How Early Was Oxygen's Rise?

Dick writes about Earth and planetary science for Science magazine.

The first living things did not require oxygen to "breathe," but early life on Earth never would have gotten much beyond pond scum without free oxygen in the atmosphere. Conventional thinking has oxygen produced by photosynthesis gaining the upper hand 2.4 billion years ago, nearly halfway into Earth history. But new laboratory results reported in tomorrow’s issue of Science challenge the late arrival of this "Great Oxidation Event."

Some researchers had long argued for oxygen's early arrival well before 3 billion years ago, but in 2000, geochemist James Farquhar of the University of Maryland, College Park, came up with a nifty technique involving sulfur isotopes that nailed the oxidation event at 2.4 billion years. The proportion of different isotopes of the same element can change during a chemical reaction. Normally, the change depends on the masses of the isotopes. But Farquhar found isotopic shifts among three sulfur isotopes before 2.4 billion years ago that hadn’t depended on isotope mass. As far as anyone knew, such "mass-independent fractionation" (MIF) could have happened only under solar ultraviolet radiation in an oxygen-free atmosphere--and MIF sulfur disappeared 2.4 billion years ago.

But theoreticians recently hinted at a potential exception: MIF sulfur might also be produced in the presence of hot, solid proteins. In the new study, geochemists Hiroshi Ohmoto and Yumiko Watanabe of Pennsylvania State University, University Park, set out to investigate this possibility. They and Farquhar--who performed the critical isotopic analyses--report that, under conditions crudely mimicking the more intense hot springs of the young Earth, two amino acids can indeed produce very small degrees of MIF.

In a carefully worded conclusion, the paper suggests that the newly discovered reactions reopen the question of early oxygenation. If an oxygen-free atmosphere is not the sole source of MIF sulfur after all, it's possible that oxygen was present in the atmosphere before the MIF signal disappeared, Ohmoto argues: "This is at least a possibility that we should be thinking about." Personally, he adds, "I feel the findings we made fit nicely to what I've been saying for years," that oxygen appeared hundreds of millions of years earlier. Farquhar, on the other hand, still considers it "much more likely" that the disappearance of the MIF signal marks the first appearance of atmospheric oxygen.

Other specialists tend to agree with Farquhar, noting that the effect in the lab is small--10% of the amount of MIF found in the geologic record--and that there's no convincing reason why the hot-spring reactions would have shut off at 2.4 billion years. "I think it is still more likely to be of atmospheric origin," says geochemist Shuhei Ono of the Massachusetts Institute of Technology in Cambridge, "so the current idea of an [early oxygen-free] atmosphere still holds."

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