Evaluating source, chemistry and climate change based upon the isotopic composition of nitrate in ice cores

Abstract

A clear negative trend is found in the nitrogen isotopic ratios (15N/14N) of nitrate over the industrial period, based on a 100-meter ice core from Summit, Greenland. This record indicates that isotopic composition of ice core nitrate reflects changes in nitrogen oxide (NOx) source emissions and that post-industrial emissions of NOx have resulted in a 12‰ (vs. air N2) decline in δ15N of atmospheric nitrate over the last ~300 years. Interestingly, over the last glacial period (as recorded in the Greenland GISP2 ice core), the δ15N of nitrate changes by ~20‰ from a typical pre-industrial Holocene value of 10‰ to a mean glacial value of 28‰, despite the lack of a significant change in nitrate concentration. The more recent ice core record clearly indicates an influence of NOx emission sources, therefore suggesting that the glacial-interglacial change in δ15N may be a record of significant variations in the contribution of NOx sources, such as lightning, biomass burning, biogenic soil emissions and oxidation of nitrous oxide in the stratosphere. In contrast to the source changes recorded by the nitrogen isotopes, the oxygen isotopic record of atmospheric nitrate (18O/16O, 17O/16O) in ice cores has implications for reconstruction of past atmospheric oxidant levels. This is because the oxygen isotopic composition of nitrate reflects oxidation of NOx to nitrate in the atmosphere by oxidants such as ozone and hydroxy and peroxy radicals. Measurements of variations in the isotopic composition of atmospheric nitrate inferred from ice cores could be used to improve interpretation of other records of δ15N such as those from tree rings and/or sediments.