Rock formations across Ireland and Scotland could be a valuable record of Snowball Earth, a key moment in Earth’s history when the planet was covered in ice.
The study found that the Port Askyg Formation, a layer of rock up to 1.1 kilometres thick, likely formed during the Sturtian Ice Age between 662 and 720 million years ago, the first of two global glacial periods thought to have triggered the development of complex life.
The study found that some of the exposed rocks found in Scotland’s Gerberax Islands are unique in that they show a transition from a previously warm, tropical environment to Snowball Earth — a transition that doesn’t exist in other rocks that formed around the same time, including in North America and Namibia.
That’s why researchers believe their discovery may be the world’s most complete record of Snowball Earth, a theory that posits that at least two extreme cooling events occurred between 2.4 billion and 580 million years ago, leaving Earth’s oceans and land surface covered in ice from the poles to the equator.
“These rocks record a time when the Earth was covered in ice. Complex multi-cellular organisms such as animals all emerged from this extreme cold, and the first evidence in the fossil record appeared soon after the Earth thawed,” said Professor Graham Shields, from UCL’s School of Earth Sciences, lead author of the study.
“Our study provides the first conclusive constraints on the age of rocks from Scotland and Ireland, confirming their global importance,” said lead author Elias Rugen, a PhD student in the School of Geosciences at University of London.
In most parts of the world, he said, ancient glaciers have scraped and eroded the rocks, causing the rock layers that record tropical environments and show transitions to be lost, “but in Scotland, miraculously, we get to see the transition,” Rugen said.
The Sturtian glaciation, which lasted 60 million years, was one of two major freezes that occurred during the Cryogenian period, between 635 and 720 million years ago.
For billions of years prior to this, life consisted only of single-celled organisms and algae. After this period, complex life emerged rapidly, and most of today’s animals are similar in fundamental ways to the types of life forms that evolved over 500 million years ago.
One theory is that the harsh nature of the cold may have caused single-celled organisms to cooperate with each other and form multicellular organisms.
The advance and retreat of ice on Earth is thought to have happened relatively rapidly over thousands of years due to the albedo effect (i.e. the more ice there is, the more sunlight is reflected back into space, and vice versa).
Shields said: “The retreat of the ice would have been devastating. Life had been accustomed to extreme cold for tens of millions of years. As soon as the planet warmed, all life would have had to go through an arms race to adapt. Those that survived became the ancestors of all animals.”
For the study, the team analysed sandstone samples from the Port Askaig Formation and the underlying, older, 70-metre-thick Garb Ayleach Formation.
The researchers said the new age constraints on the rocks may provide the evidence needed to declare the site as a marker for the beginning of the Cryogenian period.
The markers, known as Global Slope Marker Sections and Points (GSSPs), are sometimes called Golden Spikes because gold nails are driven into the rock to mark the boundary, and the sites attract tourists from around the world.
The study was led by researchers at UCL and published in the Journal of the Geological Society of London.
