Sulfate methane

The biogenic methane is generated from anaerobic degradation, accompanied by a sulfate-methane interface (SMI), which can be used to determine the upper boundary of hydrate formation depth (Pauli et al., 2005). 

Figure 7.6 Samples from ODP Leg 164 Hole 994 at Blake Bahama Ridge, showing the sub-bottom reduction in sulfate until the depth of the Sulfate-Methane Interface (SMI), and the increase in methane concentrations below the SMI. (Pauli, Personal Communication, October 25, 2001.)...

Iversen N. and J0rgensen B. B. (1985) Anaerobic methane oxidation rates at the sulfate-methane transition in marine sediments from Kattegat and Skagerrak (Denmark). Litnnol. Oceanogr. 30, 944-955. 

Sulfur reaches the sediment essentially only by means of diagenetic processes. Here, the fixation of sulfate-bound sulfur in the form of barite in the sulfate-methane transition (SMT) zone produces an interesting signal, but is quantitatively irrelevant on account of the low barium concentrations in the pore water. Sulfur precipitates in much greater amounts in shape of sulfide (Mackinawite FeS, pyrite FeS ) wherever sulfide resulting from the reduction of sulfate comes into contact with divalent iron by means of diffusion. In the core shown in Figure 3.29 this obviously proceeded at a depth of about 7 m below the sediment surface over a relatively long period of time. 

Fig. 8.5 Profiles of pore-water sulfate and methane concentrations and of rates of sulfate reduction and methane oxidation for a sediment core recovered from the Kattegat (Station B 65 m water depth). The broken horizontal line denotes the depth where sulfate and methane were at equimolar concentrations - indicating the peak of the sulfate/methane transition. From Iversen and Jorgensen (1985).

Fig. 8.6 Pore-water concentration profiles from gravity core GeoB 3714-9 from the Benguela upwelling area (2060 m water depth), South Atlantic. The shaded bar marks the sulfate-methane transition zone. The methane sample labeled C.C. was taken from the core catcher immediately after core recovery. From Niewohner et al. (1998).

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).

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).

Figure 15.10 demonstrates in its left side the initial situation, hence the situation immediately after the slide in terms of sulfate and methane concentration profiles and alkalinity. Underneath the slide, i.e at the former sediment surface, a steady-state condition with a constant gradient reaching down to the sulfate-methane transition... 

Example 3 also illustrates the modeling of a sulfate profile really measured, including a sulfate-methane transition zone in the deep part of the profile. However, in this case the profile displayed such an unusual course that its interpretation as a steady-state condition did not appear justified by any means. As a consequence, this profile rested among many other measurement data for almost ten years. Only after we understood that nonsteady-state conditions in marine sediment profiles - particularly in the continental slope region -are much more frequent than previously assumed were we able to understand this profile as well (Hensen et al. 2003). 

Starting out from this profile of initial concentrations, we then only introduced the time-correct diffusion in the model calculation, along with a correct diffusion coefficient, and the mutual degradation of sulfate and methane at the sulfate-methane transition zone. 

First, the minor, randomly occurring differences between adjoining points were smoothed out in the course of model calculation. At the same time, the upper part of the profile adapted more and more to the ocean water transition, while the lower part adapted to the low concentration of the sulfate-methane transition zone. After somewhat more than 20 years, an almost perfect adjustment to the measured profile was achieved without any other fitting technique (center diagram in Fig. 15.12). This core was obtained in 1994, and the sulfate profile was measured in the same year. The sediment avalanche consequently took place in the early 70s of the last century. The right diagram in Figure... 

A different sulfate concentration gradient is measured above the sulfate-methane transition zone than underneath the methane zone. How could this be explained, despite the fact that sulfate and methane react with each other in a ratio of 1 1 ... 

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