Air-snow transfer of nitrate on the East Antarctic Plateau - Part 1: Isotopic evidence for a photolytically driven dynamic equilibrium in summer
Erbland, J. ; Vicars, W. C. ; Savarino, J. ; Morin, Samuel ; Frey, M. M. ; Frosini, D. ; Vince, E. ; Martins, J. M. F.
Here we report the measurement of the comprehensive isotopic composition (?15N, ?17O and ?18O) 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 ?15N, ?17O and ?18O, 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 ?15N, ?17O and ?18O, 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 ?17O of atmospheric nitrate and ?17O 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 ?15N values in the atmospheric and skin layer nitrate at DC. Asymptotic ?15N(NO3?) values calculated for each snow pit are found to be correlated with the inverse of the snow accumulation rate (ln(?15N as. + 1) = (5.76±0.47) c (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 ?17O(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 ?17O is small, thus allowing the preservation of a portion of the atmospheric signal.
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