An exposed formation found in the Scottish Isles known as the Carvellocks is unique because it shows the transition from a previously warm, tropical environment to a „snowball earth”.
The study, published in the Journal of the Geological Society of London, suggests that the Port Ascaic Formation, made up of 1.1km-thick layers of rock, may have been laid down between 662 and 720 million years ago during the Sturdian glaciation – two global freezes thought to have triggered the development of the first complex, multicellular life.
An exposed formation found in the Scottish Isles known as the Carvellocks is unique because it shows the transition from a previously warm, tropical environment to a „snowball earth”.
Other rocks formed at the same time, for example in North America and Namibia, do not show this change.
Senior author Professor Graham Shields of UCL Earth Sciences said: „These rocks record the time when the Earth was covered in ice. All complex, multicellular life, such as animals, arose from this deep freeze, and the first evidence in the fossil record appeared shortly after the planet thawed.
First author Elias Rugen, a PhD candidate in UCL Earth Sciences, said: „Our study provides the first firm age constraints for these Scottish and Irish rocks, confirming their global importance.
„The rock layers exposed at Curvellogs are unique worldwide. The 70-meter-old carbonate rocks formed in tropical waters beneath the rocks laid down during the unimaginable cold of the Sturtian glaciation. These layers record a tropical marine environment that flourished with cyanobacterial life as it gradually cooled, a billion years or so of temperate climates on Earth.” Indicates the end.
A compilation of the geological background of the Port Askig Formation. (a) Generalized stratigraphy of the lower part of the Dalradian Supergroup (Stephenson et al. 2013). The older geochronology shown in the lithostratigraphic record of the Dalradian is derived from U-Pb ages from metamorphic rocks beneath the Grampian Group (Nobel et al. 1996), and the younger barrier is from rift-related U-Pb zircon ages. Volcanics at the top of the Argyll Group (Dempster et al. 2002). Two Re-Os ages have also been shown for the Appin Group Ballachulish Slate Formation (Rooney et al. 2011) and the Easdale Group Ben Eagach Schist Formation (Moles and Selby 2023). The validity of these as depositional ages is discussed in the text. (b) A stratigraphic column from the Port Ascaik Formation (Ali et al. 2018). Approximate layer positions of model horizons are shown. (c) Simplified geological map of the Dalradian Supergroup in Scotland (Shetland Islands not shown). (d) Simplified geological map of the Dalradian Supergroup in Donegal, Ireland. Source: Parts (c) and (d) modified after Thomas et al. (2004).
„Much of the world has not seen this dramatic change because ancient glaciers have eroded away the underlying rock, but in Scotland the change can be seen by some miracle.”
The Sturtian glaciation lasted approximately 60 million years and was one of the two major glaciations of the Cryogenian period (635 and 720 million years ago). For billions of years before this period, life consisted of single-celled organisms and algae.
After this period, geologically complex life emerged rapidly, and most animals today resemble in basic ways life forms that evolved 500 million years ago.
According to one theory, the hostile nature of extreme cold may have spurred the emergence of altruism, with single-celled organisms learning to cooperate with one another to form multicellular life.
Because of the albedo effect, the advance and retreat of ice across the planet was thought to happen relatively quickly, over thousands of years—that is, the more ice, the more sunlight reflected back into space, and vice versa. .
Professor Shields explained: „The retreat of the ice would have been catastrophic. Life was used to the deep freeze for tens of thousands of years. As the world warmed, all life would have had to compete in an arms race to adapt. The survivors were the ancestors of all animals.
For the new study, the research team collected sandstone samples from the 1.1-km-thick Port Askig Formation and the older, 70-meter-thick Carp Eleach Formation.
They studied tiny, highly durable minerals in the rock called zircons. Because these contain the radioactive element uranium, they can be accurately dated because they convert (decay) to lead at a uniform rate. The zircons, along with other geochemical evidence, suggest that the rocks were formed between 662 and 720 million years ago.
The researchers said the new age constraints for the rocks could provide the evidence needed to declare the site a marker for the start of the Cryogenian period.
This marker, known as the Global Boundary Stratotype Section and Point (GSSP), is sometimes referred to as the Golden Spike because a golden 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 the sites.
A panel of the International Commission on Stratigraphy, part of the International Union of Geological Sciences, visited Karvellax in July to evaluate the case for gold spikes in the archipelago. Currently, the islands can only be accessed by renting a boat or by sailing or kayaking.
The study involved researchers from UCL, the University of Milano-Bicocca in Italy and Birkbeck University in London. This work was funded by the Natural Environment Research Council (NERC).
Glacial impact and Sturdian correlation revealed by detrital zircon U-Pb ages from sandstones in the Port Ascaig Formation, Dalradian Supergroup.Journal of the Geological Society (Open Access)
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