Sulfate near sediment-water interface

There are three primary processes that have been identified that can cause undersaturation, in addition to the previously discussed undersaturation that may result during the early stages of sulfate reduction. These are early post-death microenvironments within organisms, oxidation of organic matter by processes preceding sulfate reduction, and oxidation of sulfides. These processes commonly will be most important near the sediment-water interface. 

Other reactions of probable less importance than those above leading to undersaturated conditions with respect to calcium carbonate near the sediment-water interface include nitrate reduction and fermentation (e.g., Aller, 1980). Such reactions may also be important near the sediment-water interface of continental shelf and slope sediments, where bioturbation and bioirrigation can result in enhanced transport of reactants. Generally, as water depth increases over continental slope sediments, the depth within the sediment at which significant sulfate reduction commences also increases. It is probable that the influence of reactions other than sulfate reduction on carbonate chemistry may increase with increasing water depth. 

In sUiciclastic sediments, the major source of carbonates is also primarily derived from benthic organisms. These include bivalves, other mollusks, sea urchins, and foraminifera. In these sediments there is often a zone of considerable under saturation produced near the sediment-water interface, where the oxidation of organic matter and bacterially produced sulfides can result in almost complete dissolution of sediment carbonate. Carbonates from organisms that burrow beneath this zone of intense diagenetic activity are often well preserved. In organic-rich siliciclastic sediments sulfate reduction may be very extensive, with the increase in alkalinity outweighing the decrease in pH, resulting in the precipitation of calcium carbonate. 

Emerson (1976) carried out physical-chemical analyses of interstitial waters in cores of Greifensee in north-central Switzerland. The lake has an area of 8.6 km and a mean depth of 19 m. The lake has become increasingly eutrophic, the hypolimnion being anoxic for about six months of the year. Whereas the sulfate concentration in the water is 18 mg l, it is below 2 mg l i (the limit of detection) in the interstitial waters. The pS ranges from 12 near the bottom surface to 15 at a depth of 16 m (log Chs- from —6 to —9 at pH 7.05) and identifies a zone of active sulfate reduction near the water-sediment interface. This also causes a large gradient of ferrous iron with depth because of diagenesis of iron sulfide minerals. 

Fig. 14.22 Schematic illustration of gas hydrate deposits and biogeochemical reactions in near-surface sediments on southern Hydrate Ridge. High gradients in pore water sulfate and methane are typical of methane hydrate-rich environment close to sulfate-rich seawater. At the sulfate-methane interface (also named sulphate-methane transition in earlier chapters of the book) a microbial consortium of methanothrophic archaea and sulfate-reducing bacteria (Boetius et al. 2000) perform anaerobic oxidation of methane (AOM) leading to carbonate precipitation. AOM rates influence hydrogen sulfide fluxes and gradients, which are reflected on the seafloor by the distribution of vent communities around active gas seeps and gas hydrate exposures (Sahling et al. 2002).

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