The oxides found in the earliest meteorites were formed as early as 2.7 billion years ago, indicating the effect of oxygen as it fell through the earth's atmosphere.
It is believed that free oxygen appeared on Earth thanks to early photosynthetic organisms - primarily cyanobacteria, which secreted it as a by-product of their vital activity. Dissolving in the ocean, it caused the oxidation of the most ancient minerals, and subsequently began to fill the atmosphere. For life itself, this turned out to be a difficult test: it had to adapt to existence side by side with this highly reactive oxidizer, having survived one of the most dangerous mass extinctions.
The classical estimate suggests that these grandiose events took place between 2, 3 and 2.4 billion years ago. Recently, however, works have appeared one after another, indicating a more distant period. So, only at the end of last year, geologists from South Africa, the USA and China, having studied the inclusions of oxygen in ancient minerals, came to a date of 3.2 billion years ago. The authors of the new work examined the remains of micrometeorites found in Western Australia.
Scientists from the team of Macquarie University professor Jeremy Wykes found traces of oxygen exposure in them - iron and nickel oxides - and dated them to about 2.7 billion years. A brief report on this is published by the journal Nature. The authors believe that oxidation occurred at an altitude of 75 to 90 km, when tiny - the size of a grain of sand - micrometeorites completely melted due to collisions with atmospheric particles. Only then they cooled down and hardened again, fixing the result of this influence for billions of years.
According to Wykes and his colleagues, in order for meteorites to preserve such traces, already at that time the earth's atmosphere had to contain approximately the modern (about 20%) amount of oxygen - at least at high altitudes. At the same time, the presence of pyrite and other minerals in the seabed sediments of that era should indicate the absence of oxygen exposure. This is quite possible: the authors point out that the presence of oxygen in the upper atmosphere may in no way indicate its appearance in the lower ones.
That "early" oxygen high in the atmosphere could still be of abiogenic origin, being released as a result of exposure to solar UV rays on water molecules, carbon dioxide and other volatile oxides. The denser, methane-rich lower layers of the atmosphere did not mix well with them, and while the ancient oxygen-free atmosphere reigned near the Earth's surface, oxygen was already accumulating high in the sky.