The observation of non-mass-dependent sulphur isotope ratios in sedimentary rocks more than ∼2.4 billion years old and the disappearance of this signal in younger sediments is taken as evidence for the transition from an anoxic to oxic atmosphere around 2.4 Gyr ago. But now, the preservation of a non mass-dependent signal that differs from that of preceding and following periods in the Archean is demonstrated. The findings support the original idea of an anoxic early atmosphere before 2.4 Gyr ago, and at the same time identifies variability within the isotope record that suggests changes in pre-2.4 Gya atmospheric pathways for non-mass-dependent chemistry and in the ultraviolet transparency of an evolving early atmosphere.
- James Farquhar
- Marc Peters
- Alan J. Kaufman
