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

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 δ15N, 1δ15N, 2δ15N and δ18O, respectively, since 1700 AD. Firn air data imply a decrease of about -1.1, -1.2, -1.0 and -0.6 for δ15N, 1δ15N, 2δ15N and δ18O, respectively, since 1970 AD. These results imply consistent trends from firn and ice measurements for δ15N and δ18O. 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.