Study looks finds origin-of-life clues in rocks from Archean period
By Summit Voice
FRISCO — Scientists have long been studying samples of the Earth’s atmosphere from the past by analyzing gas bubbles trapped in ice cores, but a team of European researchers have taken that process a step farther.
By examining gases in water bubbles trapped in ancient quartz from Australia, they were able to determine the composition of the atmosphere during the time life started evolving on Earth, about 2.4 to to 3.8 billion years ago, during the Archean period.
The results may help determine how life was able to take hold and develop during a time when the sun only delivered about 75 percent of the energy that it does now — and the answer may be related to concentrations of heat-trapping gases like carbon dioxide.
The weak sun during the Archean period would suggest that the Earth should have been permanently glaciated, making it more difficult for organisms to survive. But the scientists with the CRPG-CNRS University of Lorraine, The University of Manchester and the Institut de Physique du Globe de Paris say carbon dioxide levels may have been high enough compensate for less solar energy.
“During the Archean the solar energy received at the surface of the Earth was about 20 to 25 percent lower than present,” said study author, Dr Ray Burgess, from Manchester’s School of Earth, Atmospheric and Environmental Sciences. “If the greenhouse gas composition of the atmosphere was comparable to current levels then the Earth should have been permanently glaciated but geological evidence suggests there were no global glaciations before the end of the Archean and that liquid water was widespread.”
“To counter the effect of the weaker Sun, carbon dioxide concentrations in the Earth’s atmosphere would need to have been 1,000 times higher than present,” said lead author Professor Bernard Marty, from the CRPG-CNRS University of Lorraine. “However, ancient fossil soils – the best indicators of ancient carbon dioxide levels in the atmosphere – suggest only modest levels during the Archean. Other atmospheric greenhouse gases were also present, in particular ammonia and methane, but these gases are fragile and easily destroyed by ultraviolet solar radiation, so are unlikely to have had any effect.”
But another climate-warming theory — one the team wanted to test — is that the amount of nitrogen could have been higher in the ancient atmosphere, which would amplify the greenhouse effect of carbon dioxide and allow the Earth to remain ice-free.
The team analyzed tiny samples of air trapped in water bubbles in quartz from a region of northern Australia that has extremely old and exceptionally well-preserved rocks.
“We measured the amount and isotopic abundances of nitrogen and argon in the ancient air,” said Professor Marty. “Argon is a noble gas which, being chemically inert, is an ideal element to monitor atmospheric change. Using the nitrogen and argon measurements we were able to reconstruct the amount and isotope composition of the nitrogen dissolved in the water and, from that, the atmosphere that was once in equilibrium with the water.”
The researchers found that the partial pressure of nitrogen in the Archean atmosphere was similar, possibly even slightly lower, than it is at present, ruling out nitrogen as one of the main contenders for solving the early climate puzzle.
Dr Burgess added: “The amount of nitrogen in the atmosphere was too low to enhance the greenhouse effect of carbon dioxide sufficiently to warm the planet. However, our results did give a higher than expected pressure reading for carbon dioxide – at odds with the estimates based on fossil soils – which could be high enough to counteract the effects of the faint young Sun and will require further investigation.”