Did “Snowball Earth” The Once-In-A-Billion Glaciation Event Trigger Cambrian Explosion?

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About 700 million years ago, runaway glaciers covered the entire planet in ice. Harvard researchers call it ‘snowball Earth’. Credit: NASA

“There are no other comparable glacial periods on Earth. This one was really quite catastrophic,” says Graham Shields of University College London in the UK. 715 million years ago the entire planet was encased in snow and ice. This frozen wasteland may have been the birthplace of complex animals.

Scientists now believe that this crushing catastrophe drove one of the most incredible steps in evolution – Cambrian explosion. Around 540 million years ago, a host of exotic creatures suddenly appeared. They included giant woodlouse-like creatures known as trilobites, the five-eyed Opabinia, and the spiny slug-like Wiwaxia. Suddenly, Earth leapt from being dominated by single-celled bacteria to a world teeming with exotic multicellular creatures, all in a geological blink of an eye.

For Charles Darwin, trying to demonstrate his theory of natural selection, this sudden burst of evolution was a major problem. “The case must at present remain inexplicable; and may be truly urged as a valid argument against the views here entertained,” he wrote in On the Origin of Species in 1859.

To this day the Cambrian explosion remains a puzzle. But maybe a planet-encasing icy catastrophe could help explain it. Regardless of how far the ice stretched, most scientists agree that the Snowball formed suddenly. It was probably caused by rapid weathering of Earth’s continents, which sucked carbon dioxide – a planet-warming greenhouse gas – out of the atmosphere and caused temperatures to plummet.

The idea is that the ice gave a boost to microscopic plants, which released oxygen as a waste product. During the Snowball, the glaciers would have worn huge amounts of phosphorus-rich dust away from the underlying rocks. Then, when the ice retreated at the end of the Snowball, rivers washed this dust into the oceans, where it fed the microbes.

“High phosphorus levels would have increased biological productivity and organic carbon burial in the ocean, leading to a build-up of atmospheric oxygen,” says Noah Planavsky of Yale University in New Haven, Connecticut. In 2010 he identified a massive spike in phosphorus levels in sediments from around the world, just as Snowball Earth was ending.

For years, scientists have been searching for the cause of largest glaciations event in Earth’s history, widely known as “snowball Earth.” Now, researchers from Harvard University have published a new hypothesis in Geophysical Research letters, pinpointing the start of the Sturtian snowball Earth even about 717 million years ago. At the time, a huge volcanic event devastated the area from Alaska to Greenland—a fact that Francis Macdonald and Robin Wordsworth believe is more than a coincidence.

“We know that volcanic activity can have a major effect on the environment, so the big question was, how are these two events related?” MacDonald, a professor of Natural Sciences, commented.

At first, his team looked to basaltic rock, investigating whether it interacted with the CO2 in the atmosphere and cause cooling. But if that were the case, cooling would have happened over millions of years—something radio-isotopic dating has proven not to be the case. That’s when Macdonald turned to Wordsworth and asked: could aerosols emitted from the volcanoes have rapidly cooled Earth.

The answer? Yes, if the right conditions were present.

“These types of eruptions have happened over and over again throughout geological time, but they’re not always associated with cooling events,” Wordsworth noted. “So the question is: what made this even different?”

Geological and chemical studies of this region have shown that volcanic rocks erupted through sulfur-rich sediments, pushing into the atmosphere as sulfur dioxide—which happens to be very good at blocking solar radiation. The 1991 eruption of Mount Pinatubo, for example, reduced global temperatures about 1 degree Fahrenheit for an entire year. If sulfur dioxide gets past the tropopause, it will linger in the atmosphere up to a year.

“In periods of Earth’s history when it was very warm, volcanic cooling would not have been very important because the Earth would have been shielded by this warm, high tropopause,” Wordsworth notes. “In cooler conditions, Earth becomes uniquely vulnerable to having these kinds of volcanic perturbations to climate.”

The bottom line? Context and background play an essential role. Due to continental drift, 717 million years ago, the Franklin large igneous province where the eruptions took place was situated near the equator, the entry point for most of the solar radiation that keeps the Earth warm. So, if the light-reflecting gas entered the atmosphere at the right location and height, it could cause cooling.

So what made this event different than Pinatubo? The volcanoes we’re talking about spanned nearly 2,000 miles across Canada and Greenland. They can erupt more continuously and a decade or so of these eruptions could have shot enough aerosols into the atmosphere to significantly destabilize the climate.

“Cooling from aerosols doesn’t have to freeze the whole planet, it just has to drive the ice to a critical latitude. Then, the ice does the rest,” Macdonald notes.

Understanding how these changes occur will help researchers better understand how and why extinctions occurred—here and on other planets.

“We know that Earth is a dynamic and active place that has had sharp transitions,” Wordsworth notes. “There is every reason to believe that rapid climate transitions of this type are the norm on planets, rather than the exception.”

Source: Phys.org

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