The importance of relict burrow structures and burrow irrigation in controlling sedimentary solute distributions

Some areas of the seafloor, particularly the deep sea, are characterized by large numbers of apparently uninhabited or relict burrow structures formed by macrobenthic organisms. Because of low sedimentation rates or lack of physical disturbance these structures are stable for long periods of time and could potentially influence solute diffusion patterns in surface sediments. A two-dimensional diffusionreaction model which allows for diffusive rather than advective transport within stagnant, water-filled burrows demonstrates that, in the absence of advective irrigation, relict burrow structures are unlikely in most cases to significantly alter average solute distributions from those predicted by one-dimensional vertical models. This conclusion assumes changes in diffusive transport properties alone and does not account for any effects of relict structures on reaction rates or physical ventilation of deposits by bottom currents. Significant changes (≥5%) in solute distributions are generally produced only when the ratio of the halfdistance between burrows to relict burrow radii is ≤10 and sedimentary diffusion coefficients are ≤60% that in free solution. Because solute distributions in stagnant burrow waters are nearly that in surrounding sediment, sediment-water solute fluxes are also essentially unaffected by relict burrows except at extremely high abundances or fairly large differential diffusion rates between sediment and free solution. In contrast, even at low abundance, biologically irrigated or physically ventilated burrows produce major changes in solute transport and build-up patterns.