New Delhi: Billions of years ago, when oxygen first appeared on Earth, the atmosphere was not the only part of the planet undergoing radical change. The Earth’s depths were also responding to the changes, says a new study, which offers fresh clues about how life developed on the planet.
The study, published in Nature Geoscience on 14 October, was led by researchers at the University of Portsmouth. They examined ancient minerals to trace how oxygen levels changed between 3.4 billion and 2.1 billion years ago.
For much of the Earth’s early history, there was almost no oxygen in its atmosphere or oceans. This began to change nearly 2.3 billion years ago during the Great Oxidation Event, when oxygen levels suddenly rose. Scientists also suspect that small, localised pockets of oxygen existed even before this period. According to the study, these early changes were already linked to processes deep inside the planet.
Researchers studied zircon crystals that can survive for billions of years without changing. Some contained tiny grains of another mineral, apatite, which remained protected inside the zircon.
“These minerals are really exciting to scientists, as they are essentially time capsules. Because they’re so well protected, we can analyse their chemical fingerprints to get an untouched snapshot of a moment billions of years in the past. We’re looking into the early Earth, when it was a very different planet, and figuring out what we can still see of it today,” lead researcher Hugo Moreira, Fellow at the University of Portsmouth, said in a statement.
The team analysed ancient rock samples from Australia, Finland, South Africa, Brazil and Scotland. Using X-rays at the European Synchrotron Radiation Facility in France, they studied the chemical form of sulphur trapped inside the apatite crystals. Sulphur’s chemical signature can reveal the conditions in which a mineral formed, including whether oxygen was present.
The results showed that the oxygen-related chemical signature in parts of Earth’s mantle increased between 3.4 billion and 3 billion years ago. It remained consistently present throughout the period studied, reaching levels comparable to those seen today by around 2.1 billion years ago.
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A link between the surface and the depths
The findings point to a connection between the Earth’s surface and its interior through subduction, a process in which parts of the planet’s outer shell sink into the mantle and are recycled.
The research does not establish exactly when life first emerged. However, it helps scientists understand how Earth’s geological systems developed and how they may have shaped conditions for life.
“This isn’t about changing how we live tomorrow. It’s about answering the philosophical questions that are in our nature as human beings – where did we come from, and why are we here? It’s curiosity-driven research, coming from a simple drive to know things. It won’t affect a person on the street immediately, but it may help humankind in its own understanding of itself as a species,” said Moreira.