Based on studies of rock cores, geoscientists supported in part by the NASA Astrobiology Institute have determined that oxygen did not appear in Earth’s atmosphere in a single event. Instead, atmospheric oxygen came about in a long series of starts and stops.

The research was conducted using samples collected in the summer of 2007 during the Fennoscandia Arctic Russia – Drilling Early Earth Project (FARDEEP). Scientists drilled a series of shallow, two-inch diameter cores and overlapped them to create a record of 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 research was published in Science Express under lead author Lee Kump.