In the blink of a geological eye, nearly 600 million years ago, a massive ice age radically altered the planet’s climate, resulting in a “Snowball Earth,” also known as the Cryogenian Period, severely constricting the oxygen supply on the planet. Scientists at the University of Southampton have proposed that changes in Earth’s orbit may have allowed complex life to emerge and thrive during the most hostile climate episode the planet has ever experienced.
“Studying Snowball Earth glaciations can tell us just how bad it can get, in which case life as we know it would probably not survive,” says geologist Linda Sohl of Columbia University’s Center for Climate Systems Research and NASA’s Goddard Institute for Space Studies.
The Southampton researchers – working with colleagues in the Chinese Academy of Sciences, Curtin University, University of Hong Kong, and the University of Tübingen – studied a succession of rocks laid down when most of Earth’s surface was covered in ice during this severe glaciation that lasted over 50 million years.
Life Somehow Avoided a Bottleneck
“One of the most fundamental challenges to the Snowball Earth theory is that life seems to have survived,” says Dr Thomas Gernon, Associate Professor in Earth Science at the University of Southampton, and co-author of the study. “So, either it didn’t happen, or life somehow avoided a bottleneck during the severe glaciation.”
‘Glacial Oxygen Pump’ –Pockets of Oxygen in the Oceans?
An earlier 2019 study by researchers from McGill University found that the meltwater from the glaciers created pockets of oxygen in the oceans, which let life thrive until the ice age ended and they were able to emerge.
“The evidence suggests that although much of the oceans during the deep freeze would have been uninhabitable due to a lack of oxygen, in areas where the grounded ice sheet begins to float there was a critical supply of oxygenated meltwater,” the study’s lead author, Maxwell Lechte, said about the 2019 McGill study. “This trend can be explained by what we call a ‘glacial oxygen pump’; air bubbles trapped in the glacial ice are released into the water as it melts, enriching it with oxygen.”
A Link Between Snowball Earth and Animal Evolution?
“The fact that the global freeze occurred before the evolution of complex animals suggests a link between Snowball Earth and animal evolution,” Lechte added. “These harsh conditions could have stimulated their diversification into more complex forms.”
“One of the reasons why the severity of the Cryogenian ice ages are so fiercely debated is that it can be difficult to decipher environmental conditions from ancient glacial sediments, because ice sheets are such powerful forces of erosion and sedimentation,” wrote Lechte in an email to The Daily Galaxy. “Luckily, for the Sturtian glaciation, the preservation of iron-rich sediments associated with the glacial deposits offers valuable insight into marine chemistry and the hydrological system during this time. This is a valuable study, as looking for signs of orbital forcing on the deposition of iron allows us to better understand the nature of climate change during what was possibly the most extreme of ice age in Earth history.”
The Southhampton research team ventured into the South Australian outback where they targeted kilometre-thick units of glacial rocks formed about 700 million years ago. At this time, Australia was located closer to the equator, known today for its tropical climates. The rocks they studied, however, show unequivocal evidence that ice sheets extended as far as the equator at this time, providing compelling evidence that Earth was completely covered in an icy shell.
“Banded Iron Formations”
The team focused their attention on “Banded Iron Formations”, sedimentary rocks consisting of alternating layers of iron-rich and silica-rich material. These rocks were deposited in the ice-covered ocean near colossal ice sheets.
During the snowball glaciation, the frozen ocean would have been entirely cut off from the atmosphere. Without the normal exchange between the sea and air, many variations in climate that normally occur simply wouldn’t have.
‘Sedimentary Challenge’ to the Snowball Hypothesis
“This was called the ‘sedimentary challenge’ to the Snowball hypothesis,” says Professor Ross Mitchell, professor at the Chinese Academy of Sciences in Beijing, China and the lead author. “The highly variable rock layers appeared to show cycles that looked a lot like climate cycles associated with the advance and retreat of ice sheets.” Such variability was thought to be at odds with a static Snowball Earth entombing the whole ocean in ice.
“The iron comes from hydrothermal vents on the seafloor,” added Gernon. “Normally, the atmosphere oxidizes any iron immediately, so Banded Iron Formations typically do not accumulate. But during the Snowball, with the ocean cut off from the air, iron was able to accumulate enough for them to form.”
Evidence Preserved for Nearly All Earth’s Orbital Cycles
Using magnetic susceptibility – a measure of the extent to which the rocks become magnetised when exposed to a magnetic field – the team made the discovery that the layered rock archives preserve evidence for nearly all orbital cycles.
Earth’s orbit around the sun changes its shape and the tilt and wobble of Earth’s spin axis also undergo cyclic changes. These astronomical cycles change the amount of incoming solar radiation that reaches Earth’s surface and, in doing so, they control climate.
“Even though Earth’s climate system behaved very differently during the Snowball, Earth’s orbital variations would have been blissfully unaware and just continued to do their thing,” explains Mitchell.
The researchers concluded that changes in Earth’s orbit allowed the waxing and waning of ice sheets, enabling periodic ice-free regions to develop on snowball Earth.
“This finding resolves one of the major contentions with the snowball Earth hypothesis: the long-standing observation of significant sedimentary variability during the snowball Earth glaciations appeared at odds with such an extreme reduction of the hydrological cycle,” explained Mitchell.
Doubts About an Orbital “Snowball” Hypothesis
“It’s been known since the mid-1960’s from ancient permafrost structures like polygonal sand-wedges and from sand-seas (aeolianites) that large land areas were ice-free during one or the other Cryogenian glaciation in northeast Laurentia, West Africa and South Australia,” wrote Paul Hoffman, Sturgis Hooper Professor Emeritus of Geology at Harvard University in an email to The Daily Galaxy. “So it is not clear to me from the new geological evidence supporting numerical simulations that snowball ice sheets were sensitive to orbital forcing changes the picture with regards to snowball ecosystems or their evolution.”
The team’s results help explain the enigmatic presence of sedimentary rocks of this age that show evidence for flowing water at Earth’s surface when this water should have been locked up in ice sheets. Gernon states: “This observation is important, because complex multicellular life is now known to have originated during this period of climate crisis, but previously we could not explain why”.
“Our study points to the existence of ice-free ‘oases’ in the snowball ocean that provided a sanctuary for animal life to survive arguably the most extreme climate event in Earth history,” Gernon concluded.