Global ocean atmospheric deposition

Figure 3 Flux estimates of the global cadmium cycle. All fluxes are x 10 mol year . Emissions to the atmosphere are from Ref. [2], mining/smelting flux also from Ref. [16]. Atmospheric deposition and net river input (gross— loss to estuaries) to the ocean are from Ref. [18]. Loss of oceanic cadmium to marine sediments is scaled up from a cadmium accumulation rate of 0.006 nmol cm year for the Pacific from Ref. [18] and is highly uncertain. (A mass balance is not expected since the atmospheric and riverine inputs include anthropogenic increases that are recent compared to the residence time of Cd in the oceans.) Atmospheric deposition to land was calculated based on the steady-state assumption that emissions to the atmosphere equal losses to the ocean and land.

The oceans at this time can be thought of as the solution resulting from an acid leach of basaltic rocks, and because the neutralization of the volatile acid gases was not restricted primarily to land areas as it is today, much of this alteration may have occurred by submarine processes. The atmosphere at the time was oxygen deficient anaerobic depositional environments with internal CO2 pressures of about 10-2-5 atmospheres were prevalent, and the atmosphere itself may have had a CO2 pressure near lO-25 atmospheres. If so, the pH of early ocean water was lower than that of modern seawater, the calcium concentration was higher, and early global ocean water was probably saturated with respect to amorphous silica (—120 ppm). Hydrogen peroxide may have been an important oxidant and formaldehyde, an important reductant in rain water at this time (Holland et al., 1986). Table 10.5 is one estimate of seawater composition at this time. 

Figure 4 Global atmospheric deposition of Nr to the oceans and continents of the Earth in 1993 (mg N m yr (sources F. J. Dentener, personal communication Lelieveld and Dentener, 2000).

Sea-to-air fluxes of major ions are caused by bubble bursting and breaking waves at the sea surface. These processes eject sea-salts into the atmosphere, the majority of which immediately fall back into the sea. Some of these salts are, however, transported over long distances in the atmosphere and contribute to the salts in riverwater (see Section 5.3). These airborne sea-salts are believed to have the same relative ionic composition as seawater and their flux out of the oceans is estimated by measuring the atmospheric deposition rates on the continents. In terms of global budgets, airborne sea-salts are an important removal process only for Na+ and Cl" from seawater removal of other major ions by this route is trivial. 

The average amount of annual precipitation over the global ocean surface is around 411 X 10 - L with mean content of mineral salts of lOmg/L. This gives the value of dissolved salts equal to 4.1 x 10 tons/yr. We can also assume that additionally about 20% from this sum would enter the ocean with dry deposition, i.e., 0.8 x 10 tons/yr. Totally, it would be 4.9 x 10 tons/yr., of which S accounts for 0.29 x 10 tons/yr. It seems reasonable that about 10% of this sum might be transported onto Earth s land. Therefore, about 0.31 x 10 tons/yr. is supplied to the atmosphere over the global ocean of this, 0.83 x 10 tons/yr. is precipitated into the ocean as sulfates and 0.03 X 10 tons S per year is transported to the terrestrial ecosystems. 

The annual continental river discharge of Ca ions to the ocean is 0.48 x tons. The similar value is discharged in suspended form, 0.47 x 10 tons/yr. In addition, 0.048 X 10 tons/yr. is windblown off the land into the ocean. The average calcium content in the precipitation over the ocean is 0.36 mg/L (Savenko, 1976). Therefore, the total annual deposition flux of Ca over the global ocean isO. I6x lO tons, including about 20% of dry deposition, 0.03 x 10 tons/yr. (Dobrovolsky, 1994). The total amount of Ca entering the atmosphere from the global ocean is about 0.20 x lO tons annually. About 0.02 x lO" tons/yr. is transported to terrestrial ecosystems, and the rest is returned to the ocean (Table 31). 

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