Encyclopedia of Evolution.pdf - Online Reading Center

and it is not happening extensively today. However, there
was a period of ironstone formation in the late Precambrian.
Researchers claim that it occurred because the ice of
Snowball Earth isolated the oceans from the air, allowing a
volcanic buildup of iron in the ocean which could not react
with atmospheric oxygen. As the ice melted, iron deposits
again began to form.
• Iridium is rare on Earth but common in cosmic dust. It
would have accumulated on the Snowball Earth ice surface,
then deposited quickly in a concentrated layer when the ice
melted. Geologists have found a high concentration of iridium
in deposits from the end of the Marinoan Glaciation.
During the late Precambrian, there were not only a lot of
icebergs but a lot of tropical icebergs. The evidence for this is
that the magnetic fields of the deposits usually have a nearly
horizontal orientation. The magnetic field of a rock is produced
by the orientation of iron crystals (see paleomagnetism).
After the rock has hardened, the iron crystals cannot
move, and the magnetic field is frozen into place. (Frequently,
conditions such as heat or the flow of mineralized water can
cause the original magnetic field to be altered and even overprinted
by another orientation; geologists are careful to check
for this possibility.) The Earth’s magnetic field is oriented not
only north-to-south, but the field lines are nearly vertical at
the poles and horizontal at the equator.
If the tropical seas were largely covered with icebergs,
the rest of the world must have been covered with ice, making
the whole planet a snowball (with the exception of openings
in the ice over volcanoes). Life would not have become
extinct under these conditions—organisms could survive near
the volcanic openings—but there must have been a drastic
reduction in the number of species and in population sizes.
There is broad agreement among evolutionary scientists that
these glaciations occurred; the disagreement regards how
completely the Earth was covered with ice—was it really a
snowball, or were some large areas free of ice cover?
Little agreement has been reached about what might
have caused the Snowball Earth. One suggestion is that photosynthesis
by green bacteria and protists removed so much
carbon dioxide from the air that the greenhouse effect was
almost completely canceled. This allowed ice to accumulate,
which reflected more sunlight away from the Earth, which
caused more cooling, which allowed more ice to accumulate,
and so on in a self-reinforcing (positive feedback) loop. Other
scientists have suggested that all of the continents were near
the equator at that time, which reduced the ability of ocean
currents to redistribute heat into the polar regions, in which
an inexorable buildup of ice began.
Once the Snowball began to melt, the change was astonishingly
rapid. The accumulation of volcanic and meteoric
dust on the ice may have reduced albedo enough to allow
melting to begin. High concentrations of atmospheric carbon
dioxide caused a huge greenhouse effect; the world underwent
transition from its coldest to its warmest temperatures. Once
rain began to fall, it contained dissolved carbon dioxide, in
the form of carbonic acid. Once the land was exposed, then
minerals in the rocks could react with the acid rain, producing
Snowball Earth
carbonates that accumulated in the oceans. The accumulation
of carbonate was so massive that it produced a thick whitish
layer of rock immediately above (after) the layer of dropstones.
The dropstones indicated the Snowball; the carbonate
layer indicated the rapid melting of the Snowball. Carbon isotope
ratios (see isotopes) suggest that this carbonate was of
inorganic origin—directly from atmospheric carbon dioxide,
not from organisms. There were not yet any land plants to
absorb carbon dioxide through photosynthesis, but photosynthetic
protists (such as seaweeds) began to grow abundantly in
the oceans (which were fertilized by the high concentrations
of iron). These cells absorbed carbon dioxide, and the newly
exposed ocean water itself could also absorb carbon dioxide,
thus bringing the greenhouse effect slowly under control.
Some researchers believe that the last of the Snowball
events stimulated the evolution of complex life. The Ediacaran
organisms and the rapid evolution of early Cambrian
animals (see Cambrian explosion) both occurred soon after
this Snowball event. It has been suggested that because the
Precambrian world was dominated by single-celled organisms,
an extinction event such as Snowball (as well as the
buildup of oxygen that immediately followed it) was actually
necessary to allow the evolution of new adaptations, such
as the multicellular body. As paleontologist Andrew Knoll
points out, in the world of retreating ice after the last Snowball,
even poorly functioning new multicellular animal forms
had all the real estate to themselves.
One consequence of this discovery is both unsettling and
reassuring. The Earth has not always been a nice place to live.
The climate is apparently capable of going wild; and if it did
so before, it may do so again. The very presence of life itself
on this planet may spare the Earth a repeat of such drama.
Today, unlike the Precambrian, the continents are covered
with plants, which help keep both global cooling and global
warming under control. Some scientists have formalized this
concept as the Gaia hypothesis. Conservation efforts aimed
at maintaining plant cover and diversity may not just be a
good idea and the right thing to do, but may be essential to
the future of the planet.
Further Reading
Bodiselitsch, Bernd, et al. “Estimating duration and intensity of Neoproterozoic
snowball glaciations from Ir anomalies.” Science 308
(2005): 239–242. Summarized by Richard A. Kerr, “Cosmic dust
supports a Snowball Earth.” Science 308 (2005): 181.
Hoffman, Paul F. “Snowball Earth.” Available online. URL: http://
www.snowballearth.org. Accessed July 11, 2006.
Jenkins, Gregory S., Mark A. S. McMenamin, Christopher P. McKay,
and Linda Sohl, eds. The Extreme Proterozoic: Geology, Geochemistry,
and Climate. Washington, D.C.: American Geophysical
Union, 2004.
Knoll, Andrew H. Life on a Young Planet: The First Three Billion
Years of Evolution on Earth. Princeton, N.J.: Princeton University
Press, 2003.
Walker, Gabrielle. Snowball Earth: The Story of a Great Global
Catastrophe That Spawned Life as We Know It. New York:
Crown Publishers, 2003.