Abstract
Although its concentration is very low (〜10 μg/l) in ambient deep waters, particulate matter controls the oceanic distribution of reactive metals such as Al, Fe, Mn, Th and Pb through processes called "scavenging." This phenomenon has been modeled by marine geochemists largely based on radioactive disequilibria of the U/Th decay series nuclides in the water column. Nozaki et al. (1987) have proposed a new model which includes two particle size classes (small and large) as an extension of the earlier one particle phase adsorption-desorption model of Nozaki et al. (1981). The new model assumes a reversible exchange of the two classes of particles to occur through aggregation and disaggregation processes. Using Pacific data for dissolved and particulate 230Th profiles, the rate constants have been estimated for the adsorption-desorption and aggregation-disaggregation processes. However, the estimates of Nozaki et al. (1987) are dependent upon unreliable surface 230Th concentration in particles and hence the calculated values for the rate constants are highly uncertain. In this paper, another approach is described that the rate constants can be estimated without using the surface particulate 230Th value in the presence of 234Th data. Recalculation based on this approach and published 234Th data suggests that suspended small particles form aggregates on the time scale of 10 months and that the large aggregates with sinking speed higher than 100 m/day disintegrate within one week.
