Nature Geoscience

Earth Went Strangely Quiet About 2 Billion Years Ago And We Don’t Know Why

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It slowed right down.

 A new study has added evidence to the hypothesis that our planet experienced a lull in geology between 2.2 and 2.3 billion years ago, when not a lot went on as far as rock-forming processes go.

The relatively dormant phase in our planet’s history signals a significant change in tectonics, one that is fuelling discussion on exactly how continents form and could possibly provide better details on exactly where we can find new deposits of various mineral resources.

The era known as the Palaeoproterozoic covers a rather exciting time in Earth’s history, starting 2.5 billion years ago and ending around a billion years later.

Life was literally a lot simpler then. Days were four hours shorter. Our atmosphere was yet to have a lot of oxygen. There were the first global glaciation events. And the planet’s first supercontinent – a huge chunk of land called Columbia, or Nuna – was in the process of being formed.

As you might imagine, geologists are keen to understand how this far younger Earth behaved compared to today’s more mature globe.

It seems as if around 2.45 billion years ago, there was something of a quiet spell beneath the surface, one that lasted around 250 million years.

Not that everybody is convinced – other interpretations of the research suggest it was business as usual throughout the Palaeoproterozoic.

With the jury still out, more evidence is needed. Which is just what a new study led by researchers from Curtin University has provided.

A close look at the existing data as well as new rock samples collected from Western Australia, China, Northern Canada and Southern Africa has added weight to what’s described as a tectono-magmatic shutdown.

“Our research shows a bona fide gap in the Palaeoproterozoic geologic record, with not only a slowing down of the number of volcanoes erupting during this time, but also a slow-down in sedimentation and a noticeable lull in tectonic plate movement,” says Curtin University geoscientist Christopher Spencer.

Earth’s guts were a lot hotter a few billion years ago. For a while all that churning resulted in a whole lot of volcanic activity.

Whether that directly led to significant cooling, or if something else happened beneath the crust, nobody is sure.

But we can now be fairly confident that about 2.3 billion years ago, things went quiet under the lid. Volcanoes were temporarily out of fashion. Plate movements were subdued.

Earth was taking a break.

“This ‘dormant’ period lasted around 100 million years and signalled what we believe was a shift from ‘ancient-style’ tectonics to ‘modern-style’ tectonics more akin to those operating in the present day,” says Spencer.

“It’s almost as if the Earth experienced a mid-life crisis.”

After a bit of a breather, things ramped up again. Chunks of ancient crust fractured into smaller pieces called cratons, which can today be found deep inside continental plates.

“Following this dormant period Earth’s geology started to ‘wake-up’ again around 2.2 to 2.0 billion years ago with a ‘flare-up’ of volcanic activity and a shift in the composition of the continental crust,” says Spencer.

Why did the mantle ‘flare up’ again after a quiet spell? The researchers aren’t sure, but have speculated it might simply come down to a surge of accumulated heat.

Understanding the geological processes that led from ‘supercratons’ to the first supercontinent could help us understand how many of the mineral resources we rely upon formed and distributed.

More data is needed to fill in missing details on this geological ‘mid-life crisis’ model, but we can at least be grateful Earth didn’t quit its job and run off with some young moon.

Source:Nature Geoscience.

Global ocean currents explain why Northern Hemisphere is the soggier one.

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A quick glance at a world precipitation map shows that most tropical rain falls in the Northern Hemisphere. The Palmyra Atoll, at 6 degrees north, gets 175 inches of rain a year, while an equal distance on the opposite side of the equator gets only 45 inches. Scientists long believed that this was a quirk of the Earth’s geometry – that the ocean basins tilting diagonally while the planet spins pushed tropical rain bands north of the equator. But a new University of Washington study shows that the pattern arises from ocean currents originating from the poles, thousands of miles away.

Global ocean currents explain why Northern Hemisphere is the soggier one

The findings, published Oct. 20 in Nature Geoscience, explain a fundamental feature of the planet’s climate, and show that icy waters affect seasonal rains that are crucial for growing crops in such places as Africa’s Sahel region and southern India.

In general, hotter places are wetter because hot air rises and moisture precipitates out.

“It rains more in the Northern Hemisphere because it’s warmer,” said corresponding author Dargan Frierson, a UW associate professor of atmospheric sciences. “The question is: What makes the Northern Hemisphere warmer? And we’ve found that it’s the  circulation.”

Frierson and his co-authors first used detailed measurements from NASA’s Clouds and Earth’s Radiant Energy System, or CERES, satellites to show that sunlight actually provides more heat to the Southern Hemisphere – and so, by atmospheric radiation alone, the Southern Hemisphere should be the soggier one.

After using other observations to calculate the ocean heat transport, the authors next used computer models to show the key role of the huge conveyor-belt current that sinks near Greenland, travels along the ocean bottom to Antarctica, and then rises and flows north along the surface. Eliminating this current flips the  bands to the south.

The reason is that as the water moves north over many decades it gradually heats up, carrying some 400 trillion (that’s four with 14 zeroes after it) watts of power across the equator.

For many years, slanting  have been the accepted reason for the asymmetry in  rainfall.

“But at the same time, a lot of people didn’t really believe that explanation because it’s kind of a complicated argument. For such a major feature there’s usually a simpler explanation,” Frierson said.

The ocean current they found to be responsible was made famous in the 2004 movie “The Day After Tomorrow,” in which the premise was that the overturning circulation shut down and New York froze over. While a sudden shutdown like in the movie won’t happen, a gradual slowing – which the recent United Nations report said was “very likely” by 2100 – could shift tropical rains south, the study suggests, as it probably has in the past.

The slowdown of the currents is predicted because increasing rain and freshwater in the North Atlantic would make the water less dense and less prone to sinking.

“This is really just another part of a big, growing body of evidence that’s come out in the last 10 or 15 years showing how important high latitudes are for other parts of the world,” Frierson said.

Frierson’s earlier work shows how the changing temperature balance between hemispheres influences tropical rainfall. A recent study by Frierson and collaborators looked at how pollution from the industrial revolution blocked sunlight to the Northern Hemisphere in the 1970s and ’80s and shifted tropical rains to the south.

“A lot of the changes in the recent past have been due to ,” Frierson said. “The future will depend on air pollution and global warming, as well as  changes. That makes  particularly hard to predict.”