Oxygen's Turbulent History

Primordial Earth had little oxygen in its oceans and atmosphere until two major spikes in levels of the gas occurred, paving the way for complex flora and fauna. The details surrounding those events are fuzzy, but two new studies of isotope records flesh out the story.

With nitrogen isotope data, Linda V. Godfrey and Paul G. Falkowski of Rutgers University reaffirm other groups' evidence that oxygen-producing bacteria, the probable cause of the first spike, were in the oceans long before it occurred. The researchers suggest that the first spike lagged behind the bacteria's appearance because the oxygen-producing bacteria's metabolism deprived the microbes of the nitrogen needed to substantially increase their numbers and the amount of oxygen they produced (Nat. Geoscience, DOI: 10.1038/ngeo633).

Meanwhile, Robert Frei of the University of Copenhagen and colleagues report a new way to study ancient oxygen: chromium isotope levels (Nature 2009, 461, 250). Their data imply that after the first spike, oxygen levels in the atmosphere fell back to lower levels before rising again, contrary to the prevailing view that the levels were always on the rise. "We are learning that the transition from an anoxic world to an oxygenated one was bumpy," says biogeochemist Ariel Anbar of Arizona State University.