Isotopic Constraints on Past N2O Variations

Abstract:

It was previously demonstrated that N₂O produced in marine versus terrestrial environments is isotopically distinct (Kim and Craig 1993, Schilt et al. 2014). There is also enticing evidence that the N₂O site preference (position of ¹⁵N) is distinct for N₂O produced from microbial nitrification versus denitrification (Sutka et al. 2006). We are currently using large-volume ice core samples from Taylor Glacier, Antarctica to develop records of δ¹⁵N-N₂O, δ¹⁸O-N₂O, and N₂O site preference. The records will span the rapid N­₂O increases during the last deglaciation (21-11 ka) and the abrupt Northern Hemisphere warming event Dansgaard-Oeschger 8 (40-35 ka). Measurements of the N₂O site preference have not previously been made on ice core air bubbles older than ~ 3000 yr (Prokopiou et al. 2018), so our site preference measurements will represent a novel contribution. Published records of δ¹⁵N-N₂O and δ¹⁸O-N₂O also do not exist in ice older than 21 ka (Fischer et al. 2019). We intend to use the new site preference data to interpret the past N₂O increases in terms of contributions from microbial nitrification and denitrification. We will also employ an inverse solution to a box model, integrating both the site preference and bulk isotope data, to explore the full range of possible N₂O source changes implied by the isotope records (nitrification versus denitrification in marine versus terrestrial ecosystems).