Evrim Yazgin
Cosmos science journalist
Rocks ranging from Ireland to Scotland might contain the most complete record of “Snowball Earth” – a crucial period of our planet’s history which may have triggered the development of complex, multicellular life – according to a new study.
“Snowball Earth” was the world’s most extreme Ice Age. It lasted about 60 million years and began more than 700 million years ago. Snowball Earth, also called the Sturtian glaciation, was one of 2 big freezes which occurred during the Cryogenian geological period (720–635 million years ago).
For billions of years prior to the Cryogenian, the only life on our planet was single-celled organisms.
During the Sturtian glaciation ice covered the entire planet – evidence of which can be seen in rocks in places as far flung as Namibia, North America and Australia.
But exactly what caused the expansion of ice, over a relatively short period of time, across the globe remains a mystery.
Research published in the Journal of the Geological Society of London might hold some answers.
Layers of rock making up the 1.1-kilometre-thick Port Askaig Formation in Scotland were laid down between 662 and 720 million years ago. This formation, the authors of the new research say, contains one exposed outcrop which is unique because it shows the transition to snowball Earth from a previously warm, tropical environment.
The rock is on the Scottish islands called the Garvellachs (Garbh Eileaicha in Scottish Gaelic).
“The layers of rock exposed on the Garvellachs are globally unique. Underneath the rocks laid down during the unimaginable cold of the Sturtian glaciation are 70 metres of older carbonate rocks formed in tropical waters,” says first author Elias Rugen, a PhD candidate at University College London (UCL). “These layers record a tropical marine environment with flourishing cyanobacterial life that gradually became cooler, marking the end of a billion years or so of a temperate climate on Earth.
“Most areas of the world are missing this remarkable transition because the ancient glaciers scraped and eroded away the rocks underneath, but in Scotland by some miracle the transition can be seen.”
Rugen and colleagues analysed zircon minerals in sandstone from the Port Askaig and older Garbh Eileach formations. These minerals can be dated using radiometric dating because they contain uranium.
Soon after the Cyrogenian, life flourished.
“The retreat of the ice would have been catastrophic,” explains senior author Graham Shields, a professor at UCL. “Life had been used to tens of millions of years of deep freeze. As soon as the world warmed up, all of life would have had to compete in an arms race to adapt. Whatever survived were the ancestors of all animals.”
