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- Title
The Origin of Earth's Mantle Nitrogen: Primordial or Early Biogeochemical Cycling?
- Authors
Kurokawa, H.; Laneuville, M.; Li, Y.; Zhang, N.; Fujii, Y.; Sakuraba, H.; Houser, C.; Cleaves, H. J.
- Abstract
Earth's mantle nitrogen (N) content is comparable to that found in its N‐rich atmosphere. Mantle N has been proposed to be primordial or sourced by later subduction, yet its origin has not been elucidated. Here we model N partitioning during the magma ocean stage following planet formation and the subsequent cycling between the surface and mantle over Earth history using argon (Ar) and N isotopes as tracers. The partitioning model, constrained by Ar, shows that only about 10% of the total N content can be trapped in the solidified mantle due to N's low solubility in magma and low partitioning coefficients in minerals in oxidized conditions supported from geophysical and geochemical studies. A possible solution for the primordial origin is that Earth had about 10 times more N at the time of magma ocean solidification. We show that the excess N could be removed by impact erosion during late accretion. The cycling model, constrained by N isotopes, shows that mantle N can originate from efficient N subduction, if the sedimentary N burial rate on early Earth is comparable to that of modern Earth. Such a high N burial rate requires biotic processing. Finally, our model provides a methodology to distinguish the two possible origins with future analysis of the surface and mantle N isotope record. Plain Language Summary: Nitrogen (N) is the main component of Earth's atmosphere, and essential for life. The atmospheric N content influences Earth's climate and capability to retain its surface water. Primary biological production is limited by bio‐available N as well as phosphorous on modern Earth. It has been recently recognized that Earth's interior contains N comparable to that found in its atmosphere, and thus its origin is important for our understanding of Earth‐life co‐evolution. We modeled N partitioning in Earth's molten stage and long‐term cycling after Earth's solidification. Two scenarios are proposed from our modeling. One is that Earth's mantle acquired its modern N content in the earlier stage due to an excess amount of N Earth accreted, which was later lost to space following asteroid impacts. Another is that Earth's mantle acquired N via subduction of N‐rich sediments, which requires the sedimentary N burial rate on early Earth comparable to the modern value sustained by biological activity. The two scenarios can be tested with future analysis of the geochemical record of surface and mantle N. Key Points: We studied two possible origins for mantle nitrogen: the primordial and recycling scenariosThe primordial scenario requires accretion of excess nitrogen and impact erosion during late accretionThe recycling scenario needs efficient N fixation and biotic processing on early Earth
- Subjects
EARTH'S mantle; BIOGEOCHEMICAL cycles; INTERNAL structure of the Earth; ORIGIN of planets; ANALYTICAL geochemistry
- Publication
Geochemistry, Geophysics, Geosystems: G3, 2022, Vol 23, Issue 5, p1
- ISSN
1525-2027
- Publication type
Article
- DOI
10.1029/2021GC010295