An outcrop called ‘the Bubble’ on Eileach an Naoimh (Holy Isle) reveals a fascinating geological record of Earth’s ancient “snowball” phase. This massive white rock fragment, tens of meters across, was originally part of the underlying rock sequence. The layering in the carbonate rock has been tightly squeezed under immense pressure and transported by thick ice sheets to its final resting place as one of many different rock fragments within a moraine. The discovery of this evidence sheds light on a critical period in Earth’s history, providing insights into the emergence of complex life on our planet.
### Unveiling Earth’s Frozen Past
During a period roughly 635 to 720 million years ago, Earth was enveloped in thick ice, resembling a snowball. Although the planet may have seemed desolate at the time, this icy phase played a pivotal role in setting the stage for the eventual flourishing of complex life on Earth. A team of international scientists has now unearthed what they believe to be the most comprehensive geological record of this “snowball Earth” phenomenon in the Port Askaig Formation located in Scotland and Ireland. This extensive rock formation, likely deposited between 662 and 720 million years ago, just before a surge in life forms, offers valuable insights into our planet’s ancient past. The findings of this study were published on August 15 in the Journal of the Geological Society of London.
“These rocks provide a window into a time when Earth was blanketed in ice,” explained Graham Shields, a geologist from University College London and co-author of the study. “The emergence of complex, multicellular life, including animals, can be traced back to this frozen epoch, with the first evidence appearing shortly after the planet thawed.”
### Ice Ages and the Origins of Life
The distinctive rocks examined in this study date back to the Sturtian glaciation, a period lasting around 60 million years during the Cryogenian Period. Prior to the Cryogenian era, Earth had been significantly warmer, with life primarily dominated by single-celled organisms and algae. The transition from this warm climate to the deep freeze of the Cryogenian likely played a crucial role in the evolution of complex life forms. The rapid emergence of animals and other multicellular organisms following this icy phase suggests a connection between extreme cold and the development of altruism among early life forms. The ability of single-celled organisms to cooperate and form multicellular structures could have paved the way for the diverse life forms we see today.
The advance and retreat of glacial ice across Earth occurred relatively swiftly in geological terms, taking place over thousands of years. This rapid cycling is attributed to the albedo effect, where increased ice cover reflects more sunlight back into space, leading to further cooling. Shields noted, “The retreat of the ice would have been a cataclysmic event. Life that had adapted to tens of millions of years of extreme cold suddenly had to evolve rapidly in response to a warming world. The survivors of this adaptation process became the ancestors of all modern animals.”
### Unveiling the Geological Record
To uncover tangible evidence of Earth’s frozen past, the research team turned their attention to the Port Askaig Formation in Scotland and Ireland. This extensive rock formation spans the region and comprises several layers, some reaching up to half a mile in thickness. An exposed outcrop on the Garvellachs, a group of Scottish islands, offers a unique glimpse into our planet’s transition from a tropical paradise to a frozen snowball during the Sturtian glaciation. In contrast, similar rocks from the same period in North America and Africa do not exhibit this climate shift.
The layers of rock observed on the Garvellachs stand out globally for their uniqueness. Beneath the rocks deposited during the Sturtian glaciation lie 70 meters of older carbonate rocks that formed in tropical waters. Elias Rugen, a UCL PhD candidate and co-author of the study, remarked, “These layers document a tropical marine environment teeming with cyanobacterial life that gradually gave way to cooler conditions, marking the end of a billion years of temperate climate on Earth.”
The team collected samples of sandstone from the Port Askaig Formation and an older, 229-feet thick Garbh Eileach Formation beneath it. By analyzing small, durable minerals known as zircons, which contain the radioactive element uranium, geologists can accurately date rocks. The decay rate of uranium within zircons provides valuable insights into the age of the rocks, indicating that they were deposited between 662 and 720 million years ago.
The new dating of these rocks could potentially establish this site as a definitive marker for the beginning of the Cryogenian Period in the geological record. Referred to as the Global Boundary Stratotype Section and Point (GSSP), this marker serves as a reference point for delineating geological eras. The International Union of Geological Sciences is currently evaluating whether the outcropping in the Garvellachs warrants designation as a GSSP, also known colloquially as a “golden spike” due to the practice of driving spikes into rocks to mark boundaries.
By unraveling the geological mysteries preserved in the Port Askaig Formation, scientists are piecing together a comprehensive narrative of Earth’s ancient past. This remarkable record offers valuable insights into the evolution of our planet and the emergence of complex life forms that have shaped the world we know today.