Methodology:

A full description of the methods for the ISOL-ICE ice core can be found in Winton et al. (2019); doi: 10.5285/9c972cfb-0ffa-4144-a943-da6eb82431d2. Briefly, a new 120 m core was recovered from Kohnen Station, located in Dronning Maud Land (DML) on the high-elevation East Antarctic Plateau (2892 m a.s.l) in January 2017. Sections of the core were analysed for i) continuous flow analysis (CFA) of nitrate, sodium and magnesium mass concentrations and electrolytic meltwater conductivity at the British Antarctic Survey (BAS), Cambridge, and ii) discrete sections for the nitrate stable isotopic composition at the Environmental Geosciences (IGE), University of Grenoble-Alpes. Here we report the dated section of the ice core from 1455 - 1227 AD (60.80 - 79.45 m) covering the largest volcanic eruption, Samalas in 1259, over the past 1000-years. Dating was achieved by annual layer counting of measured concentrations of sodium and magnesium following previous studies at DML (Göktas et al., 2002; Weller and Wagenbach, 2007; Weller et al., 2008) constrained by well-dated volcanic eruption horizons. An age uncertainty of +/- 3 years is estimated at the base of the ice core. High-resolution sampling for nitrate isotopic analysis was carried out i) across volcanic horizons with a sample resolution of 5 - 30 cm, and ii) in 10 cm resolution baseline samples 1 m either side of the volcanic peak. A total of 119 discrete measurements of d15N-NO3- are reported here. Discrete nitrate isotope samples were pre-concentrated and analysed using the denitrifier method following Frey et al. (2009);Morin et al. (2009). The nitrogen isotopic ratio was referenced against N2-Air (Mariotti, 1983). We report 15N/14N of NO3- (d15N-NO3-) as d-values following d = (Rsample /Rstandard - 1) * 1000 where R is the elemental isotopic ratio in the sample and standard respectively. The overall accuracy of the method for d15N is 2.9 ppth.

Data collection:

Fast ion chromatography: ICS-3000, Dionex™ with a AS15-5um and AERS500-4mm suppressor.

Isotope ratio mass spectrometer (IRMS) for duel O and N analysis: Thermo Finnigan™ MAT 253 IRMS equipped with a GasBench II™ and coupled to an in-house-built NO3- interface.

Inductively coupled plasma-mass spectrometry: 7700 series, Agilent.

Conductivity: Dionex? conductivity detector.

Data quality:

Major ions: At the beginning, end and during the course of the day, a certified reference material (CRM; ERM-CA408 simulated rainwater standards; Sigma-Aldrich) was measured for quality control and yielded an accuracy of 107 %, and precision of 10 % for nitrate.

Major elements: At the beginning, end and during the course of the day, a certified reference material (CRM; ERM-CA408 simulated rainwater standards; Sigma-Aldrich) was measured for quality control and yielded an accuracy of 94 and 113 %, and precision of 10 and 13 % for sodium and magnesium.

Nitrate stable isotope ratios: Certified reference materials (IAEA USGS-32, USGS-34 and USGS-35; Böhlke et al., 1993; Böhlke et al., 2003) were prepared (matrix match 1 M NaCl solution; ACS grade) and subject to the same analytical procedures as snow and aerosol samples. For each batch of 60 samples, the overall accuracy of the method is estimated as the reduced standard deviation of the residuals from the linear regression between the measured reference materials (n = 16) and their expected values. For the ice core (n = 800), the average uncertainty values obtained for d18O, d17O, D17O and d15N were 2.8 ppth, 2.1 ppth 0.9 ppth and 2.9 ppth respectively.