A new study tracing the history of the oceans, as recorded in multibillion-year-old sediments brought up in a South African drill core, provides evidence that oxygen emerged on Earth about 300 million years earlier than is broadly agreed. One of the study's co-authors acknowledges that his paper is contentious, but it supports a number of other analyses carried out in recent years using separate methods.
The new research, published online Sunday in Nature Geoscience, focuses on the nitrogen contained in the drill core's geologic record. (Scientific American is part of the Nature Publishing Group). The nitrogen cycle is driven by life, says co-author Paul Falkowski, a biogeochemist at Rutgers University, so it carries the stamp of major biological shifts—in this case, the ability of bacteria in the upper waters of the ocean to produce oxygen through photosynthesis.
What Falkowski and his Rutgers colleague, marine geochemist Linda Godfrey, found was a shift in the prevalence of nitrogen isotopes in ocean sediments indicating oxygenation. (Isotopes are species of the same atoms with differing numbers of neutrons and hence differing atomic weights.) That shift occurred nearly 2.7 billion years ago, a few hundred million years before other substantial evidence shows the presence of oxygen in the atmosphere. (Oxygen already existed, of course, in the form of water and other molecules—the research at hand concerns so-called free oxygen, or O2.)
Woodward Fischer, a geobiologist at the California Institute of Technology who worked on the same drilling project but did not contribute to the new research, says that the timing of the evolution of oxygenic photosynthesis is an open question. "The field is really split on when that occurred," Fischer says.
Researchers generally agree that a great deal of oxygen appeared following the so-called Great Oxidation Event, somewhere around 2.4 billion years ago, Fischer says, but several other isotopic studies have also supported the theory that photosynthesizers were capable of producing oxygen much earlier, in the late stages of the Archean eon, which ended around 2.5 billion years ago. The new research "is not out there by itself," he notes. "There are a couple of different papers looking at a couple of different isotope systems or geochemical systems that have come to the same conclusion independently."
Nitrogen has two stable isotopes: nitrogen 14 and heavier nitrogen 15. The balance between them, Falkowski explains, is very sensitive to the presence of oxygen. In environments with small amounts of oxygen, nitrogen is converted by microorganisms into ammonium and then nitrate or nitrite, Falkowski says. Finally, nitrogen gas is released into the atmosphere as organisms consume the nitrate and nitrite. "When you have oxygen...nitrogen gets isotopically heavier because you blow the lighter isotope off into the atmosphere," Falkowski says. "And if you didn't have any oxygen the isotopic record would just be light."
"The bonds between atoms are stronger when heavier isotopes of the atoms are involved," Godfrey explains. The breakup of nitrate or nitrite to produce nitrogen gas proceeds more easily in compounds containing the lighter isotope—nitrogen 14—so that isotope preferentially escapes into the atmosphere. More nitrogen 15, on the other hand, tends to be left behind.
"We find that we get the heavier isotope several times in the record, hundreds of millions of years before we get the Great Oxidation Event," Falkowski says. "So that means that there must have been oxygen in the ocean, but it didn't yet get into the atmosphere. It didn't really oxidize the world yet for at least 300 million or 400 million years."
That delay is somewhat curious, as photosynthesizers would be expected to boost atmospheric oxygen levels on much shorter timescales. Under the new analysis, Fischer says, "if there is oxygenic photosynthesis around in late Archean oceans and there is essentially no oxygen accumulating [in the atmosphere], then something very funny is going on that we don't appreciate yet."
Appreciating the detailed chemistry of a world before oxygenation is a tall order, Fischer says. A totally anaerobic (oxygen-free) world, he says, might involve isotopic effects that are very different than what we see in our present oxygen-rich environment. "With all the assumptions that go into how we think the nitrogen cycle should work, we could be looking at a world that is actually very different," he says.
Falkowski says that the reliance on the nitrogen cycle made for a contentious process in publishing the paper. "What we have developed is a model where nitrogen isotopes really become sensitive to the [oxidation and reduction reactions] of the world," he says. "That's a new model, and whenever you start with a new paradigm, it's inevitable you're going to run into some cross fire."