Abstract
Abstract. Stable isotope ratios of nitrate preserved in deep ice cores are expected to provide unique and valuable information regarding paleoatmospheric 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 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 2011/2012. 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) attributed to mixing with the surrounding snow. After excluding samples impacted by the mixing process, we derived an average apparent nitrogen isotopic fractionation (15ϵapp) of −67.8 ± 12 ‰ for the snow nitrate exposed to solar UV using the nitrate stable isotope ratios and concentration measurements. For the control samples in which solar UV was blocked, an apparent average 15ϵapp value of −12.0 ± 1.7 ‰ was derived. This difference strongly suggests that solar UV photolysis plays a dominant role in driving the isotopic fractionation of nitrate in snow. We have estimated a purely photolytic nitrogen isotopic fractionation (15ϵphoto) of −55.8 ± 12.0 ‰ from the difference in the derived apparent isotopic fractionations of the two experimental fields, as both pits were exposed to similar physical processes except exposure to solar UV. This value is in close agreement with the 15ϵphoto value of −47.9 ± 6.8 ‰ derived in a laboratory experiment simulated for Dome C conditions (Berhanu et al., 2014). 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 spectral distribution 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 stable isotope ratios of snow nitrate at low-accumulation sites, where many 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.
Highlights
Nitrate (NO−3 ), the end product of the oxidation of atmospheric nitrogen oxides (NOx = NO + NO2), is one of the most abundant ions present in polar ice and snow
We investigated the role of solar UV photolysis in the postdepositional modification of nitrate mass and stable isotope ratios at Dome C, Antarctica, during the austral summer of 2011/2012
The results presented here represent a strong body of evidence that solar UV photolysis is the most relevant post-depositional process modifying the stable isotope ratios of snow nitrate at low-accumulation sites, where many deep ice cores are drilled
Summary
The interpretation of these paleorecords is problematic at most sites on the polar ice sheets, where post-depositional processes such as the desorption of nitrate species on snow grains, sublimation or condensation of water vapor and photolysis of nitrate have a major influence on the signal archived in firn and ice (Dibb et al, 1998; Honrath et al, 1999; Röthlisberger et al, 2002; Blunier et al, 2005; Frey et al, 2009; Wolff, 2013). The stable isotope ratios of nitrate are useful metrics used to constrain NOx chemistry (Savarino et al, 2007, 2013; Morin et al, 2008; Hastings et al, 2009; Vicars et al, 2013) and the post-depositional processing of nitrate in snow (Blunier et al, 2005; Frey et al, 2009; Erbland et al, 2013). Δ values are typically reported in per mill (‰), as variations in isotopic ratios for natural samples occur within a very narrow range
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