The effects of reactive transport on sulfur isotopic compositions in natural environments

Microbial sulfate reduction is one of the oldest known metabolisms to have evolved on Earth and is associated with one of the largest stable isotope fractionations recorded in both marine and terrestrial environments. Through decades of study, we are reaching a nuanced description of mass-dependent partitioning across this multi-step, eight electron transfer process, yet many complexities impede translation to natural environments. Here, we review the factors that contribute to sulfur isotope fractionation, including variations in the sulfur isotope fractionation factor associated with microbial metabolism, and demonstrate several examples of systems in which the apparent relationship between reaction rate and fractionation factor is obscured or easily misinterpreted.