Factors controlling nitrate in ice cores: Evidence from the Dome C deep ice core

In order to estimate past changes in atmospheric NOx concentration, nitrate , an oxidation product of NOx, has often been measured in polar ice cores. In the frame of the European Project for Ice Goring in Antarctica (EPICA), a high-resolution nitrate record was obtained by continuous flow analysis ( CFA) of a new deep ice core drilled at Dome C. This record allows a detaile d comparison of nitrate with other chemical trace substances in polar snow under different climatic regimes. Previous studies showed that it would be difficult to make firm conclusions about atmospheric NOx concentrations bas ed on ice core nitrate without a better understanding of the factors contro lling NO3- deposition and preservation. At Dome C, initially high nitrate c oncentrations (over 500 ppb) decrease within the top meter to steady low va lues around 15 ppb that are maintained throughout the Holocene ice. Much hi gher concentrations (averaging 53 ppb) are found in ice from the Last Glaci al Maximum (LGM). Combining this information with data from previous sampli ng elsewhere in Antarctica, it seems that under climatic conditions of the Holocene, temperature and accumulation rate are the key factors determining the NO3- concentration in the ice. Furthermore, ice layers with high acidi ty show a depletion of NO3-, but higher concentrations are found before and after the acidity layer, indicating that NO3- has been redistributed after deposition. Under glacial conditions, where NO3- shows a higher concentrat ion level and also a larger variability, non-sea-salt calcium seems to act as a stabilizer, preventing volatilization of NO3- from the surface snow la yers.