Investigating the dust-induced N2O production in ice cores using bulk and position-specific isotope analysis

Ice cores represent the only direct paleo-atmospheric archive that allow the reconstruction of greenhouse gas concentrations such as N2O. However, processes in the ice can alter the atmospheric information stored in air bubbles, for example by adding extra N2O by in situ production. This in situ production of N2O is especially severe in mineral dust-rich ice core sections corresponding to glacial periods. Understanding the production process and its link to the mineral dust content is key to systematically detecting altered samples and correcting for the in situ contribution. Isotope analysis is particularly useful for characterizing these processes and thus isolating the paleoclimatic signal from archived data.  We measured the bulk nitrogen and oxygen isotopic composition of N2O in Antarctic and Greenland ice cores from glacial periods. The isotopic signatures of N2O produced in situ, calculated using a mass balance approach, differ from that of the atmospheric N2O. In addition, enrichment or depletion in 15N and/or 18O relative to atmospheric values varies with drilling site, snow accumulation rate, and properties of the snow-ice transition. Interestingly, isotopic signatures of nitrate (NO3-) exhibit similar dependencies. It is well established that NO3- is drastically altered by post-depositional processes in low accumulation areas. Joint isotopic analysis of N2O and NO3- in samples from the EDC and EDML ice cores revealed a correlation between δ15N values of NO3- and in situ N2O, pointing to NO3- as a potential precursor for in situ production. While being linearly correlated, the nitrogen isotopic signature of NO3- is twice as enriched as in situ produced N2O. This suggests that the two N atoms of N2O originate from two distinct sources and only one is likely derived from nitrate. We additionally measured the site preference of 15N in N2O in ice core samples (SP = δ15Nα - δ15Nβ, where α is the central and β the terminal N atom in the N2O molecule). Previous work on SP suggests that SP might be indicative of the N2O formation pathway provided both N atoms are derived from the same N precursor. The SP signature in Vostok samples ranges from +57 to +242 ‰, and the δ15Nα values from +92 to +234 ‰, which is comparable to the δ15N values of NO3- at Vostok. Although similar reaction pathways were expected in different ice cores, in situ N2O from Taylor Glacier samples exhibits very different SP values from -17 to -7 ‰, with δ15Nα values from -45 to -32 ‰. Given that the difference in δ15N of NO3- is also up to 200 ‰ between these two locations, our findings suggest that the center-position nitrogen (α) of in situ N2O comes from NO3- and the terminal-position nitrogen (β) from another N-bearing compound. Thus, the SP signature seems to reflect not the N2O formation pathway but the difference in δ15N of the two nitrogen pools involved in the reaction. Gaining a thorough understanding of the N2O production in ice marks a significant advancement towards interpretation of the N2O record and possibly correction for in situ production.