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ERC

NOGAT Report Summary

Project ID: 267255
Funded under: FP7-IDEAS-ERC
Country: France

Final Report Summary - NOGAT (NOBLE GAS TRACING OF SOURCES AND SINKS OF VOLATILE ELEMENTS IN THE ATMOSPHERE)

In this project, we have investigated the origin and evolution of the terrestrial atmosphere and hydrosphere using noble gases and stable isotopes (e.g., nitrogen) as geochemical tracers. Noble gases are particularly suited for that because they are inert elements which elemental and isotopic compositions can vary only through physical processes such as phase changes, kinetic processes such as escape to space, and nuclear reactions (nucleosynthesis, radioactivity). Nitrogen to a certain extent can be considered as an inert element and, as a major atmospheric species and a key player of biogenicity, was also studied conjointly.

The analysis of extraterrestrial material in the laboratory (lunar, martian, and primitive meteorites, solar wind returned by a dedicated space mission) and in-situ (analysis of gases emitted by Comet 67P, by the Rosina instrument on board of the Rosetta spacecraft), together with the analysis of the ancient atmosphere trapped in 3.5 Ga-old rocks and of gases derived from the deep mantle, led us to propose the following scenario.

At least two cosmochemical sources contributed volatile elements to Earth. Most of water, nitrogen and carbon were contributed by inner solar system bodies akin of present-day asteroids. Relics of meteoritic gases that are found in the deepest regions of the terrestrial mantle indicate that asteroidal volatiles were trapped within the first 100 Ma (after start of solar system formation) and were isolated/preserved there since then. The atmosphere contains a remnant of cometary volatiles under the form of a specific xenon component, indicating that about at least 20 % of atmospheric noble gases were supplied to the surface of the Earth by comets, the rest being asteroidal. Comets might have also supplied a significant share of pre-biotic organics to Earth.

The high precision analysis of noble gases and nitrogen in the ancient atmosphere trapped in 3.5 Ga-old fluid inclusions indicates that:

(i) there was a major pulse of continental crust growth around 3.0 Ga ago;

(ii) The N2 partial pressure 3.5-2.7 Ga ago was comparable to, or lower than, the present-day one, implying a potential Archean CO2 pressure in the range 0.2-0.7 bar. The N isotope composition of the Archean atmosphere was also found similar to the modern one, implying efficient shielding of the atmosphere by a magnetic field at that time;

(iii) Xenon in the Archean atmosphere was isotopically intermediate between extraterrestrial Xe and modern Xe, thus providing the smoking gun evidence for xenon being lost to space through time, whereas other noble gases were not. Thus an escape process specific to xenon must have been in action, probably related to interactions of hard UV light from the young Sun with the specific electronic structure of Xe. Importantly, our study shows that xenon is the second element, after sulfur, to show a secular isotope evolution in the atmosphere through geological eons.

In order to test the possible variability of the modern atmosphere in reaction to the injection of massive hydrocarbon products, we have developed the high precision (permil level) analysis of the helium isotope composition of air. We found that this composition has been constant since 1978, and could not provide evidence of geographical variations between 79°N and 80°S. Thus atmospheric helium can be safely used as an isotope standard by worldwide laboratories.

This project led to the publication of 33 papers, of which 6 were published in Science, 3 in Nature, 3 in Science Advances, and 1 in Nature Geoscience. First author of 5 of them were the PI or his post-docs/students. These works have been cited 651 times since 2011 (h = 13, WOS).

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CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE
France
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