About 700 million years ago, before multicellular animals emerged or plants made their way to land, Earth was plunged into an ice age that lasted for 57 million years.
Now, Australian geologists believe they have figured out what caused Earth to enter its snowball era, when ice covered most continents and oceans. The answer: all-time low levels of volcanic carbon dioxide in the atmosphere.
Lead author of a study published in Geology, Dr Adriana Dutkiewicz, a sedimentologist at the University of Sydney, says: “Imagine the Earth almost completely frozen over. 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,” she says.
“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 extended ice age, also called the Sturtian glaciation, lasted 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,” says Dutkiewicz.
Glacial debris left by ancient glaciation from this period can be found in the Flinders Ranges in South Australia. A recent geological field trip there – led by co-author Alan Collins, a tectonic geologist from the University of Adelaide – prompted the team to use EarthByte computer models to dig deeper into its cause.
They connected a plate tectonic model, which shows the evolution of continents and ocean basins after the breakup of the ancient supercontinent Rodina, with a model that calculates CO2 release from underwater volcanoes along mid-ocean ridges.
The researchers found that the start of the Sturtian glaciation correlates with an all-time low in volcanic CO2 emissions, which remained relatively low for the entire ice age.
“At this time, there were no multicellular animals or land plants on Earth. The greenhouse gas concentration of the atmosphere was almost entirely dictated by CO2 from volcanoes and by silicate rock weathering processes, which consume CO2,” says Dutkiewicz.
Motions of continents (grey) and plate boundaries (orange) from 850 to 540 million years ago, a time before complex life evolved. Subduction zones are shown as toothed lines and ocean basin depth is coloured light green to dark blue. Snowflakes indicate a “snowball” ice age, with ice covering most continents and oceans. Credit: Professor Dietmar Müller and Dr Ben Mather/EarthByte Group/The University of Sydney
Co-author Professor Dietmar Müller, a geologist from the University of Sydney says geology ruled climate at this time. “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 findings help scientists better understand how sensitive the global climate is to changes in atmospheric carbon dioxide concentrations.
Dutkiewicz adds: “it is important to note that geological climate change, of the type studied here, happens extremely slowly. According to NASA, human-induced climate change is happening at a pace 10 times faster than we have seen before.”
