The time when the Earth's atmosphere turned oxygen-rich is a key milestone in our understanding of the evolution of life on the planet. It marks the point when the environment became capable of supporting much more complex life including, ultimately, us. Previous scientific evidence had pointed to this "great oxidation" event taking place about 800 million years ago. But now analysis of ancient rocks called the Stoer Group in the far north of Scotland has provided strong evidence for an even earlier rise in oxygen levels, 1.2 billion years ago or 400 million years earlier than first thought.
The discovery, published in the journal Nature, was made by University of Aberdeen geologist Professor John Parnell and his colleagues using what they dub "chemical fossils". The team extracted samples of sulphur from iron sulphide (iron pyrites, fools gold) in the rocks and compared the levels of sulphur isotopes.
Sulphur is normally found in two forms, a lighter 32S form and a heavier 34S form. Early microbial communities, operating in the absence of oxygen, evolved to reduce sulphates (SO4) to sulphides (S), preferring the lighter 32S form when they did so.
This means that rocks containing samples from these early bacterial inhabitants tend to have a relative excess of 32S (>75%) compared with 34S (<25%), which then switches around (with 34S comprising over 50%) once new oxygen-utilising microbes evolve that convert sulphides back into sulphates.
What the 1.2 billion year old Scottish rocks revealed, however, was precisely this latter result - high levels of the heavier sulphur isotope - indicating that even at this ancient age, oxygen-using bugs had already evolved. And because the rocks were part of what would have been a lake system and hence exposed to the atmosphere, the rock record the scientists have uncovered is a reliable proxy for what must have been happening worldwide.
Commenting on the implications of the discovery, John Parnell says "we are pushing back the time when there was enough oxygen in the atmosphere to support the evolution of complex life. What we've shown is that there was enough oxygen around that the atmosphere was not a barrier to that evolution."
NASA claims about 2.5 billion years ago.
This is MUCH later than current estimates... not earlier. The oxygen level became significant more than 2 billion years ago. AndroidNeox, Sun, 12th May 2013
I read the link, too - too muche of ye olde scotchhhh whiskey JimBob, Mon, 13th May 2013
Rubbish you can't have too much! Hic!
So, has Earth science come up with any sort of consensus on when oxygen level in Earth's atmosphere became significant? So far we have something between 400 million years ago and 2.5 billion years ago. AndroidNeox, Mon, 24th Jun 2013
If it happens that'd be pretty much great for the mankind i guess..the ozone level is already depleting..this will make it just great for humans.. Matildasmith, Sat, 6th Jul 2013
I think the problem here is that there are two significant events in terms of oxygen levels. The first is when oxygen appears in significant quantities in the atmosphere. This level would have been in the low single figure percent range. Then the level rises again to where it can support multi-cellular life, possibly high single figure percentage. Ophiolite, Wed, 20th Nov 2013