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

Nitrogen stable isotope ratio (delta N-15) in Greenland snow nitrate and in North American remote lake sediments has decreased gradually beginning as early as similar to 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 similar to 1970. However, our subannually resolved ice core records of delta N-15 and major ions (e.g., NO3-, SO42-) over the last similar to 200 y show that the decrease in delta N-15 is not always associated with increasing NO3- concentrations, and the decreasing trend actually leveled off similar to 1970. Correlation of delta N-15 with H+, NO3-, and HNO3 concentrations, combined with nitrogen isotope fractionation models, suggests that the delta N-15 decrease from similar to 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 similar to 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 delta N-15 similar to 1970, which, together with the leveling off of NO3- concentrations, suggests a regional mitigation of anthropogenic impact on the nitrogen cycle. Our results highlight the importance of atmospheric processes in controlling delta N-15 of nitrate and should be considered when using delta N-15 as a source indicator to study atmospheric flux of nitrate to land surface/ecosystems.