February 17, 2009
Controversy Abounds Over Oldest Traces Of Life
The argument over whether an outcrop of rock in South West Greenland contains the earliest known traces of life on Earth has been reignited, in a study to be published in the Journal of the Geological Society on 23 February. The research, led by Martin J. Whitehouse at the Swedish Museum of Natural History, argues that the controversial rocks "Ëcannot host evidence of Earth's oldest life', reopening the debate over where the oldest traces of life are located.
The small island of Akilia has long been the center of attention for scientists looking for early evidence of life. Research carried out in 1996 argued that a five meter wide outcrop of rock on the island contained graphite with depleted levels of 13C. Carbon isotopes are frequently used to search for evidence of early life, because the lightest form of carbon, 12C (atomic weight 12), is preferred in biological processes as it requires less energy to be used by organisms. This results in heavier forms, such as 13C, being less concentrated, which might account for the depleted levels found in the rocks at Akilia.
Crucial to the dating of these traces was analyzing the cross-cutting intrusions made by igneous rocks into the outcrop. Whatever is cross-cut must be older than the intruding rocks, so obtaining a date for the intrusive rock was vital. When these were claimed to be at least 3.85 billion years old, it seemed that Akilia did indeed hold evidence of the oldest traces of life on Earth.
Since then, many critics have cast doubt on the findings. Over billions of years, the rocks have undergone countless changes to their structure, being folded, distorted, heated and compressed to such an extent that their mineral composition is very different now to what it was originally. The dating of the intrusive rock has also been questioned .Nevertheless, in July 2006, an international team of scientists, led by Craig E. Manning at UCLA, published a paper claiming that they had proved conclusively that the traces of life were older than 3.8 billion years, after having mapped the area extensively. They argued that the rocks formed part of a volcanic stratigraphy, with igneous intrusions, using the cross-cutting relationships between the rocks as an important part of their theory.
The new research, led by Martin J. Whitehouse at the Swedish Museum of Natural History and Nordic Center for Earth Evolution, casts doubt on this interpretation. The researchers present new evidence demonstrating that the cross-cutting relationships are instead caused by tectonic activity, and represent a deformed fault or unconformity. If so, the age of the intrusive rock is irrelevant to the dating of the graphite, and it could well be older. Because of this, the scientists turned their attention to dating the graphite-containing rocks themselves, and found no evidence that they are any older than c. 3.67 billion years.
"ËThe rocks of Akilia provide no evidence that life existed at or before c. 3.82 Ga, or indeed before 3.67 Ga', they conclude.
The age of the Earth itself is around 4.5 billion years. If life complex enough to have the ability to fractionate carbon were to exist at 3.8 billion years, this would suggest life originated even earlier. The Hadean eon, 3.8 "“ 4.5 billion years ago, is thought to have been an environment extremely hostile to life. In addition to surviving this period, such early life would have had to contend with the "ËLate Heavy Bombardment' between 3.8 and 4.1 billion years ago, when a large number of impact craters on the Moon suggest that both the Earth and the Moon underwent significant bombardment, probably by collision with asteroids.
Full bibliographic information: M J Whitehouse, J S Myers & C M Fedo, 'The Akilia Controversy: field, structural and geochronological evidence questions interpretations of >3.8 Ga life in SW Greenland' Journal of the Geological Society, Vol. 166, 2009, pp. 1-14.
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