The isotope record of sedimentary sulfides can help resolve the history of
oxygen accumulation into the atmosphere. We measured sulfur isotopic fracti
onation during microbial sulfate reduction up to 88 degrees C and show how
sulfate reduction rate influences the preservation of biological fractionat
ions in sediments. The sedimentary sulfur isotope record suggests Low conce
ntrations of seawater sulfate and atmospheric oxygen in the early Archean (
3.4 to 2.8 billion years ago). The accumulation of oxygen and sulfate began
later, in the early Proterozoic (2.5 to 0.54 billion years ago).