Porewater profiles

Dissimilatory Reduction in Surficial Sediments. Porewater profiles from a number of sites throughout Little Rock Lake show that sulfate is always depleted below the sediment-water interface (Figure 3). Sulfate depletion in porewaters occurs not onlyin the soft gyttja but also in sandy, littoral sites with organic contents < 10%. The observed depletion of sulfate and the occurrence of H2S indicate that the sediments are anoxic immediately below the sediment-water interface and that sulfate reduction occurs in surficial sediments. 

In most of the 30 porewater profiles examined between 1984 and 1987, sulfate concentrations decreased to < 20 peq/L within 5 cm of the sediment-water interface and remained relatively constant below this depth. These data indicate that sulfate reduction occurs primarily in the upper 5 cm, a contention which is supported by results from laboratory studies in which 35SO was added to intact sediment-water cores. We generally observed steeper sulfate gradients in summer than in winter, and hypothesize that winter gradients are not as steep because microbial activity is reduced. So far, however, we have not found a statistically significant relationship between temperature and sulfate flux. 

Down-core porewater profiles of Mn2+, Fe2+, and H2S in sediments from Aarhus Bay estuary (Denmark) show sharp gradients in the subsurface peaks of Mn2+ and Fe2+, indicating the reduction of Mn and Fe oxides in the upper 2 and 4 cm, respectively... 

Figure 14.14 Porewater profiles of dissolved Fe and Mn in suboxic sediments from the Gulf of Papua. (Modified from Alongi et al., 1996.)...

McGlathery, K. J., Berg, P., and Marino, R. (in review). Using porewater profiles to assess nutrient availability in seagrass-vegetated carbonate sediments. Biogeochemistry 56, 239—263. 

In the first example, porewater measurements at different locations on the eastern flank of the Juan de Fuca Ridge near 48° N in the North East Pacific Ocean indicate a decrease in Mg + with depth (Mottl and Wheat, 1994) (Fig. 2.14), indicating chemical removal. This result is a typical example of porewater profiles that have been... 

A schematic representation of the porewater profiles that have been observed to show the sequential use of electron acceptors during organic matter degradation. Modified from Froelich et al. (1979). 

The intense interplay between the redox coupling of iron and manganese and transport by animal activity has been demonstrated in the sediments of the eastern Skagerrak between Denmark and Norway (Wang and Van Cappellen, 1996). Sediment porewater profiles from this area (Fig. 12.6) indicate that most Mn(IV) reduction is coupled to oxidation of Fe(II) which was formed during organic matter and H2S... 

Porewater profiles of O2, Fe(ll), Mn(ll), NO3- 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). Redrawn from Wang and Van Cappellen (1996). 

Porewater profiles of 504 , CH4 and SCO2 from the sediments of Scan Bay, Alaska, an anoxic fjord. Note that CH4 and SOj concentrations do not overlap substantially. Redrawn from Reeburgh (1980). 

Porewater profiles of O2, Fe, Mn, U and Re in sediments along the continental margin of northwest North America. Notice the change in depth scale. Shaded areas Indicate the top 5 cm of the sediments. Stations 3b and 4 are on the continental slope and rise less than 100 km off the coast of Washington State at water depths of I 140 and 1960 m, respectively. Stations 6 and 8 are roughly 500 and 800 km from shore in the deep sea at depths of 2810 and 3870 m, respectively. Modified from Morford et 0/. (2005). 

Jahnke, R. (1985). A model of microenvironments in deep-sea sediments formation and effects on porewater profiles. Limnol. and Oceanogr. 30, 956-965. 

The difference between the O Neil et al. (1969) palaeotemperature equation and the Bemis et al. (1998) or Lynch-Stieglitz et al. (1999) palaeotemperature equations, and their respective calculations of isotopic equilibria for the infaunal and epifaunal species, could also be due to a carbonate ion effect within the sediment porewater profile. This appears plausible, because stable carbon isotopic compositions from benthic foraminifera show that infaunal taxa calcify in different porewater chemistry than epifaunal taxa (e.g. McCorkle et al. 1997). Since a 0.2 increase in pH (directly linked to the carbonate ion effect) results in a 8 0 depletion of c. 0.22%o (Zeebe 1999), the decrease in the pH of porewaters with sediment depth (e.g. Jahnke Jahnke 2004 Zhu et al. 2006) is likely to be reflected in the stable oxygen isotopic compositions of benthic for-aminiferal tests (Bemis et al. 1998), and one would expect the of infaunal taxa to be enriched... 

Fig. 5.13 Porewater profiles of dissolved inorganic carbon (DIC), sulfate and manganese (Mn ) in the pore water measured during ODP Leg 201 at Site 1226 in the eastern tropical Pacific Ocean. Data from D Hondt, Jorgensen, Miller et al. (2003).

The methane content of the porewater profiles shows increasing concentration from the surface to 20-30 cm depth and then remains nearly constant. For a surface water temperature and sediment water temperature of 30°C the saturation of methane occurs at approximately 1 mM, which is exceeded by many of the porewaters at depths below 25 cm. Presumably this supersaturation is required for ebullition to be significant. The decrease in methane toward shallower water is probably due to consumption in the oxygenated zone and escape by diffusion and ebullition from the porewater to the atmosphere. It is interesting to note that at site JC-3 (July 91), the sediment below 25 cm has more than twice as much methane as does the sediment below a 5. patens mat (Fig. 5a). Because the mat is not completely floating all the time, we suggest that when the mat is close to or contacts the sediment it disturbs the sediment sufficiently to cause methane release. Another possibility is that the plant stem and/or roots connection of the mat to the sediment causes disturbance and release of the trapped methane. 

EPA5 (1.6%OC) bed porewater profile EPA5 (1.6%OC) water column University Lake (2.9%OC) water column Peat soil (0.65%C) water column Peat soil (10%C) water column Peat soil (19%C) water column University Lake (3%OC)-l flux to water University Lake (3%OC)-2 flux to water Bayou Manchac (2.1 %OC) flux to water University Lake (4.3%OC) flux to water... 

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