Evaluating Anthropogenic Impacts to the Nitrogen Cycle Based on the Isotopes of Nitrate in Ice Cores

Fixed nitrogen in the environment from anthropogenic sources (i.e., fossil fuel combustion and fertilizer production) has fundamentally changed the global reactive nitrogen cycle. It is difficult, however, to distinguish variability in natural sources and to diagnose how these natural sources have changed over time. The isotope ratios of ice core nitrate offers us a new tool to investigate how nitrate deposition has changed over time with changes in both natural and manmade sources, chemistry, and climate. Nitrate deposited in snow results primarily from the production of nitric acid (HNO3) in the atmosphere via reactions between nitrogen oxides (NOx) and major tropospheric oxidants such as ozone and hydroxyl compounds. As a result, the isotopic composition of nitrate deposition reflects NOx sources and chemistry. Recent advances in isotope methodologies have made it possible to characterize, at higher resolution and lower concentrations, the isotopic ratios (15N/14N, 18O/16O, 17O/16O) in nitrate. The isotope ratios of ice core nitrate offers us a new tool to investigate how nitrate deposition has changed over time with changes in both natural and anthropogenic sources, chemistry, and climate. An ice core from Summit, Greenland reveals a clear change in the nitrogen isotopic composition (δ15N) of nitrate, with pre-industrial values averaging 11–(vs. air) in comparison to an average of -1–in the last decade. The decrease observed in δ15N is noticeable as early as ~1850, while an important increase in nitrate concentration is observed starting in ~1890. Other work on ice from southern Greenland has shown that the oxygen isotopic composition of nitrate is influenced by changes in atmospheric chemistry associated with biomass burning in North America around this same time period. Globally fossil fuel combustion is the largest source of NOx emissions today, with biomass burning, biogenic soil emissions and lightning contributing significantly as well. The isotopes of nitrate offer a new tool to investigate and reconstruct the influence of NOx sources and chemistry on nitrate preserved in ice.