24 Aug 2022
24 Aug 2022
Status: this preprint is open for discussion.

Photolytic modification of seasonal nitrate isotope cycles in East Antarctica

Pete D. Akers1,2, Joël Savarino2, Nicolas Caillon2, Olivier Magand2, and Emmanuel Le Meur2 Pete D. Akers et al.
  • 1Department of Geography, Trinity College Dublin, Dublin, Ireland
  • 2Université Grenoble Alpes, CNRS, IRD, Grenoble INP, IGE, Grenoble, France

Abstract. Nitrate in Antarctic snow has seasonal cycles in its 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 a 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 still 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 along CHICTABA and in similar transitional regions remains 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.

Pete D. Akers et al.

Status: open (until 05 Oct 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • AC1: 'Comment on egusphere-2022-812', Pete D. Akers, 13 Sep 2022 reply
  • RC1: 'Comment on egusphere-2022-812', Anonymous Referee #1, 26 Sep 2022 reply
  • RC2: 'Comment on egusphere-2022-812', Guitao Shi, 27 Sep 2022 reply

Pete D. Akers et al.

Data sets

CHICTABA transect nitrate analyzing R code and data Pete D. Akers

Pete D. Akers et al.


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Short summary
Nitrate in Antarctic ice cores could be used to reconstruct past atmospheric conditions, but nitrate isotopes in deep ice are greatly altered from the seasonal nitrate cycles in the atmosphere. We sampled nitrate along an 850 km Antarctic transect that better preserves the atmospheric cycles. This shows that the degree of the isotopic transformation is related to snow accumulation rate, and nitrate isotopes can potentially reconstruct past changes in snowfall if sampled properly.