Published in the Journal of the Geological Society of London, the study reveals that the Port Askaig Formation, consisting of rock layers up to 1.1 kilometers thick, likely formed between 662 and 720 million years ago during the Sturtian glaciation. This was the first of two global ice ages believed to have played a critical role in the development of complex multicellular life.
One particularly well-preserved outcrop of the formation, located on the Scottish islands known as the Garvellachs, stands out as it captures the transition into "snowball Earth" from a previously warm, tropical environment. This transition is notably absent in similar-aged rock formations found in North America and Namibia.
"These rocks record a time when Earth was covered in ice. All complex, multicellular life, such as animals, arose out of this deep freeze, with the first evidence in the fossil record appearing shortly after the planet thawed," explained Senior author Professor Graham Shields of UCL Earth Sciences.
"Our study provides the first conclusive age constraints for these Scottish and Irish rocks, confirming their global significance," added Elias Rugen, a PhD candidate at UCL Earth Sciences and the study's first author. "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 meters of older carbonate rocks formed in tropical waters. 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."
The Sturtian glaciation, which lasted approximately 60 million years, was one of two major ice ages during the Cryogenian Period, which spanned from 635 to 720 million years ago. Before this era, life on Earth was limited to single-celled organisms and algae. Following this period, complex life forms emerged rapidly, with many modern animals sharing fundamental similarities with life forms that evolved more than 500 million years ago.
One hypothesis suggests that the extreme cold conditions may have encouraged the development of cooperation among single-celled organisms, ultimately leading to the evolution of multicellular life.
The research team collected samples of sandstone from the 1.1 km-thick Port Askaig Formation and from the older, 70-meter-thick Garbh Eileach Formation beneath it. The team analyzed zircon minerals within the rocks, which are highly durable and can be precisely dated due to the presence of radioactive uranium, which decays into lead at a steady rate. The zircon dating, along with other geochemical evidence, indicates that these rocks were deposited between 662 and 720 million years ago.
The new age constraints for the rocks may provide the necessary evidence for the site to be declared a Global Boundary Stratotype Section and Point (GSSP), a marker known as a "golden spike," which signifies the beginning of the Cryogenian Period.
A team from the International Commission on Stratigraphy, part of the International Union of Geological Sciences, visited the Garvellachs in July to evaluate the potential for establishing a golden spike on the islands. Currently, the site is accessible only by chartering a boat or by sailing or kayaking to the archipelago.
Related Links
University College London
Beyond the Ice Age
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