- Press Release
- Nov 29, 2022
Ancient pebbles provide new details about primeval atmosphere
BY Geoff Koch
Editor Note: This article was written by journalism intern 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.
"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
Stanford geologists Donald R. Lowe and Dennis K. Bird, and senior Stanford
researcher Robert E. Jones, also contributed to the research.
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."
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
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.]