Oceanic elemental scavenging

Brewer, P. G., Hao, W. M., Oceanic Elemental Scavenging, In Chemical Modeling in Aqueous Systems, American Chemical Society, 261-274 (1979). 

Table 11.3 Ocean. Deepwater Scavenging Residence Times of Some Trace Elements in the ...

In the ocean, elements that form insoluble hydroxides have relatively short residence times (e.g., A1 and Fe have residence times in the ocean of 100 and 200 years, respectively). Cations, such as Na (aq) and K (aq), and anions, such as Cl (aq) and Br (aq), have longer residence times in the ocean ( 7 x 10 to 10 years). In the atmosphere, the very stable gas nitrogen has a residence time of a million years or so, while oxygen has a residence time of 5,000-10,000 years. Sulfur dioxide, water, and carbon dioxide, on the other hand, have residence times in the atmosphere of only a few days, 10 days, and 4 years, respectively. Of course, residence times may be determined by physical removal processes (e.g., scavenging by precipitation) as well as chemical. 

For a radionuclide to be an effective oceanic tracer, various criteria that link the tracer to a specihc process or element must be met. Foremost, the environmental behavior of the tracer must closely match that of the target constituent. Particle affinity, or the scavenging capability of a radionuclide to an organic or inorganic surface site i.e. distribution coefficient, Kf, is one such vital characteristic. The half-life of a tracer is another characteristic that must also coincide well with the timescale of interest. This section provides a brief review of the role of various surface sites in relation to chemical scavenging and tracer applications. 

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. 

Th, Co, and, in some locations, Fe. Surfece-water enrichments are usually caused by rapid rates of supply to the mixed layer via atmospheric deposition or river runoff. Removal usually occurs through relatively rapid precipitation into or adsorption onto sinking particles. Trace elements controlled by scavenging tend to have short (100 to lOOOy) residence times. Since these residence times are less than the mixing time of the ocean, significant geographic gradients are common. 

The influence of river water inputs on trace metal distributions is illustrated in Figure 11.17c, which shows that the surface-water concentration of dissolved Mn in the Pacific Ocean decreases with increasing distance from the California coast. The vertical profile measured in the coastal zone (Figme 11.17b) exhibits a strong surface enrichment characteristic of scavenged trace elements. A similar vertical gradient is seen in the... 

Within the ocean, the exchange of material from the dissolved to the suspended particulate state influences the distribution of several elements. This scavenging process removes dissolved metals from solution and accelerates their deposition. The effectiveness of this process is obvious in the depth profiles of metals, especially those of the surface enrichment type. Furthermore, the removal can be expressed in terms of a deepwater scavenging residence time as indicated in Table 10. 

Table 10 The deepwater scavenging residence times of some trace elements in the oceans...

Some trace metals, such as iron and copper, have distributions that are strongly influenced by both recycling and relatively intense scavenging processes. Like nutrient-type elements, dissolved iron is observed to be depleted in remote oceanic surface waters such as high-nutrient, low-chlorophyll... 

German C. R., Klinkhammer G. P., Edmond J. M., Mitra A., and Elderfield H. (1990) Hydrothermal scavenging of rare earth elements in the ocean. Nature 316, 516-518. 

Particles represent important agents of transport in global ocean cycles of many trace elements, of carbon, and of other substances. Once introduced into the oceans, many trace elements are removed from seawater by scavenging (sorption, com-plexation, and other forms of surface reactions) to particles (Goldberg, 1954 Turekian, 1977). Scavenging and burial in marine sediments represents the principal loss process influencing the biogeochemical cycle of many trace elements in the ocean (Li, 1981). 

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