Jökull - 01.12.1984, Blaðsíða 53

Traces of Icelandic Eruptions in the Greenland Ice Sheet
C.U. HAMMER
Geophysical Isotope Laboratory, Haraldsgade 6
University of Copenhagen, 2200 Copenhagen N, Denmark.
ABSTRACT
The information in polar ice sheets on long
range transported volcanic debris is discussed.
Acidity data from the Greenland Ice Sheet are
presented, which indicate a possible relation
between the volcanic fall-out pattern on Greenland
from a past eruption and the latitude zone of the
eruption site.
A special presentation of the Icelandic eruptions
Lakagígar, 1783 A.D., Katla 1179 A.D., Eldgjá
934 A.D., Hekla I, III, IV and V is given.
Finally the potential of future volcanological
research “via" the polar ice sheets is discussed.
INTRODUCTION
The importance of the polar ice sheets in
research on past atmospheric composition stems
from the fact that they consist of well layered
frozen past precipitations. These individual pre-
cipitations are generally formed between the ice
sheet surface and 1—2 km above it, which for the
more central parts of the Greenland Ice Sheet
corresponds to some 2.5—5.5 km above sea level.
The special importance of the two major ice
sheets — the Greenland Ice Sheet and the
Antarctic Ice Sheet - relates to their vast extent,
high surface elevations and remoteness from
most aerosol sources. Not only does this secure a
certain uniform mixing ratio of atmospheric trace
substances, at levels where the precipitation
forms, but it also keeps the impurity concentra-
tion in the snow very low.
The polar ice sheets are therefore unique “lib-
raries” of past mid-tropospheric aerosol loads
over a very broad latitudinal zone. The stratos-
pheric and upper tropospheric impurities will add
to the mid-tropospheric impurity load and in this
way part of the upper atmospheric impurities is
deposited on the ice sheets.
As volcanic eruptions inject substantial
amounts of trace substances and gases into the
troposphere and in many cases also into the
stratosphere, it is not surprising that the ice
sheets have proven to offer information on the
amount and nature of long-range atmospherical
transported volcanic products.
There are of course differences in this kind of
information, which will depend on the site of
information i. e. usually from a drill site on the
ice sheet. The major differences between the
Greenland Ice Sheet and Antarctica are for inst-
ance:
1) Antarctica reveals mainly eruptions in the
southern hemisphere, while Greenland offers
information on northern hemisphere erup-
tions.
2) Due to a greater precipitation in Greenland,
as compared to Antarctica, the Greenland Ice
Sheet can be dated to a higher accuracy by
various stratigraphical methods; at least for
the past 10,000 years.
3) During the Ice Age the aerosol load increased
over both ice sheets, but while the Antarctic
snow remained fairly clean and slightly acidic,
the Greenland snow became alkaline and
showed rather high impurity concentrations,
as compared to present values. Loess and dust
from the continental shelves added strongly to
the mid-tropospheric aerosol load in the
northern hemisphere.
Such large differences are not encountered, if
we confine us to e. g. the Greenland Ice Sheet
and the Holocene period: Then only moderate
differences exist from site to site, but as will be
seen later such differences offer additional
information on the volcanic eruptions.
The possibility of extending tephra-chronology
to the large ice sheets or ice caps remote from
JÖKULL 34. ÁR 51