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.