Jökull - 01.12.1984, Side 55

1971), and they are evidently due to Antarctic
volcanoes (Kyle and Jezek 1978). Why is this so
and why are such tephra layers not present in
Greenland ice cores? — There seem to be only
two logical explanations of this fact:
1) The many volcanic eruptions during the
Holocene did not produce sufficiently large
amounts of fine tephra to add significantly to
the Greenland dust concentrations.
2) Greenland is too far “down-wind” from
“close” eruption sites as compared to Antarc-
tica.
The geographical position of Antarctica at
higher latitudes than the southern polar frontal
system and a rather circumpolar movement of the
debris laden airmasses secures a much higher
chance for fine tephra of “local” origin to remain
in the dry Antarctic air.
With respect to fine tephra from distant erup-
tions the only major difference between Green-
land and Antarctica, (to judge from fission-pro-
duced radioactive material from the nuclear bomb
tests during the years 1953-1965, especially
Sr90), is a 2-3 times higher concentration in East
Antarctica due to the low level of the yearly
precipitation, (H.B. Clausen personal com-
munication, 1983). The absence of visible ash
layers in the Greenland Ice Sheet must therefore
be due to both point 1 and 2.
If tephra is taken as a general term for fine
volcanic debris, it would include the acid volcanic
gases; in fact I can quote Sigurður Thorarinsson
(1981, p. 133) “Tephrochronological connections
have through the work on Greenland ice cores,
become global. . .” Using the word tephra in this
sense, acid tephra layers can be detected in the
ice sheets.
Before discussing the acid volcanic layers in the
Greenland Ice Sheet a few remarks on the only
visible dust layer of atmospheric “origin”,
observed in Greenland ice cores would be in
place.
A VISIBLE DUST LAYER
IN THE DYE 3 CORE
A visible yellow to brown banded layering, a
few cm thick was observed in the Dye 3 deep
core. It was preceded (in time) by a 30 cm annual
ice layer (from summer to summer) of more than
50 times the average Holocene dust concentra-
tion. Other visible layers exist in the core, but
they are near to the bedrock and consist of bot-
tom derived material.
The layer was encountered at 751,51 meters
depth and is a statistical anomaly, when com-
pared to Holocene dust concentrations in Green-
land ice cores. The layer is indicated in Fig. 1 (left
side).
The seasonal ð(lsO) variation over the year of
high dust deposition showed no unusual varia-
tion, which suggests, that no unusual
meteorological pattern, at least in the Greenland
region, was the cause of the high dust concentra-
tion. The electrical conductivity was below nor-
mal and pH measurements proved the layer to be
neutral or even slightly alkaline: The latter being
unique for Holocene ice, at least over an entire
annual ice layer.
An inspection of the dust by SEM, TEM and
microprobe techniques revealed layered as well
as unlayered microcrystalline material of high Si
content. Uncalibrated microprobe data give the
following area percentage of the Ka spectrum,
accelerator voltage 21—26 kV:
Si:69.3 Al:1.9 K:5.8 Fe:19.2 Ca:0.6
If the layer is a tephra layer it deserves some
attention, because it is dated to 174—175 A.D.,
which is not too far from the 14C date 130 A.D.
for the New Zealand Taupo eruption which has
been suggested as the cause of unusual atmos-
pheric optical phenomena during the reign of the
Chinese emperor Ling Ti — A.D. 168—189 (Wil-
son et al. 1980). Wilson et al. quote the Chinese
annals: “During the reign of Ling Ti several times
the Sun rose in the east red as blood and lacking
light, only when it had risen to an elevation of
more than two zhang was there any brightness.
When it set in the west, at two zhang above the
horizon it was similarly red. — Also during this
period, several times when the Moon rose and set
and was two to three zhang above the horizon, all
was red as blood”. (A zhang is approximately
equal to 12°)
The low conductivity of the debris layer could
be due to the special eruptive type of Taupo, but
there are complications in interpreting the layer
as volcanic! — Why is there no similar visible
layer in the Antarctic Byrd core; after all the
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