Global estimates of enhanced solute transport in marine sediments

Pore-water solute transport processes acting in addition to molecular diffusion affect sediment biogeochemistry and benthic exchange fluxes. Given the relatively few direct measurements of enhanced transport intensities, there is a need for predictive relationships to calculate enhanced transport parameters from more readily available information. Here, enhanced diffusion coefficients and nonlocal mass transfer coefficients are obtained by comparing total and molecular diffusion fluxes of oxygen across the sediment–water interface. Semiempirical relationships for these coefficients are derived as functions of benthic oxygen uptake. According to these relationships, enhanced solute transport significantly affects sediment–water column exchanges in regions with large benthic oxygen fluxes, typically on the continental shelves. On a global scale, enhanced transport contributes approximately one third of the total benthic flux of oxygen and more than half of that of phosphate. The sediment–water interface (SWI) constitutes a natural boundary in the oceans, across which the transport regimes of both solids and solutes change dramatically (Boudreau and Jørgensen 2001). For solutes, open-water turbulence gives way to molecular diffusion through the porous medium of the sediment. In the uppermost layers of marine sediments, however, biologically induced solute transport (bioirrigation) can exceed transport because of molecular diffusion (Archer and Devol 1992; Meile et al. 2001). Pore-water advection driven by pressure changes as a result of wave or tide action can also contribute significantly to solute transport fluxes in permeable sandy sediments in nearshore environments (Ziebis et al. 1996; Boudreau et al. 2001). Quantitative estimates of enhanced transport intensities are thus important in order to constrain benthic fluxes of dissolved nutrients or oxygen uptake at the seafloor. Pore-water concentrations change significantly over depth scales of millimeters to decimeters and give rise to a typical vertical zonation of pore-water chemistry. The concentration gradients are the result of a multitude of different reaction and transport processes acting simultaneously, which complicates identification of the individual processes. Concentration profiles are therefore complemented by direct measurements of reaction and transport rates. Mathematical models that explicitly couple the reaction network to the transport processes further help with interpretation of the observational data. From model simulations, it is then possible to estimate reaction rates and fluxes that can be difficult to measure directly (Soetaert et al. 1996b; Van Cappellen and Wang 1996). However, because detailed information is generally required as input, such models tend to be applied only at sites where extensive data sets have been collected, although the underlying mathematical description is generally valid. 1 Corresponding author ([email protected]).