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| Thick bands of rocks containing red, oxidised iron indicate that the atmosphere contained oxygen at least 2.4 billion years ago |
As a result the subject of when oxygen first began to accumulate in the atmosphere has been closely scrutinised, with each new piece of evidence, whether geological or genetic, adding a new facet to this ever deepening paradigm of Earth history. Even so, dates are contentious.
The geological record shows that free oxygen existed in a statistically significant quantity at least 2.4 billion years ago during the Proterozoic eon. The fossil record suggests that cyanobacteria, which would have produced oxygen as a waste product of photosynthesis, existed at least 3 billion years ago. There is evidence to suggest that they are older still. Yet there is a discrepancy between the geological evidence which confirms the existence of oxygen in the atmosphere compared to when fossil and genetic evidence suggests it should have appeared.
Various hypotheses have been put forward to explain this, in particular increased volcanic activity removing the gas through the oxidation of metals contained within lava and ash. Until recently, solid geological evidence for an oxygenated atmosphere came from the Proterozoic. Now researchers working at Trinity College Dublin have pushed its history back to the Archaean.
Palaeosols are rocks formed from an ancient soil horizon millions to billions years ago. As they are exposed to the air they record chemical changes in the environment around them and so are remarkably useful in reconstructing prehistoric environments. The Trinity College researchers sampled the Keonjhar palaeosol from the Odisha Craton in India. Using uranium radioisotopes, they dated the soils to an incredible 3.02 billion years old. What made them more special, however, was that they recorded signs of weathering.
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| The rocks in the foreground are made from the Keonjhar palaeosol, which formed 3.02 billion years ago |
'The chemical changes which occur during this weathering tell us something about the composition of the atmosphere at that time. Very few of these 'palaeosols' have been documented from a period of Earth's history prior to 2.5 billion years ago. The one we worked on is at least 3.02 billion years old, and it shows chemical evidence that weathering took place in an atmosphere with elevated oxygen levels,' said Professor Quentin Crowley, Ussher Assistant Professor in Isotope Analysis and the Environment in the School of Natural Sciences at Trinity.
South African palaeosols suggest this could have existed in the earliest days of the Proterozoic, but this was the first and only geochemical piece of evidence supporting this claim, and only at 2.92 billion years ago. Crowley's palaeosols serve to back this up and place dates even further back to the Archaean, at 3.02 billion years ago. Overall, a picture is emerging of short lived pulses of atmospheric oxygen release which occurred far earlier than the so called Great Oxidation Event 2.4 billion years ago, when oxygen became a permanent part of the atmosphere. It seems that the history of our atmosphere is not as straight forward, nor as clear cut, as was previously thought.
