Nitrogen isotopes in ice core nitrate linked to anthropogenic atmospheric acidity change

Abstract

Nitrogen stable isotope ratio (δ15N) in Greenland snow nitrate and in North American remote lake sediments has decreased gradually beginning as early as ∼1850 Christian Era. This decrease was attributed to increasing atmospheric deposition of anthropogenic nitrate, reflecting an anthropogenic impact on the global nitrogen cycle, and the impact was thought to be amplified ∼1970. However, our subannually resolved ice core records of δ15N and major ions (e.g., NO3-, SO 42-) over the last ∼200 y show that the decrease in δ15N is not always associated with increasing NO 3- concentrations, and the decreasing trend actually leveled off ∼1970. Correlation of δ15N with H+, NO3-, and HNO3 concentrations, combined with nitrogen isotope fractionation models, suggests that the δ15N decrease from ∼1850-1970 was mainly caused by an anthropogenic-driven increase in atmospheric acidity through alteration of the gas-particle partitioning of atmospheric nitrate. The concentrations of NO3- and SO42- also leveled off ∼1970, reflecting the effect of air pollution mitigation strategies in North America on anthropogenic NOx and SO2 emissions. The consequent atmospheric acidity change, as reflected in the ice core record of H+ concentrations, is likely responsible for the leveling off of δ15N ∼1970, which, together with the leveling off of NO3- concentrations, suggests a regionalmitigation of anthropogenic impact on the nitrogen cycle. Our results highlight the importance of atmospheric processes in controlling δ15N of nitrate and should be considered when using δ15N as a source indicator to study atmospheric flux of nitrate to land surface/ecosystems.