CHANGING SOURCES OF IMPURITIES TO THE GREENLAND ICE-SHEET OVER THE LAST 250 YEARS

Because the composition of precipitation reflects the composition of the atmosphere, polar ice cores provide a useful way of investigating past and present atmospheres. We have measured concentrations of major ions in nine sections of a central Greenland ice core and we found that concentrations of both SO42- and NO3- have increased dramatically over the last 250 years, up to three to four times the 18th century levels. Large changes have also occurred in the average concentrations of several other chemical species, such as NH4+, excess Cl, and Ca2+. We used a principal-component analysis to characterize variations of the season of maximum deposition mte of HNO3 and H2SO4 to the snow. We found that source fluctuations of H2SO4 are faithfully recorded in the Greenland snow and appear to switch their preferential time of deposition in the snow from summer to winter early in the 20th century. On the other hand, HNO3 is deposited preferentially during summer throughout the core, emphasizing the role of photochemistry in understanding nitrogen cycling in the Arctic. Anthropogenic inputs have clearly modified the behavior of several chemical compounds in the atmosphere.