Huronian glaciation
As rising oxygen likely destroys the methane greenhouse, Earth enters its oldest known ice age around 2.4 billion years ago, recorded in glacial rocks near Lake Huron.
The story
The Huronian glaciation is the oldest ice age known with confidence. Between about 2.4 and 2.1 billion years ago, glaciers left thick deposits in the Huronian Supergroup, a belt of rock north of Lake Huron in Ontario, Canada. Geologists read three glacial episodes there, including boulder-filled clays called tillites and lone stones dropped into fine mud by melting icebergs. Similar glacial rocks of this age turn up in Wyoming, Finland, India and South Africa, so the cold was not local.
The likely cause was life itself. Cyanobacteria had been pumping out oxygen for a long time, and in the Great Oxidation Event it finally built up in the air. Oxygen reacts with methane, a potent greenhouse gas that had probably helped keep the early planet warm under a fainter young Sun, and turns it into weaker carbon dioxide and water. As methane fell, the climate chilled. Some paleomagnetic evidence from South Africa hints that ice reached near the equator, though how complete the freeze was is still debated. Volcanic carbon dioxide slowly built up and the ice retreated.
Why it mattered
- The glaciation shows that living organisms can overturn a planet's climate, not just adapt to it.
- It is the earliest known case of the ice-and-greenhouse swings that later produced the Snowball Earth episodes.
- It gives geologists a marker in the rock record for the oxygen revolution of the Great Oxidation Event.
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