Inclusion of adsorption in the modelling of early diagenesis |
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| Authors: | Robert A Berner |
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| Institution: | Department of Geology and Geophysics, Yale University, New Haven, Conn. USA |
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| Abstract: | A general one-dimensional diagenetic equation is derived which expresses the effect of the following processes upon the depth and time distribution of a dissolved species in a modern subaqueous sediment: adsorptive or ion exchange equilibrium between pore water and grain surfaces; slower chemical or radiogenic reactions in the pore solution and on the grain surfaces; diffusion in solution and on grain surfaces; advection of grains due to depositional burial; and advection of water due both to burial and to flow past the grains. It is shown that the effects of equilibrium adsorption can be expressed in terms of a lowered diffusion coefficient only in the special case where there is negligible flow of water relative to grains, adsorption is described by a simple linear isotherm, and there is nochemical reaction occurring in the sediment (i.e. only radioactive decay or no reaction at all). Consideration of a commonly used special case of the general equation indicates that if equilibrium adsorption is strong, the depositional term in the resulting diagenetic equation may become quantitatively as important as the terms for diffusion and chemical reaction. Thus, when modelling diagenesis, deposition often cannot be ignored. A coupled pair of differential equations are also derived which express the steady state depth distribution for two ions exhibiting equilibrium ion exchange while undergoing chemical reaction, diffusion, and deposition. |
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