Evidence of magnetic isotope effects during thermochemical sulfate reduction

Thermochemical sulfate reduction experiments with simple amino acid and dilute concentrations of sulfate reveal significant degrees of mass-independent sulfur isotope fractionation. Enrichments of up to 13% for S-33 are attributed to a magnetic isotope effect (MIE) associated with the formation of thiol-disulfide, ion-radical pairs. Observed S-36 depletions in products are explained here by classical (mass-dependent) isotope effects and mixing processes. The experimental data contrasts strongly with multiple sulfur isotope trends in Archean samples, which exhibit significant S-36 anomalies. These results support an origin other than thermochemical sulfate reduction for the mass-independent signals observed for early Earth samples.