Scientists have studied substances preserved inside zircon crystals. They found that Earth’s mantle was receiving oxygen-rich material from the surface long before atmospheric oxygen reached its present-day level 2.1 billion years ago.
Oxygen in Our Planet’s History
Today, Earth’s atmosphere and its other geographical layers are enriched with oxygen. This situation is thought to have developed approximately 2.1 billion years ago. However, phys.org reports that a study published in Nature Geoscience argues that certain events must have occurred even earlier.
Molecular oxygen on our planet is generally considered to be predominantly biological in origin. The earliest organisms were anaerobic, but over time, some emerged that produced oxygen through internal processes. Eventually, its abundance exceeded a certain threshold, giving some living organisms access to a powerful new energy source. The rest died out.
Scientists estimate how much oxygen was present at the surface by examining the composition of rocks formed at the time. Yet the farther back researchers look, the fewer samples they can find that have not been altered by subsequent events. This time, however, they managed to take a closer look at zircon crystals.
What Sulfur Inside Zircon Revealed
Zircon is an exceptionally durable and ancient mineral that scientists have long used to investigate the earliest periods of our planet’s existence. This time, however, the researchers focused not on zircon itself, but on inclusions of another mineral—apatite—found in some ancient samples.
Apatite is less resistant than zircon, but it contains far more inclusions. Within it, the researchers found a particularly interesting substance: sulfur. Sulfur is sensitive to external conditions, which makes it a useful indicator of the circumstances surrounding its formation. In the samples studied, those conditions involved an oxygen-rich environment.
Zircon can preserve information about the state of Earth’s mantle at the time it formed. Judging by the chemical composition of the sulfur within it, oxygen-enriched material was already traveling from the surface into the planet’s interior through subduction zones approximately 3–3.5 billion years ago.
Scientists do not yet know exactly what this discovery reflects. Overall, however, they are not particularly surprised. They had already suspected that, before the Great Oxidation Event, the ocean contained pockets enriched with the element we all breathe today, or experienced isolated episodes of oxygenation lasting several centuries.
