Mercury in the Greenland Ice Sheet

Abstract

REPORTS of the mercury content of the Greenland Ice Cap have varied within one or two orders of magnitude. Weiss et al.¹ found an increase from about 50 ng per kg in ancient deposit layers to about 100 ng per kg in surface samples from 1964 to 1965, and concluded that there was an increase in man's global input of mercury to the atmosphere. Their deposit and surface samples were collected in the Camp Century area (77° N, 61° W, Fig. 1) in north-west Greenland. Dickson² pointed out that this finding was due to a systematic difference between sampling sites and was supported by Carr and Wilkniss³, who in samples from Camp Century found 13 ng per kg and 50 ng per kg in deposits from 1850 and 1940, and in samples taken from site 2 (77° N, 56° W), about 150km off Camp Century, found 169 ng per kg and 100 ng per kg in deposits dated 1870 and 1930. In contrast to their previous finding, Weiss et al.⁴ later found that the mercury concentration in seasonal samples of 1966–70, was similar to that of old deposits dating back to 1807 on 25 samples collected at Dye 3 station (65° N, 44° W). The average mercury concentration was 50 ng per kg of water. Herron et al.⁵ found the highest mercury concentrations yet reported from the Greenland Ice Cap ranging from 263 ng per kg to 881 ng per kg, covering ancient deposits from 1631 to surface pit samples in layers from 1972 to 1973. The 11 samples were collected at station Milcent (70° N, 45° W), and no significant increase in the mercury concentration was seen in modern times. The actual mercury level was taken as more representative of the Ice Cap than that previously found due to a new pretreatment of sample bottles. This controversy has stressed that far more information about the general level of mercmry and more reliable sampling and analytical methods at ultra low mercury levels are needed. We have collected ice core samples at two sites, Dye 3 and Crete, which show a mercury level of about 10 ng per kg, in the order of magnitude of unpolluted ocean water. This level is one to two orders of magnitude lower than previous figures. We report here that the mercury contents of the Crete core show no increased values either in recent deposits or in deposits of 1783, the year of the volcanic eruption of Laki in Iceland.