Jökull - 01.12.1984, Blaðsíða 59

whether the compared annual layers are exactly
corresponding. No date have been given for these
events in Fig. 2, because it would give a false
impression of the dating accuracy. This has, how-
ever, little importance for the R ratios, as the
annual Dye 3 acidities around these dates are
quite low and high values of R will in any case be
the result.
The 1360 signal may not be due to an eruption,
as it is one of the annual layers in the Dye 3 core,
where a possible influence of high summer tem-
peratures and/or meltlayers is suspected, though
not proven. The date has been set in brackets in
Fig. 2 and the “event” will be left out of the
discussion.
The 516 event cannot be compared to a similar
event in the Camp Century core as the latter core
is not usable for acidity measurements around
this age.
The missing indications, in the Dye 3 core, of 3
volcanic signals are not too surprising, because
such signals should in general be Iower than in
the Créte core i.e. in some cases too low for
definite detection. Still it is peculiar, that the R
values are as high as 4.7, 7.1 and 3.8 for the 3
eruptions. The R value for fission product con-
centrations (total (3 activity) is 1.8, but the fission
product profile was caused by a “mess” of nuclear
detonations at various latitudes, while we are
here considering individual eruptions which took
place at specific geographical positions. As can
be seen in Fig. 2, some Icelandic eruptions show
fairly low values of R. It seems, that Icelandic
eruptions are favorably located with respect to
high acid deposition on Greenland. This is not a
very surprising finding. However, one could not
have known “a priori”, that the Dye 3 core in
some cases would show higher acidities, than the
Créte core, but for Eldgjá, the R value is higher
than the fission product value.
If the average R value for those eruptions,
which exceeds approx. 4 pequiv. H +/kg in both
cores is calculated, it comes out as 1.7, i.e. pretty
close to the R= 1.8 for the fission products.
Even though the data mentioned above are
limited, they do give a hint for an interpretation:
The eruptions in 1642, 1602 and 623 took place in
the northern hemisphere, the debris drifted
northward, but in general the gaseous products
did not disperse widely over the hemisphere.
Perhaps they were moderately strong eruptions
in e.g. Japan, Alaska or Kamchatka. This
interpretation is further supported by the Katmai
1912 eruption, which is clearly marked in ice
cores from several locations in Mid- and North
Greenland, but is only just detectable in the Dye
3 core (not shown in Fig. 2, because the average
acidity is too small).
If the above holds to be true, we have the
possibility of classifying the eruptive signals into 3
groups:
1) Icelandic eruptions
2) Northern hemisphere eruptions north of ca.
50° North.
3) Eruptions south of ca. 50° north.
If acidity profiles from Antarctic deep cores
are available, there is the further possibility, that
we can distinguish between the eruptions south of
ca. 50° north: If a high signal occurs within the
same time period (a few years) in both the Green-
land and Antarctic profiles, the eruption most
probably took place in the equatorial region. In
fact, there is such a candidate i.e. the 1259 event,
which recentiy has been identified in both the
South Pole and the Byrd core (not published; but
dating and assessment in progress, Clausen and
Hammer, in preparation)
With these remarks I have deviated somewhat
from Icelandic eruptions, so Iet me return to the
individual acid signals of an Icelandic origin.
SOME ICELANDIC ERUPTIONS
The Lakagígar eruption
This famous Icelandic fissure eruption started
at June 8th, 1783 and has been described in detail
by Thorarinsson (1969). The eruption was mainly
a lava eruption and is estimated to have produced
a total of 12.5 km3 material — measured in lava
equivalents. It left the Greenland Ice Sheet pre-
cipitation of 1783 with strongly elevated electrical
conductivity mainly caused by high concentra-
tions of H2S04, Hammer 1977). While this ice
core information on the eruption was obtained by
analyzing melted samples, the data presented
below resulted from electrical measurements on
the solid ice (Hammer 1980b). The electrical
current being a function of the concentration of
strong acids, independent of the anions. The
Lakagígar signal is shown in Fig. 3 for 3 different
ice sheet locations. It is the highest yearly aver-
age acid signal over the entire Dye 3 core.
JÖKULL 34. ÁR 57