Saanich Inlet pore waters

Table 8.5. Sulfate concentrations Cs°4 (mmol l 1) at depth z (cm) in pore waters from the Saanich Inlet, British Columbia (Murray et al., 1978).

Figure 8.23 Sulfate concentrations in pore waters as a function of the depth below the water-sediment interface of the Saanich Inlet Murray et al. (1978). The exponential curve supports the diffusional diagenetic model.

With suitable anoxic conditions and abundant organic matter, the pore waters in sediments may be totally depleted of sulfate within a depth of 1 to 2 m. In pore waters of Saanich Inlet, British Columbia, where organic carbon contents reached 5%, Nissenbaum et al. (1972) found essentially total sulfate depletion at depths of less than 0.5 m. This depth corresponds to a time span of hundreds of years. In contrast, some off-shore cores from the JOIDES Deep Sea Drilling Programme show that, in more slowly accumulating sediments, there is negligible sulfate depletion despite apparent continuation of sulfate reduction for millions of years, and to depths of several hundred meters. A number of these extreme, as well as intermediate cases are summarised by Goldhaber and Kaplan (1974). 

If metals, particularly iron, are not available to precipitate the biogenic sulfide, then dissolved sulfide builds up in the pore waters and may even reach toxic levels. When iron is present the dissolved sulfide is significantly lower in concentration. The concentration profiles for dissolved sulfide in sediments often show a depletion in the upper layers and a maximum at a depth of a meter or less. The depletion is interpreted by Goldhaber and Kaplan (1974) to reflect reactions between iron oxide and dissolved sulfide to yield iron sulfides. As a consequence of different reactivities of iron oxides to aqueous sulfide, a depth may be reached where the sulfide production rate exceeds removal as iron sulfide. Volkov et al. (1972) reported that, in sediments off the Japan Depression, the free hydrogen sulfide concentration reaches as high as 150 mg h which is roughly 50% higher than that found in the Black Sea and is comparable to the maximum concentration observed at Saanich Inlet, British Columbia, by Nissenbaum et al. (1972). The... 

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%. 

As described in sect. 3, small sample volumes (usually 10-50 ml) and low concentrations make it extremely difficult to measure the lanthanide concentrations of pore waters. Just as lanthanide concentrations in the water column increase several-fold under conditions of low or no oxygen, lanthanides in pore waters respond to redox conditions. There are a small number of studies of lanthanides in pore waters and they focus on sub-oxic and anoxic environments of estuaries and coastal regions. These include Chesapeake Bay (Sholkovitz and Elderfield 1988, Sholkovitz et al. 1992) Buzzards Bay (Elderfield and Sholkovitz 1987, Sholkovitz et al. 1989) and Saanich Inlet (German and Elderfield 1989). The first published paper on lanthanides in pore waters is less than a decade old (Elderfield and Sholkovitz 1987). All the above studies used ID-TIMS as the method of analysis. Pore water data are compiled in table A13. 

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