First results of 15N/14N ratios in nitrate from alpine and polar ice cores

Isotopic analyses of nitrogen were performed in nitrate from alpine and polar snow and ice. Nitrate from recent alpine ice cores showed similar ¹⁵N/¹⁴N ratios and seasonal variations as continental rain nitrate. Nitrate from recent Summit (Greenland) precipitation showed also similar isotope composition as European rain but, in ice cores, increasing ¹⁵N/¹⁴N ratios with decreasing nitrate concentrations are observed as a function of depth until about the year 1950, which is the time when anthropogenic emissions of nitrogen oxides started to increase rapidly in the Northern Hemisphere. In recent Antarctic ice from the South Pole, nitrate concentrations are nearly the same as in the measured Greenland ice up to the year 1967, where the record for South Pole ice stops. No conclusions on recent nitrate pollution in the Antarctic could be given from this poorly documented core. Measured isotopic ratios for the Greenland ice core for the preanthropogenic period correspond to one Antarctic ice core (D47); both cores show similar snow accumulation rates. Isotopic ratios for other Antarctic ice cores are different from the Greenland ratio and a clear relationship is found between the isotopic composition and the snow accumulation rate with heavier ratios observed with decreasing accumulation rates. It is proposed that physical phenomena occurring in the firn and linked to the low accumulation rates of several Antarctic study sites most likely modify the initial concentration and isotopic composition of the nitrate.