ORIGINS AND VARIATIONS OF FLUORIDE IN GREENLAND PRECIPITATION

Summit-Greenland firn and ice core sections have been analyzed to determine the fluoride background level of high northern latitude precipitation and to investigate its temporal variations over various time periods. A continuous profile covering the last century and a discontinuous study of the last 27,000 years spanning the entire Holocene time period (O to 11,500 years B.P.) and the late part of the last glaciation have been achieved. This first detailed study of the fluoride content of polar ice help us obtain a better understanding of the atmospheric cycle of inorganic fluoride in remote atmospheres. First, our data suggest that blown dust is among the nonvolcanic sources that contributed to the natural fluoride background level of this high-latitude precipitation. Primary sea-salt emissions represent an insignificant contribution. Our study also indicates that high-latitude biomass burning debris which sometimes reaches the Greenland ice cap represents an additional weak source of fluoride for Greenland precipitation. Second, this work provides useful information for evaluating the impact of the volcanic activity on the fluoride budget of these high northern latitude regions. Indeed, our data demonstrate that although the natural background fluoride content of Greenland snow is very often sporadically disturbed by volcanic emissions from Iceland (numerous Hekla eruptions and the 1783 Laki eruption, for instance), located relatively close to the Greenland ice cap, large explosive eruptions (Tambora in 1815, for instance) only weakly disturb this background level. Our data suggest that even in the case of significant input within the stratosphere, fluoride is rapidly scavenged probably along with ash particles. Finally, our study of recent snow layers suggests that man-made sources (mainly coal burning) represent an important contribution which has dominated this high northern hemisphere fluoride budget for the three last decades. Furthermore, over the last IO years, a possible influence of the stratospheric reservoir builded up from the chlorofluorocarbons' degradation cannot be ruled out and in the future this growing contribution may represent a significant part of the anthropogenic fluoride input compared to the input derived from coal burning.