Sulfur proves the air had no oxygen
An isotope quirk that only happens without an ozone layer runs through every Archean rock — then stops.
- When
- Known to
- ± 200 million years
- Era
- Mesoarchean, Archean
Archean sediments show mass-independent fractionation of sulfur isotopes, a signature produced when ultraviolet light strikes sulfur dioxide high in an atmosphere with no ozone shield. It requires oxygen below about a hundred-thousandth of present levels. The signal is present throughout the Archean and vanishes abruptly around 2.4 billion years ago — which is how the timing of the Great Oxidation Event is pinned down.