From: LARRY KLAES (ljk4_at_msn.com)
Date: Fri Dec 07 2007 - 06:05:32 PST
http://www.terradaily.com/reports/Thaw_point_Snowball_Earth_was_more_a_slushball_999.html
Thaw point: 'Snowball Earth' was more a slushball
by Staff Writers
Paris (AFP) Dec 5, 2007
An extraordinary episode of global cooling hundreds of millions of years ago
that some experts say caused Earth to completely freeze over has been
miscalculated, a new study says.
Instead of "Snowball Earth," the planet really became "Slushball Earth," its
authors suggest.
The great chill -- the longest and deepest ice age in Earth's known history
-- happened during the late Neoproterozoic era, 850 to 542 million years
ago.
The evidence for the Snowball thesis comes from deep sediments in the ocean.
Scientists look through these layers to measure levels of the isotope carbon
13 (C13), deposited in plants through photosynthesis, as a telltale of
Earth's climate.
Above and below the Cryogenian layer is an abundance of C13. But the
Cryogenian layer itself has negligible levels of this isotope.
Coupled with other signs of intense glaciation, the explanation is that
Earth froze over completely -- thick ice covered the oceans and glaciers
crept down into the tropics, possibly even reaching the equator.
The brilliant white shell reflected back the Sun's rays, and thus its heat,
so well that the Snowball persisted for nearly 200 million years.
Eventually the greenhouse gas carbon dioxide (CO2), welling up from
volcanoes, escaped into the air and thankfully set the planet onto a warming
trend, and so the icy blanket was melted.
This scenario, first aired in 1989 and still fiercely contested, has now
come under assault from a new angle by a trio of physicists at the
University of Toronto, Canada.
They have devised a computer simulation of the CO2 during the late
Neoproterozoic that factors in the role of oxygen in the ocean.
Progressive cooling would have allowed atmospheric oxygen to spread more
deeply into the sea, transforming rich layers of dissolved organic carbon --
formerly created by photosynthesis -- into CO2, they say.
This CO2, released back into the atmosphere, warmed up the atmosphere
through the greenhouse effect sufficiently to induce thawing, causing sea
ice and glaciers to shrink, before a cooling cycle resumed.
In other words, the Cryogenian would have been a milder, slushier, shorter
affair, with ice-free seas in the tropics where sunlight would have
generated photosynthesis, rather than a deep, long, planetary freeze.
The ice age would not have required massive amounts of volcanic CO2 to end,
nor would it have been delayed by millions of years.
In a commentary, which like the study appears in Thursday in the British
journal Nature, US geologist Alan Kaufman questions Peltier's assumption
that levels of atmospheric oxygen were similar to today's level -- a
relatively high 21 percent of the air.
"Biological and geochemical evidence indicates that oxygen levels were low
throughout most of the Neoproterozoic, with a significant rise in breathable
air around 550 million years ago -- about the time animals first appeared on
the planet," says Kaufman.
The Snowball debate has a bearing on another great enigmatic episode in
Earth's history called the "Cambrian Explosion" -- the dramatic breakout of
biodiversity that happened after the Neoproterozoic.
The microfossil record from this time is so emphatic that no one doubts that
the Explosion happened.
Critics of the Snowball theory say that such a prolonged deep glaciation
would have destroyed all life, leaving nothing left -- or at least very
little -- to explain this sudden, riotous diversity.
Snowball supporters, though, argue that hardy microscopic biota survived
unscathed during the long chill, thriving in shallow sea waters or melt
pools on land in the tropics, or in thermal vents on the sea bed.
When the Snowball melted, these organisms grew swiftly in size and
complexity, becoming the new masters of the planet, they contend.
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