Methane pore water profiles

S., Riedinger, N., Schulz, H.D. and Boetius, A., 2003. Control of sulfate pore-water profiles by sedimentary events and the significance of anaerobic oxidation of methane for burial of sulfur in marine sediments. Geochimica et Cosmochimica Acta, 67 2631-2647. 

Fig. 15.6 Model results of the mutual decomposition of sulfate and methane as a 1 1-reaction in a diffusion controlled pore water profile. Modeling was performed according to the Press-F9-method using the standard software Excel . Details pertaining to the model and the calibration with data from a measured pore-water profile obtained from an upwelling area off Namibia (Niewohner et al. 1998) are discussed in the text.

Borowski W., Pauli C. K., and Ussier W., Ill (1996) Marine pore water sulfate profiles indicate methane flux from underlying gas hydrates. Geology 24, 655-658. 

The coincidence of maxima in the methane oxidation rate and the sulfate reduction rate in Saanich Inlet strongly suggests that the methane oxidizing agent was sulfate, either via direct reaction, or coupled indirectly through reactions with other substrates (Devol, 1983). A methane-sulfate coupled reaction diffusion model was developed to describe the inverse relationship commonly observed between methane and sulfate concentrations in the pore waters of anoxic marine sediments. When fit to data from Saanich Inlet (B.C., Canada) and Skan Bay (Alaska), the model not only reproduces the observed methane and sulfate pore water concentration profiles but also accurately predicts the methane oxidation and sulfate reduction rates. In Saanich Inlet sediments, from 23 to 40% of the downward sulfate flux is consumed in methane oxidation while in Skan Bay this value is only about 12%. 

Fig. 3.14 Concentration profiles of pore water from anoxic sediments obtained from an upwelling area off Namibia at a water depth of approximately 1300 m. The analysis of sulfide and methane was carried out in samples that were punched out with syringes from small and quickly sawed-out windows in the fresh sediment core. As for sulfide, these syringe-drawn samples were brought into an alkaline environment, whilst for methane analysis the samples were stored in head space vials for subsequent gas-chromatography analysis. The arrow points to a methane sample that originated from a sealed sediment core obtained by using a sample from the core catcher (after Niewohner et al. 1998).

In Figure 3.14 a sulfate reduction zone is documented by measured profiles of sulfate and methane in pore water. Estimate the diffusive methane flux [mol m a ] and use for the calculation the more reliable profile of sulfate concentrations. Use a temperature of 5°C and a sediment porosity of ([) = 0.7. 

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).

Niewohner et al. (1998) describe profiles of pore water which were obtained from upwelling areas off the shores of Namibia. In these pore-water samples, sulfate reacted with methane in a ratio of 1 1. A similar example has already been introduced in Chapter 3 (Figure 3.31, also cf. Chapter 5), in context with problems occurring in methane analytics. In this example, the measured values will now be presented in Figure 15.6 together with the result of a model designed according to the Press-F9- method . 

Fig. 15.10 Model calculation of sulfate in pore water, alkalinity and methane in an explicit numeric solution of Pick s Second Law, accounting for the reaction between sulfate and methane and the alkalinty which is affected thereby. The adjustment to the measured profiles depends on the time passed since the slide occurrence and thus permits the reconstruction of the time of its occurrence which took place about 300 years ago.

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