How ancient Scottish rocks throw ‘snowball Earth’ theory up in the air

During the ”snowball Earth” period about 700m years ago, Earth’s climate shut down. The planet was encased in ice and insulated from seasonal variations: spring, summer, autumn and winter all stopped. Or at least that was the theory.

Recent examination of some ancient rocks from the west coast of Scotland have now overturned that thinking, suggesting there were periods during snowball Earth when the climate woke up.

Close-up views of thin, repeating rock layers known as varves, each thought to represent a single year of sedimentation during the snowball Earth period. Photograph: Prof Thomas Gernon/University of Southampton

Thomas Gernon and Chloe Griffin, from the University of Southampton, along with other colleagues, studied rocks from the remote islands of the Garvellachs that were deposited during the snowball Earth period.

Under a microscope they were able to examine 2,600 exquisitely preserved layers, recording year by year changes in climate. Variations in layer thickness revealed climate cycles similar to those that can be seen today, such as solar cycles and El Niño oscillations.

Another closeup view of varves. By analysing thousands of these layers, the researchers identified climate cycles operating during Earth’s deep-freeze period. Photograph: Prof Thomas Gernon/University of Southampton

Their findings, which are published in Earth and Planetary Science Letters, suggest such occurrences were rare during snowball Earth and that these particular rocks recorded a slushy interlude lasting a few thousand years, when a small fraction of ocean thawed and the climate briefly woke up.

But the findings are more than a mere curiosity. They shed light on how sensitive Earth’s climate system really is and offer important pointers as to how Earth might respond to major disturbances in the future.

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