A rock formation over Ireland and Scotland may be a rare record of “snowball Earth” — a pivotal moment in planetary history when the globe was covered in ice.
The Port Askaig Formation, which consists of layers of rock up to 1.1 km (0.7 mi) thick, was probably deposited between 662 m and 720 m years ago during the Sturtian glaciation, research suggests. This was the first of two global freezing points that are thought to have triggered the development of complex life.
According to the study, one section of exposed rock, found on Scotland’s Garvellachs Islands, is unique as it shows the transition in snowball Earth from a previously warm, tropical environment. Other rocks that formed at a similar time, such as some in North America and Namibia, miss this transition.
Therefore, researchers believe their findings may be the world’s most complete record of snowball Earth – a theory that suggests the Earth’s oceans and land surfaces were covered in ice, from the poles to the equator, during at least two extreme cooling events between 2.4 billion and 580 m. years ago.
Prof Graham Shields, a senior author of the study, from University College London (UCL) Earth Sciences, said: “These rocks record a time when the Earth was covered with ice. All complex, multicellular life, such as animals, arose from this deep freeze, with the first evidence in the fossil record appearing shortly after the planet thawed.”
First author Elias Rugen, a PhD candidate at UCL Earth Sciences, said: “Our study provides the first conclusive age constraints for these Scottish and Irish rocks, confirming their global significance.”
He said that most areas of the world lack the layers in the rocks that record a tropical environment and mark the transition, because the ancient glaciers scraped and swept away the rocks underneath. “But in Scotland the transition can be seen by some miracle,” Rugen said.
The Sturtian glaciation, which lasted about 60 m years, was one of two major freezes that occurred during the Cryogenian period between 635 m and 720 m years ago. For billions of years before that, life consisted only of single-celled organisms and algae. After this period, complex life arose rapidly, with most animals today similar in fundamental ways to the types of life forms that evolved more than 500m years ago.
One theory is that the hostile nature of the extreme cold may have spurred single-celled organisms to cooperate with each other and form multicellular life.
The advance and retreat of the ice across the planet is thought to have happened relatively quickly, over thousands of years, due to the albedo effect – that is, 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 catastrophic. Life was used to tens of millions of years of deep freezing. Once the world warmed, all life would have to compete in an arms race to adapt. Whatever survived were the ancestors of all animals.”
For the study, the research team analyzed samples of sandstone from the Port Askaig Formation as well as from the older, 70 meter thick Garbh Eileach Formation below.
The researchers said the new age constraints for the rocks could provide the evidence needed for the site to be declared a marker for the beginning of the Cryogenic period. This marker, known as a global boundary stratotype section and point (GSSP), is sometimes referred to as a gold point – as a gold point is driven into the rock to mark the boundary – and these sites attract visitors from around the world.
The study, led by UCL researchers, is published in the Journal of the Geological Society of London.
