Abstract. Stable isotope ratios of nitrate preserved in deep ice cores are expected to provide unique and valuable information regarding paleo-atmospheric processes. However, due to the post-depositional loss of nitrate in snow, this information may be erased or significantly modified by physical or photochemical processes before preservation in ice. We have investigated the role of solar UV photolysis in the post-depositional modification of nitrate mass and stable isotope ratios at Dome C, Antarctica during the austral summer of 2011/12. Two 30 cm snow pits were filled with homogenized drifted snow from the vicinity of the base. One of these pits was covered with a plexiglass plate that transmits solar UV radiation, while the other was covered with a different plexiglass plate having a low UV transmittance. Samples were then collected from each pit at a 2–5 cm depth resolution and a 10 day frequency. At the end of the season, a comparable nitrate mass loss was observed in both pits for the top-level samples (0–7 cm). At deeper levels (7–30 cm), a significant nitrate mass loss (ca. 30%) was observed in the UV-exposed pit relative to the control field. From the nitrate stable isotope ratios and concentration losses measured in the snow nitrate exposed to solar UV, we have derived average apparent isotopic fractionations (15ϵ,18ϵ and 17E) of −67.8 ± 12‰, 12.5 ± 6.7‰ and 2.2 ± 1.4‰ for δ15N, δ18O, and Δ17O, respectively. These values are fairly stable throughout the season and are in close agreement with the apparent fractionations measured in natural snow at Dome C. Meanwhile, for the control samples in which solar UV was blocked, an apparent average 15ϵ value of −12.0 ± 1.7‰ was derived. The difference in the apparent 15ϵ values obtained for the two experimental fields strongly suggests that solar UV photolysis plays a dominant role in driving observed nitrate mass loss and resulting isotopic fractionation. We have also observed an insensitivity of 15ϵ with depth in the snowpack under the given experimental setup. This is due to the uniform attenuation of incoming solar UV by snow, as 15ϵ is strongly dependent on the shape of the incoming light flux. Together with earlier work, the results presented here represent a strong body of evidence that solar UV photolysis is the most relevant post-depositional process modifying the mass and stable isotope ratios of snow nitrate at low accumulation sites where most deep ice cores are drilled. Nevertheless, modeling the loss of nitrate in snow is still required before a robust interpretation of ice core records can be provided.
Precipitation regime and stable oxygen isotopes at Dome C, East Antarctica – a comparison of two extreme years 2009 and 2010