Oceans stay low in oxygen
Below a thin oxygenated skin, the sea remains anoxic — and complex life stays small.
- When
- Known to
- ± 200 million years
- Era
- Mesoproterozoic, Proterozoic
Trace-metal and iron-speciation data indicate mid-depth oceans remained largely anoxic and locally sulfidic throughout the Mesoproterozoic, with atmospheric oxygen possibly only a few percent of present levels. Sulfide removes molybdenum and other metals that nitrogen-fixing enzymes require, potentially capping the nitrogen supply. It is the leading explanation for why eukaryotes existed for a billion years without producing anything large.