When did Earth begin to become oxygen rich?

When the Earth first formed into a solid sphere with an atmosphere, that atmosphere was deficient in oxygen. The question of when and how the planet became oxygen rich is an interesting and important one and recent research challenges some old ideas about the earliest appearance of oxygen.

The “Great Oxygenation Event” that infused the gas into our atmosphere is commonly thought to have occurred around 2.4 billion years ago, when a rise in cyanobacteria released a huge amount of oxygen through photosynthesis. But it’s been difficult to get any more precise in terms of dating the first appearance of this life-giving gas; after all, how are scientists supposed to detect such a small quantity of oxygen from so long ago?

The evidence for this massive oxygen increase lies in banded iron formations and sulfur isotopes, among other evidence. The isotopic signature of sulfur from more than 2.4 billion years ago, for example, shows damage from ultraviolet radiation. After that point, there are fewer of these changes in the rock, perhaps because atmospheric oxygen partially blocked incoming UV rays.

In 2007, a team of researchers turned to trace amounts of oxidation in sulfur and molybdenum —chemical elements found in rocks from the Mount McRae shale deposits in Australia — that date to roughly 2.5 billion years ago. When they used a chemical stratigraphy technique to analyze the powdered rock specimens, it revealed a “whiff of oxygen” more than 50 million years before the supposed Great Oxygenation Event.

In a newer study published in Science Advances this year, however, Sarah Slotznick, an assistant professor of Earth sciences at Dartmouth College, and her colleagues used different high-resolution techniques to reexamine these rock specimens.

[T]his fine-grained analysis seemed to prove that the first traces of oxygen occurred hundreds of millions of years after the rocks were deposited — not prior to the Great Oxygenation Event.

Slotznick explains that sulfur isotopes and organic material also show that Earth was extremely oxygen-deficient around 2.5 billion years ago. She says the new research is evidence that it’s worth revisiting other studies that point to whiffs of oxygen before the Great Oxygenation Event.

The issue is not yet settled. As is often the case in science, when there is disagreement, it takes some time to arrive at a consensus. There will be more studies, chipping away at the differences until a consensus finally emerges.

This illustrates the difficulty of identifying how the first self-replicating molecule came into being, signaling the beginning of the transition from non-life to life, around 3.5 billion years ago. Such a question would be difficult at the best of times but scientists have to deal with an environment that was very different from what we have now. In order to experimentally test hypotheses, they have to speculate on what conditions were like then and that adds a great deal of complexity to the problem.


  1. John Morales says

    The “Great Oxygenation Event” that infused the gas into our atmosphere is commonly thought to have occurred around 2.4 billion years ago […]

    […] it revealed a “whiff of oxygen” more than 50 million years before the supposed Great Oxygenation Event.

    Wow. A whole 2% difference!

  2. moarscienceplz says

    Yeah, like John Morales said.
    Unless I’m missing something, this seems to be a disagreement about 5 parts in 240, hardly what I would call a “challenge to old ideas”.
    I think it’s great that we have different teams using different methodologies to explore this question, but it looks like they are pretty much agreeing with each other.

  3. ardipithecus says

    Yeah, me too. I would expect the margin of error on these measurements to be greater than the difference between them.

  4. Reginald Selkirk says

    A small burst of oxygen occurred 50 million years early, but it was hoarded by the 1%.

  5. Pierce R. Butler says

    IANA chemist, but I know of two ways free oxygen might appear in the atmosphere: photosynthesis and electrolysis.

    Could there have been a period of heavy lightning storms before the big algae blooms?

  6. birgerjohansson says

    Reginald Selkirk @ 4
    The billionaires started anoxic. So, they evolved from those big underwater worms at mid-ocean ridges? Seems about right.

  7. seachange says

    Geology has undergone a sea-change from an art that is descriptive and declarative to a science that is precise and allows us to kinda sorta predict things about the natural world we live in. Most of this has happened during my lifetime, and in the decades after I got my degree.

    Now if it eats you it’s biology if it stinks it’s chemistry if it sets the atmosphere on fire it’s physics, and all of these sciences have temporal effects that are easy to detect and experiment with in the life times of human beings. There’s a drop of pitch falllllllling somewhere in a university in Great Britain somewhere since 1927. That is what geology is like.

    Being able to say 2% about anything, expecially when speaking about billions of years, *is* exciting. To us.

  8. lorn says

    IMHO it is hard to imagine volcanoes without positing that some oxygen is released. Lots of minerals are oxides and it seems reasonable that if you heat them they might release some oxygen.

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