ISSN 2330-717X

Where Does The Nitrogen We Breathe Come From?

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Researchers from the Center for Petrographic and Geochemical Research (CRPG), a joint research unit of the Université de Lorraine and the Center for National Scientific Research, (CNRS), in collaboration with the Institut de Physique du Globe of Paris (IPGP) have taken samples of gases in various terrestrial volcanic sites to determine the origin of our atmosphere. Their study was published in Nature.

With its 78% of dinitrogen (N2) and its 21% of oxygen (O2), Earth’s atmosphere is a unique mixture in the solar system*. Dioxygen was produced by some of the earliest living organisms, but where does dinitrogen come from? Does it come from the earth’s mantle thanks to volcanism? Was it present in the earliest moments of our planet? To try to answer these questions, Jabrane Labidi, researcher at the IPGP, Michael Broadley, David Bekaert and Bernard Marty, researchers at the CRPG, Peter Barry of the Woods Hole Oceanographic Institution, as well as their British and Italian colleagues, took samples of gases in various volcanic sites on Earth, in particular in Eifel (Germany) and Yellowstone (USA).

This sampling was made possible thanks to a new instrumentation unique in the world, developed by Edward Young at the University of California at Los Angeles (USA) which highlighted a slight anomaly in the composition of atmospheric nitrogen**. This anomaly identifies precisely the contribution of nitrogen in volcanic fluids rising to the surface of the Earth. These results, published on April 16, 2020 in Nature***, have shown that dinitrogen from magmas formed in the Earth’s mantle does not exhibit this anomaly. The authors were thus able to characterize the properties of nitrogen present in the Earth’s mantle and show that that of the atmosphere does not only come from degassing by volcanoes. Atmospheric nitrogen may have been provided by extraterrestrial bodies (asteroids that gravitate between Mars and Jupiter or comets) of a different nature than the small planets from which Earth was formed.

* On Venus as on Mars, the atmosphere is essentially composed of CO2.

** A chemical element can exist in different versions, called isotopes, which are distinguished by their mass. Thus, three isotopic variants of dinitrogen exist in nature: 14N14N, 14N15N and more rarely 15N15N. It is the abundance of the latter that has enabled to determine the mantle and atmospheric origins of nitrogen.

*** Hydrothermal 15N15N abundances constrain the origins of mantle nitrogen, J. Labidi, P.H. Barry, D.V. Bekaert, M.W. Broadley, B. Marty, T. Giunta, O. Warr, B. Sherwood Lollar, T.P. Fischer, G. Avice, A. Caracausi, C.J. Ballentine, S.A. Halldórsson, A. Stefánsson, M. D. Kurz, I.E. Kohl, E. D. Young. Nature, 580, 367–371 (2020). https://doi.org/10.1038/s41586-020-2173-4

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