Changes in the Isotopic Signature of Atmospheric Nitrous Oxide and Its Global Average Source During the Last Three Millennia

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 δ¹⁵N^av _PI = (9.5 ± 0.1)‰, δ¹⁸O_PI = (47.1 ± 0.2)‰, δ¹⁵N^α _PI = (17.8 ± 0.4)‰, and δ¹⁵Ν^β _PI = (1.2 ± 0.4)‰. From PI to modern times all isotope signatures decreased with a total change of δ¹⁵N^av = (−2.7 ± 0.2)‰, δ¹⁸O = (−2.5 ± 0.4)‰, δ¹⁵N^α = (−2.0 ± 0.7)‰, and δ¹⁵Ν^β (−3.5 ± 0.7)‰. Interestingly, the temporal evolution is not the same for δ¹⁵N^av and δ¹⁸O. δ¹⁸O trends are relatively larger during the early part, and δ¹⁵N^av 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 δ¹⁵N^av _{source,anthrop} = (−15.0 ± 2.6)‰, δ¹⁸O_{source,anthrop} = (30.0 ± 2.6)‰, δ¹⁵N^α _{source,anthrop} = (−4.5 ± 1.7)‰, and δ¹⁵N^β _{source,anthrop} = (−24.0 ± 8.4)‰. 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.