domingo, 16 de octubre de 2011

The Global Glaciation of "Snowball Earth"--How Life Survived

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Global glaciation likely put a chill on life on Earth hundreds of millions of years ago, but new research indicates that simple life in the form of photosynthetic algae could have survived in a narrow body of water with characteristics similar to today's Red Sea.  This "Snowball Earth" hypothesis suggests that Earth was entirely covered by ice during the Cryogenian period, which took place from about 790 to 630 million years ago.

"Under those frigid conditions, there are not a lot of places where you would expect liquid water and light to occur in the same area, and you need both of those things for photosynthetic algae to survive," said Adam Campbell, a University of Washington doctoral student in Earth and space sciences.

A long, narrow body of water such as the Red Sea, about 6.5 times longer than it is wide, would create enough physical resistance to advancing glacial ice that the ice sheet likely could not make it all the way to the end of the sea before conditions cause the ice to turn to vapor. That would leave a small expanse of open water where the algae could survive.

"The initial results have shown pretty well that these kinds of channels could remain relatively free of thick glacial ice during a 'snowball Earth' event," Campbell said. He examined the issue using an analytical model that applied basic principles of physics to a simple set of atmospheric conditions believed to have existed at the time.

Many scientists believe Earth became a giant snowball two or three times between 800 million and 550 million years ago, with each episode lasting about 10 million years. These all preceded the Cambrian explosion about 530 million years ago, when life on Earth rapidly expanded, diversified and became more complex.

But simple photosynthetic plankton turn up in the fossil record before and after the "snowball Earth" events, leading scientists to wonder how that could happen if Earth's oceans were completely encased in ice.

Campbell said it is assumed the algae survived these episodes, "unless they re-evolved each time, which creates a whole different problem for evolutionary biology."

He chose the Red Sea as an example because it is formed from a tectonic process called continental rifting, a process known to have existed at the time of the snowball Earth events, and it lies in an arid region between Egypt and the Arabian Peninsula.

Campbell noted that in a snowball Earth event, the open water in such a sea wouldn't have lasted long if it didn't have a way of being replenished – if, for example, the glacial ice acted as a dam and cut off the influx of additional sea water. The open water had to exist on the order of 10 million years for the algae to survive.

"Over 10 million years, you could evaporate the deepest lake in the world," Campbell said. "If you're in a desert, you'd have to have a supply of sea water."

In an earlier study, LSU scientist Huiming Bao, along with colleagues from UCLA and China, discovered some of the first atmospheric evidence in support of the “Snowball Earth” hypothesis.
Bao and his group used a new parameter called “sulfate oxygen-17 anomaly” to measure atmospheric records found in mineral sulphate deposits.

“My group specializes in measuring these anomalies – very few other groups do,” said Bao. “This puts us in an extremely good position for uncovering previously unknown information.”

These oxygen-17 anomalies are usually not measured by scientists who study Earth rocks because they were originally believed to be exclusively extra-terrestrial in nature, coming only from specific types of asteroids. Over the years, Bao’s group has worked on many extremely dry deserts on Earth and shown that there are a large range of oxygen-17 anomalies among desert salts that record atmospheric reactions.

All of the previous documented anomalies are positive, meaning that there is an excess in oxygen-17 isotopes. This finding, however, reveals a large depletion in oxygen-17 content in some of the sulfate minerals. These are the first oxygen-17 depletions, or negative anomalies, found in Earth minerals. What is even more striking is the timing of the negative anomalies – there is a spike in the depletion right at the time when a global glaciation came to an abrupt end approximately 635 million years ago.

To account for the data, Bao and his colleagues proposed that this depletion spike was caused by an extremely high atmospheric carbon dioxide concentration at that time, at least 40 times the modern level. That is what the “Snowball Earth” hypothesis predicted when the entire oceans were frozen over for millions of years. Bao and his colleagues will still have to rule out other scenarios before calling their evidence a “smoking gun” for the theory. “But we have found a new way to look into the details of very old glaciations events that other approaches couldn’t,” said Bao. “Using this new parameter, we should be able to read from the rock record the dynamics of the glaciations as well as the impact to biosphere, atmosphere and hydrosphere of our Earth system.”

In light of the increasing environmental stresses humans have placed on Earth, Bao said that there is a critical need to understand how a complex system like Earth’s can be expected to react.

Source: The Daily Galaxy

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