What turned Earth into a giant snowball 700m years ago?

Australian geologists have used plate tectonic modelling to determine what most likely caused the Earth to experience an extreme ice-age climate more than 700 million years ago. Their study, published in the journal Geology, helps our understanding of the functioning of the Earth’s built-in thermostat that prevents the Earth from getting stuck in overheating mode, while also showing how sensitive global climate is to atmospheric carbon concentration.

“Imagine the Earth almost completely frozen over,” said lead author Dr Adriana Dutkiewicz, from The University of Sydney. “That’s just what happened about 700 million years ago; the planet was blanketed in ice from poles to equator and temperatures plunged. However, just what caused this has been an open question.

“We now think we have cracked the mystery: historically low volcanic carbon dioxide emissions, aided by weathering of a large pile of volcanic rocks in what is now Canada; a process that absorbs atmospheric carbon dioxide.”

The project was inspired by the glacial debris left by the ancient glaciation from this period that can be spectacularly observed in the Flinders Ranges in South Australia. A geological field trip to the Ranges, led by co-author Professor Alan Collins from The University of Adelaide, prompted the team to use The University of Sydney’s GPlates plate tectonic software to investigate the cause and the exceptionally long duration of the so-called Sturtian glaciation, which stretched from 717 to 660 million years ago.

“Various causes have been proposed for the trigger and the end of this extreme ice age, but the most mysterious aspect is why it lasted for 57 million years — a time span hard for us humans to imagine,” Dutkiewicz said.

Professor Dietmar Müller points to carbonates overlying glacial deposits in the Flinders Ranges. Image credit: Dr Adriana Dutkiewicz.

The team went back to a plate tectonic model that shows the evolution of continents and ocean basins at a time after the breakup of the ancient supercontinent Rodina. They connected it to a computer model that calculates CO₂ degassing of underwater volcanoes along mid-ocean ridges — the sites where plates diverge and new ocean crust is born.

They soon realised that the start of the Sturtian glaciation precisely correlates with an all-time low in volcanic CO₂ emissions. In addition, the CO₂ outflux remained relatively low for the entire duration of the ice age.

“At this time, there were no multicellular animals or land plants on Earth,” Dutkiewicz said. “The greenhouse gas concentration of the atmosphere was almost entirely dictated by CO₂ outgassing from volcanoes and by silicate rock weathering processes, which consume CO₂.”

“Geology ruled climate at this time,” added co-author Professor Dietmar Müller, from The University of Sydney. “We think the Sturtian ice age kicked in due to a double whammy: a plate tectonic reorganisation brought volcanic degassing to a minimum, while simultaneously a continental volcanic province in Canada started eroding away, consuming atmospheric CO₂.

“The result was that atmospheric CO₂ fell to a level where glaciation kicks in — which we estimate to be below 200 parts per million, less than half today’s level.”

The team’s work raises intriguing questions about Earth’s long-term future. A recent theory proposed that over the next 250 million years, Earth would evolve towards Pangea Ultima, a supercontinent so hot that mammals might become extinct. However, the Earth is also currently on a trajectory of lower volcanic CO₂ emissions, as continental collisions increase and the plates slow down. So perhaps Pangea Ultima will turn into a snowball again.

“Whatever the future holds, it is important to note that geological climate change, of the type studied here, happens extremely slowly,” Dutkiewicz said. “According to NASA, human-induced climate change is happening at a pace 10 times faster than we have seen before.”

Top image credit: iStock.com/DaryaDanik