The Archean sulfur cycle and the early history of atmospheric oxygen.

The isotope record of sedimentary sulfides can help resolve the history of oxygen accumulation into the atmosphere. We measured sulfur isotopic fractionation during microbial sulfate reduction up to 88 degrees C and show how sulfate reduction rate influences the preservation of biological fractionations in sediments. The sedimentary sulfur isotope record suggests low concentrations 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).