Diffusive flux sulfate

Measurements of S cycling in Little Rock Lake, Wisconsin, and Lake Sempach, Switzerland, are used together with literature data to show the major factors regulating S retention and speciation in sediments. Retention of S in sediments is controlled by rates of seston (planktonic S) deposition, sulfate diffusion, and S recycling. Data from 80 lakes suggest that seston deposition is the major source of sedimentary S for approximately 50% of the lakes sulfate diffusion and subsequent reduction dominate in the remainder. Concentrations of sulfate in lake water and carbon deposition rates are important controls on diffusive fluxes. Diffusive fluxes are much lower than rates of sulfate reduction, however. Rates of sulfate reduction in many lakes appear to be limited by rates of sulfide oxidation. Much sulfide oxidation occurs anaerobically, but the pathways and electron acceptors remain unknown. The intrasediment cycle of sulfate reduction and sulfide oxidation is rapid relative to rates of S accumulation in sediments. Concentrations and speciation of sulfur in sediments are shown to be sensitive indicators of paleolimnological conditions of salinity, aeration, and eutrophication. 

Lake Sulfate Reductiona Diffusive Flux Rate of S Accumulation References... 

A major factor governing diffusive fluxes of sulfate into sediments is lake sulfate concentration. A linear relationship exists between lake sulfate concentrations and diffusive fluxes calculated from pore-water profiles (Figure 5). The relationship extends over a range of 3 orders of magnitude in sulfate... 

Figure 5. Data from the literature (56, 80, 99, 164, 195, 220, 222, 223, 243) indicate that diffusive fluxes of sulfate (calculated from 40 pore-water profiles measured with pore-water equilibrators) are linearly related to concentrations of sulfate in the overlying lake water. The correlation is significant (p < 0.05) both with (r2 = 0.991) and without (r2 = 0.42) the two lakes with high sulfate concentrations. The strong correlation suggests that variations in the depth interval within which sulfate is consumed and in the minimum sulfate concentration defining the gradient are relatively unimportant in determining the flux, compared to variations in sulfate concentrations defining the upper end...

Despite the strong relationship between sulfate concentrations and diffusive fluxes, there is no universal relationship between lake sulfate concentrations and concentrations of S in sediments (Figure 1A cf. 24, 26). Concentrations of S in sediments are the net result of inputs from seston, diffusive inputs, recycling to the water, and dilution by other materials. Mathematically this quantity may be expressed as... 

Total S content cannot indicate whether increased carbon inputs to sediments cause increased diffusion of sulfate into sediments or restrict reoxidation and release of S from sediments, because the net effect is the same. In a survey of 14 lakes, Rudd et al. (80) did not observe a strong correlation between organic matter content per volume and net diffusive flux of sulfate. However, in English lakes the lowest C S ratios occur in the most productive lakes (24) whether this represents enhanced influx or retarded release is not clear. Among 11 Swiss lakes, ratios of C to S sedimentation rates are relatively constant and substantially below C S ratios in seston net S fluxes... 

Seston-S deposition probably is a more important process than dissimilatory reduction in lakes with low [SO42 ]. As lakewater sulfate concentrations increase, seston deposition reaches a plateau limited by the overall primary production rate and the maximum algal S content, but diffusive fluxes continue to increase in direct proportion to [SO42 ]. Thus, in highly acidic lakes (pH 3 5 [SOjt2 J > 100 peq/L), such as McCloud Lake, Florida and Lake 223, Ontario, dissimilatory sulfate reduction probably is the major sulfate sink. Nriagu and Soon (131 concluded that endproducts of dissimilatory reduction and elevated sediment S content would not be observed below S mg/L (240 / eq/L), but we see clear evidence of dissimilatory reduction in Little Rock Lake at concentrations of approximately SO /teq/L. 

In-lake processes remove approximately half of the sulfate inputs from the water column of Little Rock Lake. Two processes, seston deposition and dissimilatory reduction, are responsible for sulfate retention. For the preacidified lake, seston deposition probably is the dominant sink, accounting for 70% of net retention. Preliminary data and theoretical considerations suggest that the diffusive flux of sulfate to sediments will increase during experimental acidification, and we believe that dissimilatoty reduction is the dominant sulfate sink in lakes with elevated sulfate concentrations. 

The profile of Mg2+ in Figure 8.25 indicates downward diffusion of this constituent into the sediments. Mass balance calculations show that sufficient Mg2+ can diffuse into the sediments to account for the mass of organogenic dolomite formed in DSDP sediments (Baker and Bums, 1985 Compton and Siever, 1986). In areas of slow sedimentation rates, the diffusive flux of Mg2+ is high, and the pore waters have long residence times. Dolomites form under these conditions in the zone of sulfate reduction, are depleted in 13c, and have low trace element contents. With more rapid sedimentation rates, shallowly-buried sediments have shorter residence times, and dolomites with depleted 13C formed in the sulfate-reduction zone pass quickly into the underlying zone of methanogenesis. In this zone the DIC is enriched in 13C because of the overall reaction... 

Various workers have estimated the rate of pyrite formation. Berner (1972) summed the sulfur accumulation rates of various sediment types in proportion to their areal coverage and found a flux of about 10% of the river flux. Li (1981) carried out a similar calculation and finds 30% of the river flux, probably indicative of the uncertainty of the approach. Toth and Lerman (1977) established that the decrease of sulfate with depth in sediment pore waters is a function of sedimentation rate. This information was used to estimate the diffusive flux of sulfur into sediments driven by pyrite formation, again a value about 10% of the river flux. Apparently, pyrite... 

Fig. 3.6 Sulfate profile in pore water from sediments of the Amazon deep sea fan at a water depth of about 3500 m. A linear concentration gradient can be distinctly derived from the sediment surface down to a depth of about 5.4 m. The gradient change, and thus a change in the diffusive flux, is strongly limited to a depth interval of at the most 10 to 20 cm (after Schulz et al. 1994).

Table 8.2 Role of methane as a carbon source for sulfate reduction in marine sediments. The compiled data show cumulative sulfate reduction rates measured by radiotracer technique, either over the entire sulfate zone, or in the upper 0-15 cm combined with modeling below that depth. The contribution of methane was calculated from the diffusion flux of methane up into the lower sulfate zone. In other data sets where sulfate reduction rates are determined only by modeling, or where also methane oxidation was measured by radiotracer technique, the calculated % of SRR from CH is higher than shown here. (SRR = sulfate reduction rate). ...

Estimated Diffusive Flux of Ammonium, Phosphate, and Sulfate as Influenced by Soil or Sediment Porosity... 

The estimated diffusive flux values for soils with the range of porosities are shown in Table 14.2. The molecular diffusion of ammonium, phosphate, and sulfate is affected by soil porosity, with greater flux observed in soils of high porosity and low tortuosity. 

The greatest reduction rate of sulfate occurs in surface layer sediment interstitial waters, and it is regulated by the content of organic matter in sediment and influenced by the season. S with -2 state mainly results from the diffusion from sedimentary interstitial waters to overlying waters, the diffusing flux of HS in the Huanghe River Estuary regions reached 8.95 i,mol/(m -d) (Qiu et al., 1999). 

Tg year x. Lein and Ivanov [71] have estimated the total sulfide burial in the Black Sea of 2.4 Tgyear 1 including about 1 Tgyear-1 that is buried in the anoxic zone. Using these data and integrated over the upper 20 cm of sediment sulfate reduction rates, Neretin and co-authors [75] concluded that the annual sulfide flux into the water column from sediments of the anoxic zone is between 3 and 5 Tgyear x. The value is likely to be overestimated due to spatial differences in pyrite burial rates and possible sulfide diffusion downward into the deeper sediment layers. 

For sulfate diffusion, this overall flux may be separated into two distinct terms, one for sulfate and one for sulfate (Equations (18a) and (18b)) ... 

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