Snowball Earth: Our planet’s largest ice age probably didn’t have a single cause

For Earth to become as cold as it was during the “Snowball Earth” era, and stay there, required a combination of a reorganization of Earth’s continents and the weathering of a vast volcanic province, scientists have argued. Another team, coincidentally publishing at the same time, blames an asteroid. However, at this stage they have less evidence to point to.

The periods known as Snowball Earth made our recent ice ages resemble a minor cold snap. Not only were there huge glaciers on the equator, but in one case the cold lasted an estimated 57 million years. It is still a mystery how it survived.

While some scientists are pondering that question, others want to know how such an event could have happened. Changes in Earth’s orbit, such as those that have caused recent cold spells, are not nearly enough. However, there is at least one common feature between the big event and its pale imitations: low carbon dioxide levels.

As the main cause of changes in Earth’s temperature, there is no doubt that carbon dioxide levels were very low during the Snowball Earth periods, but that leaves open the question of why.

“We now think we have solved the mystery,” Dr. Adriana Dutkiewicz of the University of Sydney said in a statement. “Historically low emissions of volcanic carbon dioxide, aided by weathering of a large pile of volcanic rock in what is now Canada.”

“At that time, there were no multicellular animals or land plants on Earth,” Dutkiewicz noted. “The greenhouse gas concentration in the atmosphere was almost entirely determined by CO2 outgassing by volcanoes and by weathering processes of silicate rocks, which consume CO22.”

Evidence of the Sturtian Snowball Earth is found in rocks worldwide. Here, Dr Adriana Dutkiewicz studies examples in the Flinders Range, not far from the original discovery site.

Image credits: Professor Dietmar Müller/University of Sydney

Evidence for the weathering part is easy to find. The Franklin Igneous Province was by far the largest volcanic province on Earth at the time. Once growth and associated carbon emissions stopped, exposure to air and water inevitably led to chemical changes that would have absorbed carbon dioxide from the atmosphere.

However, that in itself would not be enough. Once the Earth became covered in ice, it reflected more sunlight back into space, temporarily keeping temperatures low. On the other hand, all that ice would also have slowed weathering. Under normal conditions, this would have caused carbon dioxide levels to rise, warming the planet again. The fact that this has not happened, Dutkiewicz and co-authors argue, indicates that CO has been being released for tens of millions of years.2 must have been as slow as its removal.

They attributed this to continental movements at the time. The longest Snowball Earth, known as the Sturtian after the rift where evidence first emerged, coincided with a period when only 9 megatonnes of carbon were released annually. At that slow rate, even minimal weathering beneath thick ice layers would have been enough to keep atmospheric carbon dioxide low. This low emission rate reflects a relative shortage of underwater volcanoes at the mid-ocean ridges, which in turn is a consequence of the breakup of Rodinia, the supercontinent before Pangea.

Dutkiewicz looked one step further back in the chain and told IFLScience that the responsibility lies with mantle convection processes, but “the problem [of what causes them] is not well understood. This is an area of ​​intense research and has been for a long time.”

Nearly the entire Earth froze at least once more, during the Marinoan Ice Age. Although Dutkiewicz believes there is some similarity between the causes of the two ice ages, she told IFLScience: “Marinoan was much shorter and occurred much later. The Sturtian is the one who requires explanation.

At the same time that Dutkiewicz and colleagues published their work, another paper reported that an asteroid impact of a similar magnitude to the ‘dinosaur killer’ could have left the world in a similarly frozen state. The authors of that paper conclude that such an event would have much more temporary effects during a warm period, such as the Cretaceous. However, if carbon dioxide levels were already low, this could cause something much bigger.

Dutkiewicz acknowledged to IFLScience that there may have been factors beyond those her team identified, but added that the other paper “is purely a climate modeling study hypothesizing whether an asteroid impact of similar magnitude to the Chicxulub impact could have caused a global ice age. There is no evidence that such an asteroid impact actually created ‘Snowball Earth’. It’s a highly speculative scenario. In any case, even if such an impact had occurred, an ice age is unlikely to last very long (geologically speaking).”

Dutkiewicz’s work is open access in geology. The Asteroid Hypothesis is open access in Science Advances.

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