NEW EVIDENCE PUTS 'SNOWBALL EARTH' THEORY OUT IN THE COLD

Science Daily, 25 March 2007

 HYPERLINK "javascript:ol('http://www.sciencedaily.com/releases/2007/03/070323104746.htm');" http://www.sciencedaily.com/releases/2007/03/070323104746.htm

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

this so-called 'Snowball Earth' era.

Analyses of glacial sedimentary rocks in Oman, published online in Geology, have

produced clear evidence of hot-cold cycles in the Cryogenian period, roughly

850-544 million years ago. The UK-Swiss team claims that this evidence

undermines hypotheses of an ice age so severe that Earth's oceans completely

froze over.

Using a technique known as the chemical index of alteration, the team examined

the chemical and mineral composition of sedimentary rocks to search for evidence

of any climatic changes. A high index of alteration would indicate high rates of

chemical weathering of contemporary land surfaces, which causes rocks to quickly

decompose and is enhanced by humid or warm conditions. Conversely, a low

chemical index of alteration would indicate low rates of chemical weathering

during cool, dry conditions.

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. Leader of the study, Professor Philip Allen of Imperial College London's

Department of Earth Science and Engineering, explains: 

"If the Earth had become fully frozen for a long period of time, these climatic

cycles could not exist -- the Earth would have changed into a bleak world with

almost no weather, since no evaporation from the oceans could take place, and

little snowfall would be possible. In fact, once fully frozen, it is difficult

to create the right conditions to cause a thaw, since much of the incoming solar

radiation would be reflected back by the snow and ice. The evidence of climatic

cycles is therefore hostile to the idea of 'Snowball Earth'."

Professor Allen adds that understanding how Earth's climate has changed in the

past provides important data for current climate change models. He says:

"This isn't just curiosity about the past - we are living in a time of climate

change and there is a huge debate going on over what the natural variability of

the climate is. Knowledge of climate change in deep time provides clues to the

way in which our climate system works under extreme conditions. But these

extreme conditions were probably not a full global freeze. It is equally

important to understand a picture of global climate retaining open ocean between

the tropics." 

This challenge to the 'Snowball Earth' opens intriguing questions about how the

Earth came so close to climate disaster but managed to avoid it, according to

Professor Allen.

"This was the most severe glaciation experienced by the planet over the last

billion years, and the big question is - how can ice get all the way to the

tropics but not finish the job?" he says. "The total icy shutdown that we came

so close to would have dealt a severe blow to early life and most likely would

have resulted in a completely different evolutionary pathway. The reasons for

Earth's near-miss with global refrigeration remains an important scientific

question to resolve."

The team's findings come from analyses of sedimentary rock from the Huqf

Supergroup, Oman's oldest sedimentary sequence that spans around 200 million

years of the Neoproterozoic era. 

Note: This story has been adapted from a news release issued by Imperial College

London.

Copyright 2007, Science Daily