Jökull - 01.12.1984, Blaðsíða 54

volcanic regions is therefore only natural, when
considering how far from eruption sites visible
tephra layers can be traced. For example, the
“Greetings from Iceland” travelled a long way,
Thorarinsson (1981a). The many tephra layers
encountered in peatbogs, soils or sea sediments
around the globe often serve as useful reference
horizons and contribute evidence in describing
the character of the individual eruptions.
The tephra studies of Sigurður Thorarinsson
are well known as well as the many applications
of such studies (see e. g. Thorarinsson (1981 b).
A disadvantage of extending the studies to the
polar ice sheets is the narrowing of the applica-
tions, because the ice sheets “per se” offer no
possibilities for archaeological studies, or soil
erosion studies etc. unless some adventurous
eskimo or viking did get lost on the Greenland
Ice Sheet.
What kind of volcanic material do we find in
the Greenland Ice Sheet? It is strong acids associ-
ated with the acid gaseous products of volcanic
eruptions and not fine grained tephra (Hammer
1977). This does not mean that fine tephra is
absent in the ice, but it is not abundant and
therefore extremely difficult and time consuming
to search for it in a systematic way. Why is this
so?
TEPHRA DEPOSITION
ON ICE SHEETS
During the last decade the subject of long
range transported fine tephra deposition on the
Greenland and Antarctic ice sheets has been
controversial. Even though volcanic impurities in
the Greenland Ice Sheet are mainly water soluble
strong electrolytes, especially strong acids (Ham-
mer et al. 1980 a), I will discuss the tephra deposi-
tion on the two large ice sheets, because it may be
relevant for future studies as well as throw some
light on the volcanic production and the
long-range transport of the most fine grained
material.
In this paper I will consider fine tephra to
consist of solid particles less than approx. 2-5
|im in diameter. This definition has the advan-
tage, that it coincides with the upper limit of
particle sizes in the Greenland Ice Sheet; which
can be defined as the size range, where the mass
per unit of particle radius drops drastically. In
other words the bulk of the particle mass consists
of particles having radii less than 2-5|xm.
Such a definition also agrees, almost “a priori”,
with the upper limit of the Junge particle distribu-
tion (see e. g. Junge 1963, p. 118) in the mid-tro-
pospheric air - at least over the latitude zone in
question. A violent volcanic eruption in e. g.
Iceland will of course produce a lot of larger
particle sizes, but one should keep in mind the
strong influence of the weather system, which
acts to keep the long range transported tephra
particles within the above stated size range. The
above definition has close ties to what is observed
in the ice.
How much fine grained tephra is actually pro-
duced in eruptions? Very little is known about it,
as this fine tephra is almost by definition so fine,
that it cannot be traced in peat bogs etc. (at least
not without laborious work and much difficulty).
— In fact the clean ice sheets of Greenland and
Antarctica are the natural places to search for it!
No visible tephra layers have been observed in
the Greenland ice cores (one visible layer, consis-
ting of atmospherically transported particles, was
observed in the Dye 3 core, see later). Some 10—
100 mg of ash or continental dust per kg of ice is
needed in order to give a coloring of the ice core;
somewhat depending on the size distribution of
the particles. More quantitative techniques have
been used in order to measure the micro-particle
concentrations in the ice. Even, when using Coul-
ter and light scattering technique, (Hammer
1977) no ice layers of higher than average particle
concentration could be safely reconciled with vol-
canic eruptions. Using a single particle counting
laser technique (counting particles down to
0.05pm over the ice covering acid fallout from the
1815 Tambora eruption and the 1783 Lakagígar
eruption), did not reveal any increase in particle
concentration over the relevant years (Hammer,
unpublished).
The average dust concentration over the
Holocene ice is some 50—100 pg/kg and it seems
surprising, that no eruptions in North America or
Iceland contributed significantly to the annual
particle concentrations in e. g. the Créte and the
Dye 3 core. (The Créte core is from Central
Greenland and reaches 1400 years back in time,
while the Dye 3 core in South Greenland covers
more than 50,000 continuous years).
In Antarctica tephra layers have been observed
e. g. in the Byrd core (Gow and Williamson
52 JÖKULL 34. ÁR