Perhaps "Snowball Earth" wasn't a never-ending frozen wasteland after all.

 

Earth during the Marinoan Ice Age might have been more slushy than we thought. (Philipp Tur/Getty Images)

Earth experienced an ice era unlike any other before complex life emerged from the oceans to colonise land. Fingers of ice reached far from the poles on two distinct occasions, turning the planet into a frozen waste.

 

It has been disputed just how far ice could travel. Some claim the Earth was fully covered in a thick layer of ice as far as the equator due to the runaway deep freeze, which allegedly affected all latitudes.

 

Some claim that equatorial regions may have had little ice. There may have been areas of exposed ocean, enough for oxygen and light to penetrate the waters and support complex life, according to evidence buried in the fossil record.

 Illustration of Snowball Earth model with open waters in both low- and mid-latitude oceans. (Dr Huyue Song)

 Accordingly, a recent study by scientists from China and the UK is the most recent to contend that "Snowball Earth" wasn't entirely coated in ice and may have even displayed habitable open-ocean conditions far from the equator.

 

 

It's unclear exactly why Earth abruptly entered a prolonged cold spell 700 million years ago. Possibly a reduction in sunshine or a reduction in greenhouse gases, followed by feedback where ice reflects heat and makes more ice.

While this may have sparked an explosion in biodiversity, complicated life would have had a difficult time surviving in icy waters shut off from the outside world during the extreme ice age. In a world devoid of oxygen, simple anaerobes and deep-dwelling chemotrophs ought to have ruled life.

Yet the fossil evidence contradicts that. Black shales that are submerged in South China's Nantuo Formation contain remnants of the sediments the Marinoan glaciers left behind. They contain microscopic remnants of remarkably complex organisms, which experts have identified as a type of algae.

 

Because life on Snowball Earth depends on sunshine to survive, it is possible that some areas of the ocean were once free of ice, allowing life to gaze up at the sun and absorb its rays.

The team behind this latest investigation wanted to find more information by examining the chemistry of the Nantuo Formation shale, which would help determine whether some parts of the surface stayed ice-free during this significant period in Earth's history.

For instance, information on redox reactions that characterised oxygen levels at the interface between sediment and the water above was obtained by looking at the iron content of the material. The team was able to gain a better understanding of the aerobic nitrogen cycle happening close to the water's surface by analysing the mixture of nitrogen isotopes.

 

It appears that at least a portion of the planet's surface was ice-clear towards the end of the Marinoan, providing a warm oasis for photosynthesizing organisms. This conclusion is supported by the findings of numerous other studies.

Importantly, these ice-free islands of open water would have emerged at mid-latitudes, far from the equator, based on the location of the fossil beds in South China more than half a billion years ago.

 

Even though it's conceivable that meltwater ponds in the glaciers could have offered comparable access to oxygen and sunshine, the researchers contend that it's unlikely that such lakes would have had enough organic matter to keep the carbon and nitrogen cycles active.

 

Earth's more complex life forms would have shelters from the hostile darkness if they were more of a "slushball" than a fully frozen ball of ice, enabling them to recover more quickly once the planet warmed.

Reference: sciencealert.com