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

The ¹⁵N/¹⁴N and ¹⁸O/¹⁶O 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 ¹⁵N/¹⁴N and ¹⁸O/¹⁶O 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 ¹⁵N/¹⁴N 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 ¹⁸O/¹⁶O is best explained by a greater contribution of HNO₃ production from hydrolysis of N₂O₅, 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 ¹⁵N/¹⁴N data may indicate glacial/interglacial changes in the relative contributions from different natural sources of NO_x on a hemispheric or global scale.