At the end of the Proterozoic, the Earth was covered in a large-scale glaciation. It is believed to have played a major role in reshaping landscapes and accelerating the evolution of species. But many details remain unclear. In a new study, European scientists have dated a rock complex in Scotland and Ireland that preserves the most complete picture of that period.

In the distant past, two major glaciations occurred on Earth one after another: the Stert and Marinoan glaciations. The first (717-658 million years ago) was the most severe. There was ice even at the equator. In geology, this period is called the Cryogenian, and the hypothesis that describes it is “Snowball Earth”.

Some believe that in cryogenics the planet was completely frozen, oceans, rivers froze, immobile glaciers bound everything. Others, on the contrary, believe that glaciations were local, and the surface was actively eroded, significantly changing the landscape of the continents.

In the 1970s scientists proposed that the Port Askaig rock complex, exposed on the Isle of Islay and elsewhere in Scotland, was formed during the Sturt glaciation. It is one of the most complete stratigraphic sections of that period. The thickness of the formation (1.1 kilometres) and the signs of glacial advance and retreat contradict the hypothesis of a long-term permanent ice cover.

To clarify the question, scientists from Great Britain, led by University College London, analysed the dating of detrital zircons from the Port Askaig Formation. It is most represented in the Scottish islands north and south of County Donegal, Ireland. Stratigraphically, it occupies an intermediate position between the larger Dalred rock series. It is underlain by the Gar Eilekh Formation.

Scientists took 11 samples from sandstones of the Port Askaig and Gar Eileh formations, isolated more than 2,000 detrital zircons from them and dated them using the uranium-lead method. A related paper has been published Geology Association Journal.

The ages of the minerals in eight samples corresponded to the Stertian glaciation. In the others they are much older and date back to the Archaean period; according to the authors this indicates that the old Laurentian continent was destroyed by a glacier. The glaciation was therefore partial and mobile. Deep transformations of the landscape are also evidenced by the distribution of zircons of different ages up to the Askeig Harbour formation.

One of the outcrops on the Scottish Garwell Islands is of particular interest. It preserves a record of the change from the warm tropical climate before the Cryogenian to the glaciation. Other races of similar age, such as North America and Namibia, do not have such a transition.

Elias Rugen, the first author of the article, explained that for the first time they have managed to obtain convincing data on the age of these breeds in Scotland and Ireland, confirming their global importance.

“The breeds at Harwell are unique. These are 70 metres of ancient carbonates that formed in tropical waters, beneath layers of sediments. They have preserved traces of a warm marine environment where cyanobacterial life thrived and gradually shifted to a colder environment. “The billion-year period of Earth’s temperate climate is over,” Rugen said.

Knowing the exact age of the rocks, scientists can now mark the beginning of the cryogeny. Perhaps it will be the new “golden nail”. It is the name of the reference section that indicates the lower limit of the geological layer.