THE INFLUENCE OF CLAY MINERALS ON THE EVOLUTION OF MUDSTONE PORE FLUIDS DURING MICROBIAL IRON REDUCTION

Investigations of early chemical diagenesis in mudstones are primarily focused on microbially induced reactions that alter pore fluid pH and carbonate saturation, promoting either carbonate dissolution or precipitation. While these reactions are critical in elucidating early mudstone diagenesis, previous research has ignored the potential impact of clay minerals and their acid-base properties.

Here, we present a chemical model for microbial Fe reduction that accounts for the buffering capacity of clay mineral-rich sediments. The model is based on equations describing speciation, mass balance, and charge balance, and predicts the evolution of pH and carbonate saturation as a function of moles Fe³⁺ reduced. Model accuracy is tested experimentally by adding the Fe reducing bacteria Shewanella oneidensis MR-1 to a sediment slurry while regularly measuring pH and the concentration of Fe²⁺ during the reaction progress. Sediments composed of 56% clay-sized particles and 59% clay minerals were obtained from the Nankai Trough as part of International Ocean Discovery Program Expedition 322. Stepwise titrations of the Nankai sediment in nanopure water provide a pKa value of ~4.5 and a total acidity of ~0.14 mmol g^-1, both key input parameters for the model.

Model results show that with an initial pH of 6.5 and 0.5 mM Fe³⁺ being consumed by microorganisms the pH is 0.3 log units lower than microbial Fe reduction alone, but if the reaction begins at a pH of 8 there is no change in pH after the consumption of 0.5 mM Fe³⁺. This is because deprotonation of clay minerals in the Nankai sediment occurs at a pH of ~4.5, which is 2-4 log units below the normal pore fluid pH range making it ineffective as a buffer at higher initial pH’s. We anticipate that culture experiments will demonstrate the utility of our model, and perhaps by incorporating other sediments with higher pKa values the buffering capacity of clay minerals can be better recognized. This heightened understanding of the acid-base properties of clay mineral-rich sediments has the profound implication that authigenic carbonate cementation could be limited or prevented during microbial Fe reduction when the initial pore water pH is within a few log units of the sediment pKa.