The first traces of free oxygen
Oxygen appears in trace amounts hundreds of millions of years before it fills the air.
- When
- Known to
- ± 150 million years
- Era
- Mesoarchean, Archean
Chromium, iron and molybdenum isotopes in Mesoarchean sediments record free oxygen at roughly three ten-thousandths of present levels — vanishingly small, but not zero. Oxygen was being produced locally in shallow-water microbial mats and consumed almost immediately by dissolved iron and volcanic gases. These transient whiffs show the machinery was running long before the atmosphere changed.