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Edited by Thure E. Cerling, University of Utah, Salt Lake City, UT, and approved August 28, 2014 (received for review June 5, 2014)
Significance
In this paper, we account for the wide range (approximately a few thousand permil) of nitrogen isotopic composition measured in solar system materials. Several theoretical models have been proposed to explain the nitrogen isotopic enrichments measured in meteorites (especially in organic matter) and in cometary ice (NH3 and/or HCN). These models include ion−molecular isotope exchange reactions and isotope self-shielding in the disk. However, a major limit is that there are no experiments to substantiate any model. We measured and found massive N-isotopic fractionations during vacuum UV photodissociation of N2, perhaps one of the largest isotope effects ever measured, and present mechanistic evidence for the wide distribution in nitrogen isotopic compositions.
Abstract
Nitrogen isotopic distributions in the solar system extend across an enormous range, from −400‰, in the solar wind and Jovian atmosphere, to about 5,000‰ in organic matter in carbonaceous chondrites. Distributions such as these require complex processing of nitrogen reservoirs and extraordinary isotope effects. While theoretical models invoke ion-neutral exchange reactions outside the protoplanetary disk and photochemical self-shielding on the disk surface to explain the variations, there are no experiments to substantiate these models. Experimental results of N2 photolysis at vacuum UV wavelengths in the presence of hydrogen are presented here, which show a wide range of enriched δ15N values from 648‰ to 13,412‰ in product NH3, depending upon photodissociation wavelength. The measured enrichment range in photodissociation of N2, plausibly explains the range of δ15N in extraterrestrial materials. This study suggests the importance of photochemical processing of the nitrogen reservoirs within the solar nebula.
Footnotes
- ↵1To whom correspondence should be addressed. Email: subrata{at}ucsd.edu.
Author contributions: S.C. and M.H.T. designed research; S.C., B.H.M., T.L.J., and R.D.L. performed research; S.C. and T.L.J. contributed new reagents/analytic tools; S.C., B.H.M., and M.A. analyzed data; and S.C., M.A., R.D.L., and M.H.T. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1410440111/-/DCSupplemental.
Nitrogen isotopes in solar system
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