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Ancient oceans were surprisingly cold: but isolated hydrothermal activity might have nurtured life.

About 3.5 billion years ago, Earth's oceans were cool, not inhospitably hot as previously thought. In fact, the entire planet was probably locked in a cold snap that lasted at least 30 million years, a new study concludes. The findings, published 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 roughly 30 million years after the deep ocean rocks.

De Wit and Furnes analyzed hundreds of rock samples for the concentration of oxygen-18 isotopes, an indicator of what the temperature was like when the rocks formed. The researchers also found other temperature clues. In the younger rocks, they discovered diamictite, a clay-rich sedimentary rock typically formed in glacial environments. And in the older rocks, they found gypsum, which 3.5 billion years ago would have formed only in deep, cold seas. Together, these findings suggest that both the shallow and deep waters were cool. Ambient ocean temperatures must have been close to zero degrees Celsius, de Wit says.

Paleomagnetic data point to a colder-than-expected global environment, too, de Wit and Furnes found. As volcanic rock cools, minerals in the rock capture the prevailing magnetic pole direction, which reverses every few hundred thousand years. Scientists use the data to estimate the latitude at which rocks formed. In this case, the rocks bearing evidence of low temperatures formed near the tropics at a latitude of 20[degrees] to 30[degrees]. "Because there was ice near sea level at low latitudes," de Wit says, "the oceans and atmosphere were globally likely to be cold."

What's more, de Wit and Furnes figured out why previous researchers had interpreted ocean temperatures to be roughly 30[degrees] C to 80[degrees] C back then. Two periods of searing-hot hydrothermal activity had cooked both the seafloor cherts and the surface glacial sediments. In the older seafloor sediments from the Barberton, the team discovered hard evidence for hydrothermal vents. Earlier studies had focused mainly on oxygen isotopes from limited samples that happened to have been strongly affected by this hydrothermal activity, and those researchers had not recognized that the results revealed local, not ambient, ocean temperatures.

That, de Wit says, was like looking at data from Yellowstone hot springs and extending them to an entire ocean. By sampling a much broader area, he and Furnes determined that the superheating effects of hydrothermal activity had been strictly local. "You really have to map carefully and do a lot of isotope follow-up work to test it all," he says.

The study has implications for how life may have evolved. While hot oceans would have been largely inhospitable, de Wit says that hydrothermal fields in a cool ocean would have provided a nurturing environment, just as scientists see today around deep ocean hydrothermal vents.

"This study is quite significant," says Yale University geochemist Ruth Blake, who has also studied the Barberton. The researchers "present compelling new evidence that advances our understanding of one of the most highly debated periods in Earth's history."

Caption: Billions of years ago, localized hydrothermal activity, not globally warm oceans, heated and altered this South African chert (rocks in the foreground) that formed on the seafloor.

Caption: Among the evidence pointing to cold oceans 3.5 billion years ago are varved sediments (shown). Varves are seasonal bands that typically form today when lakes freeze each winter.


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Title Annotation:EARTH & ENVIRONMENT
Author:Geiger, Beth
Publication:Science News
Date:Apr 2, 2016
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