Index Geophysics

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.

Bernard, S.

Röckmann, T.

Kaiser, J.

Barnola, J.-M.

Fischer, H.

Blunier, T.

Chappellaz, J.

2018-06-28

Text

application/pdf

10.5194/acp-6-493-2006

https://acp.copernicus.org/articles/6/493/2006/

eISSN: 1680-7324

eng