EARTH

The Iron Stops Settling

~1.85 billion years ago

Proterozoic · Paleoproterozoic · Orosirian

For more than a billion years, the ocean had been laying down its alternating bands of iron and silica. Around 1.85 billion years ago the deposits cease, everywhere, never to resume. Two explanations still compete: either the deep sea filled with hydrogen sulfide, which stripped the dissolved iron out as pyrite, or the Sudbury impact, falling at just that moment, stirred the ocean from top to bottom and left deep water in which iron could no longer stay dissolved. Either way, the greatest iron deposits in history had just received their final layer.

Why it matters

The end of these stripes is the end of an ocean full of dissolved iron, and that iron sea was the only world life had ever known.

Dating & uncertaintyDisputed

Two explanations compete and both are still current: the deep ocean turned sulfidic, so iron precipitated as pyrite instead of oxide; or the Sudbury impact mixed the whole water column and left it faintly oxygenated. Iron, sulfur and carbon systematics support the first; the coincidence of dates supports the second.

Sources

1. Poulton et al. (2004), Nature · The transition to a sulphidic ocean approximately 1.84 billion years ago
2. Slack & Cannon (2009), Geology · Extraterrestrial demise of banded iron formations 1.85 billion years ago
3. Lyons et al. (2014), Nature · The rise of oxygen in Earth's early ocean and atmosphere

Earth history as one day

14:13:129 h 47 min before midnight

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