An improved method for delta ¹⁵N measurements in ice cores

Abstract. The use of isotopic ratios of nitrogen gas (δ¹⁵N) trapped in ice cores as a paleothermometer to characterise abrupt climate changes is becoming a widespread technique. The versatility of the technique could be enhanced, for instance in quantifying small temperature changes during the last glacial period in Antarctic ice cores, by using high precision methods. In this paper, we outline a method for measuring δ¹⁵N to a precision of 0.006\permil (1σ, n=9) from replicate ice core samples. The high precision results from removing oxygen, carbon dioxide and water vapour from the air extracted from ice cores. The advantage of the technique is that it does not involve correction for isobaric interference due to CO⁺ ions. We also highlight the importance of oxygen removal from the sample, and how it influences δ¹⁵N measurements. The results show that a small amount of oxygen in the sample can be detrimental to achieving an optimum precision in δ¹⁵N measurements of atmospheric nitrogen trapped ice core samples.