By Sid Perkins
Analyses of ancient sulfide minerals and the modern organisms that create sulfides are giving scientists a better idea of what Earth’s atmosphere and oceans may have been like billions of years ago. The findings may also explain a paradox that has long puzzled solar astronomers.
In one of the new studies, scientists looked at the ratio of isotopes in sulfide particles trapped in diamonds unearthed from a mine in Botswana. Radioactive dating shows that those gems formed about 2.9 billion years ago, says Mark H. Thiemens, a chemist at the University of California, San Diego.
In the Dec. 20, 2002 Science, Thiemens and his colleagues argue that their data–in particular, a higher-than-normal proportion of sulfur-33 in the inclusions–can only be explained by certain atmospheric chemical reactions that are stimulated by specific wavelengths of ultraviolet light. Today, those wavelengths are screened from most of Earth’s atmosphere by ozone (O3) and oxygen (O2). If there had been more than a trace of oxygen in the atmosphere 2.9 billion years ago, the sulfide inclusions would have contained isotope ratios typical of today’s compounds.
Coauthor Kevin D. McKeegan says that it’s “rather surprising” that a diamond formed deep within the planet contains information about Earth’s ancient atmosphere.
In another report in the same issue of Science, other researchers infer the composition of the early atmosphere in a different way–from the isotope ratios in sulfides produced by aquatic microorganisms that feed on dissolved sulfates.
Donald E. Canfield of the University of Southern Denmark in Odense and his coworkers studied living microbes taken from both freshwater lake sediments and coastal marine sediments. They also measured isotope ratios in sulfides produced by Archaeoglobus
fulgidus, a single-celled organism that thrives around deep-sea hydrothermal vents.
When dissolved sulfates were plentiful in the experiment, the microbes produced sulfide compounds with isotope ratios that differed significantly from those in the sulfates. However, when the scientists provided concentrations of dissolved sulfates of less than 20 parts per million, the sulfur-isotope ratios in the microbe-generated sulfides didn’t vary markedly from those in the sulfates.
That’s telling because sediments deposited more than 2.5 billion years ago don’t show large differences between the isotope ratios of their sulfates and sulfide minerals produced by primordial microbes, says Canfield.
The new lab results suggest that ancient oceans had only low concentrations of dissolved sulfates–a sign that there probably wasn’t much oxygen in the air to react with the abundant sulfur dioxide that was being spewed into the sky by volcanoes.
The early scarcity of dissolved sulfates and atmospheric oxygen has big implications.
For one, says Canfield, microbial communities were probably dominated by organisms that produced methane, a planet-warming greenhouse gas that traps heat more than 20 times as effectively as carbon dioxide does.
A methane-rich atmosphere on early Earth, in turn, could explain the so-called faint-young-sun paradox, says Uwe H. Wiechert of the Institute for Isotope Geology and Mineral Resources in Zurich. Current models of solar evolution suggest that when our planet first formed 4.5 billion years ago, the sun produced only about 70 percent as much radiation as it does now. Without a large greenhouse effect, solar luminosity at that time wouldn’t have been enough to keep the oceans from freezing solid.
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