The existence of life on Earth is said to be by chance, and our planet could have been just like any other world in the solar system if not for a few unlikely events happening at the right time. Enumerable geochemical and physical transformations turned the planet into a lively paradise we see today, but a few random events kick-started the chain of life!
A vital element that makes aerobic life like ours thrive on this rock is a gas: oxygen. Earth’s atmosphere did not always have sufficient oxygen to support life. There were episodes when oxygen was present for a short time before fading again. Scientific studies show that it was only about 2.4 billion years ago when the gas began to spread widely in our atmosphere.
But this story behind the emergence of oxygen in large numbers remains a mystery, and several scientific shreds of evidence have given credit to various sources.
Moreover, mysteries thicken over the possible trigger for the emergence of short intervals of oxygen. They also could have led to the abundant presence of the gas we see today. A study published in August 2021 attributed Earth’s slow rotation that eventually increased the oxygen concentrations in the atmosphere. Another new study has proposed an alternative hypothesis.
First whiff of oxygen
The new study credits volcanoes for this mega boon of oxygen on Earth, adding to other potential clues. The study hypothesised that volcanoes could be responsible for making Earth an oxygen-rich planet about 2.4 billion years ago. The study also shows that about 50 to 100 million years before this excessive oxygenation event, there was a presence of oxygen in the atmosphere for a short duration.
This short oxygen phase eventually paved the way to oxygen being abundant in the Earth’s atmosphere. It remained a perplexing problem as to what exactly caused the birth of oxygen for such a short term. In this new study, the team has found volcanoes behind as the most potential source.
The team explored 2.5-billion-year-old rocks from the Mount McRae Shale formation in Western Australia to investigate the earliest sources of oxygen. This rock belongs to the period just before the Great Oxygenation Event.
How did the team prove that the rocks are from a volcanic site? Or how it led to a spike of oxygen in the atmosphere?
Addressing first, the rock analysis revealed a presence of mercury, which is typically released from volcanic explosions. Usually, volcanoes spit out large amounts of mercury into the upper atmosphere, where it lingered for a longer duration—per se one to two years—before falling onto Earth’s surface. There are several geological records of that time that support volcanic activity.
“Sure enough, in the rock below the transient spike in oxygen, we found evidence of mercury, both in its abundance and isotopes, that would most reasonably be explained by volcanic eruptions into the atmosphere,” said co-author Roger Buick, a UW professor of Earth and Space Sciences.
The study shows this spike in mercury levels for a few million years just before the short-term presence of oxygen. And from this originates a complex process that answers the second part! As we know, volcanic emissions lead to the formation of nutrient-rich lava and volcanic ash fields. These nutrients are usually washed away via winds and rain into oceanic bodies. Further, the presence of nutrients fuels the growth of oxygen-producing cyanobacteria and other single-celled life forms.
“During weathering under the Archaean atmosphere, the fresh basaltic rock would have slowly dissolved, releasing the essential macronutrient phosphorus into the rivers. That would have fed microbes that were living in the shallow coastal zones and triggered increased biological productivity that would have created, as a byproduct, an oxygen spike,” explains Jana Meixnerová, one of the authors of the study.
In the short span, the eruption possibly might have led to an explosion in the population size of marine microorganisms, which created the first whiffs of the gas by converting carbon dioxide and water into oxygen. Thanks to the enumerable process, the Earth is the ‘chosen one’ in the entire solar system!
The results of this study have been published in the journal Proceedings of the National Academy of Sciences and can be accessed here.