Organic matter 509 oxidation rate

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 6 The rate of organic matter oxidation as a function of depth below the sediment-water interface at two locations on the northwest Atlantic margin at 70° W (Martin and Sayles, submitted). The calculated organic C oxidation rates are based on fits to pore-water O2, NO, and NH4 profiles. The apparent discontinuity between the rates of oxidation by O2 and NOJ is most likely an artifact of the modeling procedure used to calculate the rates. Fits to dissolved O2 profiles were entirely independent of fits to NO profiles at sites where NOJ profiles could be used to calculate both the rate of oxidation by O2 and that of oxidation by NO, magnitudes of oxidation rates were similar to those shown here, but no discontinuity was present (Martin and Sayles, submitted). The small overlap in the zones of oxidation by O2 and NOJ is most likely due to the relatively poor resolution ( 0.5 cm) of the pore-water NOJ...

Hammond et al. (1996) used fits of the model of Equation (1) to pore-water O2 and NOJ profiles in the central equatorial Pacific to define the rate of organic matter oxidation as a function of depth below the sediment-water interface. Their model was built on a two-exponential description of the O2 consumption or NO production rate (Equation (2)). They assumed that the fraction of organic matter decomposition represented by the exponential with a short scale length was the highly reactive fraction, the fraction represented by the exponential with the longer scale length was a less reactive fraction, and that there was a third fraction that was unreactive on the... 

Plots of alkalinty versus Tqo2 and Ca " " versus alkalinity demonstrate conclusively the occurrence of metabolic dissolution. However, they do not show the ratio of the rates of dissolution and carbon oxidation well. The reason is that calcite dissolution is rapid relative to organic matter oxidation. Therefore, pore waters that have become undersaturated due to oxic metabolism re-equilibrate with sedimentary calcite very rapidly. The sedimentary layer in which the release of metabolic acids is not matched by dissolution is expected to be thin, so that the slope... 

Orr, W.L. and Gaines, A.G., Jr., 1973. Observations on rate of sulfate reduction and organic matter oxidation in the bottom waters of an estuarine basin the upper basin of the Pettaquamscutt River (Rhode Island). In Advances in Organic Geochemistry. Editions Technip, Paris, France, pp. 791—812. 

In the last decade, however, in-situ techniques have been developed to overcome these problems. Profiling lander systems were deployed to record the pore water microprofiles of oxygen, pH and pCOj, and Ca whereas benthic chambers were deployed to measure solute fluxes across the sediment-water interface directly. Very often, reactive-transport models are used to explain the interrelation between measured microprofiles, to predict overall calcite dissolution rates by defining the dissolution rate constants, and to distinguish between dissolution driven by organic matter oxidation and by the undersaturation of the bottom water. 

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