Hydroxides , residence times ocean

Thorium generally exists as a neutral hydroxide species in the oceans and is highly insoluble. Its behavior is dominated by a tendency to become incorporated in colloids and/or adhere to the surfaces of existing particles (Cochran 1992). Because ocean particles settle from the water column on the timescale of years, Th isotopes are removed rapidly and have an average residence time of = 20 years (Fig. 1). This insoluble behavior has led to the common assertion that Th is always immobile in aqueous conditions. While this is generally true in seawater, there are examples of Th being complexed as a carbonate (e.g.. Mono Lake waters, Anderson et al. 1982 Simpson et al. 1982) in which form it is soluble. 

Neutral hydroxide Si(OH)4 is predominant in the natural water, the content of anion Si(0H)30 is in a lesser degree. The continental river water discharge is responsible for 0.2 x 10 tons of soluble silicon species. The mass of Si compounds in the ocean is 4, 110 x 1tons, and the residence time of Si in the marine waters is 20,550 years. The transport of silicon from terrestrial to oceanic ecosystems is not counterbalanced by the reverse transport. In addition to the soluble species, the content of silicon in river particulate matter is about 120 /rg/L. This gives the elemental transport of 4.8 x 10 tons/yr. The total estimate of river water fluxes from the global land area to the ocean is 5.0 x 10 tons/yr. Aeolian migration of silicon is responsible for 0.47 X 10 tons per year. It means the annual global land losses (river and wind fluxes) are 5.47 x 10 tons (Dobrovolsky, 1994). 

Table 1 Summary of the hydroxide speciation, concentrations, and residence times for the refractory elements in the oceans...

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. 

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