Most of the carbon dioxide and other gases emitted by volcanoes were generated by organic reserves that formed hundreds of millions of years ago, which indicates a deep connection between the evolution of life on Earth and the geological processes in its interior, according to an article published in the journal Science.
“This discovery makes us think about a complete reassessment of the entire history of the evolution of life. Our observations show that all estimates of the level of oxygen in the Earth's atmosphere in the distant past may be completely incorrect, since volcanic emissions of CO2 could radically change the isotopic composition of the limestone deposits on which they were measured,”explains Marie Edmonds from Cambridge University (UK).
On Earth, as scientists explain, there is a kind of "carbon cycle". Huge amounts of this element are constantly buried at the bottom of the Earth's oceans, where they get along with the remains of animals and plants, and no less large amounts of CO2 enter the atmosphere along with volcanic emissions and other manifestations of geological activity.
Before the onset of the Anthropocene, the "era of man," these processes were one of the main conductors of the Earth's climate, as they determined whether the concentration of CO2 in the atmosphere was increasing or was falling. In the distant past, an imbalance in carbon "loss" and "gain" associated with increased volcanic activity or the release of organic rocks to the surface, already caused mass extinctions and once deprived the planet of all its oxygen reserves.
Edmonds and her colleagues found that this connection between the evolution of life, the carbon cycle, and volcanoes is actually much deeper than geologists have assumed, studying the chemical and isotopic composition of the eruptions of about 80 volcanoes from around the Earth.
As scientists note, geologists have noticed for a long time that the composition of gases and magma emitted by different volcanoes can be very different, which was associated with differences in mineral and isotopic composition in those deep regions of the mantle, from where the lava moved to the surface of the planet.
After analyzing the concentration of helium and carbon isotopes in the emissions of these volcanoes, scientists noticed something unusual - their share depended on which region of the Earth these foci of geological activity were located in. For example, carbon dioxide emitted by continental volcanoes contained much more "heavy" carbon-13 and less "light" helium-3 than emissions from oceanic volcanoes.
The reason for this, as scientists note, was hidden in the distant geological past, and was associated with traces of life that existed on Earth hundreds of millions of years ago. The fact is that a significant share of volcanic emissions of carbon dioxide was generated not by the mantle, but by organic deposits moving towards the mantle along with the earth's crust, and not by the reserves of inorganic carbon that have existed in the interior of the planet since its formation.
Promotional video:
Such a discovery came as a big surprise and a problem for geologists, since earlier scientists believed that the proportion of carbon isotopes in volcanic emissions are constant values that have hardly changed throughout the entire existence of the Earth. Scientists used this property of volcanic emissions in order to estimate the amount of oxygen in the Earth's atmosphere in the distant past, and to assess how the ecological conditions on the planet changed in different eras.
Accordingly, the movement of continents, their mergers and disintegrations should have greatly changed the proportion of CO2 isotopes in volcanic emissions, as well as in the atmosphere and in sedimentary rocks formed from the remains of living beings. This all greatly complicates the restoration of the picture of how life developed on Earth, but at the same time indicates a close connection between its evolution and the behavior of volcanoes, the authors of the article conclude.
