A team of researchers led by Chinese Academy of Sciences’ Professor Ross Mitchell has studied a succession of rocks laid down when most of Earth’s surface was covered in ice during severe ‘snowball Earth’ glaciations, about 720 to 635 million years ago (Cryogenian period).
An artist’s impression of a ‘snowball Earth.’ Image credit: NASA.
Professor Mitchell and colleagues ventured into the South Australian outback where they targeted kilometer-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 the scientists 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.
They 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.
“This was called the ‘sedimentary challenge’ to the snowball hypothesis,” Professor Mitchell said.
“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,” said Dr. Thomas Gernon, a researcher at the University of Southampton.
“Normally, the atmosphere oxidizes any iron immediately, so banded iron formations typically do not accumulate.”
“But during the snowball glaciations, with the ocean cut off from the air, iron was able to accumulate enough for them to form.”
Using magnetic susceptibility, a measure of the extent to which the rocks become magnetized when exposed to a magnetic field, the authors 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 the planet’s spin axis also undergo cyclic changes.
Known as Milankovitch cycles, 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,” Professor Mitchell said.
The team 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,” Professor Mitchell said.
The 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.
“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,” Dr. Gernon said.
“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.”
The findings were published in the journal Nature Communications.
R.N. Mitchell et al. 2021. Orbital forcing of ice sheets during snowball Earth. Nat Commun 12, 4187; doi: 10.1038/s41467-021-24439-4