Pore water profiles

As we saw with the steady-state water-column application of the one-dimensional advection-diffusion-reaction equation (Eq. 4.14), the basic shapes of the vertical concentration profiles can be predicted from the relative rates of the chemical and physical processes. Figure 4.21 provided examples of profiles that exhibit curvatures whose shapes reflected differences in the direction and relative rates of these processes. Some generalized scenarios for sedimentary pore water profiles are presented in Figure 12.7 for the most commonly observed shapes. 

However, in contrast to microbiological experiments and near-surface studies, modelling of sulfate reduction in pore water profiles with in the ODP program has demonstrated that natural populations are able to fractionate S-isotopes by up to more than 70%c (Wortmann et al. 2001 Rudnicki et al. 2001). Brunner et al. (2005) suggested that S isotope fractionations of around -70%c might occur under hyper-sulfidic, substrate limited, but nonlimited supply of sulfate, conditions without the need of alternate pathways involving the oxidative sulfur cycle. 

Numerous measurements of pore-water chemistry have been made in LRL throughout the experiment (4, 17, 59). Typical vertical pore-water profiles (Figure 6) indicate that the sediments are acting as sinks for sulfate... 

Figure 6. Sediment pore-water profiles for various IAG-related parameters in treatment basin (T, solid line) and reference basin (R, dashed line), 5-m sites,...

The pore-water profiles also indicate a possible treatment effect. In the treatment basin before acidification to pH 4.7, much higher levels of ANC (alkalinity) and higher pH were found in the pore water just 1-2 cm below the sediment-water interface (59). In contrast, pore-water pH profiles obtained in the same site in the summers of 1990 and 1991 show pH < 5.0 in the upper 5-10 cm of sediment. Corresponding profiles for a site in the reference basin did not show such a depression (Figure 6a ref. 4). 

Occurrence and Rates of Sulfate Reduction. Sulfate reduction is widespread in lakes, as evidenced by depletion of sulfate in sediment pore waters. Pore-water profiles showing depletion of sulfate have been published for more than 35 lakes (e.g., 98, 99). An absence of sulfate depletion in pore waters does not indicate an absence of sulfate reduction. Sulfate depletion was not evident in pore waters of McNearney Lake, but stable isotope measurements indicated that low rates of sulfate reduction must occur (1). Sulfate depletion was noted in 15 of 17 lakes in northeastern North America and Norway (80, 98), but uptake of 35S042 occurred even in two lakes in which no sulfate depletion was observed. Sulfate production and reduction can occur concurrently, and the former may exceed the latter. 

Figure 4. A, Pore-water profiles (October 15, 1990) in Lake Sempach typically indicate that sulfate is consumed within the upper 3 cm. B, Diffusion rates for sulfate calculated from the profile in panel A (T = 5°C) indicate that the rate is maximal just below the interface, but all rates are less than 2 nmol/cm2 per hour. C, Sulfate reduction rates measured with 15S in intact cores on the same date are 2 orders of magnitude greater and do not exhibit the same depth profile as diffusion rates. Error bars indicate the standard deviation among 10-15 replicates. The sulfate profile in panel A was measured by centrifuging pore water from cores identical to those in which sulfate reduction was measured. D, The 35S measurements indicate that 50% of the areal sulfate reduction occurs below 2-cm depth and 25% occurs below 5-cm depth. The pore-water profile indicates that negligible...

A major factor governing diffusive fluxes of sulfate into sediments is lake sulfate concentration. A linear relationship exists between lake sulfate concentrations and diffusive fluxes calculated from pore-water profiles (Figure 5). The relationship extends over a range of 3 orders of magnitude in sulfate... 

Figure 5. Data from the literature (56, 80, 99, 164, 195, 220, 222, 223, 243) indicate that diffusive fluxes of sulfate (calculated from 40 pore-water profiles measured with pore-water equilibrators) are linearly related to concentrations of sulfate in the overlying lake water. The correlation is significant (p < 0.05) both with (r2 = 0.991) and without (r2 = 0.42) the two lakes with high sulfate concentrations. The strong correlation suggests that variations in the depth interval within which sulfate is consumed and in the minimum sulfate concentration defining the gradient are relatively unimportant in determining the flux, compared to variations in sulfate concentrations defining the upper end...

Pore-water profiles are frequently interpreted according to this concept. For example, White et ah (35) described a conceptual model of biogeo-chemical processes of sediments in an acidic lake (cf. Figure 4). They discussed the numbered points in Figure 4 as follows Diffusion of dissolved oxygen across the sediment-water interface leads to oxidation of ferrous iron and to an enrichment of ferric oxide (point 1). Bacterial reductive dissolution of the ferric oxides in the deeper zones releases ferrous iron (point 2). The decrease in sulfate concentration stems from sulfate reduction, which produces H2S to react with ferrous iron to form mostly pyrite in the zone below the ferric oxide accumulation (point 3). 

Diagenesis of Microbially Reduced Sulfur. Postdepositional transformations play an important role in controlling the extent of recycling of microbially reduced S. Pore water profiles from many freshwater systems clearly show that H2S is a short-lived intermediate in sulfate reduction which does not accumulate in sediments (14.16 41-431. However, the conventional paradigm for sulfur diagenesis, in which H2S is initially immobilized by iron monosulfides that later are diagenetically altered to pyrite and elemental S (e.g., 2Q)> does not apply to all freshwater systems. Instead, organic S and CRS (chromium reducible S, which is believed to represent pyrite + S° after preliminary acid distillation to remove AVS), are important initial endproducts of dissimilatoiy reduction. 

Leermakers, M., Y. Gao, C. Gabeille, et al. 2005a. Determination of high resolution pore water profiles of trace metals in sediments of the Rupel River (Belgium) using DET (diffusive equilibrium in thin films) and DGT (diffusive gradients in thin films) techniques. Water Air Soil Pollut. 166 265-286. 

The pore water profiles, solid phase chemistry, and pyrite petrography in three closely spaced cores from salt marsh subenvironments show complex trends and rapid variations. 

Figure 6.12 Pore-water profiles of oxygen, nitrate and ammonium along a zonal section off the Washington State continental margin at approximatly 47°N latitude extending 650 km offshore. Data from Hartnett and Devol (2003) and Emerson and Devol unpublished.

Maris C. R. P. and Bender M. L. (1982) Upwelfing of hydrothermal solutions through ridge flank sediments shown by pore-water profiles. Science 216, 623—626. 

Figure 5 Pore-water profiles of O2, Fe(II), Mn(II), NO, and NH from sediments of the near-shore waters of Denmark. Symbols represent data from Canfield et al. (1993) and lines are model results from Wang and Van Cappellen (1996) (after Wang and Van Cappellen, 1996) (reproduced by permission of Elsevier from Geochim.

Figure 7 Pore-water profiles of SO4, CH4, and DIG from the sediments of Scan Bay Alaska an anoxic fjord (after Reeburgh, 1980) (reproduced by permission of Elsevier from Earth Planet. Sci. 1980, 47, 345-352).

Dan McCorkle of Woods Hole Oceanographic Institution conceived of yet another way to confirm that pore-water respiration CO2 was largely neutralized by reaction with CaCOj. As summarized in Figure 7, he made ratio measurements on XCO2 from pore-water profiles and found that the trend of with... 

Perhaps the most desirable method of obtaining concentration data for pore-water profiles is that of an in situ measurement since all sampling methods appear to suffer from some artifacts. The number of pore-water constituents that has been measured in situ in deep-sea sediments is quite... 

Jo and j2 have units of pmol cmp yr while ji and j3 are expressed in 1/cmsed- For O2, P(x) has the same form and, in principle, jo and j 2 for O2 are related to the similar quantities for NO through the stoichiometric relationship described above. Martin and Sayles (2003) applied this equation to a set of 60 pore-water O2 and NO profiles, obtained by in situ and shipboard sampling methods, from three regions in the equatorial Atlantic (Ceara Rise, Cape Verde Plateau, and Sierra Leone Rise Martin and Sayles, 1996 Hales and Emerson, 1996 Sayles and Martin, unpublished data), the central equatorial Pacific (6°S-10°N, 130° W Martinet of., 1991), and the AESOPS transect in the Southern Ocean (66° S, 170° W to 56° S, 170° W Sayles et al, 2001). The sites considered span water column depths from 3,100 m to 5,200 m and total sedimentary organic carbon oxidation rates of 3-50 pmol cm yr Examples of the fits of the model above to pore-water profiles, used to define values for jo, jj, jj, and j3, were shown in Figures 3 and 4. 

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