Abstract
Abstract. Here we report the measurement of the comprehensive isotopic composition (δ15N, Δ17O and δ18O) of nitrate at the air–snow interface at Dome C, Antarctica (DC, 75°06' S, 123°19' E), and in snow pits along a transect across the East Antarctic Ice Sheet (EAIS) between 66° S and 78° S. In most of the snow pits, nitrate loss (either by physical release or UV photolysis of nitrate) is observed and fractionation constants associated are calculated. Nitrate collected from snow pits on the plateau (snow accumulation rate below 50 kg m−2 a−1) displays average fractionation constants of (−59±10) ‰, (+2.0±1.0) ‰ and (+8.7±2.4)‰ for δ15N, Δ17O and δ18O, respectively. In contrast, snow pits sampled on the coast show distinct isotopic signatures with average fractionation constants of (−16±14) ‰, (−0.2±1.5) ‰ and (+3.1±5.8) ‰, for δ15N, Δ17O and δ18O, respectively. Our observations corroborate that photolysis (associated with a 15N / 14N fractionation constant of the order of –48 ‰ according to Frey et al. (2009) is the dominant nitrate loss process on the East Antarctic Plateau, while on the coast the loss is less pronounced and could involve both physical release and photochemical processes. Year-round isotopic measurements at DC show a~close relationship between the Δ17O of atmospheric nitrate and Δ17O of nitrate in skin layer snow, suggesting a photolytically driven isotopic equilibrium imposed by nitrate recycling at this interface. Atmospheric nitrate deposition may lead to fractionation of the nitrogen isotopes and explain the almost constant shift of the order of 25 ‰ between the δ15N values in the atmospheric and skin layer nitrate at DC. Asymptotic δ15N(NO3−) values calculated for each snow pit are found to be correlated with the inverse of the snow accumulation rate (ln(δ15N as. + 1) = (5.76±0.47) ċ (kg m−2 a−1/ A) + (0.01±0.02)), confirming the strong relationship between the snow accumulation rate and the degree of isotopic fractionation, consistent with previous observations by Freyer et al. (1996). Asymptotic Δ17O(NO3−) values on the plateau are smaller than the values found in the skin layer most likely due to oxygen isotope exchange between the nitrate photoproducts and water molecules from the surrounding ice. However, the apparent fractionation in Δ17O is small, thus allowing the preservation of a portion of the atmospheric signal.
Highlights
Ocean ScienceNitrate (NO3−) is the final product of the oxidation of nitrogen oxides (NOx = NO + NO2) in the atmosphere and one of the most abundant ions found in Antarctic snow (Wolff, 1995)
The 15N / 14N fractionation constant estimated for the physical release of nitrate suffers from a number of caveats, we note that it is significantly different from the estimated value for UV photolysis in DC conditions (−48 ‰, Frey et al, 2009), allowing us to disentangle the nitrogen isotopic effects of both processes
The different temperatures used in this experiment should not be seen as relevant for different Antarctic site conditions but rather as a way to force nitrate mobility through snow metamorphism and to achieve significant nitrate mass loss in order to allow for the calculation of fractionation constants
Summary
Nitrate (NO3−) is the final product of the oxidation of nitrogen oxides (NOx = NO + NO2) in the atmosphere and one of the most abundant ions found in Antarctic snow (Wolff, 1995). The physical release of HNO3 (via evaporation and/or desorption, often referred to as “evaporation”) and the photolysis of NO3− in the UV range (280 to 350 nm) have been proposed to explain nitrate mass loss from snow (Rothlisberger et al, 2002; Grannas et al., 2007). The relative importance of these photochemical and physical loss processes at a given site is a key issue determining the nitrate transfer function at the air–snow interface
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