Newswise — Public awareness of the global warming effects of carbon dioxide (CO2) may be a relatively recent phenomena, but such warming has been pivotal to life on our planet for billions of years.
The latest evidence of the ancient importance of CO2 can be found in a new study by University of Maryland geologist Jay Kaufman that will be published in the September 18 issue of the journal Nature.
Using samples from individual fossils of an ancient relative of algae, Kaufman and colleague Shuhai Xiao of Virginia Tech provide the first empirically-based estimates of the concentration of CO2 of in the atmosphere some 1.4 billion years ago. Their study finds that CO2 concentration was 10 to 200 times today's levels. This finding indicates the gas played a major role in keeping the earth warm and likely dominated over another greenhouse gas, methane, after the atmosphere and oceans became pervasively oxygenated between 2 billion and 2.2 billion years ago.
"The Sun was not as luminous then, so it did not provide as much light and heat as it does now," said Kaufman. "Our new findings confirm models of how much greenhouse gas was required to keep Earth warm enough so that the oceans didn't freeze during this time."
Funded primarily by NASA as part of its exobiology program, this work also provides the best evidence to date of the antiquity of the Calvin cycle, the photosynthetic cycle by which plants convert light energy and CO2 into cellular tissue.
In an accompanying "News and Views" article Stephen Mojzsis, with the Center for Astrobiology at the University of Colorado, cites Kaufman and Xiao's "novel approach" to gaining information on a period in the planet's history about which only meager geological evidence is available.
"Geobiology is conceived of as an innovative and inspiring venture in Earth science," writes Mojzsis. "By linking metabolism with CO2 concentrations, the paper by Kaufman and Xiao provides a new tool for improving our understanding of Proterozoic atmospheres."
The Proterozoic period began 2.5 billion years ago and ended 543 million years ago. Scientists think that during this period many of the most exciting events in the evolutionary history of the Earth occurred, including the appearance of abundant living organisms (probably mostly bacteria and other early single- and multi-celled organisms) and significant oxygen in the atmosphere.
One of the ocean-dwelling organisms producing oxygen during the later part of the Proterozoic period was Dictyosphaera delicata, a multi-celled plant not much bigger than the dot in this i. To estimate ancient levels of atmospheric CO2, Kaufman and Xiao measured ratios of two different forms, or isotopes, of carbon present in individual microfossils.
"It was a painstaking process to get individual organisms," Kaufman said. "I was able to take a camel hair brush and, using one hair of the brush, pick up one of these microfossils, which had been removed from its substrate [rock] using hydrofluoric acid. The acid dissolves the inorganic minerals but not organic matter."
Numerous microscopic samples of fossilized cellular material were knocked out of each organism using high-energy beams of ions from an ion probe. The sample material was analyzed with a mass spectrometer.
Kaufman is widely known for his contributions to the research indicating that Earth has been almost entirely covered in ice several times within the last billion years. Kaufman and other scientists believe that each of these "snowball earth" periods were ended by a warming of the Earth that resulted from a buildup in the atmosphere of greenhouse gases, particularly carbon dioxide.
"High CO2 levels in the Proterozic atmosphere estimated from analyses of individual microfossils" Alan J. Kaufman, Department of Geology, University of Maryland and Shuhai Xiao, Department of Geological Sciences, Virginia Polytechnic Institute and State University, Nature 425, 279-281 (18 September 2003).
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