Earth’s atmosphere is about 21 % oxygen – this fact is pretty much in every handbook of natural sciences. However, oxygen levels changed dramatically over the lifetime of the Earth. In fact, scientists from Australia are now thinking that levels of oxygen actually varied much more than previously believed, rising as high as 25 % and dropping to 10 % in the period of 2 billion years.
It is widely accepted that there were two leaps in the levels of oxygen in Earth’s atmosphere – one 2.3 billion years ago and another one 0.8 billion years ago. Scientists thought that outside of these two moments, oxygen levels remained largely the same. However, now scientists analysed changes in the composition of a group of trace metals in the ocean and found that over the last two billion years oxygen rose to very high levels (greater than 25 %) and fell to very low levels in between (less than 10 %). Scientists identified 8 oxygen cycles with less than 10 % oxygen at the end of each. At the end of every cycle there was a mass extinction.
Major mass extinction events at 450, 370, 250 and 200 million years ago corresponded with dramatic drops in oxygen below 10%. Scientists say that the oxygen cycles were driven by the supercontinent cycles of drifting and colliding continents. Collisions of continents increased the level of oxygen, because eroding mountains fed the oceans with trace elements. Increased concentrations of oxygen boosted evolution and allowed life forms to thrive. However, long periods of calmness with no collisions between continental shelves led to decrease in oxygen levels, which later contributed to mass extinctions. However, varying oxygen levels had a much bigger influence than that.
Decreasing atmospheric oxygen facilitated extensive black shale sedimentary basins that trapped large lead-zinc-silver deposits in northern Australia. We can imagine these processes were happening in other locations as well. Professor Ross Large, leader of the research team, said: “The picture is now emerging that nearly all major Earth processes in geological history were cyclical in nature, with several orders of cycles superimposed. Supercontinent cycles led to mountain building cycles, which stimulated nutrient cycles, oxygen cycles, evolutionary cycles, climate cycles and mass extinction cycles”.
Understanding how oxygen levels changed in the past could help us predict similar changes in the future. It is also interesting to see how variations in atmospheric oxygen concentration influenced evolution and even some geological changes.
Source: University of Tasmania