Forecast for Early Earth: Haze followed by Sun, then Haze, then Sun. . .

COLLEGE PARK, Md. - Earth's early atmosphere periodically flipped back and forth between hazy and sunny in a way that would have had a profound effect on the climate of our young planet, says a new study by scientists at the University of Maryland, Newcastle University and NASA.

The research published March 18 in the journal Nature Geoscience, indicates that some 2.5 billion years ago - just prior to the oxygenation of Earth that led to its development of complex life - the atmosphere seesawed between an orange haze laden with hydrocarbon particles and sunny, mostly hydrocarbon-free skies.

"The trigger for these events appears to be atmospheric changes in a potent greenhouse gas, methane," says University of Maryland geochemist James Farquhar, a coauthor of the study. "These high concentrations of methane, produced by microbial ocean life, caused the haze and an 'anti-greenhouse' effect. This is one of the earliest examples of the tight climatic coupling between Earth and its inhabitants."

Lead author Aubrey Zerkle of Newcastle University, began the study while a research scientist at the University of Maryland, working with UMD's Farquhar and Simon Poulton of Newcastle University. They analyzed the geochemistry of ancient sedimentary rocks formed from ocean sediments deposited between 2.65 and 2.5 billion years ago in what is now South Africa. They found evidence of local production of oxygen by microbes in the oceans, but say that carbon and sulphur isotopes indicate that little of that oxygen entered the atmosphere.

"Models previously have suggested that the Earth's early atmosphere could have been warmed by a layer of organic haze," says Zerkle. "Our geochemical analyses of marine sediments from this time period provide the first evidence for such an atmosphere. However, instead of evidence for a continuously 'hazy' period, we found the signal flipped on and off, in response to microbial activity," she says.

The geochemical analyses of Zerkle, Farquhar and Poulton were supported by modeling of the ancient atmosphere performed by colleagues at the NASA Astrobiology Institute, led by Mark Claire (currently at the University of East Anglia) and Shawn Domagal-Goldman. Their models demonstrated how the transitions could be caused by changes in the rate of methane production by microbes.

According to the research team, the conditions which enabled the bi-stable organic haze to form permanently ended when the atmosphere became oxygenated some 100 million years after the sediments were laid down.

"What most surprised us about these findings is that it seems to indicate the atmospheric events were discrete in nature, flip-flopping between one stable state into another," says Farquhar. "This type of response is not all that different from the way scientists think climate operates today, and reminds us how delicate the balance between states can be."

Journal Article:

A bistable organic-rich atmosphere on the Neoarchaean Earth
Aubrey Zerkle, Mark Claire, Shawn Domagal-Goldman, James Farquhar and Simon Poulton
Nature Geoscience
DOI:10.1038/NGEO1425

Media Contacts:

James Farquhar
Department of Geology and Earth System Sciences Interdisciplinary Center
University of Maryland
301-405-5043
jfarquha@Glue.umd.edu

Aubrey Zerkle
School of Civil Engineering and Geosciences
Newcastle University
44 191 222 6916
aubrey.zerkle@newcastle.ac.uk

Mark Claire
School of Environmental Sciences
University of East Anglia
44 160 359 1409
M.Claire@uea.ack.uk

Lee Tune, University Communications
University of Maryland
240-328-4914
ltune@umd.edu

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