Modeling Pb-210-derived mixing activity in ocean margin sediments: Diffusive versus nonlocal mixing

The influence of sediment mixing on activity versus depth profiles of the radionuclide (210)pb in the upper 20 cm of the sediments has been investigated along a depth transect (208 m-4500 m, 17 stations) in the OMEX study area (Goban Spur, NE Atlantic Ocean). A hierarchical family of bioturbation/nonlocal exchange models was derived. Each member of the hierarchy includes all processes of the previous model, and adds a one- or two-parameter process. The significance of the additional parameters is tested using a one-tailed F-test. It was found that (1) in five cases there is a significant improvement when direct injection of part of the flux into deeper sediment layers (nonlocal exchange) is added to the diffusive mixing model. (2) In these five cases, the best model required only two additional parameters, compared to the diffusive mixing model. More elaborate models, including additional parameters did not result in a significantly better fit. (3) In four cases, the inclusion of diffusive mixing (bioturbation) to an advection/decay model does not result in a significant better fit of modeled Versus measured (210)pb activity-depth profiles. Using the simplest nonlocal exchange model, the amount of particulates that are directly injected at depth into the sediment was estimated and compared with the amount incorporated at the sediment surface. Along the OMEX transect, between 8-86% of the total flux enters the sediment by nonlocal exchange rather than by mere bioturbation/advection at the sediment surface. The importance of nonlocal exchange decreases with increasing water depth. To allow comparison with other measurements, we have also calculated the diffusive mixing coefficient using the classical bioturbation model. The sediments in the OMEX area have low bioturbation coefficients, especially at the deeper sites. Finally our models have also been used to reproduce and to explore some aberrant (210)pb profiles reported in the literature.