Jökull


Jökull - 01.12.1984, Side 62

Jökull - 01.12.1984, Side 62
Fig. 5. Acid fallout from the Eldgjá eruption in 934 A.D. as seen in 3 Greenland ice cores. Sampling as in Fig. 3. Mynd 5. Súr úrkoma frá gosinu í Eldgjá 934 í þremur ískjörnum frá Grœnlandi. explosive phase in the eruption, which must have been more pronounced than in the case of the Lakagígar eruption. The long duration of the fallout is also, at least partly, supporting this point of view. Another interesting feature is the chemical composition of the acid: During the Dye 3, 1980 deep drilling season the Eldgjá signal was encountered and samples were collected on loca- tion for chemical analysis by M. Herron over the ice segment, where the acidity increased due to the eruption, Fig 5. The results are interesting, because they show a quite different acid composi- tion, than the Lakagígar eruption, as well as a change of chemical composition with time, (Her- ron 1982). The deposited acids are first consisting of mainly HF, then in the peak acidity H2S04 dominates and finally a mixture of HF, H2S04 and HCl. These chemical data must be interpreted with some care, because in the Créte acid record there is an additional high level of HCl in the Eldgjá fallout (Hammer, 1980b and M. Herron, personal communication 1981). This partly explains the very pronounced maximum in the Créte acidity profile. The interpretation of the difference between the Dye 3 and Créte acid composition is probably to be found in either aeolian differentia- tion and/or in the actual phases of the eruption. In both cases meterorological conditions at the time of the eruptive activity, could have played a role. As there can be little doubt, that we are deal- ing with the same event in both cores, one must conclude, that “fingerprinting” the eruption by chemical analysis of the anions is not a straight-forward procedure, though it may be helpful. In Fig 5 part of the Eldgjá acid signal in the Camp Century core is shown. The data are incomplete due to bad quality of the core. Only data, which are comparable with the quality of Dye 3 and Créte data are shown. The high acidi- ties and the dating of the ice segment to 925 ± 15 justify the notion “Eldgjá signal”; thus the Eldgjá eruption has been revealed in all the 3 Greenland ice cores, which reach back to the time of the eruption. The Katla 1179 eruption According to Thorarinsson (personal com- munication, 1979) the only information we have of a Katla eruption in 1179 derives from an old script, in which it is mentioned, that a quarrel took place in 1179 between the bishop Þorlákur Þórhallsson and the mightiest temporal chief on Iceland, Jón Loptson: The quarrel came up due to a jökulhlaup in Höfði (i.e. from Katla), which had destroyed several farms of which two had churches. The jökulhlaup must have been large, indicating an eruption in Katla, but no tephra layer has yet been identified with certainty. The short duration of the acid fallout, the precise dating of the signal and its occurrence in all 3 ice cores, Fig. 6, “smells” like an Icelandic eruption. This is almost, what one can derive from the ice cores, had it not been for the Dye 3 R value of the eruption. The R value is quite low and the peak value of the acidity profile is almost equal to the Créte value. It must have been a fairly strong eruption with respect to gas release, but not very explosive with respect to sending large amounts of acid gases into the stratosphere. Is it too much to call it the Katla 1179 eruption? At least I will do so, until someone disproves it. In order to give the reader an impression of the non-volcanic acid background a period of years before the 1179 eruption, is also presented in Fig. 6. The some- what high acidities around 1168 (Créte) could be due to an unknown eruption, as there is a fairly 60 JÖKULL 34. ÁR
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