Jökull - 01.12.1984, Page 56

Taupo eruption took place in New Zealand? — In
the northwest Greenland Camp Century deep
core no visible layer was observed (This core
suffers from severe thermal shock, due to drill-
ing, at these depths and acidities cannot be mea-
sured, but a visible layer would anyway be seen if
present). There are other complications: The visi-
ble banding, which contains the largest particles
(aggregates of some 100 pm) concludes the
period of high dust level, which is peculiar if the
layer derives from a volcanic eruption, as one
would expect the largest particles and highest
concentrations to occur in the beginning of the
period due to gravitational settling. Indication of
melting was present in the visible layer, which
may explain the presence of large aggregates.
The microprobe data do not lend support to
the idea, that the layer contains large amounts of
cosmic dust. The layer could well be interpreted
as being due to continental dust as suggested by
Froggatt (1981, p. 491), but the composition
could not arise from e.g. forest or bush fires
(Froggatt 1981, p. 491).
Was the layer due to unusual meteorological
conditions in 174—175 A.D. or due to an indirect
effect of a cosmic body?
The phenomenon stirs the imagination, even if
it in the end proves to be, what it looks iike i.e. a
statistical anomaly in the northern hemisphere
environmental and meteorological system. That
the ice layer is dated to within the time period of
emperor Ling Ti’s reign suggests, that the corres-
ponding high dust concentration in the air was
not just a Greenland phenomenon, but for the
moment we do not know what caused it and how
widespread it was.
HIGH ACIDITY LAYERS IN THE DYE
3 CORE AND SINGLE ERUPTIONS
In Fig. 1 is shown the acidity along the Dye 3
deep core. As the data treatment of the Dye 3
acidity has not yet been finished the average
acidity over Holocene has been given as approx.
1,2 pequiv. H+/kg of ice. The pre-Holocene ice
(below 1786 m from surface) is in general slightly
alkaline, which prevents detection of volcanic
eruptions by the acidity method.
The volcanic acidity signals, shown as peaks on
the figure, are for practical reasons exaggerated
with respect to the depth interval they cover. The
peaks give the average acidity over the year of
maximum acid concentration from a particular
large eruption. Only values larger than approx. 4
pequiv. H+/kg (preliminary estimate) have been
shown in order to ensure, that the values are well
above the natural non-volcanic acid background.
The background is of course not constant, as
shown in the figure, but its variation is well below
the 4 pequiv. H+/kg.
Summer melting or high temperatures in the
Dye 3 region during a few days of summer (some
+ 5°C) can cause high acidities, which are not
related to volcanism. Usually the melting and
warm temperatures only influence the acidity
over some 10—20% of an annual layer and the
yearly average acidity will still be well below 4
pequiv. H+/kg.
In a few cases the ice core contained strong
percolation paths i.e. refrozen meltwater along
an irregular vertical path, which by chance was
confined within the core. The percolated meltwa-
ter usually refreezes, when it reaches the pre-
vious denser and colder winter snow, and is seen
as percolations in the core more or less like a
meltlayer. The presence of percolation paths in
the ice core is easily seen, when the annual layers
are not too thin. Only 3 such paths, which in fact
showed high acidities, are left out of the figure.
There is a chance, that a large eruption has
marked the ice with high acidity in the annual
layer where such a percolation path is present,
but it is not likely to occur, because in Fig. 1,20
peaks are shown over a period of approx. 10,000
years and only 3 high acidity peaks had to be left
out. The peaks shown in Fig. 1 are therefore,
most probably, all due to volcanic eruptions.
The Dye 3 acidity profile is ideal with respect
to continuity and details along the core. The
entire core was measured in the field, mm by
mm, and a core recovery better than 99.9%
secures, that no strong acidity layers were lost
e.g. due to missing ice.
The Dye 3 location, or for that matter South
Greenland, is not the best place to detect acid
fallout from volcanic eruptions, as fission product
profiles over the period 1953-1965 show almost
half the concentrations of more northerly loca-
tions, while the acid non-volcanic background is
nearly the same as e.g. in North- and Central
Greenland. This is due to a generally higher
precipitation at South Greenland latitudes (over
the entire hemisphere) combined with a stronger
54 JÖKULL 34. ÁR