Sulfidic oceans
Beneath an oxygenated surface layer, the deep sea turns toxic with hydrogen sulfide.
- When
- Known to
- ± 150 million years
- Era
- Paleoproterozoic, Proterozoic
With oxygen present at the surface but scarce below, sulfate washed off the oxidising continents feeds sulfate-reducing bacteria in the deep ocean, producing hydrogen sulfide. Large parts of the mid-depth ocean become euxinic — anoxic and sulfidic — a state that persists for much of the following billion years. Sulfide ties up trace metals such as molybdenum that nitrogen-fixing organisms need, which may have throttled the biosphere for a very long time.