Jökull - 01.12.1979, Blaðsíða 38

Wind erosion
One of the things that surprises foreign geologists
visiting Iceland is the great role played by wind
erosion in this oceanic island with its humid cold-
temperate climate. It is estimated that since the
beginning of settlement at least 20.000 km2, or
nearly 50% of the area then vegetated, has since
been deprived of its soil cover by wind and water.
Birch woods covered at least 20.000 km2 eleven
centuries ago, nów only about 1.200 km2 remain.
Opinions have differed as to the main cause of the
destruction of the birch woods and the soil erosion
by wind and water. Some students of the problem
have put the blame. mainly on deteriorating
climate and volcanic activity. Others have main-
tained that man and his grazing livestock are the
main cause. With the aid of dated tephra layers it
has been possible to measure the rate of thickening
of the soil cover and this rate is an indicator of the
soil erosion going on, as the windborne material is
partly deposited on the vegetation covered areas
and bound by the vegetation. The diagram in Fig.
4 is a typical one. It is based on a soil profile from
which have been extracted the tephra layers which
are not part of the thickening of the loessial soil as
such. Up to a certain point on the diagram the rate
of thickening is rather even, although increasing
somewhat with the climatic deterioration at the
beginning of Subboreal time about 500 B. C. But a
great increase comes shortly after the arrival of man
and his livestock to this isolated country which
before that time had no grazing mammals.
Tephrochronological correlation
Because of the limited size of Iceland much of the
tephra produced there is deposited on the ocean
surrounding the country. Ocean currents
occasionally carry Icelandic tephra to distant
shores. Airborne tephras from Iceland have been
identified in peat soils in Scandinavia, among them
the light Hekla layers H3 and H4, Cl4-age respec-
tively about 4000 and about 2800 years. Measuring
of the O16/18 ratios has enabled dating of cores from
the Greenland ice more than 100.000 years back-
wards in time and for the last one and a half mil-
lenium the dating is exact almost to the year. Ice
layers of high acidity, due to the fallout of aerosols
from large eruptions in the past, can be detected
by measuring the electric conductivity of melted
samples. One of the first eruptions so detected was
the Lakagigar eruption of 1783. Among other Ice-
landic eruptions detected is a very big eruption in
934 A. D., which almost certainly is the eruption
that formed the great chasm Eldgjá. Tephrochro-
nological teleconnections have now become global
and enable exact datings reaching backwards in
time far beyond written records in many volcanic
areas.
SELECTED REFERENCES
Friedman, J. D., C. E. Johansson, N. Oskarsson, H.
Svensson, S. Thorarinsson and R. S. Williams,
1971: Observations on Icelandic polygon sur-
faces and palsa areas. Photo interpretation and
field studies. Geogr. Ann. Stockh. 53. Ser. A:
115—145.
Hammer, C. U., 1979: Acidity of polar ice cores in
relation to absolute dating, past volcanism and
radioechoes. Journ. Glaciol. (in press).
Persson, Chr., 1966: Försök till tefrokronologisk
datering av nágra svenska torvmossar.
(Attempt at tephrochronological datings of
some Swedish peat bogs). Geol. Fören. Stockh.
Förh. 88: 361 — 394.
Steinthorsson, S., 1977: Tephra layers in a drill core
from the Vatnajökull ice cap. Jökull 27: 2—27.
Thorarinsson, S., 1944: Tefrokronologiska studier
pá Island. (Tephrochronological studies in Ice-
land). Geogr. Ann. Stockh. 26: 1 —127.
Thorarinsson, S., 1962: L’érosion éolienne en
Islande a la lumiere des études tephrochrono-
logiques. Rev. Géomorphol. Dynamique 13:
107 — 134.
Thorarinsson, S., 1967: The eruptions of Hekla in
historical times. The eruption of Hekla 1947 —
1948 I. Soc. Sci. Islandica: 170 pp + 13 Pl.
Thorarinsson, S., 1970: Tephrochronology and
medieval Iceland. In Beyer, R. (Ed.): Scientific
Methods in Medieval Archaeology: 295 — 328.
Univ. of California Press.
Westgate, J. A. and C. M. Gold (Eds.), 1974: World
bibliography and index of Quaternary teph-
rochronology: 528 pp. Printing Service Depart-
ment. The University of Alberta.
36 JÖKULL 29. ÁR