A Danish-Japanese research team has unearthed the mystery behind why the Archean world was not frozen solid. Published in the Proceedings of the National Academy of Sciences (PNAS), the findings provide evidence that greenhouse gas was responsible for keeping water liquid on Earth.
The chemists from the University of Copenhagen in Denmark and the Tokyo Institute of Technology in Japan found that because the Sun was 30% weaker then than it is now, greenhouse gas was able to keep the Earth from becoming a ball of ice. Carbonyl sulphide, a result of the sulphur disgorge during many years of volcanic activity, was the missing link.
'Carbonyl sulphide is and was the perfect greenhouse gas. Much better than carbon dioxide,’ explained Professor Matthew S. Johnson of the Department of Chemistry at the University of Copenhagen. 'We estimate that a blanket of carbonyl sulphate would have provided about 30% extra energy to the surface of the planet. And that would have compensated for what was lacking from the Sun,’ he added.
The researchers looked at the ratio of sulphur isotopes in ancient rocks in order to determine the extra support the Sun got to keep the Earth warm so long ago. According to them, the mix of isotopes did not result from geological processes.
'There is really no process in the rocky mantle of Earth that would explain this distribution of isotopes,’ Professor Johnson said. 'You would need something happening in the atmosphere.’
The chemists irradiated sulphur dioxide with various wavelengths of sunlight and found that sunlight passing through carbonyl sulphide generated the wavelengths that produced the 'weird’ isotope mix.
'Shielding by carbonyl sulphide is really a pretty obvious candidate once you think about it, but until we looked, everyone had missed it,’ the researcher pointed out. 'What we found is really an archaic analogue to the current ozone layer – a layer that protects us from ultraviolet radiation. But unlike ozone, carbonyl sulphide would also have kept the planet warm. The only problem is it didn’t stay warm.’
The generation of life on the planet resulted in increasing amounts of oxygen, which in turn fuelled atmospheric oxidation. As a result, the sulphur emitted by volcanoes was no longer converted to carbonyl sulphide but to sulphate aerosols, which cause a cooling effect on Earth.
Professor Johnson and his colleagues produced a computer model of the ancient atmosphere, and the models, together with laboratory experiments, indicated that the drop in carbonyl sulphide levels and the rise of sulphate aerosols taken together would have generated an icy Earth; researchers had speculated that this is what took place near the end of the Archean eon 2,500 million years ago.
'Our research indicates that the distribution and composition of atmospheric gasses swung the planet from a state of life-supporting warmth to a planet-wide ice-age spanning millions of years,’ Professor Johnson commented. 'I can think of no better reason to be extremely cautious about the amounts of greenhouse gasses we are currently emitting to the atmosphere.’
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Category: Miscellaneous
Data Source Provider: Proceedings of the National Academy of Sciences (PNAS); University of Copenhagen
Document Reference: Ueno Y., et al. (2009) Geological sulfur isotopes indicate elevated OCS in the Archean atmosphere, solving the faint young sun paradox. Proceedings of the National Academy of Sciences. DOI:10.1073/pnas.0903518106.
Subject Index: Coordination, Cooperation; Earth Sciences; Scientific Research
RCN: 31225
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