Unlocking the secrets of Snowball Earth

Approximately 720 million years ago, Earth is hypothesised to have been gripped by a planetary-scale glaciation, with ice spanning almost from the poles to the equator. This phenomenon, known as a Snowball Earth interval, took place during the Cryogenian period (circa 720 – 635 million years ago, Ma). The planetary-scale freeze is thought to have been driven by ice-sheet expansion triggering a climatic tipping point that led to runaway ice-albedo feedbacks: the ice sheets reflected incoming solar radiation back to space, causing climate cooling and continued ice-sheet formation.  

The Cryogenian period is also thought to have set the stage for the emergence of complex, multicellular life, with animal and algal-based ecosystems beginning to appear once the ice sheets retreated. Much of the evidence for this pivotal period in Earth’s history has been lost through the destructive processes of time, but rocks from a cluster of Scottish islands have now shed light on some of its mysteries. 

These rocks appear to capture the transition from a preceding warm, tropical environment into a new icy world

PhD researcher Elias Rugen from University College London, together with Graham Shields and colleagues, have identified the world’s most complete known record of Snowball Earth in rock formations on the Garvellach islands, Inner Hebrides. These rocks, part of the Port Askaig Formation, appear to be unique because they capture the transition from a preceding warm, tropical environment into a new icy world, and may therefore record the mechanisms behind this dramatic period of global cooling and its impact on the evolution of life. 

Illustration of Snowball Earth (© Science Photo Library)

Thermostat breakdown 

Earth fluctuates between two primary climatic states: greenhouse and icehouse. As Elias explains, Snowball Earth represents a rare third climatic state that the planet has experienced at least twice in its history, during the Sturtian (circa 717 – 660 Ma) and Marinoan (circa 655 – 632 Ma) glaciations, both in the Cryogenian period. 

“The reason it is so rare for Earth to find itself in the Snowball climate state is that the planet has several feedback mechanisms that prevent it from becoming too hot or too cold. A Snowball Earth therefore requires the breakdown of Earth’s natural thermostat.”  

It is critical to understand what can cause Earth’s natural regulatory systems to fail with such spectacular consequences. However, Elias explains that because of the erosive nature of glaciations, rocks deposited during a Snowball Earth interval typically have an erosive contact with their underlying rock formations, obliterating the earlier rock record and leaving a time gap. Remarkably, Elias and his team were able to confirm that rocks in the Port Askaig Formation on the Garvellach islands preserve a transitional contact between carbonate rocks formed in tropical shallow seas (during the Tonian period) and the glacial rocks that formed during Snowball Earth. 

“This transition appears to be a unique record of the final years before glacial onset, making it an extremely special place geologically speaking.” 

Without any age constraints on the Port Askaig Formation, it was impossible to tell whether this transition marks the critical initial passage of Earth into a Snowball state at the start of the Cryogenian period (the Sturtian glaciation) or a later episode of extensive, planetary-scale glaciation (such as the Marinoan glaciation). To explore this, the research team sampled sandstones from the 1.1 km-thick glacial succession to obtain the ages of the enclosed zircon crystals based upon their uranium and lead isotopic compositions.  

“Zircons form in magma chambers and are then scattered around the planet via volcanic eruptions and/or erosion and transport. Using laser ablation mass spectrometry, we analysed the uranium and lead isotopic compositions of zircons from the Port Askaig Formation. Because uranium decays to lead at a known and constant rate, we could determine when the zircons were formed and thereby constrain maximum depositional ages for the sandstones.” 

The new dates – the first for the Port Askaig Formation – confirm that these rocks were indeed formed during the Sturtian glaciation. The formation therefore provides not only “a globally unique record of Earth’s descent into Snowball Earth”, but also a record that could be critical to our understanding of evolution. 

Snowball Earth represents a rare third climatic state that the planet has experienced at least twice in its history

“For billions of years prior to the Cryogenian period, ecosystems were dominated by single-celled organisms, but after this period, complex life developed rapidly with definitive animal fossils found in the ensuing Ediacaran period (circa 635 – 539 Ma). It therefore appears that these glaciations may have been a pivotal turning point in Earth system evolution.” 

Golden spike 

The Port Askaig Formation on the Garvellach islands is now a leading candidate to become a global reference point – a Global Stratotype Section and Point (GSSP) – that defines the lower boundary of the Cryogenian period on the International Chronostratigraphic Chart. Elias and his colleagues will present their formal case to a sub-group of the International Commission on Stratigraphy, who are scheduled to vote on this against other leading candidate sites in 2025. If successful, a ‘golden spike’ will be hammered into the succession, something Elias says would be a significant moment for one of his colleagues, Tony Spencer, who has worked tirelessly on this research for decades. 

“None of this work would have been possible were it not for the relentless determination and enthusiasm of Tony Spencer. He was one of the first people to recognise the significance of these rocks during his PhD in the 1960s and still today he trudges around the islands for weeks at a time each summer, developing deeper insights. His knowledge and enthusiasm are infectious and none of this work would be coming to light if it weren’t for him. I take a lot of inspiration from Tony and hope to continue working on these rocks throughout my career.”  

Acknowledgements 

Elias Rugen thanks Ian Fairchild (University of Birmingham, UK) who pioneered the observations that demonstrated the significance of the transition into the Port Askaig Formation and Snowball Earth glaciation, and Graham Shields (University College London, UK) who first took Elias to the Garvellach islands and continues to help inspire ideas for this research. 

Elias Rugen

Doctoral Researcher at University College London, UK. 

Interview by Hannah Bird, a Doctoral Researcher in Micropalaeontology, Oceanography and Climate Science at the University of Birmingham, and member of the Geoscientist contributors team. 

Further reading 

• Rugen, E.J. et al. (2024) Glacially influenced provenance and Sturtian affinity revealed by detrital zircon U–Pb ages from sandstones in the Port Askaig Formation, Dalradian Supergroup. J. Geological Society 181; doi.org/10.1144/jgs2024-029 

• Shields, G. (2024) Born of Ice and Fire: How Glaciers and Volcanoes (with a Pinch of Salt) Drove Animal Evolution. Yale University Press, 352 pp. 

• Subcommission on Cryogenian Stratigraphy;
  cryogenian.org/defining-cryogenian 

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