A comparison of closed- and open-system models for porewater pH and calcite-saturation state

Abstract

We compare the theoretical predictions of a closed- and an open-system model for the evolution of pH and calcite-saturation state during the sequential oxidation of organic matter by O2, NO3−, and SO4=. The closed-system model is similar to previous thermodynamic models found in the geochemical literature (e.g., Ben-Yaakov, 1973). The open-system model allows for differential diffusion of dissolved species, exchange with overlying waters, fast acid/base reactions, and a variety of spatially distributed sources and sinks. In particular, dissolution and precipitation of minerals are included either as local equilibrium processes as in the case of calcite dissolution, or as depth-dependent exponentials as for FeS and CaCO3 precipitation. The model calculations reveal that closed and open systems have qualitatively similar behavior with respect to pH and carbonate saturation, Ωc. However, a quantitative comparison establishes that the closed-system model represents usually an upper limit on pH in the oxic zone of sediments, while it always sets a lower bound on pH in the zone of SO4−− reduction. The changes in Ωc in closed and open systems during oxic decay are more complex than those of pH. The closed system will present an upper limit on Ωc when the initial pH is high (i.e., 8.0), but does not exhibit limiting behavior if the initial pH is low (i.e., 7.0). The closed-system model always places a lower limit on Ωc during sulfate reduction. Both models predict that CaCO3 dissolution can buffer the potential pH fall due to oxic CO2 production; however, quantitative evaluation of this effect requires a diagenetic model with realistic dissolution kinetics. Our investigations also demonstrate that vigorous mono-sulfide precipitation from an FeOOH-iron source can mitigate the fall in pH normally associated with organic matter decay as this mineral-forming reaction consumes an important protolytic species, H2S. On the other hand, the precipitation of CaCO3 has only a modest lowering effect on pH. The oxidation of total dissolved ammonia and sulfide by O2 represents a potentially strong source of protons. Counter-intuitively, such oxidation reactions at the base of the oxic layer will raise the pH at a given O2 concentration because of a concomitant decrease in the O2-penetration depth. In addition, the O2 demand of the sediments can switch almost entirely from organic matter decay to reduced byproduct oxidation as the amount of sulfate reduction increases. Byproducts oxidation leads, however, to lower calcite-saturation states, and this can lead to more dissolution of fossil material near the sediment-water interface.