Oxygen pore water profiles

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). 

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 Data from Hartnett and Devol (2003) and Emerson and Devol unpublished.

Fignre 6.5 shows typical pore water profiles of oxygen and nitrate measured in organic rich sediments off Namibia summing up the net reactions described above. Due to nitrification, the highest nitrate concentrations are reached approximately at the oxygen penetration depth. At abont 3 cm depth, nitrate is consumed in the process of denitrification. The nitrate profile indicates an npward flnx into the bottom water and a downward flnx to the zone of denitrification. Both profiles are verified by the application of Equations 6.1 and 6.2 within the computer model CoTAM/CoTReM (cf. Chapter 15) as indicated by the solid and dashed lines. 

Fig. 8.17 Biogeochemical profiles of sulfur, manganese and iron species in a coastal marine sediment (Aarhus Bay, Denmark, 16 m water depth). A) Oxygen and nitrate profiles measured with and NO microsensors. B) Pore water profiles of dissolved manganese, iron and H S. C) Profiles of solid phase oxidized manganese and iron and of pyrite. D) Distribution of sulfate reduction rates (SRR) measured hy S-technique. The broken line at 4 cm depth indicates the transition between the suboxic zone and the sulfidic zone. Data in A) were measured at the same site but a different year than data in B)-D). (Data from Kjaer 2000 and Thamdrup et al. 1994a reproduced from Jorgensen and Nelson 2004).

Fig. 15.5 Model of the decomposition of organic substance by dissolved oxygen and dissolved nitrate in a diffusion controlled pore water profile. Modeling was performed according to the Press-F9-method with the spreadsheet software Excel . Details pertaining to this model are explained in Table 15.5 and in the text.

The microstructure of a catalyst layer is mainly determined by its composition and the fabrication method. Many attempts have been made to optimize pore size, pore distribution, and pore structure for better mass transport. Liu and Wang [141] found that a CL structure with a higher porosity near the GDL was beneficial for O2 transport and water removal. A CL with a stepwise porosity distribution, a higher porosity near the GDL, and a lower porosity near the membrane could perform better than one with a uniform porosity distribution. This pore structure led to better O2 distribution in the GL and extended the reaction zone toward the GDL side. The position of macropores also played an important role in proton conduction and oxygen transport within the CL, due to favorable proton and oxygen concentration conduction profiles. 

Pore-water nitrate profiles in marine sediments typically show one of three profile shapes. In sediment with rapid rates of organic matter oxidation relative to rates of solute supply from the overlying water, both oxygen and nitrate concentrations decrease more or less exponentially from overlying water concentrations at the sediment—water interface to zero, with oxygen depletion preceding or simultaneous with nitrate depletion at shallow sediment depth (see 105 m and 440 m profiles in Fig. 6.12). These types of profiles are common in continental shelf and upper slope sediments, and are due to relatively large carbon rain to the sediments (relatively... 

Figure 4 Pore-water oxygen concentrations at three sites on the Ceara Rise, western equatorial Atlantic. In each plot, model pore-water O2 profiles are shown in which model parameters are calculated from fits to pore-water NO3 profiles from the same sites, with stoichiometric adjustments based on (a) RKR stoichiometry (dotted lines) and (b) Anderson and Sarmiento (1994) stoichiometry (solid lines) (source Hales and Emerson, 1997).

Reimers C. E. (1987) An in situ microprofiling instrument for measuring interfacial pore water gradients methods and oxygen profiles from the North Pacific Ocean. Deep-Sea Res. 34, 2019-2035. 

Fig. 3.5 A quite successful oxygen profile in a marine sediment. This profile was measured by Glud et al. (1994) in highly reactive sediments off the western shoreline of Africa using the Profilur lander in situ. The most pronounced concentration gradient (chain line) lies directly below the sediment surface. Down to a depth of only about 25 mm below the sediment surface, the oxygen dissolved in pore water is entirely depleted.

Fig. 3.7 Nitrate profile of pore water obtained from sediments of the upwelling area off the coast of Namibia. The profile displays the shape which is typical of nitrate profiles, with a maximum at a depth which is determined by the decomposition of organic material and the oxidized nitrogen released from it after having reacted with the dissolved oxygen. The gradients indicated document a flux upward into the bottom water and a flux downward into zones where nitrate functions as an electron acceptor in the oxidation of other substances.

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