Extending records of the isotopic compositions of atmospheric N₂O back to 1800 A.D. from air trapped in snow at the South Pole and the Greenland Ice Sheet Project II ice core

A historical record of changes in the N₂O isotope composition is important for a better understanding of the biogeochemical cycles involving atmospheric N₂O. Here we combine measurements of trapped gases in the Greenland Ice Sheet Project II (GISP II) ice core and the interstitial air from the snowpack at the South Pole to reconstruct atmospheric N₂O isotope records back to 1785 A.D. Results from six measurements of ice with gas ages between 1785 and 1819 A.D. indicate that the δ¹⁵N and δ¹⁸O of atmospheric N₂O was 10.1 ± 0.6 and 47.4 ± 1.0‰, respectively. These values are 1.9 and 2.9‰ higher than present-day values as measured in this study. A simple two-box atmospheric model was constructed to simulate the various sources and sinks of atmospheric N₂O over the last 250 years. Model results suggest that the total anthropogenic N₂O emissions in 1995 were between 4.2 and 5.7 Tg N yr^-1. The ice core and box model results were compared to δ¹⁵N and δ¹⁸O measurements of air trapped in the snowpack at South Pole spanning the 20th century. Our results suggest that δ¹⁵N and δ¹⁸O of atmospheric N₂O have dropped by 1.7 and 0.9‰, respectively, during the last century. These data support previous predictions of decreasing atmospheric isotope ratios that are related to a ∼30% increase in total N₂O emissions which are primarily related to agricultural activities.