Effects of bioturbation on solutes and solids in marine sediments

Bioturbation, the mixing of solutes and solids in sediments caused by movem ents of fauna, was studied through tracer experiments and numerical modelin g. The generally accepted mathematical formulation of transport by bioturba tion 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 s olutes, 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 compa ring interpretations of measured concentration-depth profiles and fluxes Of O-2. For solids, D-B was estimated from modeling the depth-distributions o f glass beads, which were added to the sediment surface in the same cores a s used for the bromide tracer experiments. In addition, D-B, also for solid s, was determined by interpretations of 2 measured Pb-210 depth profiles. W e validated our findings through sensitivity analyses and comparisons to ot her studies. As part of this process we tested if irrigation, the pumping a ctivity of tube-dwelling animals, could influence our results. It is common ly assumed that the same D-B value applies to both the bioturbation of solu tes and solids. Our analyses, however, show clearly that the effects of bio turbation 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 sol utes and a value that is 15 to 20 times smaller for solids. The results als o show that the transport of solutes by bioturbation is equally as importan t as molecular diffusion in the upper sediment layers (few cm). Since the d ensity and species composition of fauna in the studied sediment were compar able to those at many other near-shore marine sites, we believe that our re sults are general for many sediments. We suggest that the recognized mathem atical formulation of bioturbation as a diffusive process be extended to in clude 2 different biodiffusivities, one for solutes and one for solids,