『Abstract
The gradual discovery that late Neoproterozoic ice sheets extended
to sea level near the equator poses a palaeoenvironmental conundrum.
Was the Earth's orbital obliquity＞60゜(making the tropics colder
than the poles) for 4.0 billion years following the lunar-forming
impact, or did climate cool globally for some reason to the point
at which runaway ice-albedo feedback created a ‘snowball’ Earth?
The high-obliquity hypothesis does not account for major features
of the Neoproterozoic glacial record such as the abrupt onsets
and terminations of discrete glacial events, their close association
with large (＞10‰) negative δ13C shifts in seawater
proxies, the deposition of strange carbonate layers (‘cap carbonates’)
globally during post-glacial sea-level rise, and the return of
large sedimentary iron formations, after a 1.1 billion year hiatus,
exclusively during glacial events. A snowball event, on the other
hand, should begin and end abruptly, particularly at lower latitudes.
It should last for millions of years, because outgassing must
amass an intense greenhouse in order to overcome the ice albedo.
a largely ice-covered ocean should become anoxic and reduced iron
should be widely transported in solution and precipitated as iron
formation wherever oxygenic photosynthesis occurred, or upon deglaciation.
The intense greenhouse ensures a transient post-glacial regime
of enhanced carbonate and silicate weathering, which should drive
a flux of alkalinity that could quantitatively account for the
world-wide occurrence of cap carbonates. The resulting high rates
of carbonate sedimentation, coupled with the kinetic isotope effect
of transferring the CO2 burden to the ocean,
should drive down the δ13C of seawater, as is observed.
If cap carbonates are the ‘smoke’ of a snowball Earth, what was
the ‘gun’? In proposing the original Neoproterozoic snowball Earth
hypothesis, Joe Kirschvink postulated that an unusual preponderance
of land masses in the middle and low latitudes, consistent with
palaeomagnetic evidence, set the stage for snowball events by
raising the planetary albedo. Others had pointed out that silicate
weathering would most likely be enhanced if many continents were
in the tropics, resulting in lower atmospheric CO2
and a colder climate. Negative δ13C shifts of 10-20‰
precede glaciation in many regions, giving rise to speculation
that the climate was destabilized by a growing dependency on greenhouse
methane, stemming ultimately from the same unusual continental
distribution. Given the existing palaeomagnetic, geochemical and
geological evidence for late Neoproterozoic climatic shocks without
parallel in the Phanerozoic, it seems inevitable that the history
of life was impacted, perhaps profoundly so.』
Introduction
Alternative theories for low-latitude glaciation
Snowball Earth
Testing the snowball hypothesis
Test 1: carbon isotopes
Test 2: cap carbonates
Test 3: iron formations
Questioning the snowball Earth
Would glaciers flow?
Would seawater change?
Would eukaryotes survive?
Snowball events in Earth history
Conclusions
Acknowledgments
References