Around 700 million years ago, the world is thought to have experienced its most severe ice age – a period evocatively described by scientists as Snowball Earth.
It threatened the survival of much of the planet’s primitive living things, like oxygen-dependent marine life – including the earliest animals, such as simple sponges.
But new geological evidence uncovered by scientists at McGill University in Montreal has found that glacial meltwater provided a crucial lifeline at the time to microscopic organisms known as eukaryotes.
“The evidence suggests that although much of the oceans during the deep freeze would have been uninhabitable due to a lack of oxygen, in areas where the grounded ice sheet begins to float there was a critical supply of oxygenated meltwater,” said McGill University sedimentologist Maxwell Lechte in a press statement.
Previously, scientists thought that oxygen-dependent life may have been restricted to meltwater puddles on the surface of the ice, but the new study published Monday in the journal Proceedings of the National Academy of Sciences provides evidence of “oxygen oases” below the ice where the glacier meets the sea. These would have allowed primitive life forms to wait out the ice age.
Lechte, who is also a postdoctoral researcher in the Department of Earth and Planetary Sciences at McGill University, said that air bubbles trapped in the glacial ice were released into the water as it melts, enriching it with oxygen, a process he described as a “glacial oxygen pump.”
The researchers studied iron-rich rocks left behind by glacial deposits in Australia, Namibia and California, which they said offered a window into the environmental conditions during the ice age.
By examining the chemistry of the iron formations in these rocks, the researchers were able to estimate the amount of oxygen in the oceans around 700 million years ago and understand the effects this would have had on all oxygen-dependent marine life.
Lechte said that not only did life survive Snowball Earth, but the massive glaciation that engulfed the planet could have played a role in the evolution of more complex lifeforms.
“The fact that the global freeze occurred before the evolution of complex animals suggests a link between Snowball Earth and animal evolution,” Lechte said. “These harsh conditions could have stimulated their diversification into more complex forms.”