Jökull - 01.12.1979, Blaðsíða 46

6 On climatic changes in Iceland
LEIFUR A. SlMONARSON
Science Institute, University of Iceland, Reykjavík
The fossil record of Iceland goes back as far as its
oldest rocks which have been radiometrically dated
at about 16 m.y. The lava pile of Iceland formed
almost exclusively above sea level and as a result
the fossil bearing strata are primarily of terrestrial
origin. A number of sedimentary horizons con-
taining identifiable plant remains occur inter-
stratified with the lavas. Studies of their fossil floras'
were initiated more than a hundred years ago and a
Miocene age was correctly assigned to the oldest of
them. This was later rejected in favour of an Eocene
age and that view persisted into the nineteen-
sixties. Radiometric dating of the lavas which came
up in the mid-sixties made it clear that the floras
could be no older than Miocene, as originally pro-
posed by Oswald Heer. The global cooling that has
occurred on the earth since the Miocene is well
documented in the record of fossil floras, owing to
the regular spacing apart of plant bearing horizons
within the lava pile.
Marine fossiliferous strata occur in a few places,
but the age range — Pliocene up to Holocene — is
much shorter than with the fossil floras. Singular
amongst the marine deposits are the Tjörnes beds
which contain fossil faunal assemblages represent-
ing the entire Pliocene epoch. The Pleistocene
faunal record is much less coherent.
Holocene climatic trends have been recon-
structed on the basis of both marine fossil evidence
and on palynological studies of bog profiles where
radiocarbon dating and tephrochronology are in-
dispensible aids.
Miocene — Pliocene climate
Palaeobotanical studies of Tertiary interbeds in
Iceland indicate more or less symmetrical horizons
on both sides of the Quaternary formations. The
oldest horizon, older than 14 m.y., is found along
the northwest coast and comprises the Selárdalur
flora (for location see Fig. 1). It consists of a mixed
warm-temperate forest of conifers and deciduous
trees with Sequoia, Pinus,Juglans, A Inus, Fagus, Ulmus,
Tilia and Vitis. Taxodiaceae dominate over
Pinaceae and angiosperms over conifers. The well-
known Brjánslækur flora belongs to the next hori-
zon, about 14—13 m.y. The most thermophilic
taxa, as Vitis, have disappeared, whereas Abies,
Sequoia, Comptonia, Betula prisca Ettingshausen, Acer,
Alnus, Magnolia and Sassafras are prominent.
Pinaceae dominate over Taxodiaceae. From 13 —
10 m.y. Pinaceae increased and conifers came to
dominate over angiosperms. This is probably the
oldest horizon in East Iceland (Gerpir). In the next
horizon, about 10—9 m.y., comprising the Húsa-
víkurkleif and Tröllatunga floras in West Iceland
and the Hólmatindur flora in East Iceland, Poly-
podiaceae, Osmunda, Salix, Juglans, Betula, Acer,
Magnolia and Carya predominate. From 9—8 m.y.
Alnus, Betula, Acer, Pterocarya, Fagus and Corylus are
the most prominent taxa known from Mókolls-
dalur in Northwest Iceland.
Icelandic Tertiary floras older than 8 m.y. are
warm-temperate and show close affinity with the
retent flora in the Eastern Deciduous Forests of
North America. The oldest horizons indicate
annual mean temperature higher than 10°C and
frosts were probably rare. Apparently the
precipitation was more or less constant throughout
the year. A slight cooling may be responsible for the
disappearance of Vitis 14 m.y. ago and Magnolia 9
m.y. ago.
The temperate Hredavatn flora in West Iceland
is about 7 m.y. old and Fagus and other warm-
temperate indicators are rare or absent, whereas
Betula, Salix and conifers are prominent.
Apparently the climate grew cooler during the
Upper Miocene.
The cooling trend continued in the Pliocene and
from 6—3 m.y. Betula and Salix shrubs and grasses
became more and more common when the forest
declined, as indicated by the Sleggjulækur flora in
West Iceland and the Pliocene Tjörnes flora in
44 JÖKULL 29. ÁR