Jökull - 01.12.1985, Síða 4

ably close. Discrepancies in estimates which occur for
some smaller eruptions must be viewed in the context of
many contributing potential errors, such as inadequate
modelling of global fallout from ice-core data, under-
estimates of erupted mass of magma, compositional
variation during eruption, presence of sulfur and
halogen-bearing mineral or iiquid phases in magmas,
and other factors, all of which require further study.
Recently we have progressed from viewing the vol-
canic aerosols as „dust“ composed mostly of fine silicate
ash (Axelrod 1981) to the realization that it is largely
sulfuric acid (Castleman etal. 1974). The new petrologic
data (Devine et al. 1984) and re-examination of the ice-
core evidence has resulted in the further realization that
sulfur is only one of the major volatile elements and that
chlorine and possibly fluorine compounds may be
equally abundant in certain events, although they may
not undergo gas-to-particle conversion and thus may
not have the same residence time and climatic effect as
H2S04. The geological record, as read in petrologic
studies of glass inclusions, shows that the composition
of released volcanic volatiles is highly variable from one
eruption to another, and is largely dependent on the
geochemistry of magma in the specific source volcano.
In this paper we discuss petrologic estimates of released
volcanic volatile mass from several recent eruptions and
compare these estimates with direct measurements of
erupted aerosol mass, where available, as well as ice-
core data.
TAMBORA 1815
The great 1815 Tambora eruption in Indonesia pro-
duced phonolitic tephra, estimated between 87.5 and 90
km3 dense-rock equivalent (Rampino and Self 1982
Stothers 1984). No new field studies of the distal tephra
fall deposit have been carried out since 1929 (Neeb
1943) and the current volume estimate is very poorly
constrained. The aerosols from the eruption produced
widespread optical and meteorological phenomena,
causing a mean Northern Hemisphere tropospheric
temperature decrease of 0.4 to 0.7°C (Stothers 1984).
From analysis of sulfur, chlorine and fluorine in glass
inclusions in phenocrysts and matrix glass of 1815 Tam-
bora tephra, we have estimated (Table 1) a total vol-
canic volatile yield of about 4xl014 g acids from this
eruption. By comparison, Hammer et al. (1980) esti-
mated a total of 1.5xl014 g acids (H2S04+HX) on basis
of fallout on Greenland ice-sheet. This estimate has
been increased to about 2xl014 g total acids by recon-
sideration of the ice-core background value (Stothers
1984). The difference between the petrologic estimate
of volatile mass and ice-core based estimate of the
Tambora aerosol is thus about a factor of two, which is a
good agreement, considering the uncertainties
involved, particularly in estimation of the mass of total
erupted tephra.
Rampino and Self (1982) claim that our petrologic
estimates of total sulfur yield from Tambora 1815 and
other eruptions are underestimates when compared to
the ice-core data. This claim is unfounded and is
apparently based on the presumption that the ice-core
acidity is due solely to H2S04 aerosol, whereas in fact,
the mass of acids in the ice core layer calculated by
Hammer et al. (1980) is presented as (H2S04+HX). The
importance of the other acids (HX) is clearly demon-
strated in the example of Tambora (Table 1) and in
other cases discussed below.
The most unexpected aspect of the petrologic esti-
mate for the Tambora volcanic volatile is that the
atmospheric yield of both chlorine and fluorine was
higher than that of sulfur. Detailed chemical analyses of
the Tambora acidity layer in well dated Greenland ice-
cores are needed to corroborate this finding. Very high
concentrations of C1 are indeed present in the early 19th
century level of an undated ice-core from north-west
Greenland, where C1 is up to 830 mg/g, compared to the
10 mg/g background level (Herron 1982) and may be
due to high-latitude fallout of the volcanic aerosol from
the Tambora eruption.
LAKI 1783
The well-known Laki basaltic fissure eruption in 1783
in Iceland was one of the major volcanic-aerosol-pro-
ducing events of the historical period (Sigurdsson
1982a). The magma was predominantly erupted to form
lava flows and hence the total erupted mass is well
known, unlike in the case of most explosive eruptions,
where tephra is very widely dispersed and the volume
poorly known. The petrologic estimate of the volcanic
volatile is 9.19xl013 g total acids (Devine et al. 1984)
consisting predominantly of H2S04 (98%), with minor
HCl (2%). By comparison, Hammer et al. (1980) esti-
mated 10xl013 g global total acid fallout (H2S04+HX)
from the Laki eruption on basis of acidity in a Green-
land ice-core (71°N). They furthermore estimated that
the acidity layer consisted of approximately 70% and
60% S02'4 in the Crete (Hammer 1980) and Dye 3
(.Hammer 1977) ice-cores, respectively, the remainder
being Cl". More recently, however, Herron (1982) has
shown, by direct chemical analysis of the acid layer from
Laki in the Milcent core that about 95% is accounted
for by S02‘4 and only 5% by Cl', or very close to the
proportions predicted by our petrologic estimate.
Another important feature of the ice-core data on
2 JÖKULL 35. ÁR