Jökull

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Jökull - 01.12.1984, Qupperneq 65

Jökull - 01.12.1984, Qupperneq 65
comprehensive description of volcanic activity during the Holocene. More refined techniques and new ice core data may solve the problem of identification, but for the moment only sugges- tions are possible. The H IV and H V eruptions These eruptions are C 14 dated to 4600 B.P. and 7400 B.P! (Larsen and Thorarinsson 1977; H. Tauber personal communication, 1983). They produced 9 and 3 km3 of tephra, estimated as freshly fallen. The H IV eruption is a good candi- date, with respect to an acid fallout signal on Greenland. A thorough discussion of these erup- tions and traces of them in ice cores must await the final dating of the Dye 3 core, but they have previously been proposed as the cause of two of the high acidity signals in the Camp Century record (Hammer et al. 1980 a). The latter record is problematic, especially for dates before ca. 4000 B.P., and the H IV and H V acid productions are best treated as part of a discussion on the validity of the Camp Century acidity record. THE CAMP CENTURY ACIDITY RECORD - A REVIEW AND SOME REVISIONS While the acidity of the Dye 3 core was mea- sured in the field and under almost ideal condi- tions, this was not the case for the Camp Century core. When the strongest acid signals detected in the Camp Century core were published (Hammer et al. 1980 a), we were quite confident of the validity of the highest signals and felt, that the moderately high acidity peaks could be trusted. Today, experience and more refined techniques have taught us to be a little more careful, when dealing with ice cores stored over long time periods. In 1982 we had the opportunity to remeasure some of the important segments, of what re- mained, of the Camp Century core. The techni- que we used was a more refined one and our cleaning of the core, prior to measuring, proved to be sufficient. Not all important segments could be remeasured, as too little core remained. The results were informative, though somewhat dis- turbing: Out of the 18 highest acidity signals detected in the core (before 40 A.D.) 3 mode- rately high signals had to be deleted, 4 signals had to be lowered somewhat and 7 signals could not be remeasured correctly due to bad core quality. Four out of the 5 biggest signals were still among the biggest. One moderately high signal was higher than previously measured. Unfortunately one of the signals, which had to be deleted was the previously suggested traces of the H V 5470 B.C. eruption. The revised Camp Century findings are shown in Fig. 9. Note, that the figure only show those high acidity signals, which are now safely identi- fied in the Camp Century core; it is not a compre- hensive figure and all doubtful measurements have been left out. The remeasurement of the core was not entirely concentrated on revisions. The detection of traces of the Lakagígar —, un- known 1259 — and Eldgjá-eruptions and the important confirmation of the highest acidities were reassuring findings. Perhaps only those who have worked with the remains of the now 16 year old Camp Century core understand the problems involved in work- ing with it. As only one deep ice core from Greenland existed a few years ago, we tried to “squeeze” as much information out of it as possi- ble; evidently we made some mistakes, when using a technique which was in its infancy at the time of measuring. The revision will be important, when the dat- ing of the acid signals in the new Dye 3 core is completed, because with only two existing deep cores from Greenland, it will be a prerequisite for a comparison of the high acid signals in the two records, that the compared data are correct. Fig. 9 offers a Camp Century record which can be used for such a comparison — even though it is not exhaustive. VOLCANOLOGY AND POLAR ICE CORES In this article I have concentrated on Icelandic eruptions, even though they have been seen on a “general background” of volcanic traces in the Greenland Ice Sheet. The ice sheets are unique with respect to the possibilities of estimating the long-range transport of volcanic debris, but there are many aspects of their analysis and potential use in volcanology, which I have only touched upon. In the near future there is a need to obtain data from more locations, including Antarctica. JÖKULL 34. ÁR 63
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