Sulfate zones

The above reactions start at the two surfaces of the plates and form two zones rich in PbS04, which grow towards the core of the plate (Fig. 3.3). Cured pastes are yellow in colour (basic lead sulfates and PbO are hydrated), while sulfated zones are grey. This difference in colouring allows easy determination of the growth rate of the sulfate zones towards the interior of the plate. The rate of movement of the reaction layer (FjuO into the bulk of the cured paste depends on the following parameters ... 

In marine sediments below the sulfate zone, methanogenesis is the predominant terminal pathway of organic carbon degradation. Methane may also be formed from acetate or from organic Cj-compounds such as methanol or methylamines ... 

In comparison to all other heterotrophs, the microorganisms oxidizing methane and other Cj compounds such as methanol, have a unique metabolic pathway which involves oxygenase enzymes and thus requires O. Only aerobic methane-oxidizing bacteria have been isolated and studied in laboratory culture, yet methane oxidation in marine sediments is known to take place mostly anaerobically at the transition to the sulfate zone. Microbial consortia that oxidize methane with sulfate have in particular been studied at methane seeps on the sea floor and the communities can now also be grown in the laboratory (Boetius et al. 2000 Orphan et al. 2001 Nauhaus et al. 2002) Anaerobic methane oxidation is catalyzed by archaea that use a key enzyme related to the coenzyme-M reductase of methanogens, to attack the methane molecule (Kruger et al. 2003 see Sect. 5.1). The best studied of these ANME (ANaerobic MEthane... 

Below the sulfate zone, however, there are no available electron acceptors left other than CO, and methane accumulates here as the main terminal product of organic matter degradation. 

Below the sulfate zone, methanogenesis is the main terminal pathway of organic carbon mineralization. Methane is produced exelusively by anaerobic archaea that utilize a narrow speetrum of substrates for the process (Whitman et al. 

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). ...

Fig. 8.8 Schematic profiles of sulfate and methane in marine sediments. A) In sediments with low methane flux, sulfate reduction based on oxidation of sediment organic matter predominates throughout the sulfate zone. B) In sediments with high methane flux, sulfate reduction based on anaerobic oxidation of methane tends to straighten out the sulfate profile.

Fig. 8.10 Geochemical data for core GeoB 1023-4 recovered off north Angola (17°09.6 S, 10°59.9 E, 2047 m water depth). Barium and sulfate pore-water concentration profiles as well as the distribution of solid-phase barium indicate the precipitation of authigenic barite at a front slightly above the depth of complete sulfate consumption. Below the sulfate/methane transition barite becomes undersaturated and is thus subject to dissolution due to the total depletion of pore-water sulfate. Dissolved barium diffuses upwards into the sulfate zone where the mineral barite becomes supersaturated and so-called authigenic or diagenetic barite precipitates at a front at the base of the sulfate zone. Modified from Gingele et al. (1999), after Kolling (1991).

These processes are illustrated in Fig. 8.16. Sulfate that penetrates down into the sediment from the overlying sea water is reduced to H S by sulfate reducing bacteria that use the deposited organic material as their energy source. Also methane diffusing up from below feeds sulfate reduction in the lower sulfate zone. At depth in... 

By a relatively thin oxic and suboxic zone, e g. in organic-rich sediments or in oxygen-deficient enviromnents, relatively more organic material is buried down into the sulfate zone. Bioturbation enhances the burial of relatively fresh organic material into the sulfate reduction zone. At the same time, however, bioturbation and bioirrigation deepen the oxic-suboxic zones and thereby push down the zone of sulfate reduction. 

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