An international team of geologists discovered in South Africa the oldest known "oxygen pockets" - a place where already 2.97 billion years ago, 430 million before the oxygen revolution, the first photosynthetic organisms lived and gave off oxygen.
There was very little oxygen in the atmosphere of the young Earth; Today's 20% of this gas in the air is the result of photosynthetic plants and bacteria. Cyanobacteria, unicellular photosynthetic organisms, were the first to release oxygen. At first, the oxygen released by them was spent only on the oxidation of rocks, but about 2.5 billion years this process ended, and oxygen began to accumulate in the air and dissolve in the ocean; this event is called the oxygen revolution (or catastrophe, since as a result of the enrichment of the oxygen atmosphere, species adapted to life in a reducing, rather than oxidizing atmosphere) died.
The jump in the oxygen content in the atmosphere of the ancient Earth is determined by the distribution of sulfur isotopes in sedimentary rocks. As a group of geochemists from Caltech discovered in the mid-nineties, the distribution of sulfur isotopes changed dramatically after the oxygen revolution due to the appearance of an ozone layer that sheltered the Earth from solar ultraviolet radiation, which increased the reactivity of light sulfur isotopes and created the so-called mass-independent distribution.
Scientists learn about the activity of the first photosynthetic organisms before the oxygen revolution by the composition of sedimentary rocks that slowly accumulated in the "oxygen pockets". These are areas near large bacterial mats, around which the oxygen concentration was closer to modern than on the rest of the planet. Several such "oxygen pockets" are known, ranging in age from 2.5 to 2.7 billion years; they were found on all continents except Antarctica. Apart from carbon (from carbon dioxide), hydrogen and oxygen (from water), they need other elements like sulfur and nitrogen. Cyanobacteria obtained sulfur by reducing it from sulfates in the soil. The rocks on which cyanobacteria lived and divided, like the rocks formed after the oxygen revolution, are characterized by a mass-independent distribution of sulfur isotopes.
Last year, an international team of geologists from the University of Tübingen in Germany discovered a rock in the Pongola Nature Reserve in South Africa, the distribution of sulfur isotopes in which indicates a high concentration of oxygen in the atmosphere already 2.97 billion years ago, long before the oxygen revolution took place. The distribution of sulfur isotopes in the rocks of the Pongola Formation makes the Pongolian "oxygen pocket" the oldest known today.
The research is published in the journal Nature Geochemistry.
Ksenia Malysheva