Sulfate concentration ocean

As a specific example, consider oceanic sulfate as the reservoir. Its main source is river runoff (pre-industrial value 100 Tg S/yr) and the sink is probably incorporation into the lithosphere by hydrogeothermal circulation in mid-ocean ridges (100 Tg S/yr, McDuff and Morel, 1980). This is discussed more fully in Chapter 13. The content of sulfate in the oceans is about 1.3 X lO TgS. If we make the (im-realistic) assumption that the present runoff, which due to man-made activities has increased to 200 Tg S/yr, would continue indefinitely, how fast would the sulfate concentration in the ocean adjust to a new equilibrium value The time scale characterizing the adjustment would be To 1.3 X 10 Tg/(10 Tg/yr) 10 years and the new equilibrium concentration eventually approached would be twice the original value. A more detailed treatment of a similar problem can be found in Southam and Hay (1976). 

The case of bacterial reduction of sulfate to sulfide described by Berner (1984) provides a useful example. The dependence of sulfate reduction on sulfate concentration is shown in Fig. 5-4. Here we see that for [SO ] < 5 mM the rate is a linear function of sulfate concentration but for [SO4 ] > 10 itiM the rate is reasonably independent of sulfate concentration. The sulfate concentration in the ocean is about 28 mM and thus in shallow marine sediments the reduction rate does not depend on sulfate concentration. (The rate does depend on the concentration of organisms and the concentration of other necessary reactants - organic carbon in this case.) In freshwaters the sulfate concentration is... 

Only a few evaporites have been found that are more than 800 miUion years old, indicating that most of the salt formed prior to this period has been recycled via uplift and weathering. No evaporites of Archean age have as yet been discovered. The oldest known chemical sediments were deposited 3.45 bybp in what is now western Australia. They appear to have precipitated as shallow-water carbonates. This suggests that sulfate concentrations during the Archean were much lower than present day, probably because of limited oxygenation of the atmosphere and ocean. 

Miller, J. M. Tellus. in press) have examined the transport of North American sulfur emissions across the north Atlantic Ocean to Europe. In a review of available precipitation-sulfate data from the north Atlantic and adjacent coastal regions, they report a concentration field consistent with known source distributions and meteorological factors. The excess sulfate concentration of marine background... 

Rubey (1950) formulated questions concerning the history of the ocean, and Holland (1972) has attempted to solve the problem of ocean development, which is fundamental. Conway (1942,1943,1945) discussed whether the ocean had from its very beginning in the Precambrian the same volume and composition as today. Holland (1972) examined the geological records provided by evaporite deposits and affirmed that the composition of the ocean never was fundamentally different. Holland s calculations set definite limits for variations in pH, C02 pressure and related CO3 and HCO3 concentration, and Mg++, Ca++ and sulfate concentrations and ratios to each other, but do not give the total amount of sea water present at each geological age. 

Using sulfur-35. Sulfur-35 ( S) is a naturally produced radioactive tracer (half-life = 87 d) that can be used to trace the movement of atmospherically derived sulfate in the environment. It is formed in the atmosphere from cosmic ray spallation of " °Ar (Peters, 1959), and deposits on the Earth s surface in precipitation or as dryfall. It can be used both to trace the timescales for movement of atmospheric sulfate through the hydrosphere and, in ideal cases, to trace the movement of young (<1 yr) water. It is an especially useful tracer in regions away from the ocean where sulfate concentrations are relatively low. 

Sulfur isotope studies have also provided insights into the transition from Archean low Po to higher values in the Proterozoic. In the same studies that revealed extremely low Archean ocean sulfate concentrations, it was found that by —2.2 Ga, isotopic compositions of sedimentary sulfates and sulfides indicate bacterial sulfate reduction under more elevated seawater sulfate concentrations compared with the sulfate-poor Archean (Habicht et al., 2002 Canfield et al., 2000). As described above, nitrogen isotope ratios in sedimentary kerogens show a large and permanent shift at —2.0 Ga, consistent with denitrification, significant seawater nitrate concentrations, and thus available atmospheric O2. 

The magnitude of the various processes varies with locality. Over vegetated continental areas typically a fifth of the sulfur is dry-deposited as SO2. In areas of high SO2 concentrations much higher amounts can be deposited. Where sulfate concentrations in particulate material are high, dry deposition rates can be greater. The balance of wet and dry deposition of sulfate particles over the ocean is uncertain, while some authors suggest dry deposition dominates others favor wet deposition (Wameck, 1999). 

X 10 Tg S. If we make the (unrealistic) assumption that the present run-off (200 Tg S/year) would continue indefinitely, how long would it take for the sulfate concentration in the ocean to adjust to this new flux The answer is ro 1.3 x 10 Tg/10 Tg/ year 10 years. A more detailed treatment of a similar problem can be found in Southam and Hay (1976). 

Sulfate reduction rates in marine shelf sediments commonly lie in the range of 1-100 nmol cm May (Jorgensen 1982). Since the sulfate concentration in the pore water is around 28 mM or 20 pmol cm the turn-over time of the sulfate pool is in the order of 1-50 years. A purely chemical experiment would thus require a month to several years of incubation. This clearly illustrates why a radiotracer technique is required to measure the rate within several hours. In sediment cores from the open eastern equatorial Pacific obtained by the Ocean Drilling Program it has been possible to push the detectability of this radiotracer method to its physical limit by measuring sulfate reduction rates of <0.001 nmol cm d in 9 million year old sediments at 300 m below the seafloor (Parkes et al. 2005). 

Later the concentration of sulfate gradually increased in the seawater. Initially its concentration was too low to form anhydrite and all the sulfate entering the hydrothermal system got reduced. This formed a stabilizing buffer on the sulfate concentration in seawater because the sulfate flux into the oceanic crust was proportional to its concentration. 

Assuming that in oceanic air sulfate concentrations can range from 0.040 to 1.0 ixg m and cloud liquid water contents range from 0.10 to 2.5 g m , calculate the range of pH values of cloud water over the oceans. Assume that the sulfate is H2SO4 and neglect other effects on the acidity. Do you think that the upper pH value you have calculated would be achieved in nature If not, why ... 

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