| Summary: | A historical record of changes in the N 2 O isotope composition is important for a better understanding of the global N 2 O 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 18 O and position dependent 15 N isotopic composition of N 2 O. By comparing these data to simulations carried out with a firn air diffusion model, we have reconstructed the temporal evolution of the N 2 O 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 δ 15 N, 1 δ 15 N, 2 δ 15 N and δ 18 O, respectively, since 1700 AD. Firn air data imply a decrease of about -1.1, -1.2, -1.0 and -0.6 for δ 15 N, 1 δ 15 N, 2 δ 15 N and δ 18 O, respectively, since 1970 AD. These results imply consistent trends from firn and ice measurements for δ 15 N and δ 18 O. The trends for the intramolecular distribution of 15 N are less well constrained than the bulk 15 N trends because of the larger experimental error for the position dependent 15 N measurements. The decrease in the heavy isotope content of atmospheric N 2 O can be explained by the increasing importance of agriculture for the present atmospheric N 2 O budget.
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