New evidence suggests that it took about 100 million years for oxygen levels in the atmosphere and oceans to reach a point where the explosion of life could occur, dispelling theories that it was a quick process.
The new findings, which were published in the journal Nature Communications, uncovered a clear link between the evolution of life and the evolution of our climate. Up to now scientists were unsure how long it took Earth’s atmosphere to become oxygenated. It had been suggested that a change in animal behavior lead to a relatively quick increase in oxygen levels. However, the new research suggests that early life on Earth was slowly turbocharged by an increase in oxygen levels, rather than animal behavior leading to an oxygen spike.
Lead author of the study, Dr. Philip Pogge von Strandmann said,”We want to find out how the evolution of life links to the evolution of our climate. The question on how strongly life has actively modified Earth’s climate, and why Earth has been habitable for so long is extremely important for understanding both the climate system, and why life is on Earth in the first place.”
The team tracked oxygen levels globally from 720- 520 million years ago by using new tracer techniques in rocks found in Canada, the U.S and China. Rocks that fell to the bottom of the sea at different times from those locations were used for analysis. The results show that it took 100 million years for the amount of oxygen in the atmosphere to climb from less than 1% to over 10% (today’s level is 21%). The imprints of oxygenation in the rocks were understood by measuring selenium isotopes in each rock.
This is arguably the most significant oxygenation event in Earth’s history because it ushered in an age of complex life that still continues today.
“We took a new approach by using selenium isotope tracers to analyse marine shales which gave us more information about the gradual changes in oxygen levels than is possible using the more conventional techniques used previously. We were surprised to see how long it took Earth to produce oxygen and our findings dispel theories that it was a quick process caused by a change in animal behaviour,” said Pogge von Strandmann.
During the period of slow oxygenation, there were three instances where temperatures dropped drastically only to rise again. Researchers think the influx of nutrients into the ocean from glacial melting added to oxygen levels in the oceans. Increased nutrients means more ocean plankton which neutralizes carbon, adding to the more favorable conditions for life.
The findings also significantly push back the date of when the explosion of life supposedly occurred. Previously, it was thought that life really got its legs about 580 million years ago, following the Gaskiers glaciation – but this study pushes that date back to about 635 million years ago during the Marinoan glaciation.
“Oxygen was like a slow fuse to the explosion of animal life. Around 635 million years ago (Ma), enough oxygen probably existed to support tiny sponges. Then, after 580 Ma, strange creatures shaped like pizzas lived on a lightly oxygenated seafloor. Fifty million years later, vertebrate ancestors were gliding through oxygen-rich seawater. Tracking how oxygen increased is the first step towards understanding why it took so long. Ultimately, a grasp of geologic controls on oxygen levels can help us understand whether animal-like life might exist or not on Earth-like planets elsewhere,” concluded Dr. Pogge von Strandmann.