Sulfate diffusive fluxes into sediments

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... 

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... 

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... 

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. 

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... 

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). ...

It should be indicated at this point as well that the calculated diffusive sulfate flux from the bottom water into the sediment, and from there into a depth of about 5.4 m, is the unequivocal consequence of the profile shown in Figure 3.6. It also follows that this sulfate is degraded in the depth of 5.4 m within a depth interval of at the most 10 to 20 cm thickness. The calculated Corg amount that undergoes conversion again depends on the assumption made by Froelich et al. (1979) that indeed the whole of sulfate reacts with organic carbon. Several studies demonstrated that this must... 

The dC/dx values are 3, 0.3, and -3 jg cm for ammonium, phosphate, and sulfate, respectively. Positive flux indicates upward diffusion from soil or sediment to overlying water column, whereas negative flux is from water column into soil or sediment. 

Dissolved sulfate normally is added to the sediment column by two mechanisms (1) burial of sulfate-containing seawater with the sediment and (2) diffusion of dissolved sulfate from overlying seawater or sediments into sulfate-depleted pore waters (here, ignoring addition of sulfate by advection or bioturbation). The flux of sulfate due to burial of seawater is (Berner, 1980)... 

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