Air-snow transfer of nitrate on the East Antarctic Plateau – Part 1: Isotopic evidence for a photolytically driven dynamic equilibrium

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. For each of the East Antarctic snow pits in most of which nitrate loss is observed, we derive apparent fractionation constants associated with this loss as well as asymptotic values of nitrate concentration and isotopic ratios below the photic zone. Nitrate collected from snow pits on the plateau have average apparent 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 apparent fractionation constants of (−16±14)‰, (−0.2±1.5)‰ and (+3.1±5.8)‰, for δ15N, Δ17O and δ18O, respectively. From a lab experiment carried out at DC in parallel to the field investigations, we find that the 15N/14N fractionation associated with the physical release of nitrate is (−8.5±2.5)‰, a value significantly different from the modelled estimate previously found for photolysis (−48‰, Frey et al., 2009) when assuming a Rayleigh-type process. Our observations corroborate that photolysis 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. The 3–4 weeks shift observed for nitrate concentration in these two compartments may be explained by the different sizes of the nitrate reservoirs and by deposition from the atmosphere to the snow. Atmospheric nitrate deposition may lead to fractionation of the nitrogen isotopes and explain the almost constant shift on the order of 25‰ between the δ15N values in the atmospheric and skin layer nitrate at DC. Asymptotic δ15N(NO3−) values and the inverse of snow accumuation rates are correlated (ln(δ15N(as.) + 1) = (5.76±0.47) · (kg m−2 a−1/A) + (0.01±0.02)) confirming the strong relationship between the snow accumulation rate on the residence time of nitrate in the photic zone and the degree of isotopic fractionation, consistent with with previous observations by Freyer et al. (1996). Asymptotic Δ17O(NO3−) values on the plateau are smaller compared to the values found in the skin layer most likely due to oxygen isotope exchange between the nitrate photo-products and water molecules from the surrounding ice. However, the overall fractionation in Δ17O is small thus allowing the preservation of an atmospheric signal.