Abstract
Abstract. The seasonal and annual representativeness of ionic aerosol proxies (among others, calcium, sodium, ammonium and nitrate) in various firn cores in the vicinity of the NEEM drill site in northwest Greenland have been assessed. Seasonal representativeness is very high as one core explains more than 60% of the variability within the area. The inter-annual representativeness, however, can be substantially lower (depending on the species) making replicate coring indispensable to derive the atmospheric variability of aerosol species. A single core at the NEEM site records only 30% of the inter-annual atmospheric variability in some species, while five replicate cores are already needed to cover approximately 70% of the inter-annual atmospheric variability in all species. The spatial representativeness is very high within 60 cm, rapidly decorrelates within 10 m but does not diminish further within 3 km. We attribute this to wind reworking of the snow pack leading to sastrugi formation. Due to the high resolution and seasonal representativeness of the records we can derive accurate seasonalities of the measured species for modern (AD 1990–2010) times as well as for pre-industrial (AD 1623–1750) times. Sodium and calcium show similar seasonality (peaking in February and March respectively) for modern and pre-industrial times, whereas ammonium and nitrate are influenced by anthropogenic activities. Nitrate and ammonium both peak in May during modern times, whereas during pre-industrial times ammonium peaked during July–August and nitrate during June–July.
Highlights
Ice cores provide a valuable archive for past climate and environmental changes, through gases and isotopes, and through aerosol-derived chemical constituents
In addition to the lower wind speeds during the months of high NO−3 and H2O2 concentrations, one could speculate that a reason for the high representativeness could be the diffusion of H2O2 in the ice and the post-depositional loss of NO−3, which might both be enhanced during warmer months
The firn cores and the snow pit analysed in this work agree well with data available in the literature from other firn cores and snow pits
Summary
Ice cores provide a valuable archive for past climate and environmental changes, through gases and isotopes, and through aerosol-derived chemical constituents. Chemistry measurements on ice cores provide information about past climatic conditions for the drilling region as well as for the source regions and give insight into past changes of largescale circulation patterns Chemical constituents in ice cores are regarded as representative on inter-annual and seasonal timescales. It is well known that they contain noise introduced by glaciological processes, arising from the movement of surface snow by wind scouring and ablation or atmospheric processes such as unevenly distributed deposition. Glaciological noise processes were modelled and quantified for accumulation and δ18O by means of standard Fourier spectral methods in the mid-1980s by Fisher et al (1985). In 2006, Karlöf et al (2006) used wavelet analysis to decompose signal from noise on Electrical Conductivity Measurements (ECM) and δ18O time series from Antarctic firn cores, separated by distances between 3.5 and 7 km, coming to the conclusion that ECM shows a significant common signal if averaged over timescales from 1 to 3 years, whereas δ18O showed no statistically significant common signal
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