How much greenhouse gas was there in the ancient atmosphere? A 1993 model by Jim Kasting of Pennsylvania State University estimates that carbon dioxide (CO2) levels in the Earth's early atmosphere must have been 10 times to as much as 10,000 times today's level, in order to compensate for the young (and fainter) sun. Now, a measurement of the fossil record using a new instrument has confirmed a portion of the model. Atmospheric CO2 level 1.4 billion years ago was at least ten to 200 times greater than today, according to the new research.
The findings are reported in the September 18, 2003, issue of Nature by Alan Jay Kaufman of the geology department at the University of Maryland and Shuhai Xiao of the geosciences department at Virginia Tech ("High CO2 levels in the Proterozoic atmosphere estimated from analyses of individual microfossils").
The researchers determined the CO2 level by using the carbon ion microprobe housed at the Carnegie Institute in Washington, D.C. They conducted their studies on the microscopic fossil Dictyosphaera delicata from Proterozoic shales in northern China. "This was a eukaryotic photosynthesizer - it had a nucleus and made organic matter from CO2 - about one-tenth of a millimeter in size," says Xiao. "It had the ability to become dormant in bad times, when it formed a robust wall to protect itself. That tough wall is what is preserved in the fossil record."
All modern eukaryotic photosynthesizers use a similar biochemical pathway to convert CO2 into organic matter. "We assume the old guy used the same biochemical pathway," says Xiao. Therefore, they would be able to measure the type of carbon in the fossil in order to determine the CO2 concentrations in the ocean and the atmosphere.
"We zapped into the fossil using a 10 micrometer ion beam, which destroys a small amount of the organic material and ejects carbon ions, which we analyzed," says Xiao.
The critical measure was the amount of carbon-12 (12C) versus carbon-13 (13C). D. delicata formed their organic wall from dissolved CO2 in the ocean. Carbon dioxide formed with 12C is preferred because it is lighter. The higher ratio of 12C in the tissue would indicate higher levels of CO2 available in the water. Since D. delicata lived in the surface ocean, which is at equilibrium with the atmosphere, the amount of CO2 in the atmosphere could also be calculated.
Carbon dioxide today is 350 parts per million (ppm) or .035 percent - compared to 270 ppm before industrialization - a less than 30 percent increase. But 1.4 billion years ago, CO2 was more than 10 to 200 times today's level.
"This gives us a geological context for CO2 evolution and climate change," says Xiao. While Kaufman and Xiao's study confirms the model, "We need more data points to fill the gaps and test the model for the first four billion years," Xiao says.
There are many data points to confirm the model from the last half billion years, but the Kaufman and Xiao study provide only the second data point between a half billion years ago and 4.5 billion years ago. Rob Rye at University of Southern California and colleagues looked at ancient soil from 2.7 billion years ago and determined there was barely enough CO2 to compensate for the weaker Sun - the lowest range of the Kasting model. "But there were probably significant amounts of other greenhouse gases, such as methane, 2.7 billion years ago," says Xiao.
The Earth's atmosphere became more oxidized by 2.2 billion years ago, after which methane became a less significant greenhouse gas, "But, by the period of our study, there was plenty of CO2," says Xiao.
Xiao and Kaufman began their collaboration at Harvard, where Kaufman was a post-doctoral associate while Xiao was a graduate student. The research was supported by NASA Exobiology, NSF Geology and Paleontology, and China Ministry of Science and Technology 973 programs. Xiao was a faculty member at Tulane University before joining the Virginia Tech faculty this fall.
There is also an article on this research in the News and Notes section of Nature and a University of Maryland news release with information from Dr. Kaufman will be available on that university's web site once the embargo lifts.