Abstract. The nitrogen stable isotopic composition in nitrate (δ15N-NO3-) measured in ice cores from low-snow-accumulation regions in East Antarctica has the potential to provide
constraints on past ultraviolet (UV) radiation and thereby total column
ozone (TCO) due to the sensitivity of nitrate (NO3-) photolysis to UV radiation. 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 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-
mass concentration and δ15N-NO3- in the atmosphere,
skin layer (operationally defined as the top 5 mm of the snowpack), 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 on the East Antarctic Plateau. Additionally, we
apply the conceptual 1D model of TRansfer of Atmospheric
Nitrate Stable Isotopes To the Snow (TRANSITS) to assess the impact of
NO3- recycling on δ15N-NO3- and NO3- mass concentrations archived in snow and firn. We find
clear evidence of NO3- photolysis at DML and confirmation of
previous theoretical, field, and laboratory studies that UV photolysis is
driving NO3- recycling and redistribution at DML. Firstly, strong
denitrification of the snowpack is observed through the δ15N-NO3- signature, which evolves from the enriched snowpack (−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 snowpack. Based on the TRANSITS model, we find that NO3- is
recycled two times, on average, before it is archived in the snowpack below
15 cm and within 0.75 years (i.e. below the photic zone). Mean annual
archived δ15N-NO3- and NO3- mass
concentration values are 50 ‰ and 60 ng g−1,
respectively, at the DML site. We report an e-folding depth (light
attenuation) of 2–5 cm for the DML site, which is considerably lower than
Dome C. A reduced photolytic loss of NO3- at DML results in less
enrichment of δ15N-NO3- than at Dome C mainly due to
the shallower e-folding depth but also due to the higher snow accumulation
rate based on TRANSITS-modelled sensitivities. Even at a relatively low snow
accumulation rate of 6 cm yr−1 (water equivalent; w.e.), the snow
accumulation rate at DML is great enough to preserve the seasonal cycle of
NO3- mass concentration and δ15N-NO3-, in
contrast to Dome C where the depth profiles are smoothed due to longer
exposure of surface snow layers to incoming UV radiation before burial. TRANSITS sensitivity analysis of δ15N-NO3- at DML
highlights that the dominant factors controlling the archived δ15N-NO3- signature are the e-folding depth and snow
accumulation rate, with a smaller role from changes in the snowfall timing
and TCO. Mean TRANSITS model sensitivities of archived δ15N-NO3- at the DML site are 100 ‰ for
an e-folding depth change of 8 cm, 110 ‰ for an annual
snow accumulation rate change of 8.5 cm yr−1 w.e., 10 ‰ for a change in the dominant snow deposition season
between winter and summer, and 10 ‰ for a TCO change of
100 DU (Dobson units). 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.
A review of the brittle ice zone in polar ice cores