Suboxic condition

Local water column paleoredox. The offset between recent euxinic and suboxic sediments (Fig. 6) suggests that such data could provide an additional tool to differentiate between euxinic and suboxic conditions in ancient sediments. Such application requires independent constraint on 5 Mo, as might be obtained from analyses of coeval Mn oxide sediments. 

Because phosphate is released during remineralization with no decrease in O2, the A02/AP0 produced via denitrification should be lower than that predicted by the aerobic respiration of Redfield-Richards planktonic detritus. To reach the suboxic conditions required for denitrification requires the aerobic respiration of a considerable amount of POM and, hence, release of phosphate. Thus, A02/AP0 ratios less than 138 are most likely to be found in waters with high phosphate concentrations. The prevalence of denitrification in deep waters is suggested by their low (14.7) average N-to-P ratio (Figure 8.3). Areas where the OMZ are pronoimced, such as coastal upwelling areas, have particularly low N-to-P ratios as shown in Figure 10.7. 

The nodules that form at rates on the order of tens of millimeters per million years appear to have been produced primarily by postdepositional remobilization under oxic conditions. These nodules have relatively high copper and nickel contents. The nodules that accrete at the fastest rates (200 mm per million years) appear to have formed primarily via postdepositional remobilization under suboxic conditions. Despite these rapid accretion rates, the suboxic diagenesis -type nodules account for only half of those found in areas where biological productivity is high. The other half appear to have been formed primarily by oxic remobilization. Hydrogenous precipitation appears to play the dominant role in forming only a small percentage of the nodules. 

Marine nitrifiers tend to be slow growers compared to heterotrophs. This reflects the relatively low energy yield from ammonia oxidation. The nitrifiers tend to be photoin-hibited, making them less likely to compete with the photoheterotrophs for O2. While O2 is a requirement, some nitrifiers are capable of growing under suboxic conditions. 

Redox potential discontinuity layer a distinct layer in sediments associated with distinct coloration, indicative of difference between oxic and suboxic conditions. 

Equilibria involving reductive dissolution reactions add to the complexity of mineral solubility phenomena in just the way that pE-pH diagrams are more complicated than ordinary predominance diagrams, like that in Fig. 3.7. The electron activity or pE value becomes one of the master variables whose influence on dissolution reactions must be evaluated in tandem with other intensive master variables, like pH or p(H4Si04). Moreover, the status of microbial catalysis under the suboxic conditions that facilitate changes in the oxidation states of transition metals has to be considered in formulating a thermodynamic description of reductive dissolution. This consideration is connected closely to the existence of labile organic matter and, in some cases, to the availability of photons.26... 

Bange (2006a) conclusions are in line with a recent seasonal study of N2O in the water column of the Bedford Basin (Northwest Atlantic Ocean), which showed that water-column nitrification was the dominant N2O formation pathway (Punshon and Moore, 2004). N2O consumption was only temporarily observed when dissolved O2 was considerably depleted (2.5 pmol L ) shifting the system from oxic to suboxic conditions (Punshon and Moore, 2004). 

Sarma, V. V. S. S. (2002). An evaluation of physical and biogeochemical processes regulating perennial suboxic conditions in the water column of the Arabian Sea. Global Biogeochem. Cycles 16(4), doi 10.1029/2001GB001461. 

Vogel et al., 1987). It seems likely to us that there is a biological transformation of these compounds by marine bacteria, particular as marine bacteria can transform CHaBr (see Section 6.03.3.3.2). Not only are these compounds likely to be removed from oceanic and coastal waters under anoxic and suboxic conditions, but given that compounds such as CH4 and N2O are thought to be produced in suboxic micro-environments within the water column (see Section 6.03.3.2.9), it seems reasonable to assume that the same sites might be areas of chlorofluorocarbon removal. 

A recent report provides evidence that under anoxic laboratory conditions, MnOa solids can enhance the oxidative attack of FeS2 and FeS by dissolved Fe(III), whereas in the absence of Mn02 (and under sterile conditions), FeS2 is not affected by dissolved Fe(lll) or even NO3 (Shippers and Jorgensen, 2001). This reaction does not need molecular oxygen to proceed, and could potentially take place even in anoxic aquifers. Further study is needed on the stability of sulfides generally, and As-bearing sulfides in particular, under anoxic and suboxic conditions. 

The reduction of Mn occurs under suboxic conditions (Murray et al., 1995), and it has been assumed that the enhanced upward difftision of soluble Mn(ll) in the water column possibly controls and interferes with the downward diffusion of the dissolved Cd, Cu, and Zn species. 

Similar to the possibility of concurrent reduction of sulfate and ferric iron by a culture of a single bacteria (Coleman et al. 1993 see section 7.4.3.4) other iron reducing bacteria were found to additionally maintain dissimilation with more than one electron acceptors under suboxic conditions (Lovley and Phillips 1988) or even under oxic conditions (Myers and Nealson 1988a). In the presence of Fe(III) and Mn(IV) strain MR-1 was found to reduce both but additional manganese reduction occurred due to the immediate abiotic reaction with released Fe (Myers and Nealson 1988b). The interactions of biotic and abiotic reactions are shown in Fig. 7.17. 

Diagenesis Release of particulate metals into the dissolved phase under suboxic conditions. 

Suboxic Condition lacking free oxygen, but not extremely reducing. 

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