Radionuclides scavenging/removal rates

Some of these particles eventually sink to the seafloor, thus removing metals from the ocean. This process of surface adsorption followed by settling is referred to as particle scavenging. The rate and degree to which a dissolved metal is scavenged from the ocean depends on (1) its elemental nature, (2) the abimdance of particulate matter, (3) the concentrations of other solutes that can compete fc>r adsorption sites, and (4) the depth of the water column. Metal scavenging rates have been inferred from the concentrations of naturally occurring radionuclides, such as " Th, Th, and Th. 

The removal rate of particle-reactive radionuclides on particles with a pore size of 0.4 xm or greater can be made using an empirically determined scavenging rate constant (Xs), which equals 1/residence time—where the residence time (t) of an element is defined as the ratio of the element s standing stock to the removal rate or supply. 

Recognizing that scavenging of dissolved radionuclide tracers is a reversible process was a milestone in understanding the removal of particle-reactive substances from the ocean. This principle must be taken into account in using natural radionuclides as tracers of particle transport, and when using natural radionuclides to infer the rates of processes responsible for scavenging particle-reactive substances (e.g., trace metals) from the ocean. 

In a study of sediment trap samples collected at a depth of 3,200 m in the Sargasso Sea, Bacon et al. (1985) found that the fluxes of several radionuclides exhibited a seasonal cycle in phase with the total mass flux of particles. Mass flux, in turn, was found to be closely coupled to the seasonal cycle of primary production in surface waters of this region (Deuser et al., 1981). Seasonally varying fluxes of radionuclides produced in the deep sea (e.g., Th, Pa, and °Pb) are inconsistent with the view that these nuclides are removed by scavenging to small particles which constitute the bulk of particle mass in the deep sea and which are inferred to sink at an average rate of several hundred meters per year (Section 6.09.3). A seasonal cycle in the flux of these nuclides implies that scavenging in the deep sea responds rapidly to changes in the export of particles from surface waters. 

---