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3.5 billion years ago, oceans were cool, not hot


About 3.5 billion years ago, Earth’s oceans were cool, not inhospitably hot as previously thought. In fact, the entire planet at the time was probably locked in a cold snap that lasted at least 30 million years, a new study concludes. The findings, published online February 26 in Science Advances, could change the view of Earth’s ancient climate and life’s earliest years.

“This is the first evidence that over the entire [last] 3.5 billion years, Earth has operated within a temperature range that suits life,” says Maarten de Wit, a geologist at Nelson Mandela Metropolitan University in Port Elizabeth, South Africa.

Evidence for this big chill was found in South Africa’s Barberton Greenstone Belt, which contains some of the oldest, best preserved rocks on Earth. Along with Harald Furnes, a geologist at the University of Bergen in Norway, de Wit spent six years mapping and sampling the Barberton. The researchers studied volcanic rock and a kind of silica called chert that formed deep underwater. They also studied shallower sedimentary and volcanic rocks deposited 30 million years after the deep ocean rocks.

The researchers analyzed hundreds of these rock samples for the concentration of oxygen-18 isotopes, an indicator of what the temperature was like when the rocks formed. They also discovered in the younger rocks diamictite — a clay-rich sedimentary rock typically formed in glacial environments — and in the older rocks gypsum, which 3.5 million years ago would have formed only in deep, cold seas. These findings suggest that both the shallow environment and deep waters were cool. Ambient ocean temperatures must have been close to 0°Celsius, de Wit says.  ….

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