Two teams of researchers report that traces of oxygen appeared in Earth’s atmosphere from 50 to 100 million years before what is known as the Great Oxidation Event. This event happened between 2.3 and 2.4 billion years ago, when many scientists think atmospheric oxygen increased significantly from the existing very low levels.
Scientists analyzed a kilometer-long drill core from Western Australia, representing the time just before the major rise of atmospheric oxygen. They found evidence that a small but significant amount of oxygen was present in Earth’s oceans and atmosphere 2.5 billion years ago. The findings appear in a pair of research papers in the September 28 issue of the journal Science.
“We seem to have captured a piece of time during which the amount of oxygen was actually changing — caught in the act, as it were,” said Ariel Anbar, an associate professor at Arizona State University, Tempe, and leader of one of the research teams.
The goal of both research teams was to learn more about the environment and life in the oceans leading up to the Great Oxidation Event. The researchers did not expect to find evidence of oxygen earlier than what was previously known.
“The core provides a continuous record of environmental conditions, analogous to a tape recording,” explained Anbar. He and his research group analyzed the amounts of the trace metals molybdenum, rhenium and uranium. The quantity of these metals in oceans and sediments depend on the amount of oxygen in the environment. The other research group, led by Alan Kaufman of the University of Maryland, College Park, Maryland, analyzed sulfur isotopes. Its distribution also relies on the abundance of oxygen.
“Studying the dynamics that gave rise to the presence of oxygen in Earth’s atmosphere deepens our appreciation of the complex interaction between biology and geochemistry,” said Carl Pilcher, director of the NASA Astrobiology Institute at NASA’s Ames Research Center, Moffett Field, California, which co-funded the study. “Their results support the idea that our planet and the life on it evolved together.”
One possible explanation for the Great Oxidation Event is the ancient ancestors of today’s plants first began to produce oxygen by photosynthesis. However, many geoscientists think organisms began to produce oxygen much earlier, but the oxygen was destroyed in reactions with volcanic gases and rocks.
“What we have now is new evidence for some oxygen in the environment 50 to 100 million years before the big rise of oxygen,” Anbar said. “Our findings strengthen the notion that organisms learned to produce oxygen long before the Great Oxidation Event, and that the rise of oxygen in the atmosphere ultimately was controlled by geological processes.”
