The best evidence yet may have just been found: ScienceAlert

For almost 60 million years, our home planet was probably frozen into a large snowball.

Now scientists have discovered evidence of Earth’s transition from a tropical underwater world full of photosynthetic bacteria to a frozen wasteland – all preserved in the layers of giant boulders on a chain of Scottish and Irish islands.

The team, led by researchers at University College London (UCL), examined more than 2,000 zircon grains from eleven sandstone samples taken from depths of up to 200 metres from the 1.1 kilometre-thick Port Askaig Formation and the older, underlying 70-metre-thick Garbh Eileach Formation.

These formations are part of the Dalradian Supergroup of Scotland and Ireland, a chain of geological formations that stretches northeast from Donegal in Ireland through central Scotland, surfacing at places like the Scottish island of Garbh Eileach, where the researchers found their evidence.

Zircon grains deposited in sedimentary layers can be used to determine the age of a rock layer. When it is formed, zircon repels any lead that might become embedded in its structure. However, zircon always contains a certain amount of uranium, which decays to lead at a constant rate over time, even when embedded in lead-hostile zircon.

Lead finds in zircons indicate the decay of uranium, which in turn provides an excellent time indicator.

Using this technique, it was determined that the rocks in the Port Askaig and Garbh Eileach formations were formed between 720 and 662 million years ago, during a period when the Earth was undergoing drastic climate changes, known as the Sturtian Ice Age.

This was the first of two global “freezes” that may have marked the beginning of multicellular life on Earth, so it’s pretty exciting (and of course practical) to find such a well-preserved geological archive from this period so close to the surface.

“Our study provides the first conclusive dating constraints on these Scottish and Irish rocks and confirms their global importance,” says Elias Rugen, PhD student in Earth Sciences at UCL.

Before the upper layers of rock were deposited during the “unimaginable cold” of the Sturtian Ice Age – some believe it was a Snowball Earth event – the older layer of carbonate rock formed in tropical waters, Elias explains.

“These layers document a tropical marine environment with thriving cyanobacterial life that gradually cooled, marking the end of a temperate climate on Earth that lasted about a billion years,” he says.

“In most parts of the world, this remarkable transition is not visible because ancient glaciers have scraped and eroded the underlying rock. But in Scotland, this transition is miraculously visible.”

The age constraints they set for these rocks could mean that the site is considered the official starting point of the Cryogenian.

“These rocks are evidence of a time when the Earth was covered in ice. All complex, multicellular life, such as animals, arose from this deep cold, and the first evidence in the fossil record appeared shortly after the planet thawed,” says geochemist Graham Shields of UCL.

“The retreat of the ice would have been catastrophic. Life was accustomed to tens of millions of years of deep cold. Once the Earth warmed, all life would have had to compete in an arms race to adapt. What survived were the ancestors of all animals,” says Shields.

This research was published in Journal of the Geological Society of London.