Oxygen penetration depth

By using the unsteady-state model, the maximum oxygen penetration depth for highly packed immobilised cells has been reported to be in the range of 50-200 xm. 

Stations 23 and 24, located in the LSLE, have been subjected to hypoxic conditions since the 1980s. With depletion of oxygen in the bottom waters, the sediment oxygen penetration depth decreased, and Fe oxides, concentrated in the oxic sediment layer, were reductively dissolved and released adsorbed arsenic. Hence, the low oxygen levels during the last 25 years in the bottom waters is reflected in more reducing conditions in the sediment and increases in both dissolved and HA-extractable Fe and As. 

The sediment oxygen penetration depth as a function of water coiumn depth in the Atlantic Ocean (open symbols) and Pacific Ocean (closed symbols). Source-. From Martin, W. R., and F. L. Sayles (2003). Treatise on Geochemistry, Elsevier. 

The oxygen penetration depth in the ocean calculated from bottom water oxygen concentration and the particulate rain rate of organic matter to ocean sediments. Penetration depths less than I -2 cm are concentrated in the continental margin regions. From Morford and Emerson (1999). 

Denitrification starts when oxygen is almost depleted (below the oxygen penetration depth) by inducing an enzymatic system of nitrate reductase and nitrite reductase by facultative aerobic bacteria, which can only use nitrogenous oxides if oxygen is - nearly - absent. Measurements carried... 

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. 6.16 Plots of (a) the ratio between diffusive oxygen uptake rates (DOU) ex-situ and in-situ and (b) oxygen penetration depth ex-situ and in-situ versus water depth from stations off the continental slope off Southwest Africa (from Glud et al. 1994). Solid lines indicate linear regressions. With increasing water depth fluxes appear to be overestimated and oxygen penetration underestimated when measured ex-situ.

Fig. 6.20 Diffusive oxygen fluxes (a) and oxygen penetration depths (b) for a number of sites in the South Atlantic and the Canaries (from Wenzhofer and Glud 2002). Oxic respiration decreases with increasing water depth resulting in higher oxygen penetration into the sediment.

Since microelectrode measurements are limited to a few centimeters of sediment depth, oxygen penetration depths have been difficult to obtain in strongly oligotrophic areas until the late 1990s. The invention of optode techniques, however, allows measurements up to several decimeters into the sediment (Fig. 6.22). The example is from a station located in the oligotrophic western equatorial Atlantic. 

Fig. 6.21 Diffusive oxygen uptake vs. oxygen penetration depths for the same sites as shown in Fig. 6.20. The correlation follows the function found hy Cai and Sayles (1996).

Convective gas phase transport in natural unsaturated soil systems may be induced by variations in temperature, air pressure and groundwater, by wind pressure or by seepage water movement after rain events. All these processes are of minor importance in lab column experiments and have not been implemented in DiffModV. The only convective movement in the gaseous phase considered in DiffMod is the ground air recharge due to gas volume decrease caused by oxygen consumption. The effects of this type of convection have to be taken into consideration as they result in an increase of the initial oxygen penetration depth or the thickness of the pyrite oxidation zone and thus induce an increase of the total pyrite decomposition rate on the column by 10 to 15 %. 

The programme DiffModV was applied to predict the oxygen penetration depth and the associated depyritization depth of a 43 year old dump body. In situ measurements of the oxygen distribution and the determination of pyrite contents within the dump body supported the model results. 

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