The appearance of oxygen in the Earth's atmosphere probably did not occur as a single event, but as a long series of starts and stops, according to geoscientists who investigated rock cores from the FAR DEEP project. The Fennoscandia Arctic Russia - Drilling Early Earth Project--FAR DEEP--took place during the summer of 2007 near Murmansk in Northwest Russia.

The project, part of the International Continental Scientific Drilling Program, drilled a series of shallow, two-inch diameter cores and, by overlapping them, created a record of stone deposited during the Proterozoic Eon--2,500 million to 542 million years ago.

"We've always thought that oxygen came into the atmosphere really quickly during an event," said Lee Kump, a geoscientist at Penn State University.

"We are no longer looking for an event. Now we're looking for when and why oxygen became a stable part of the Earth's atmosphere."

The researchers report in this week's issue of the journal Science Express that evaluation of these cores, in comparison with cores from Gabon previously analyzed by others, supports the conclusion that the Great Oxidation Event, the appearance of free oxygen in Earth's atmosphere, played out over hundreds of millions of years. Kump is the lead author of the Science Express paper.

Oxygen levels gradually crossed the low atmospheric oxygen threshold for pyrite--an iron sulfur mineral--oxidation by 2,500 million years ago, and the loss of what scientists call mass-independently fractionated (MIF) sulfur by 2,400 million years ago.

Then oxygen levels rose at an ever-increasing rate through the Paleoproterozoic, achieving about one percent of the present atmospheric level.

"The definition of when an oxygen atmosphere occurred depends on which threshold you are looking for," said Kump. "It could be when pyrite becomes oxidized, when sulfur MIF disappears, or when deep crustal oxidation occurs."

When the MIF sulfur disappeared, the air on Earth was still not breathable by animal standards.

When red rocks containing iron oxides appeared 2,300 million years ago, the air was still unbreathable.

"At about one percent oxygen, the groundwater became strongly oxidized, making it possible for water seeping through rocks to oxidize organic materials," said Kump.

Initially, any oxygen in the atmosphere, produced by the photosynthesis of single-celled organisms, was used up when sulfur, iron and other elements oxidized.

When sufficient oxygen accumulated in the atmosphere, it permeated the groundwater and began oxidizing buried organic material, oxidizing carbon to create carbon dioxide.

"Insights into Earth's carbon cycle offer tantalizing clues to the history of atmospheric oxygen levels, and Kump and others have revealed unrecognized details of the timing and mechanism of the Great Oxidation Event," said Enriqueta Barrera, program director in the National Science Foundation's Division of Earth Sciences, which funded the research.

The cores from the FAR-DEEP project were compared with samples from Gabon using the ratio of carbon isotopes, or variants, 13 and 12 to see if the evidence for high rates of oxygen accumulation existed worldwide.

Both the FAR-DEEP project's cores and the Gabon cores show large deposits of carbon in the form of fossilized petroleum.

Both sets of cores also show similar changes in carbon 13 through time, indicating that the changes in carbon isotopes occurred worldwide and that oxygen levels throughout the atmosphere were high.

"Although others have documented huge carbon isotope variations at later times in Earth history associated with stepwise increases in atmospheric oxygen, our results are less equivocal because we have many lines of data all pointing to the same thing," said Kump.

"These indications include not only carbon 13 isotope profiles in organic mater from two widely separated locations, but also supporting profiles in limestones and no indication that processes occurring since that time have altered the signal."

Working with Kump on the project were geoscientists Michael Arthur of Penn State; Christopher Junium of Syracuse University; Alex Brasier and Anthony Fallick of the Scottish Universities Environmental Research Centre; Victor Melezhik, Aivo Lepland and Alenka Crne at the Norwegian Geological Survey; and Genming Luo, China University of Geosciences.