Sulfur isotope systematics of a euxinic, low-sulfate lake: Evaluating the importance of the reservoir effect in modern and ancient oceans

The sulfur (S) isotope difference between sedimentary sulfate and sulfide phases preserved in sedimentary rocks (Delta S-34) 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 parts per thousand), but only at sulfate concentrations >similar to 200 mu M. In this study, we examine S isotope systematics in a modern, low-sulfate euxinic lake (similar to 100-350 mu M) and find that the calculated kinetic isotope effect associated with microbial sulfate reduction (epsilon S-34) is relatively large (similar to 23.5 parts per thousand), but preserved Delta S-34 values are considerably smaller (4.7 parts per thousand-9.9 parts per thousand). Delta S-34 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 Delta S-34 values increase as a function of sulfate levels, even when sulfate concentrations are > 200 mu M 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.