Argon and nitrogen isotopes of trapped air in the GISP2 ice core during the Holocene epoch (0-11,500 B.P.): Methodology and implications for gas loss processes

Argon and nitrogen isotopes of air in polar ice cores provide constraints on past temperature and firn thickness, with relevance to past climate. We developed a method to simultaneously measure nitrogen and argon isotopes in trapped air from the same sample of polar ice. This method reduces the time required for analysis, allowing large numbers of measurements. We applied this method to the entire Holocene sequence of the GISP2 ice core (82.37-1692.22 m) with a 10-20 year sampling interval (670 depths). delta Ar-40 and delta N-15 show elevated values in the oldest part of the dataset, consistent with a thicker firn layer and increased temperature gradient in the firn due to the legacy of the abrupt warming at the end of the Younger Dryas interval and the gradual warming during the Preboreal interval (11.5-10.0 ka). The Preboreal Oscillation and the 8.2k event are clearly recorded. The data show remarkable stability after the 8.2k event. Available data suggests that post-coring gas loss involves two distinct types of fractionation. First, smaller molecules with less than a certain threshold size leak through the ice lattice with little isotopic fractionation. Second, gas composition changes via gas loss through microcracks, which induces isotopic fractionation. These two gas loss processes can explain most trends in our data and in other ice core records. (C) 2008 Elsevier Ltd. All rights reserved.