Iron and Manganese Redox Cycles

The intensity of the redox cycling and thus the importance for oxidation and reduction reactions in the sediment is terminated by either one of the following conditions 1. In case of the absence of any efficient oxidant (e g. O ) in the upper-most layer or bottom water no oxidation will occur and the redox cycling cannot be maintained. 2. In case of the absence of a reactive fraction (bioavailable or rapidly reducible by HS , see section 7.4.3.1) in the lower layer no reduction will occur and the redox cycle will cease. 3. A vertical transport mode must be maintained between the zone of oxidation and the zone of reduction. As advection is usually very much slower than the downward transport by bioturbation the intensity of bioturbation terminates the transport between the redox-zones. 

In case of a very low sedimentation rate relative to mixing (A 2. Dj ) Eq. 7.17 can be rearranged to calculate the biodiffusion coefficient, D,  

The above restrictions for the calculations of the sedimentation rate and the biodiffusion coefficient imply the use of radioactive isotopes with different half-lifes (t = 0.693 A, ) for different purposes and depositional environments. The higher the sedimentation rate, the shorter should be the half-life of the radioactive isotope. The more intense bioturbation in the surface layer, the shorter should be the half-life of the applied radioactive isotope be. For coastal and shelf sediments sedimentation rates of several decimeters to few meters per 1000 years are typical and can be determined by Pb (t,j = 

1 d) are applicable for the determination of the mixing intensity. Th (t = 75,200 y) is a commonly used radioactive isotope in oceanographic sciences to trace processes over longer periods of times. The above isotopes are rapidly scavenged by particles once they are formed from the decay of some parent isotopes and settle to the sea floor. Due to analytical reasons post-depositional processes can be traced for a time 

As mentioned in the beginning of this section bioturbation and advection by sedimentation cause particle transport in the sediment. In Fig. 7.20 three scenarios are schematically shown representing constant molecular diffusive and 

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