Wednesday, July 09, 2008

Snowball Earth

Readers of this blog may forget that I am actually a geologist by academic training, if not precisely by current vocation. Yet, a geologist I am, and I still try and keep current on advances in the science.

Recently, geologists announced they found strong evidence that a half-dozen major basins in India were formed a billion or more years ago, making them at least 500 million years older than commonly thought. According to a University of Indiana professor, "The required revision is enormous -- 500 million years or about 11 percent of total Earth history." An associate professor at the University of Florida noted "In modern geology, to revise the age of basins like this by 500 million years is pretty unique."

(Oh, really? Last year, geologists from the University of Alberta found that the Queen Maud block of Arctic Canada collided a minimum of 500 million years earlier than previously thought. The Queen Maud block is a large bedrock terrain that is said to occupy a key tectonic position in Northern Canada. The U of A team reached the remote Northern Canadian location by helicopter and discovered that the age of the rocks in basins within the terrain challenged the previous models.)

The Indian findings appear to remove one of the major obstacles to the Snowball Earth theory that a frozen Earth was once entirely covered in snow and ice from pole to pole, and might even lend some weight to a controversial claim that complex life originated hundreds of million years earlier than most scientists currently believe.

The Snowball Earth theory posits that the Earth was completely covered in snow and ice from about 635 million to 700 million years ago. While much geological evidence has been found to support that theory worldwide, the Vindhyan and other Purana basins of India lacked numerous telltale signs, such as striated or scratched boulders formed when ice drags small pebbles over bedrock and boulder beds derived from glaciers. As a result, the basins represented a prominent obstacle to the theory. The new study removes that obstacle because it pushes back the origins of the basins to well before Snowball Earth would have occurred.

The Indian basins are located south of New Delhi in the northern and central regions of India. They are slight, mostly flat depressions in the Earth's crust that span thousands of square miles. For decades, most geologists believed the basins formed 500 million to 700 million years ago when the Earth's crust stretched, thinned and then subsided. Apparent fossils retrieved from the basin seemed to have originated between 500 million and 700 million years ago.

But a Florida graduate student dated rock retrieved from one of the basin to about 1.07 billion years ago. As a result, geologists cored dozens of rocks collected from 56 sites in the basins. Zircon age dating from all of the samples tested indicated the rocks formed about 1.02 billion years ago. A separate 2007 study dated rocks from another Purana basin to 1.02 billion years ago, another 500-million-year revision.

The new age dates not only push back the origins of the basins to well before Snowball Earth would have occurred, but also call into question the hypothesis that the basins formed as the supercontinent Rodinia broke up. Rodinia is thought to have separated into the modern continents about 700 million years ago, but the revisions make the basins too old for that split.

The Florida research could also support a Swedish paleontologist's controversial dating of multicellular creatures ("Ediacara" fauna) from an older part of the basin to 1.6 billion years. But of all the implications of this research, the notion that Ediacaran-like organisms may be much older than 580 million years is probably the most speculative.

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