Jökull


Jökull - 01.12.1984, Side 60

Jökull - 01.12.1984, Side 60
Fig. 3. Acid fallout from the Lakagígar 1783 A.D. eruption as seen in 3 Greenland ice cores: Camp Century (A), Créte (B) and Dye 3 (C). The dates have been obtained by stratigraphical dating methods. The sampling corresponds to 12 samples over the calculated annual layer thick- ness at the depth in question. More or less than 12 samples per stratigraphically dated year indi- cate respectively higher and lower precipitation than the average annual precipitation at the loca- tion. Mynd 3. Súr úrkoma frá Skaftáreldum 1783 í þremur ískjörnum á Grœnlandi, Camp Century (A), Créte (B) og Dye 3 (C). In Fig. 3 the annual layers have been separated by means of the oxygen isotope seasonal record; the annual layers are therefore not separated into exact calendar years, but into oxygen isotope years, from minimum to minimum of the ð(lsO seasonal variation — close to a calendar year. For the moment no method exists by which the true yearly shift can be established, but using the minima of the seasonal oxygen profile is a close enough approximation. (Dansgaard 1964, p. 445, Fig 4) Even though the oxygen profiles are smoothed with time and depth in the core due to diffusion, they can in many cases be reestablished by a back-diffusion technique (Johnsen 1977). The dating can also be supported by other strati- graphical dating methods (Hammer et al. 1978), which are less prone to diffusion. In the case of high accumulation areas i.e. more than approx. 20 cm precipitation per year (in water-equivalent) the difference between the true shift of the year and the ð(lsO) definition of the shift will in general only change the “true” yearly average acidities little; unless a high volca- nic acid signal is confined to a few winter months. The latter is not very likely, because strong volca- nic signals tend to last more than a few months. The Lakagígar signal is of short duration i.e. approx. 1 year. The immediate appearance of the Lakagígar signal in the early summer snow of 1783 is interes- ting (interpretation from the ð(lsO) profile). because S02 injected into the stratosphere by a volcano takes some months to be transformed into H2S04 which has a residence time of approx. 1 year in the polar stratosphere. The above fin- dings agree with our present knowledge on the eruption: The Lakagígar eruption was only a moderately explosive eruption and the large amount of liberated S02 was to a large degree confined to the troposphere, which speeds up its transformation to sulfate (Junge 1963, p. 68), the acid deposition on the ice sheet declined quickly, when the eruptive activity ceased. Thorarinsson 's minimum estimates on the acid production (changing his S02 values to H2S04 values) gives 21.1 mill. tons of H2S04 + HCl (1.2 mill. tons), while my own estimates based on the Créte core, gives 100 mill. tons. In a correspond- ence with Thorarinsson 1977 he wrote — — Using ca. 1% water percent in basalt magma, as some do, and the same fractional composition of the gas as in Surtsey, we get a max. of ca. 40 mill. tons or the same order of magnitude as your estimate.” (translated to English). My estimate may be a little on the high side, because the Icelandic eruptions seem to give a somwhat higher acid deposition on Greenland, than one would expect from the fission product data. Selected data from the latter profiles, combined with the global fallout pattern, was used to relate ice core acid data to the acid production by an eruption (see e.g. Hammer et al. (1981)). Presently the Lakagígar event has been detected as highly elevated acidity in the follow- ing ice cores: Dye 3, Dye 2 (South Greenland), Milcent, Créte (Mid Greenland), Camp Century, North Central, Hans Tavsen (North Greenland), Devon Island, Ellesmere Island (North-East Canada), (D.A. Fisher personal communication, 1982). 58 JÖKULL 34. ÁR
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