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 famil y 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 pa rameters is tested using a one-tailed F-test. It was found that (1) in five cases there is a significant improvement when direct injection o f 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 diffus ive mixing model. More elaborate models, including additional paramete rs did not result in a significantly better fit. (3) In four cases, th e inclusion of diffusive mixing (bioturbation) to an advection/decay m odel does not result in a significant better fit of modeled Versus mea sured (210)pb activity-depth profiles. Using the simplest nonlocal exc hange model, the amount of particulates that are directly injected at depth into the sediment was estimated and compared with the amount inc orporated 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 impor tance of nonlocal exchange decreases with increasing water depth. To a llow comparison with other measurements, we have also calculated the d iffusive mixing coefficient using the classical bioturbation model. Th e sediments in the OMEX area have low bioturbation coefficients, espec ially at the deeper sites. Finally our models have also been used to r eproduce and to explore some aberrant (210)pb profiles reported in the literature.