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

Abstract

Nitrogen stable isotope ratio (δ(15)N) 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 δ(15)N and major ions (e.g., NO3(-), SO4(2-)) over the last ∼200 y show that the decrease in δ(15)N is not always associated with increasing NO3(-) concentrations, and the decreasing trend actually leveled off ∼1970. Correlation of δ(15)N with H(+), NO3(-), and HNO3 concentrations, combined with nitrogen isotope fractionation models, suggests that the δ(15)N 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 SO4(2-) also leveled off ∼1970, reflecting the effect of air pollution mitigation strategies in North America on anthropogenic NO(x) 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 δ(15)N ∼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 δ(15)N of nitrate and should be considered when using δ(15)N as a source indicator to study atmospheric flux of nitrate to land surface/ecosystems.

Keywords: acid deposition; clean air act; fossil fuel emissions; industrial; proxy.

Fig. 1.

Subannually resolved δ¹⁵N() (top) and nitrate concentration data (bottom) in the Summit, Greenland ice core. Heavy curves are five-point (∼2 y) running averages.

Fig. 2.

Annual concentration of H⁺ (A), (B), (C), and δ¹⁵N() (D) from 1772 to 2006. Large H⁺ and peaks in A and B indicate volcanic eruptions. The blue curve in C represents the calculated acid form nitrate (HNO₃) concentrations; the blue and red curves in D are the modeled δ¹⁵N() trends throughout the record calculating with ε = −8.5 ‰ and −21 ‰, respectively, assuming ideal conditions. Vertical dashed lines mark the years of interest.