How life on our planet survived Snowball Earth

Today, the world is warming. But from about 720 to 635 million years ago, temperatures swerved the other way as the planet became encased in ice during the two ice ages known as Snowball Earth .

Within just a few thousand years or so, ice stretched from the poles to the tropics. Life lived in the oceans at the time, and the ice entombed that life, cutting it off from both the sun and the atmosphere.

“This is the one time when Earth’s natural thermostat broke,” said Noah Planavsky, a Yale biogeochemist.

“The question was: How did life actually make it through this?”

Glaciations can drive mass extinctions. Yet life somehow survived these deep freezes. In research published on Monday, Dr Planavsky and his colleagues report the discovery of oases just beneath the ancient ice sheets that likely helped life persevere.

Snowball Earth came to an end over a half-billion years ago, but its marks still exist in remote corners. In 2015, Max Lechte and Malcolm Wallace drove to one of those corners in the South Australian outback.

They trekked over redcoloured rock that had formed in the oceans during the snowball glaciations, and their colour caught Dr Lechte’s eye, so he took a few samples. Then, in 2015 and 2016, he traveled to Namibia and Death Valley in California and found more red rocks that formed at the same time.

The rocks’ colour signalled that they are rich in iron, which means they turned red for the same reason that old cars with iron exteriors turn red — They rusted.

If the iron rocks below the ancient oceans rusted, then there was oxygen in those oceans. And if there was oxygen, then oxygen-breathing life-forms had a lifeline they could cling to. But how that oxygen got into the oceans in the first place was a mystery.

Dr Lechte and his team found that the iron in rocks that formed far out in the open oceans rusted much less than the iron in rocks that formed closer to land.

Today, beneath ice sheets in Antarctica , glacial meltwater flows into the Southern Ocean. That water melts from ice that can have air bubbles trapped inside it, and those bubbles can seed the meltwater streams with oxygen. On Snowball Earth, such oxygen-laden streams flowed into the oceans around the edges of continents and sustained life.