NOAA/WDS Paleoclimatology - Polar Ice Cores 3,000 Year Nitrous Oxide d15N and d18O Data

Nitrous oxide (N2O) is a strong greenhouse gas whose mole fraction in the atmosphere has increased over the industrial period. We present a new set of isotope measurements of N2O in air extracted from ice cores covering the last 3,000 years. For the preindustrial (PI) atmosphere, we find an average N2O mole fraction of (267 ± 1) nmol/mol and average tropospheric N2O isotopic values of d15NavPI = (9.5 ± 0.1)permil, d18OPI = (47.1 ± 0.2)permil, d15NalphaPI = (17.8 ± 0.4)permil, and d15NBetaPI = (1.2 ± 0.4)permil. From PI to modern times all isotope signatures decreased with a total change of d15Nav = (-2.7 ± 0.2)permil, d18O = (-2.5 ± 0.4)permil, d15Nalpha = (-2.0 ± 0.7)permil, and d15NBeta (-3.5 ± 0.7)permil. Interestingly, the temporal evolution is not the same for d15Nav and d18O. d18O trends are relatively larger during the early part, and d15Nav trends are larger during the late part of the industrial period, implying a decoupling of sources over the industrial period. Using a mass balance model, we determined the isotopic composition of the total average N2O source. Assuming that the total present source is the sum of a constant natural source and an increasing anthropogenic source, this anthropogenic source has an isotopic signature of d15Navsource,anthrop = (-15.0 ± 2.6)permil, d18Osource,anthrop = (30.0 ± 2.6)permil, d15Nalphasource,anthrop = (-4.5 ± 1.7)permil, and d15NBetasource,anthrop = (-24.0 ± 8.4)permil. The 15N site preference of the source has increased since PI times, which is indicative of a relative shift from denitrification to nitrification sources, consistent with agricultural emissions playing a major role in the N2O increase.