Constraints on N₂O budget changes since pre-industrial time from new firn air and ice core isotope measurements

Serge Bernard (1) , T. Röckmann (2) , J. Kaiser (2, 3) , J.-M. Barnola (1) , H. Fischer (4) , T. Blunier (5) , J. Chappellaz (1)

1 LGGE - Laboratoire de glaciologie et géophysique de l'environnement
2 MPIK - Max Planck Institute for Nuclear Physics
3 Department of Geosciences [Princeton]
4 AWI - Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung = Alfred Wegener Institute for Polar and Marine Research = Institut Alfred-Wegener pour la recherche polaire et marine
5 CEP - Climate and Environmental Physics [Bern]

A historical record of changes in the N₂O isotope composition is important for a better understanding of the global N₂O atmospheric budget. Here we have combined measurements of trapped gases in the firn and in ice cores of one Arctic site (North GReenland Ice core Project – NGRIP) and one Antarctic site (Berkner Island). We have performed measurements of the ¹⁸O and position dependent ¹⁵N isotopic composition of N₂O. By comparing these data to simulations carried out with a firn air diffusion model, we have reconstructed the temporal evolution of the N₂O isotope signatures since pre-industrial times. The decrease observed for all signatures is consistent from one pole to the other. Results obtained from the air occluded in the ice suggest a decrease of about -2.8‰, -2.4‰, -3.2‰ and -1.6‰ for d¹⁵N, ¹d¹⁵N, ²d¹⁵N and d¹⁸O, respectively, since 1700 AD. Firn air data imply a decrease of about -1.1‰, -1.2‰, -1.0‰ and -0.6‰ for d¹⁵N, ¹d¹⁵N, ²d¹⁵N and d¹⁸O, respectively, since 1970 AD. These results imply consistent trends from firn and ice measurements for d¹⁵N and d¹⁸O. The trends for the intramolecular distribution of ¹⁵N are less well constrained than the bulk ¹⁵N trends because of the larger experimental error for the position dependent ¹⁵N measurements. The decrease in the heavy isotope content of atmospheric N₂O can be explained by the increasing importance of agriculture for the present atmospheric N₂O budget.

Domaines: Océan, Atmosphère

Serge Bernard, T. Röckmann, J. Kaiser, J.-M. Barnola, H. Fischer, et al.. Constraints on N₂O budget changes since pre-industrial time from new firn air and ice core isotope measurements. Atmospheric Chemistry and Physics Discussions, 2005, 5 (4), pp.7547-7575. ⟨hal-00327958⟩