GoKawiilPlanet Earth has some pretty great qualities going for it. (Negative reviews mostly revolve around the staff and clientele.) Pretty high on the list of positives is a richly oxygenated atmosphere. But that’s something that evolved and built up over a couple billion years, only eventually resulting in a world conducive to animal life like us.
Scientists have many ideas about what could have caused oxygen to increase, and it seems that a number of them are probably correct. No one thing in isolation seems to explain it. Life is part of the story, with photosynthetic life pumping out oxygen. The chemistry of the solid Earth also had a role to play, both through supporting photosynthetic life and through reactions that can shuttle oxygen between the atmosphere and rocks deep inside the Earth.
A new study led by Wei Shi of the Chengdu University of Technology suggests that evidence of changes in the subduction of tectonic plates—the process by which they disappear down into Earth’s interior—lines up with the timing of jumps in oxygen levels.
Cooling off
The Earth has gradually cooled over time, and the scant remnants of its earliest history show us that major geologic processes evolved quite a bit as a result. Early in its history, cold, dense surface rock would have sunk through hot mantle rock in ways that bear little resemblance to modern plate tectonics. And the continents around us are 4.5 billion-year-long construction projects, so imagination is required to picture what was present early on.
It wasn’t a smooth, linear evolution—there seem to be transition points in that geologic history. The oxygenation of Earth’s atmosphere wasn’t linear, either. It started with a jump during the Great Oxygenation Event about 2.4 to 2.0 billion years ago. But then it stalled out until resuming between 800 and 500 million years ago. A third increase between 450 and 250 million years ago brought us up to modern oxygen levels.