| Summary: | The nitrogen (δ 15 N) and triple oxygen (δ 17 O and δ 18 O) isotopic composition of nitrate (NO 3 − ) was measured year-round in the atmosphere and snow pits at Dome C, Antarctica (DC, 75.1° S, 123.3° E), and in surface snow on a transect between DC and the coast. Comparison to the isotopic signal in atmospheric NO 3 − shows that snow NO 3 − is significantly enriched in δ 15 N by >200‰ and depleted in δ 18 O by <40‰. Post-depositional fractionation in Δ 17 O(NO 3 − ) is small, potentially allowing reconstruction of past shifts in tropospheric oxidation pathways from ice cores. Assuming a Rayleigh-type process we find fractionation constants ε of −60±15‰, 8±2‰ and 1±1‰, for δ 15 N, δ 18 O and Δ 17 O, respectively. A photolysis model yields an upper limit for the photolytic fractionation constant 15 ε of δ 15 N, consistent with lab and field measurements, and demonstrates a high sensitivity of 15 ε to the incident actinic flux spectrum. The photolytic 15 ε is process-specific and therefore applies to any snow covered location. Previously published 15 ε values are not representative for conditions at the Earth surface, but apply only to the UV lamp used in the reported experiment (Blunier et al., 2005; Jacobi et al., 2006). Depletion of oxygen stable isotopes is attributed to photolysis followed by isotopic exchange with water and hydroxyl radicals. Conversely, 15 N enrichment of the NO 3 − fraction in the snow implies 15 N depletion of emissions. Indeed, δ 15 N in atmospheric NO 3 − shows a strong decrease from background levels (4±7‰) to −35‰ in spring followed by recovery during summer, consistent with significant snowpack emissions of reactive nitrogen. Field and lab evidence therefore suggest that photolysis is an important process driving fractionation and associated NO 3 − loss from snow. The Δ 17 O signature confirms previous coastal measurements that the peak of atmospheric NO 3 − in spring is of stratospheric origin. After sunrise photolysis drives then redistribution of NO 3 − from the snowpack ...
|
|---|