Photolytic control of the nitrate stable isotope signal in snow and atmosphere of East Antarctica and implications for reactive nitrogen cycling
1CNRS – Institut National des Sciences de l'Univers (INSU), France
2Laboratoire de Glaciologie et Géophysique de l'Environnement, Université Joseph Fourier-Grenoble, St Martin d'Hères, France
3Laboratoire d'Etude de Transferts en Hydrologie et Environnement, Université Joseph Fourier-Grenoble, St Martin d'Hères, France
*now at: British Antarctic Survey, Natural Environment Research Council, Cambridge, United Kingdom
Abstract. The nitrogen (δ15N) and triple oxygen (δ17/18O) isotopic composition of nitrate (NO3−) was measured year-round in the atmosphere and snow pits at Dome C (DC, 75.1° S, 123.3° E), and in surface snow on a transect between DC and the coast. Snow pit profiles of δ15N (δ18O) in NO3− show significant enrichment (depletion) of >200 (<40) ‰ compared to the isotopic signal in atmospheric NO3−, whereas post-depositional fractionation in Δ17O(NO3−) is small, allowing reconstruction of past shifts in tropospheric oxidation pathways from ice cores. Assuming a Rayleigh-type process we find in the DC04 (DC07) pit fractionation factors ε of −50±10 (−71±12) ‰, 6±3 (9±2) ‰ and 1±0.2 (2±0.6) ‰, for δ15N, δ18O and Δ17O, respectively. A photolysis model reproduces ε for δ15N within the range of uncertainty at DC and for lab experiments reported by Blunier et al. (2005), suggesting that the current literature value for photolytic isotopic fractionation in snow is significantly underestimated. Depletion of oxygen stable isotopes is attributed to photolysis followed by isotopic exchange with water and hydroxyl radicals. Conversely, 15N enrichment of the NO3− fraction in the snow implies 15N depletion of emissions. Indeed, δ15N in atmospheric NO3− shows a strong decrease from background levels (4.4±6.8‰) to −35.1‰ in spring followed by recovery during summer, consistent with significant snow pack emissions of reactive nitrogen. Field and lab evidence therefore suggest that photolysis dominates fractionation and associated NO3− loss from snow in the low-accumulation regions of the East Antarctic Ice Sheet (EAIS). The Δ17O signature confirms previous coastal measurements that the peak of atmospheric NO3− in spring is of stratospheric origin. After sunrise photolysis drives then redistribution of NO3− from the snowpack photic zone to the atmosphere and a snow surface skin layer, thereby concentrating NO3− at the surface. Little NO3− is exported off the EAIS plateau, still snow emissions from as far as 600 km inland can contribute to the coastal NO3− budget.