Rocks formed before 2.4 Gyr have not been found to contain a mass-independently fractionated sulphur isotopes (MIF-S) chemical signature. This suggests that either atmospheric oxygen levels fluctuated dramatically before 2.4 Gyr, or that MIF-S is not a reliable indictor of when atmospheric oxygen levels were low.
- Hiroshi Ohmoto
- Yumiko Watanabe
- Bruce E. Taylor