Drivers of late Holocene ice core chemistry in Dronning Maud Land: The context for the ISOL-ICE project

Abstract. Quantifying the natural variability of the stratospheric ozone layer and understanding the underlying factors that control natural total column ozone (TCO) variability are required to put modern observations into historical context and evaluate the effectiveness of climate and TCO protection policies. Within the framework of the Isotopic Constraints on Past Ozone Layer in Polar Ice (ISOL-ICE) project, we present initial ice core results from the new ISOL-ICE ice core covering the last millennium from the high-elevation Dronning Maud Land (DML) located under the Antarctic spring stratospheric TCO minimum, and discuss the implications for interpreting the stable isotopic composition of nitrogen in ice core nitrate (δ¹⁵N(NO₃^-)) as a surface ultra-violet radiation (UV) and TCO proxy. To interpret the ice core δ¹⁵N(NO₃^-) record, an understanding of past snow accumulation changes, as well as aerosol source regions and present-day drivers of their variability are required. We therefore report here the ice core age-depth model, the snow accumulation and ice chemistry records, and correlation analysis of these records with climate variables over the observational era (1979–2016). The ISOL-ICE ice core covers the last 1349 years from 668 to 2017 C.E. ± 3 years extending previous ice core records from the region by two decades and shows excellent reproducibility with those records. The extended ISOL-ICE record of last two decades showed a continuation of the methanesulphonate (MSA) increase from ~1800 to present while there were less frequent large deposition events of sea salts relative to the last millennium. The correlation analysis, combined with the finding that sea salts do not carry a sea ice signature to the site, highlight that sea salt and MSA aerosol concentrations are primarily related to atmospheric transport over the extended two-decade period and not to changes in sea ice source strength. Correlation of the snow accumulation record with climate variables over the observational era showed that precipitation at ISOL-ICE is predominately derived from the South Atlantic with onshore winds delivering marine air masses to the site. The snow accumulation rate was stable over the last millennium with no notable trends over last two decades relative to the last millennium. Interannual variability in the accumulation record, ranging between 2 and 20 cm a⁻¹ (w.e.), would influence the ice core δ¹⁵N(NO₃^-) record. The mean snow accumulation rate of 6.5 ± 2.4 cm a^-1 (w.e.) falls within the range suitable for reconstructing surface mass balance from ice core δ¹⁵N(NO₃^-) highlighting that the ISOL-ICE ice core δ¹⁵N(NO₃^-) can be used to reconstruct either the snow accumulation rate or surface UV if the ice core δ¹⁵N(NO₃^-) is corrected for the snow accumulation influence.