Effects of bioturbation on solutes and solids in marine sediments

Bioturbation, the mixing of solutes and solids in sediments caused by movements of fauna, was studied through tracer experiments and numerical modeling. The generally accepted mathematical formulation of transport by bioturbation as a diffusive process was applied and values of the biodiffusivity (D-B) were estimated for both dissolved and solid constituencies in the same sediment, Two independent estimates were found for each constituency, For solutes, D-B was determined from incubated sediment cores after addition of bromide to the overlying water and subsequent modeling of the bromide depth-distributions in the sediment. D-B for solutes was also estimated by comparing interpretations of measured concentration-depth profiles and fluxes Of O-2. For solids, D-B was estimated from modeling the depth-distributions of glass beads, which were added to the sediment surface in the same cores as used for the bromide tracer experiments. In addition, D-B, also for solids, was determined by interpretations of 2 measured Pb-210 depth profiles. We validated our findings through sensitivity analyses and comparisons to other studies. As part of this process we tested if irrigation, the pumping activity of tube-dwelling animals, could influence our results. It is commonly assumed that the same D-B value applies to both the bioturbation of solutes and solids. Our analyses, however, show clearly that the effects of bioturbation on solutes are many fold stronger than on solids, as reflected in the estimated D-B value of 4.6 +/- 1.0 x 10(-6) (1 SE) cm(2) s(-1) for solutes and a value that is 15 to 20 times smaller for solids. The results also show that the transport of solutes by bioturbation is equally as important as molecular diffusion in the upper sediment layers (few cm). Since the density and species composition of fauna in the studied sediment were comparable to those at many other near-shore marine sites, we believe that our results are general for many sediments. We suggest that the recognized mathematical formulation of bioturbation as a diffusive process be extended to include 2 different biodiffusivities, one for solutes and one for solids,