Glacial/interglacial changes in the isotopes of nitrate from the Greenland Ice Sheet Project 2 (GISP2) ice core

The 15 N/ 14 N and 18 O/ 16 O ratios of nitrate in the Greenland Ice Sheet Project 2 (GISP2) (Summit, Greenland) ice core are much higher in ice from the last glacial period than in the pre‐industrial Holocene, despite the lack of a significant glacial/interglacial change in nitrate concentration. While both the 15 N/ 14 N and 18 O/ 16 O records are anticorrelated with snow accumulation rate, neither is satisfactorily explained by accumulation changes or post‐depositional processes. The similarity in the glacial/interglacial change in 15 N/ 14 N from several different Greenland ice cores and the large amplitude of this change relative to observed seasonal variation raise the possibility that the isotopes of nitrate in ice cores indicate a large‐scale glacial/interglacial change in the isotopic composition of atmospheric NO x . The glacial/interglacial change in 18 O/ 16 O is best explained by a greater contribution of HNO 3 production from hydrolysis of N 2 O 5 , which has implications for reconstruction of past atmospheric oxidant levels. Although isotope effects associated with NO x photochemistry and nitrate scavenging have not been fully characterized, the 15 N/ 14 N data may indicate glacial/interglacial changes in the relative contributions from different natural sources of NO x on a hemispheric or global scale.