An improved method for delta15N measurements in ice cores

The use of isotopic ratios of nitrogen gas (d15N) trapped in ice cores as a paleothermometerto 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 d15N to a precision of 0.006%° (1s, 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 d15N measurements. The results show that a small amount of oxygen in the sample can be detrimental to achieving an optimum precision in d15N measurements of atmospheric nitrogen trapped ice core samples.