Life exploded on Earth after a 100 million year oxygen buildup
Life exploded on Earth, specifically animal life, after a 100 million year oxygen buildup that reached its peak approximately 600 million years ago, says a team of scientists from the UK, USA and Denmark, led by University College London (UCL).
Scientists have long wondered about the chicken-and-egg question – whether animal life exploded before or after oxygen levels rose, i.e. did the explosion bring about the oxygen increase or was it the other way round? That is, until now.
Nobody was sure how long it took for Earth’s atmosphere and oceans to become oxygenated until now either.
First came the rise in oxygen levels, and then the explosion in animal life. (Image: student.societyforscience.org)
Study leader, Dr. Philip Pogge von Strandmann, Senior Lecturer in Isotope Geochemistry at UCL’s Department of Earth Sciences, and colleagues wrote about their study and findings in the academic journal Nature Communications.
The authors say their study shows that the rise in oxygen levels started considerably earlier than previously thought, and occurred sporadically over a very long period.
Therefore, they have concluded that the early animal evolution was driven by rising amounts of oxygen, rather than that a change in animal behaviour led to oxygenation.
Helps explain why there is life on Earth
Dr. 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 the 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.”
Scientists from Bristol University (UK), UCL (UK), the University of Leeds(UK), Utah State University (USA), the University of Washington(USA), and the University of Southern Denmark tracked what had been occurring with oxygen levels worldwide from 770 million to 520 million years ago using new tracers in rocks across China, Canada and the United States.
The buildup in oxygen levels led to an age of abundant animal life that continues today.
Samples of rocks that had been laid down at the bottom of the sea at different times were taken from various locations to get a global picture of the oxygen levels of Earth’s atmosphere and oceans.
100 million year oxygen climb
The researchers measured selenium isotopes in the rocks and found that it took one hundred million years for atmospheric oxygen levels to climb from below 1% to more than 10% of levels present today.
This was likely the most significant oxygenation event in the history of our planet because it heralded an age of animal life that has continued until this day.
Dr. Pogge von Strandmann, said:
“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.”
During the three periods studied, three large glaciations – the Marinoan (~635 million years ago), the ‘snowball Earth’ Sturtian (~716 million years ago), and the smaller Gaskiers glaciation (~580 million years ago) – occurred whereby Earth’s land was iced over and most of the oceans from the poles to the tropics were frozen.
Dr. Philip Pogge von Strandmann’s research focuses on the biogeochemical cycling of elements, and how that interacts with current and past climates. As such he is interested in chemical weathering, soil formation chemical processes, and the effects of both on ocean chemistry. In particular, this aims to understand how the Earth’s climate has responded to extreme perturbations in the past, and how we can use this information to understand future global warming. (Image: www.ucl.ac.uk/earth-sciences)
Melting glaciers pushed nutrients into the sea
During these three periods, temperatures plunged and rose again, causing glaciers to melt and an influx of nutrients into the sea, which scientists believe caused oxygen levels to increase deep in the oceans.
More nutrients in the oceans meant more plankton, which bury organic carbon in seafloor sediments after they die. When carbon was buried oxygen increased, which changed conditions on Earth dramatically.
Until this study, most scientists thought oxygenation occurred after the relatively small Gaskiers glaciation melted.
The authors say their findings push it much earlier, to the Marinoan glaciation, after which animals started to flourish in the more favourable conditions – this led to the first big expansion of animal life.
Co-author Professor David Catling, who works at the University of Washington’s School of Earth and Space Sciences, added:
“Oxygen was like a slow fuse to the explosion of animal life. Around 635 Ma, enough oxygen probably existed to support tiny sponges. Then, after 580 Ma, strange creatures, as thin as crêpes, 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.”
In an Abstract in the journal, the authors concluded:
“Overall, increased ocean oxidation and atmospheric O2 extended over at least 100 million years, setting the stage for early animal evolution.”
The study was funded by the Natural Environment Research Council.
Citation: “Selenium isotope evidence for progressive oxidation of the Neoproterozoic biosphere,” Philip A. E. Pogge von Strandmann, Eva E. Stüeken, Tim Elliott, Simon W. Poulton, Carol M. Dehler, Don E. Canfield & David C. Catling. Nature Communications 6, Article number: 10157. Deccember 18, 2015. DOI:10.1038/ncomms10157.