Changes in Earth's orbit enabled the emergence of complex life

Date:
July 7, 2021
Source:
University of Southampton
Summary:
'Snowball Earth' is the most extreme climate event in Earth's
history, when it was completely engulfed in ice. The theory of
its existence has faced two challenges - how life survived and
variations in rock formations from the time implying changes to the
climate cycle. New study shows that changes to Earth's orbit caused
the ice sheets to advance and retreat, providing ice-free 'oases'
for animal life and explaining variations in rock formations.



FULL STORY ========================================================================== Scientists at the University of Southampton have discovered 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.


==========================================================================
The researchers -- working with colleagues in the Chinese Academy of
Sciences, Curtin University, University of Hong Kong, and the University
of Tu"bingen - - studied a succession of rocks laid down when most of
Earth's surface was covered in ice during a severe glaciation, dubbed
'Snowball Earth', that lasted over 50 million years. Their findings are published in the journal Nature Communications.

"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." The 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.

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.



========================================================================== "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." 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 Professor 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.

Professor Mitchell explained, "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." 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. Dr. 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," Dr Gernon concluded.

========================================================================== Story Source: Materials provided by University_of_Southampton. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Ross N. Mitchell, Thomas M. Gernon, Grant M. Cox, Adam R. Nordsvan,
Uwe
Kirscher, Chuang Xuan, Yebo Liu, Xu Liu, Xiaofang He. Orbital
forcing of ice sheets during snowball Earth. Nature Communications,
2021; 12 (1) DOI: 10.1038/s41467-021-24439-4 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210707112419.htm

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