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Solute-specific pore water irrigation: Implications for chemical cycling in early diagenesis

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2005-05-01
Meile, C
Berg, P
Van Cappellen, P
Tuncay, Kağan
In one-dimensional (1D) early diagenetic models, bioirrigation is typically represented by a nonlocal mass transfer or bioirrigation coefficient, alpha. Usually, all pore water species are assigned the same alpha. Here, we show that this assumption can lead to significant errors in estimates of bioirrigation intensities. Using a simplified early diagenetic reaction network, we compute the 3D concentration fields of major pore water species around a vertical burrow, as well as the solute fluxes across the burrow wall. From these results, corresponding 1D vertical alpha profiles are derived. The alpha profiles show pronounced differences from one solute to another. Dissolved O-2 systematically exhibits the highest alpha values, while fast oxygenation kinetics near the burrow wall result in near-zero alpha values for aqueous Fe2+. For nitrate, use of a species-averaged alpha profile may even lead to an erroneous prediction of the direction of the irrigation flux across the water-sediment interface. The large differences in alpha profiles reflect the variable effects of biogeochemical processes on pore water concentration fields of reactive solutes near the burrow wall. Even for inert solutes, however, determination of alpha can be ambiguous. Transient simulations mimicking the intrusion into the sediment of an inert tracer during an incubation experiment yield apparent mixing intensities that depend on the incubation time.