"Snowball Earth" Challenged
The Imperial College of London released research that the theory that Earth once underwent a prolonged time of extreme global freezing has been dealt a blow by new evidence that periods of warmth occurred during the "Snowball Earth'" events.
Before 50 million years ago, Earth had no regular ice ages, but when they occurred they were colossal. There is evidence that as many as four major episodes of glaciation on a scale that makes the last Ice Age, the Pleistocene epoch 0f 2.5 million to 10,000 years ago seem like a June Day.
So much of the planet was covered by ice during two Precambrian ice ages -one 2.4 billion years ago and one 850-544 million years ago, the Cryogenian -that Dr. Joesph Kirschvinck of Cal Tech dubbed them "Snowball Earth" events. New evidence was published by Harvard geologist Paul Hoffman showing that ice extended to the equatorial latitudes about 700 million years ago.
Astronomers once thought that a previously warm world's descent into a "snowball event" would be irreversible as the planet gets more and more thickly coated by ice the fraction of sunlight reflected back into sapce increases and solar heating of the surface declines. Yet it is clear that the planet was able to esacpe the deep freeze, not once but several times through volcanic emissions of gases such as carbon dioxide into the atmoswphere producing a warming greenhouse effect.
Here's how Kirschvink has described the escape from the "icehouse" condition (in Rare Earth by Peter Ward and Donald Brownlee): the escape "was only accomplished by the buildup of volcanic gases, particularly carbon dioxide, mosatly from undersea volcanic activity. Deglaciation..must have been spectacular, with nearly 30 million years of carbon dioxide, ferrous iron, and long buried nutrients suddenly exposed to fresh air and sunlight. For a brief time, the Earth's oceans would have been as green as Irish clover, and the sudden oxygen spikes may have sparked early animal evolution."
Analyses of glacial sedimentary rocks in Oman, published online in the journal Geology, have produced clear evidence of hot-cold cycles in the Cryogenian period. The UK-Swiss team claims that this evidence undermines hypotheses of an ice age so severe that Earth's oceans completely froze over.
The researchers found three intervals with evidence for extremely low rates of chemical weathering, indicating pulses of cold climate. However these intervals alternate with periods of high rates of chemical weathering, likely to represent interglacial periods with warmer climates.
These warmer periods mean that, despite the severe glaciation of this time in Earth history, the complete deep-freeze suggested by 'Snowball Earth' theories never took place, and that some areas of open, unfrozen ocean continued to exist. These findings have important implications for our understanding of how past life on Earth interacted with its changing planetary environment. Posted by Casey Kazan.
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