Scottish and Irish rocks confirmed as rare recording of ‘Snowball Earth’

A rock formation spanning Ireland and Scotland could be the world’s most complete record of ‘Snowball Earth’, a crucial moment in planetary history when the globe was covered in ice, a new study by researchers at University College London has revealed.

The study, published in the Journal of the Geological Society of Londondiscovered that the Port Askaig Formation, composed of rock layers up to 1.1 km thick, probably formed 662 to 720 million years ago during the Sturtian glaciation, the first of two global freezes thought to have triggered the development of complex multicellular life.

An exposed outcrop of the formation, discovered on the Scottish islands called Garvellachs, is unique because it shows the transition to a ‘Snowball Earth’ from a previously warm tropical environment.

Other rocks that formed around the same time, for example in North America and Namibia, do not show this transition.

Professor Graham Shields, lead author of the study and Professor of Earth Sciences at UCL, said: “These rocks provide evidence of a time when the Earth was covered in ice. All complex, multicellular life, such as animals, arose from this deep freeze, and the first fossil evidence appeared 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 importance.

“The rock layers exposed on the Garvellachs are unique in the world. Beneath the rocks laid down during the unimaginably cold Sturtian glaciation lie 70 metres of older carbonate rocks formed in tropical waters. These layers provide evidence of a tropical marine environment with thriving cyanobacterial life that became progressively colder, marking the end of about a billion years of a temperate climate on Earth.

“Most parts of the world have not seen this remarkable transition because ancient glaciers scraped and eroded the rocks beneath, but in Scotland, miraculously, the transition can be seen.”

The Sturtian Ice Age lasted about 60 million years and was one of two major freezing periods during the Cryogenic Period (635–720 million years ago). For billions of years before this period, life consisted only of single-celled organisms and algae.

After this period, complex life appeared rapidly, in geological terms, with most of today’s animals being fundamentally similar to the types of life forms that evolved more than 500 million years ago.

One theory is that the hostile nature of extreme cold may have caused the emergence of altruism, with single-celled organisms learning to cooperate with each other, thus forming multicellular life.

The advance and retreat of ice on the planet was thought to have occurred relatively quickly, over thousands of years, due to the albedo effect: the more ice there is, the more sunlight is reflected back into space, and vice versa.

Professor Shields explained: “The retreat of the ice would have been catastrophic. Life had been accustomed to tens of millions of years of deep freezing. As soon as the planet warmed, all life would have had to participate in an arms race to adapt. Those that survived were the ancestors of all animals.”

For the new study, the research team collected samples of sandstone from the 1.1km-thick Port Askaig Formation, as well as the older 70m-thick Garbh Eileach Formation below it.

They analyzed tiny, extremely strong minerals in the rock called zircons. These can be dated precisely because they contain uranium, a radioactive element that transforms (decays) into lead at a constant rate. The zircons, along with other geochemical evidence, suggest that the rocks were deposited between 662 and 720 million years ago.

The researchers said the new age constraints for the rocks could provide the evidence needed for the site to be declared a marker of the start of the Cryogenic period.

This marker, known as the Global Boundary Stratotype Section and Point (GSSP), is sometimes called a gold spike because a gold spike is driven into the rock to mark the boundary.

GSSPs attract visitors from all over the world and, in some cases, museums have been established on these sites.

A group from the International Commission on Stratigraphy, part of the International Union of Geological Sciences, visited the Garvellachs in July to assess the possibility of a golden point on the archipelago. Currently, the islands are only accessible by chartered boat, sailboat or kayak.

The study involved researchers from UCL, the University of Milan-Bicocca in Italy, and Birkbeck University of London.