An international team of geologists and geochemists has published a study in the journal Nature Geoscience that significantly shifts the timeline for the appearance of early signs of oxygenation on Earth. The key tool of the research was zircon crystals up to 3.4 billion years old, inside of which scientists discovered a sealed apatite mineral. It was the chemical composition of sulfur in this apatite that proved to be a "tombstone" of the ancient atmosphere: the form of sulfur compounds accurately reflects the amount of free oxygen at the moment the mineral formed. The results showed that local "oxygen oases" existed on the planet much earlier than previously assumed — in the interval between 3 and 3.4 billion years ago.
How Zircon's "Time Capsule" Works
The choice of zircon as an object of study is no accident. This mineral possesses exceptional chemical stability and is capable of surviving in a practically unchanged form for billions of years, enduring even the most aggressive geological processes. Inside individual zircon crystals, scientists discovered inclusions of apatite that were hermetically sealed within the host matrix. Such "encapsulation" protected the apatite from any subsequent chemical impacts — weathering, metamorphism, hydrothermal processing. In thickness, the studied crystals are thinner than a human hair, yet inside them microscopic particles of elements are concentrated, carrying information about the composition of the atmosphere during an era when Earth radically differed from its modern appearance.
Sample Geography and Analytical Methodology
The study was truly international in character. Samples of ancient rocks were collected from five locations across the planet: Scotland, Finland, South Africa, Brazil, and Australia. All samples up to 3.4 billion years old were transferred to the University of Portsmouth, and the final stage of analysis was carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble, using powerful X-ray radiation to accurately determine the valence state of sulfur in the apatite. Methodological reliability of the work was ensured by comparing the same rock type formed over a span of billions of years: this made it possible to exclude systematic data distortions and separate the primary oxygen signal from artifacts of later geological history.
"Oxygen Oases" a Billion Years Before the Great Oxidation Event
For a long time, a model was entrenched in the scientific community according to which Earth's atmosphere and oceans contained virtually no free oxygen until the so-called Great Oxidation Event (GOE), dated to approximately 2.3 billion years ago. The new study revises this picture. The analysis of sulfur compounds in apatite indicates that already in the interval of 3–3.4 billion years ago, local zones with elevated oxygen content existed on the planet's surface — those very "oxygen oases." This does not mean that the atmosphere as a whole was saturated with O₂, but the presence of even areally limited, yet chemically confirmed zones of free oxygen indicates that biogenic or photochemical oxygen production began a billion years earlier than believed.
Surface-Mantle Connection: The Role of Subduction
Equally important is the fact that during this same period, Earth's mantle was already reacting with the oxygen present on the surface through the process of subduction — the sinking of tectonic plates deep into the mantle with the recycling of surface rocks. This means that the deep layers of the planet and its surface were functionally connected and interacted much earlier than previous models supposed. The researchers emphasize that plate tectonics played a key role in creating and maintaining conditions that made Earth habitable. The discovery has no direct practical application in everyday life, but it brings science closer to answering a fundamental question — exactly how our planet transformed from an oxygen-free world into an environment capable of supporting a complex biosphere.
