The Nitrogen Budget of Earth

By Benjamin W. Johnson, C. Goldblatt. 2015

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Abstract We comprehensively compile and review N content in geologic materials to calculate a new N budget for Earth. Using analyses of rocks and minerals in conjunction with N–Ar geochemistry demonstrates that the Bulk Silicate Earth (BSE) contains ~ 7 ± 4 times present atmospheric N (4 × 10 18  kg N, or PAN), with 27 ± 16 × 10 18  kg N. Comparison to chondritic composition, after subtracting N sequestered into the core, yields a consistent result, with BSE N between 17 ± 13 × 10 18  kg to 31 ± 24 × 10 18  kg N. Embedded in the chondritic comparison we calculate a N mass in Earth's core (180 ± 110 to 30 ± 180 × 10 18  kg) as well as present discussion of the Moon as a proxy for the early mantle. Significantly, our study indicates that the majority of the planetary budget of N is in the solid Earth. We suggest that the N estimate here precludes the need for a “missing N” reservoir. Nitrogen–Ar systematics in mantle rocks and primary melts identify the presence of two mantle reservoirs: MORB-source like (MSL) and high-N. High-N mantle is composed of young, N-rich material subducted from the surface and identified in OIB and some xenoliths. In contrast, MSL appears to be made of old material, though a component of subducted material is evident in this reservoir as well. Taking into account N mass and isotopic character of the atmosphere and BSE, we calculate a δ 15 N value of ~ 2%. This value should be used when discussing bulk Earth N isotope evolution. Additionally, our work indicates that all surface N could pass through the mantle over Earth history, and in fact the mantle may act as a long-term sink for N. Since N acts as a tracer of exchange between the atmosphere, oceans, and mantle over time, clarifying its distribution in the Earth is critical for evolutionary models concerned with Earth system evolution. We suggest that N be viewed in the same light as carbon: it has a fast, biologically mediated cycle which connects it to a slow, tectonically-controlled geologic cycle.

Subfields: Astronomy and Astrophysics, Geochemistry and Petrology

View Paper: https://www.semanticscholar.org/paper/5a23054f2aa01273fc0dba7ac64fc962cd911c02

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In Astronomy and Astrophysics

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