The sulfur (S) isotope difference between sedimentary sulfate and sulfide phases preserved in sedimentary rocks (Δ34S) has been utilized to reconstruct marine sulfate concentrations and inferentially the redox evolution of Earth’s surface. These interpretations are largely based on experimental studies that indicate that microbial sulfate reduction is accompanied by a substantial kinetic isotope effect (up to 66‰), but only at sulfate concentrations >~200 μM. In this study, we examine S isotope systematics in a modern, low-sulfate euxinic lake (~100–350 μM) and find that the calculated kinetic isotope effect associated with microbial sulfate reduction (ε 34S) is relatively large (~23.5‰), but preserved Δ34S values are considerably smaller (4.7‰–9.9‰). Δ34S values in this system are controlled by the fraction of the sulfate reservoir that is consumed during sulfate reduction and the location of pyrite formation. This reservoir effect strongly influences the S isotope composition of sulfide preserved in the rock record such that Δ34S values increase as a function of sulfate levels, even when sulfate concentrations are >200 μM and the kinetic isotope effect is expressed. These findings have important implications for reconstructing the chemical evolution of the ocean-atmosphere system throughout Earth history—not just for the Precambrian.