Deposition, recycling and archival of nitrate stable isotopes between the air-snow interface: comparison between Dronning Maud Land and Dome C, Antarctica
The nitrate (NO3−) isotopic composition δ15N-NO3− 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 δ15N-NO3− and NO3− mass concentrations. As NO3− undergoes a number of post-depositional processes before it is archived in ice cores, site-specific observations of δ15N-NO3− 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 NO3− concentration and δ15N-NO3− 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 δ15N-NO3− and NO3− in the snow pack. We find clear evidence of NO3− photolysis at DML, and confirmation of our hypothesis that UV-photolysis is driving NO3− recycling at DML. Firstly, strong denitrification of the snow pack is observed through the δ15N-NO3− 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 NO3− from the snow pack. Secondly, constrained by field measurements of snow accumulation rate, light attenuation (e-folding depth) and atmospheric NO3− mass concentrations, the TRANSITS model is able to reproduce our δ15N-NO3− observations in depth profiles. We find that NO3− 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 δ15N-NO3− and NO3− concentration values are 50 ‰ and 60 ng g−1 at DML. NO3− photolysis is weaker at DML than at Dome C, due primarily to the higher DML snow accumulation rate; this results in a more depleted δ15N-NO3− signature at DML than at Dome C. Even at a relatively low snow accumulation rate of 6 cm yr−1 (water equivalent; w.e.), the accumulation rate at DML is great enough to preserve the seasonal cycle of NO3− concentration and δ15N-NO3−, in contrast to Dome C where the profiles are smoothed due to stronger photochemistry. TRANSITS sensitivity analysis of δ15N-NO3− at DML highlights that the dominant factors controlling the archived δ15N-NO3− 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 δ15N-NO3− from DML. Ice core δ15N-NO3− 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 δ15N-NO3− records.
Authors: Winton, V. Holly L., Ming, Alison, Caillon, Nicolas, Hauge, Lisa, Jones, Anna E., Savarino, Joel, Yang, Xin, Frey, Markus