Ancient climate explained by pebbles

BY GEOFF KOCH

Analysis of 3.2-billion-year-old pebbles has yielded perhaps the oldest geological evidence of Earth's ancient atmosphere and climate. The findings, published in the April 15 issue of the journal Nature, indicate that carbon dioxide levels in the early atmosphere were substantially above those that exist today and above those predicted by other models of the early Earth. The research implies that carbon dioxide, perhaps aided by another greenhouse gas such as methane, helped to keep the planet warm enough for life to form and evolve.

Drill core samples taken from a gold mine in South Africa contain 3.2-billion-year-old pebbles and weathering rinds that provide clues about climate conditions. Courtesy of Angela Hessler

"The early mix of greenhouse gases is relevant to the evolution of atmospheric oxygen and the conditions in which life arose," said Angela Hessler, a geology professor at Grand Valley State University, who completed the research as a doctoral student at Stanford University. "A more detailed picture of early Earth might serve as a proxy for exploring the history of nearby planets in the solar system."

Stanford geologists Donald R. Lowe and Dennis K. Bird, and senior Stanford researcher Robert E. Jones, also contributed to the research.

Early life

Scientists have long agreed that some sort of greenhouse effect started relatively soon after the Earth was formed 4.6 billion years ago. Microbial life appeared as early as 3.8 billion years ago when the sun was 25 percent dimmer than it is today. Absent some process to retain and amplify heat, the planet would have been a frozen wasteland and the first bacteria would not have appeared so soon.

The exact mix of these ancient greenhouse gases is poorly understood, in large part because of the paucity of data. Weathering rinds -- discolorations near the surface of pebbles that give evidence of reactions that occurred billions of years ago with the primeval atmosphere -- offer useful evidence. But the steady churning of the Earth's crust through plate tectonics ensured that most of these pebbles, and all their accompanying information, have long since been recycled.

The researchers say the pebbles they analyzed, found in drill cores taken at the Royal Sheba gold mine in South Africa, were rolled into smooth, round shapes in a 3.2 billion-year-old river or stream system. "This outcrop [in South Africa] is unique in its preservation," Bird said. "Few remain that haven't been modified in some way by tectonic and metamorphic processes."  

Early atmosphere

Hessler's geochemical analysis of the rinds, which include an iron-rich carbonate, allowed the team to determine the minimum amount of carbon dioxide in the atmosphere when the carbonate was formed. This amount is several times higher than the amount of carbon dioxide in the atmosphere today, consistent with the current understanding that life evolved in a dramatically different environment than exists today.

Other research by the Stanford group suggests that carbon dioxide levels gradually declined during the 500 million years after the formation of the pebbles. As the continents became stable, and surface weathering and photosynthesis evolved, it is likely that carbon dioxide was more readily removed from the atmosphere.

Methane, another greenhouse gas produced by decaying biomass, may have combined with carbon dioxide to maintain warm or even hot surface temperatures. Earlier work by the study's authors suggests that surface temperatures on the 3.2-billion-year-old Earth may have topped 60 degrees Celsius (140 degrees Fahrenheit).

Geologic samples with evidence of atmospheric chemistry in the Archaean Eon, the first two billion years of Earth's history, are separated by 500 million years. So despite the new information in the Nature study, attempts to understand Earth's ancient history still involve lots of inferences and educated guesses.

The authors, whose research was funded by the NASA Exobiology Program and the National Science Foundation, agree on the need for more hard evidence. "There can be little doubt about the importance of empirical geologic observations for constraining future climate models of Earth's early atmosphere," they wrote.

Geoff Koch is a journalism intern at the Stanford News Service.