STABLE ISOTOPIC COMPOSITION OF SULFATE IN MEROMICTIC MAHONEY LAKE AND IMPLICATIONS FOR THE EARLY OCEAN

We present a study of paired sulfur and oxygen isotopic analysis of dissolved sulfate in meromictic Mahoney Lake, British Columbia. Mahoney is a small (20 ha), shallow (15m), pristine lake, with no outflow. A dense plate of purple sulfur bacteria (Amoebobacter purpureus) is established at the chemocline (6 m water depth) where light and sulfide levels are sufficient to support anaerobic sulfide oxidation. Mahoney is a sulfate-dominated brine with measured chloride levels in excess of 70 mM throughout the water-column and sediment pore waters. Elevated rates of sulfate reduction below the chemocline are evidenced by vertical profiles of total dissolved sulfide levels in excess of 30 mM and total alkalinity greater than 10,000 mg/L. The chemical stratification and abundance of purple sulfur bacteria make Mahoney Lake an ideal natural laboratory for studies of anaerobic sulfide oxidation and the ultimate formation of sulfate. d³⁴S_SO4 is governed by the isotopic composition of the parent fluid and the isotope effects imparted during microbial sulfate reduction and chemical and biological oxidation. The isotopic composition of secondary sulfate oxygen is derived from a combination of atmospheric oxygen and ambient water. Our research into the isotopes of sulfate dissolved in the water-column and sediment pore waters combined with the sulfur isotope compositions of solid phase sulfur (AVS, pyrite, and organic) will be used to trace sulfur redox transformations within a highly sulfidic, permanently stratified lake. More generally, we will track the oxygen isotope effects linked to intense biological cycling of sulfur, with a strong component of phototropic microbial sulfide oxidation. This pathway of sulfate production and the resulting oxygen isotope fingerprint in a complex natural setting should inform our studies of the earliest ocean and the coupled redox state of the ocean-atmosphere system.