The sulfur signal disappears
An isotope fingerprint present in every older rock stops abruptly — the sharpest line in the record.
- When
- Known to
- ± 30 million years
- Era
- Paleoproterozoic, Proterozoic
Mass-independent fractionation of sulfur isotopes requires an atmosphere with essentially no oxygen and no ozone. Its abrupt disappearance around 2.4 billion years ago is the single cleanest marker of the Great Oxidation Event, and it is how the timing is fixed. Recent work shows the signal vanishes at slightly different times on different continents, which has prompted arguments that the event needs redefining.