Photolytic modification of seasonal nitrate isotope cycles in East Antarctica

Abstract. Nitrate in Antarctic snow has seasonal cycles in nitrogen and oxygen isotopic ratios that reflect its sources and atmospheric formation processes,
and as a result, nitrate archived in Antarctic ice should have great potential to record atmospheric chemistry changes over thousands of
years. However, sunlight that strikes the snow surface results in photolytic nitrate loss and isotopic fractionation that can completely obscure the
nitrate's original isotopic values. To gain insight into how photolysis overwrites the seasonal atmospheric cycles, we collected 244 snow samples
along an 850 km transect of East Antarctica during the 2013–2014 CHICTABA traverse. The CHICTABA route's limited elevation change, consistent
distance between the coast and the high interior plateau, and intermediate accumulation rates offered a gentle environmental gradient ideal for
studying the competing pre- and post-depositional influences on archived nitrate isotopes. We find that nitrate isotopes in snow along the transect
are indeed notably modified by photolysis after deposition, and drier sites have more intense photolytic impacts. Still, an imprint of the original
seasonal cycles of atmospheric nitrate isotopes is present in the top 1–2 m of the snowpack and likely preserved through archiving in
glacial ice at these sites. Despite this preservation, reconstructing past atmospheric values from archived nitrate in similar transitional regions
will remain a difficult challenge without having an independent proxy for photolytic loss to correct for post-depositional isotopic
changes. Nevertheless, nitrate isotopes should function as a proxy for snow accumulation rate in such regions if multiple years of deposition are
aggregated to remove the seasonal cycles, and this application can prove highly valuable in its own right.
