The oldest soils
Rock weathered in place 3.3 billion years ago records an atmosphere with no oxygen in it.
- When
- Known to
- ± 100 million years
- Era
- Paleoarchean, Archean
Paleosols — fossil soil horizons — from Western Australia preserve the chemistry of rock exposed to Archean air and rain. Iron behaviour in them constrains how much oxygen the atmosphere held, and the answer is almost none. They are also direct evidence of subaerial land surfaces, rainfall and a working weathering cycle.