Deposition, recycling and archival of nitrate stable isotopes between the air-snow interface: comparison between Dronning Maud Land and Dome C, Antarctica

Abstract

<p><strong>Abstract.</strong> The nitrate (NO<sub>3</sub><sup>−</sup>) isotopic composition δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> of polar ice cores has the potential to provide constraints on past ultraviolet (UV) radiation and thereby total column ozone (TCO), in addition to the oxidising capacity of the ancient atmosphere. However, understanding the transfer of reactive nitrogen at the air-snow interface in Polar Regions is paramount for the interpretation of ice core records of δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> and NO<sub>3</sub><sup>−</sup> mass concentrations. As NO<sub>3</sub><sup>−</sup> undergoes a number of post-depositional processes before it is archived in ice cores, site-specific observations of δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> and air-snow transfer modelling are necessary in order to understand and quantify the complex photochemical processes at play. As part of the Isotopic Constraints on Past Ozone Layer Thickness in Polar Ice (ISOL-ICE) project, we report new measurements of NO<sub>3</sub><sup>−</sup> concentration and δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> in the atmosphere, skin layer (operationally defined as the top 5 mm of the snow pack), and snow pit depth profiles at Kohnen Station, Dronning Maud Land (DML), Antarctica. We compare the results to previous studies and new data, presented here, from Dome C, East Antarctic Plateau. Additionally, we apply the conceptual one-dimensional model of TRansfer of Atmospheric Nitrate Stable Isotopes To the Snow (TRANSITS) to assess the impact of photochemical processes that drive the archival of δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> and NO<sub>3</sub><sup>−</sup> in the snow pack. We find clear evidence of NO<sub>3</sub><sup>−</sup> photolysis at DML, and confirmation of our hypothesis that UV-photolysis is driving NO<sub>3</sub><sup>−</sup> recycling at DML. Firstly, strong denitrification of the snow pack is observed through the δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> signature which evolves from the enriched snow pack (−3 to 100 ‰), to the skin layer (−20 to 3 ‰), to the depleted atmosphere (−50 to −20 ‰) corresponding to mass loss of NO<sub>3</sub><sup>−</sup> from the snow pack. Secondly, constrained by field measurements of snow accumulation rate, light attenuation (e-folding depth) and atmospheric NO<sub>3</sub><sup>−</sup> mass concentrations, the TRANSITS model is able to reproduce our δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> observations in depth profiles. We find that NO<sub>3</sub><sup>−</sup> is recycled three times before it is archived (i.e., below the photic zone) in the snow pack below 15 cm and within 0.75 years. Archived δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> and NO<sub>3</sub><sup>−</sup> concentration values are 50 ‰ and 60 ng g<sup>−1</sup> at DML. NO<sub>3</sub><sup>−</sup> photolysis is weaker at DML than at Dome C, due primarily to the higher DML snow accumulation rate; this results in a more depleted δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> signature at DML than at Dome C. Even at a relatively low snow accumulation rate of 6 cm yr<sup>−1</sup> (water equivalent; w.e.), the accumulation rate at DML is great enough to preserve the seasonal cycle of NO<sub>3</sub><sup>−</sup> concentration and δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup>, in contrast to Dome C where the profiles are smoothed due to stronger photochemistry. TRANSITS sensitivity analysis of δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> at DML highlights that the dominant factors controlling the archived δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> signature are the snow accumulation rate and e-folding depth, with a smaller role from changes in the snowfall timing and TOC. Here we set the framework for the interpretation of a 1000-year ice core record of δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> from DML. Ice core δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> records at DML will be less sensitive to changes in UV than at Dome C, however the higher snow accumulation rate and more accurate dating at DML allows for higher resolution δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> records.</p>

Stratospheric Ozone Changes From Explosive Tropical Volcanoes: Modeling and Ice Core Constraints

“…We do not see this positive correlation in the background variability in the ISOL‐ICE ice core (Figures 4c and 4e; R 2 =0.04, p <10 −3 with data from 5 years either side of the volcanic eruptions is not used). The is weakly anticorrelated to the accumulation rate (Figure 4d and 4e); R 2 =0.2, p <10 −4 again with 5 years either side of the volcanic eruptions removed) as would be expected from spatial transects across Antarctica (Erbland et al, 2015, 2013; Noro et al, 2018; Shi et al, 2018), and sensitivity tests of variable accumulation rate on the signal at the DML site (Winton, Ming et al, 2019).…”

“…The accumulation rate is variable at the DML site (2.5 to 11 cm year −1 water equivalent) (Oerter et al, 2000; Sommer et al, 2000) and there is no trend over the last 1,000 years. We speculate that changes in the accumulation rate will lead to changes in e ‐folding depth over time which can account for part of the variability of the signal (Winton, Ming et al, 2019), with a smaller contribution from extreme precipitation events (Turner et al, 2019). The e ‐folding depth of the local snowpack depends on snow physical properties and contributes to the signal eventually preserved in local firn and ice (Winton, Ming et al, 2019).…”

“…We speculate that changes in the accumulation rate will lead to changes in e ‐folding depth over time which can account for part of the variability of the signal (Winton, Ming et al, 2019), with a smaller contribution from extreme precipitation events (Turner et al, 2019). The e ‐folding depth of the local snowpack depends on snow physical properties and contributes to the signal eventually preserved in local firn and ice (Winton, Ming et al, 2019). Unfortunately, the variability of e ‐folding depth in the past is not known and may be a source of additional noise in the signal.…”

“…Winton, Ming et al (2019) carried out a comprehensive field and modeling study of the air‐snow transfer of at the low snowfall accumulation site at Kohnnen Station in Dronning Maud Land (DML), East Antarctica as part of the ISOL‐ICE (ISotopic constraints of past Ozone Layer in polar ICE) project. At the DML site, is recycled 3 times before it is archived in the snowpack below a depth of 15 cm and within 0.75 year.…”

“…Sensitivity analysis with a 1‐D air‐snow model, TRANSITS (TRansfer of Atmospheric Nitrate Stable Isotopes To the Snow) (Erbland et al, 2015), of at DML showed that the dominant factors controlling the archived signature are the snow accumulation rate and e ‐folding depth of the surface snowpack for incident UV, with a smaller role from changes in the snowfall timing and TCO. The Winton, Ming et al (2019) study sets the framework for the interpretation of a record from the new ISOL‐ICE ice core drilled in January 2017 at Kohnen Station in DML, henceforth referred to as the DML site, following the terminology in Winton, Ming et al (2019). The DML region experiences low annual accumulation rates (<10 g cm −2 year −1 ) but ice cores from the area still record seasonal‐, centennial‐, and millennial‐scale variability in glaciochemistry (Göktas et al, 2002; Oerter et al, 2000; Sommer et al, 2000), as well as highly resolved tropical volcanic eruptions (Hofstede et al, 2004; Severi et al, 2007).…”