Comparison of analytical methods used for measuring major ions in the EPICA Dome C (Antarctica) ice core

In the past, ionic analyses of deep ice cores tended to consist of a few widely spaced measurements that indicated general trends in concentration. The ion-chromatographic methods widely used provide well-validated individual data, but are time-consuming. The development of continuous flow analysis (CIA) methods has allowed very rapid, high-resolution data to be collected in the field for a wide range of ions. In the European Project for Ice Coring in Antarctica (EPICA) deep ice-core drilling at Dome C, many ions have been measured at high resolution, and several have been analyzed by more than one method. The full range of ions has been measured in five different laboratories by ion chromatography (IC), at resolutions of 2.5-10 cm. In the field, CIA was used to measure the ions Na+, Ca2+, nitrate and ammonium. Additionally, a new semi-continuous in situ IC method, fast ion chromatography (FIC), was used to analyze sulphate, nitrate and chloride. Some data are now available to 788 in depth. In this paper we compare the data obtained by the three methods, and show that the rapid methods (CIA and FIC) give an excellent indication of trends in ionic data. Differences between the data from the different methods do occur, and in some cases these are genuine, being due to differences in speciation in the methods. We conclude that the best system for most deep ice-core analysis is a rapid system of CIA and FIC, along with in situ meltwater collection for analysis of other ions by IC, but that material should be kept aside for a regular check on analytical quality and for more detailed analysis of some sections