gradients with measured solute fluxes, are influenced by both the size and spacing of burrows. Because of the range of solute concentrations within the microenvironment at any given depth it is not strictly valid to make detailed solubility calculations on the basis of average pore water concentrations within the bioturbated zone.