OXYGEN ISOTOPE COMPOSITION OF CARBONATE-ASSOCIATED SULFATE AND THE SOURCES AND CYCLING OF SULFATE IN THE EARLY OCEAN

The oxygen content of Earth's atmosphere increased about 2.4 billion years ago. Some of the most compelling evidence for this fundamental biospheric transition comes from sulfur isotope fractionations, both mass-dependent and independent, preserved within sedimentary sulfate and sulfide minerals. We are exploring ancient sulfate as a paleoredox tracer by tapping into the comparatively unknown systematics of how the isotope composition of oxygen varies in relation to the biogeochemical cycle of sulfur.

We start from the experimentally validated premise that during oxidation of reduced sulfur (e.g., pyrite) much if not most of the oxygen in the resulting sulfate derives from water molecules present in the oxidizing environment. In most models, an increased sulfate flux to the ocean ca. 2.4 billion years ago is primarily a product of oxidation of sulfide minerals on the continents beneath an oxygen-containing atmosphere. In the presence of rainwater and soil waters, the oxygen within the sulfate is diagnostically depleted in the heavy isotope relative to seawater. Once this sulfate enters the ocean, bacterial sulfate reduction (BSR) and other cycling within the marine environment can heavily overprint the oxygen's continental composition, but the specifics are poorly understood. We are exploring BSR overprints through a comprehensive suite of batch culture experiments.

Because sulfate minerals are often sparse in the early geologic record, we are using an additional oxygen isotope proxy: carbonate-associated sulfate or CAS. Our initial results confirm that sulfate trapped within carbonate minerals can preserve the oxygen isotope composition of seawater sulfate. Because the full strength of the CAS proxy for oxygen is largely untested, we are calibrating the method by tracking possible isotopic variability across a diverse array of depositional and diagenetic environments in modern Florida Bay, where CAS seems to faithfully record seawater isotopic signals, and in a sulfidic lake in British Columbia, where anoxygenic, phototrophic bacterial sulfide oxidation analogous to that of the early ocean is pervasive. Armed with an improved experimental context and greater confidence in the CAS method, we are carefully generating oxygen isotope data across the critical intervals of initial atmospheric oxygenation.