EARTH × LIFE

When the Deep Sea Turned Toxic

~1.8 to 0.8 billion years ago

Proterozoic · Paleoproterozoic · Statherian

With oxygen thin in the air and sulfate washing off the continents, bacteria turned swathes of the deep sea into water laced with hydrogen sulfide. Iron that had stayed dissolved in those depths settled out, the banded iron formations came to an end, and the metals life needs, molybdenum and copper among them, grew scarce.

Why it matters

A sea poor in molybdenum is a sea where nitrogen is hard to pull from the air. That may be why complex life waited a billion years.

Dating & uncertaintyDisputed

The original model had the whole deep ocean turn sulfidic after the banded iron formations stopped. Trace-metal budgets now suggest sulfidic water covered only a small fraction of the sea floor, with most of the deep ocean anoxic but iron-rich. The picture has moved from one global state to a patchwork.

Sources

1. Canfield (1998), Nature · A new model for Proterozoic ocean chemistry
2. Reinhard et al. (2013), PNAS · Proterozoic ocean redox and biogeochemical stasis
3. Lyons et al. (2014), Nature · The rise of oxygen in Earth's early ocean and atmosphere

Earth history as one day

14:29:049 h 31 min before midnight

if Earth’s 4.54 billion years were compressed into a single day