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

The N-15/N-14 and O-18/O-16 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 N-15/N-14 and O-18/O-16 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 N-15/N-14 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 NOx. The glacial/interglacial change in O-18/O-16 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 NOx photochemistry and nitrate scavenging have not been fully characterized, the N-15/N-14 data may indicate glacial/interglacial changes in the relative contributions from different natural sources of NOx on a hemispheric or global scale.