Irrigation pore water profile

Pore-water profiles were used together with solid-phase dissolution rates in diagenetic models to determine first-order anoxic precipitation rate constants for both Mn and Fe. A two-dimensional cylindrical coordinate model was employed to account for the effects of biogenic irrigation of burrows on pore-water Mn " distributions. Two-dimensional diffusion can result in a decrease in Mn " with depth that would be interpreted as evidence for precipitation and cause overestimation of precipitation rates in a one-dimensional model. 

Aller (1984) created a mechanistic model for the multi-dimensional transport of dissolved pore-water species by animals. He observed that ammonia profiles caused by sulfate reduction in the top-ten-centimeter layer of Long Island Sound sediments could not be interpreted by onedimensional diffusion (Equation (3)). The multidimensional effects of irrigation were reproduced mathematically by characterizing the top layer of... 

These changes apparently result from a combination of lower production rates of Fe " and Mn ", together with the relatively increased importance of biogenic transport at this time compared to summer. Production rates decrease because of decreased microbial activity associated with lower water temperatures and depleted food sources [e.g., Eqs. (6.1) and (6.2)]. This allows the transport effects of biogenic reworking and irrigation to dominate the form of the fall solute profiles relative to those of the summer period. Similar changes are also observed for other pore-water constituents (Part I). 

If there are no or very few irrigated burrows present in the sediment, lateral diffusion is not significant and the r dependence of Eq. (6.12) can be ignored. In that case, the equation becomes the more traditional onedimensional transport-reaction equation used to model pore-water solute profiles where advection is relatively unimportant (Berner, 1971 1980 Lerman, 1979). Both the cylindrical microenvironment model and the onedimensional Cartesian coordinate model will be used here to quantify the Mn distributions at NWC and DEEP. 

Fig. 19. Comparison of the one- and two-dimensional models for Mn distribution in the top 0-18 cm of sediment at NWC. The production rate in both cases is that found for core NWC-4. The anoxic precipitation rate is assumed to be zero. The effective cylinder geometry used in the two-dimensional model is that determined for NH4 in Part I r, = 0.14 cm, rj = 4.5 cm. The basal gradient is constrained to be zero. The diffusion geometry created by irrigated burrows results in a vertical pore-water solute profile exhibiting apparent precipitation.

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