Reactive Transport Modelling of pH Dynamics in Aquatic Sediments

Reactive transport models can account for the complex set of transport and reaction processes that control the geochemical conditions in aquatic sediments. A diagnostic indicator of reaction processes is pore water pH. Thus, the combination of high resolution pH profiles and reactive transport models has the potential to unravel the interplay of the major biogeochemical processes occurring in sediments. Existing models do not satisfactorily reproduce measured pH profiles, however. This points to an incomplete understanding, or inadequate representation of the processes that affect the proton balance in sediments. In order to quantify the factors controlling pore water pH distributions, we adopt a systematic approach to modelling the relevant processes. In this contribution, the role of particulate deposition fluxes and calcite dissolution are evaluated. We use the Biogeochemical Reaction Network Simulator (BRNS), which provides an efficient and flexible modelling environment in that modifications of model formulations are easily implemented and tested. The kinetic formulations of primary and secondary redox reactions, aqueous carbonate and sulfide equilibrium reactions, and calcite dissolution kinetics constitute the reaction network. These reactions are coupled to the transport processes: sedimentation, diffusion, and bioturbation in one dimension. The buffering effect of calcite dissolution on the pH profiles is evaluated quantitatively under contrasting conditions of inorganic carbon fluxes reaching the sediment-water interface. Two simulation environments are considered, namely deep-sea sediments above and below the lysocline, for which published data exist. These environments display similar redox conditions, yet distinctly different pH profiles. The calculated pH profiles are found to be a highly non-linear function of the solid (organic and inorganic) deposition fluxes. By varying the organic carbon flux, a close correlation between the oxygen penetration depths and the depths of pH minima is observed. This correlation is independent of the pH buffering by calcite dissolution.