Did CO2 Overload Destroy the Early Earth World?

Zircons are tiny mineral grains smaller than a speck of sand, but they may have answered a very big question: Why have no rock samples have ever been found dating back to the first 500 million years of Earth’s existence? Previous explanations have included destruction by barrages of meteorites, as well as the possibility that the early Earth was a sizzling sea of magma.

"Currently, no rocks remain from before about 4 billion years ago," he says. "Some people consider this as evidence for very high temperature conditions on the ancient Earth," notes UW-Madison geologist Takayuki Ushikubo.

However, new research suggests that neither the meteorite or the magma scenarios were the case, but rather that the ancient climate was so amazingly abrasive and harsh, that it scoured—even destroyed—the early continents that once existed. Analysis of ancient zircons offers a glimpse in time back as far as 4.4 billion years ago, when the planet was a only 150 million years old. These tiny crystals are extremely resistant to chemical changes, making them the standard for determining the age of ancient rocks, says.

In a new paper published online this week in the journal Earth and Planetary Science Letters, a team of scientists led by UW-Madison geologists outlines how rocky continents and liquid oceans did actually exist at least as far back as 4.3 billion years ago, but were assaulted by heavy weathering from an intensively acidic climate.

Takayuki Ushikubo, the first author on the new study, says that atmospheric weathering may be the answer to the long-standing geological enigma.

Ushikubo and colleagues used a sophisticated new instrument called an ion microprobe to analyze isotope ratios of the element lithium in zircons from the Jack Hills in western Australia. By comparing these chemical fingerprints to lithium compositions in zircons from continental crust and primitive rocks similar to the Earth's mantle, they found evidence that the young planet already had the beginnings of continents, relatively cool temperatures and liquid water by the time the Australian zircons formed.

"At 4.3 billion years ago, the Earth already had habitable conditions," Ushikubo says.

The zircons' lithium signatures also hold signs of rock exposure on the Earth's surface and breakdown by weather and water, identified by low levels of a heavy lithium isotope. "Weathering can occur at the surface on continental crust or at the bottom of the ocean, but the [observed] lithium compositions can only be formed from continental crust," says Ushikubo.

The findings suggest that extensive weathering may have destroyed the Earth's earliest rocks, he says.

"Extensive weathering earlier than 4 billion years ago actually makes a lot of sense," says Valley. "People have suspected this, but there's never been any direct evidence."

Carbon dioxide in the atmosphere can combine with water to form carbonic acid, which falls as acid rain. The early Earth's atmosphere is believed to have contained extremely high levels of carbon dioxide — maybe 10,000 times as much as today.

"At [those levels], you would have had vicious acid rain and intense greenhouse [effects]. That is a condition that will dissolve rocks," Valley says. "If granites were on the surface of the Earth, they would have been destroyed almost immediately — geologically speaking — and the only remnants that we could recognize as ancient would be these zircons."

Fortunately, the climate eventually became more hospitable for land to reform and life to flourish. But interestingly enough, according to the EPA and other scientific and environmental organizations, acid rain has made a comeback in modern times, and poses a serious risk to human health.

Acid rain was first found in modern times in Manchester, England. In 1852, Robert Angus Smith identified the relationship between acid rain and man-made atmospheric pollution. However, it wasn't until the late 1960s that scientists began widely observing and studying the phenomenon. Canadian Harold Harvey was among the first to research a "dead" lake caused by acid precipitation. Since then numerous studies have confirmed that acid precipitation has harmful effects on humans, forests, and structures.

In recent years, many governments have introduced laws to reduce these emissions in an effort to prevent any further acidification of the environment, and in some areas the problem has improved, although in general—especially in developing nations—acid precipitation still poses a significant risk to human health and our environment.

While natural phenomenon like volcanoes can also cause acid rain, the EPA states that principal cause of acid rain today has been identified as sulfur and nitrogen compounds resulting from human sources, such as electricity generation, factories and motor vehicles. Just one more good reason for us to make the switch to clean energy technologies.

Posted by Rebecca Sato.

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Sources:
http://www.news.wisc.edu/15317
http://bqs.usgs.gov/acidrain/
http://www.epa.gov/acidrain/effects/forests.html
http://www.epa.gov/acidrain/effects/index.html

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