Investigation of post-depositional processing of nitrate in East Antarctic snow: isotopic constraints on photolytic loss, re-oxidation, and source inputs

Abstract

Abstract. Snowpits along a traverse from coastal East Antarctica to the summit of the ice sheet (Dome Argus) are used to investigate the post-depositional processing of nitrate (NO − 3 ) in snow. Seven snowpits from sites with accumulation rates between 24 and 172 kg m −2 a −1 were sampled to depths of 150 to 300 cm. At sites from the continental interior (low accumulation, < 55 kg m −2 a −1 ), nitrate mass fraction is generally > 200 ng g −1 in surface snow and decreases quickly with depth to < 50 ng g −1 . Considerably increasing values of δ 15 N of nitrate are also observed (16-461 ‰ vs. air N 2 ), particularly in the top 20 cm, which is consistent with predicted fractionation constants for the photolysis of nitrate. The δ 18 O of nitrate (17-84 ‰ vs. VSMOW (Vienna Standard Mean Ocean Water)), on the other hand, decreases with increasing δ 15 N, suggestive of secondary formation of nitrate in situ (following photolysis) with a low δ 18 O source. Previous studies have suggested that δ 15 N and δ 18 O of nitrate at deeper snow depths should be predictable based upon an exponential change derived near the surface. At deeper depths sampled in this study, however, the relationship between nitrate mass fraction and δ 18 O changes, with increasing δ 18 O of nitrate observed between 100 and 200 cm. Predicting the impact of post-depositional loss, and therefore changes in the isotopes with depth, is highly sensitive to the depth interval over which an exponential change is assumed. In the snowpits collected closer to the coast (accumulation > 91 kg m −2 a −1 ), there are no obvious trends detected with depth and instead seasonality in nitrate mass fraction and isotopic composition is found. In comparison to the interior sites, the coastal pits are lower in δ 15 N (−15-71 ‰ vs. air N 2 ) and higher in δ 18 O of nitrate (53-111 ‰ vs. VSMOW). The relationships found amongst mass fraction, δ 15 N, δ 18 O and 17 O ( 17 O = δ 17 O-0.52 × δ 18 O) of nitrate cannot be explained by local post-depositional processes alone, and are instead interpreted in the context of a primary atmospheric signal. Consistent with other Antarctic observational and modeling studies, the isotopic results are suggestive of an important influence of stratospheric ozone chemistry on nitrate formation during the cold season and a mix of tropospheric sources and chemistry during the warm season. Overall, the findings in this study speak to the sensitivity of nitrate isotopic composition to post-depositional processing and highlight the strength of combined use of the nitrogen and oxygen isotopes for a mechanistic understanding of this processing.

Spatial variations in snowpack chemistry, isotopic composition of NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;−&lt;/sup&gt; and nitrogen deposition from the ice sheet margin to the coast of western Greenland

“…Photolysis is associated with large fractionation of N ( 15 ε between −48 and −56 ‰) and O ( 18 ε = −34 ‰) which tends to increase δ( 15 N) and δ( 18 O) in the remaining snowpack NO − 3 if the NO x produced is removed from the system Erbland et al, 2013;Berhanu et al, 2015;Geng et al, 2015). In situ recycling of NO − 3 can also reduce δ( 18 O) and ( 17 O) due to oxygen isotope exchange with water Shi et al, 2015), which has a different isotopic signature than atmospheric oxidants. This means that the negative 18 ε is not expressed in the residual snow NO − 3 and, in fact, the apparent overall oxygen isotope fractionation can be positive (between 9 and 13 ‰, Berhanu et al, 2015).…”

Isotope Fractionation of Nitrate During Volatilization in Snow: A Field Investigation in Antarctica

“…Frey et al (2009) proposed a theoretical framework to estimate the isotopic fractionation constant (ε) for NO 3 − via photolysis, and the calculated ε generally agreed with field observations at Dome C (Blunier et al, 2005;Frey et al, 2009). Following photolytic loss of NO 3 − from snow, 15 N is enriched in the remaining NO 3 − , but the heavy oxygen isotopes have been observed to be depleted due to reoxidation and oxygen-exchange reactions between the photoproducts and H 2 O/OH (McCabe et al, 2005;Shi et al, 2015). Following photolytic loss of NO 3 − from snow, 15 N is enriched in the remaining NO 3 − , but the heavy oxygen isotopes have been observed to be depleted due to reoxidation and oxygen-exchange reactions between the photoproducts and H 2 O/OH (McCabe et al, 2005;Shi et al, 2015).…”

“…The findings suggest that the isotopic effects of NO 3 − volatilization in snow are closely related to temperature.which is thought to inversely depend on snow accumulation rate (Grannas et al, 2007;Röthlisberger et al, 2002;Shi et al, 2015;Weller et al, 2004;Zatko et al, 2016). Here we discuss results from experiments in field Antarctic snow investigating isotopic fractionation of nitrate due to volatilization.…”

“…which is thought to inversely depend on snow accumulation rate (Grannas et al, 2007;Röthlisberger et al, 2002;Shi et al, 2015;Weller et al, 2004;Zatko et al, 2016). The recent combination of concentration and isotopic composition of NO 3 − has yielded a more detailed understanding of postdepositional processing in snow and the ability to discern past sources and chemistry from ice cores.…”

Potential genesis and implications of calcium nitrate in Antarctic snow

“…However, there are many uncertainties and speculations over the sources of nitrate, its seasonality, and post-depositional losses (e.g. Traversi et al, 2014;Shi et al, 2015).…”