Intense bursts of volcanic activity between 3.5 and 3.2 billion years ago delivered massive quantities of sulphur dioxide to the atmosphere, suggests a paper published online this week in Nature Geoscience. The breakdown of these sulphur dioxide aerosols by UV radiation can explain the unusual sulphur isotope trends reported from rocks of this age.

Pascal Philippot and colleagues measured the sulphur isotope values of sulphides in 3.5-billion-year-old volcanic ash deposits from South Africa. A comparison with the isotopic composition of co-occurring sulphates in this region is most consistent with a volcanic sulphur source that was altered by UV radiation. They suggest that the pulses of volcanic activity they detect were related to a major episode of continental crust growth.