Náttúrufræðingurinn - 2012, Side 24

Náttúrufræðingurinn
24
Summary
Fossils and sediments in Iceland
The paper presents an overview of fos-
sils and sedimentary horizons in the
Icelandic succession of volcanic rocks.
The geological history of Iceland is char-
acterized by volcanic processes includ-
ing eruptions of lava and tephra. It is
generally assumed that all rocks above
the present sea level in Iceland have
been formed after Iceland became a sep-
arate island in the North Atlantic follow-
ing the extinction of the Aegir Ridge and
initiation of rifting volcanism along the
currently active Kolbeinsey and Reykja-
nes Ridges at c. 27 Ma. Studies of lava
sequences in Iceland have revealed that
a given site in a zone of accumulation
may be expected to receive a lava flow
every 5–10 thousand years. This pro-
vides ample time for the development of
vegetation and soil, as well as deposi-
tion of sediments in basins associated
with tectonic rifting or denudation.
Numerous horizons of plant fossils pre-
served as leaf impressions in reddish
sediments interpreted as soils or as both
microfossils and macrofossils in more
local lacustrine deposits. Locally, the ter-
tiary plant remains occur as lignite
seams, containing tree trunks and
branches, including the genera Pinus,
Sequoia, and Glyptostrobus, but decidu-
ous genera are also preserved. In many
cases, the lava flows have overrun for-
ests, and molds and casts of trees have
been preserved in the lavas. One of the
oldest plant horizons in Iceland at
Selárdalur and Botn in Vestfirðir dates
from c. 15 Ma and contains evidence of
a warm-temperate climate with a mixed
coniferous and deciduous forest.
Successively younger plant horizons to-
wards the presently active volcanic
zones show changes in the plant assem-
blages reflecting a cooling trend from
warm-temperate to temperate climate.
The affinity of the flora is distinctly
North-American before 10 Ma with an-
nual mean air temperature close to 15°C.
Late Miocene cooling is evidenced, be-
coming severe towards the end of the
Pliocene with the first regional glacia-
tion of Iceland at 2.5–2.6 Ma. The
Quaternary fossil flora in Iceland has
mainly been preserved in interglacial
lake deposits, e.g. in Bakkabrúnir,
Víðidalur, North Iceland (about 2 Ma),
in Stöð, Snæfellsnes, West Iceland (about
2 Ma), and in Svínafell, South Iceland
(about 0.7 Ma). The most common tree
genera are Betula, Salix, and Alnus.
Terrestrial animal fossils are rare in
Icelandic sediments, but include sponges,
insects and freshwater molluscs. The
only known terrestrial mammal find is
from Vopnafjörður, Northeast Iceland,
where bones of a young deer have been
found (3–3.5 Ma). Interglacial deposits in
Elliðavogur contain fresh water crusta-
ceans and insects. These deposits also
have preserved bird footprints. Polar
beer teeth have been found in Lateglacial
deposits in North Iceland, and eiderduck
bones from the Lateglacial have been
found in Melabakkar, West Iceland.
The marine Pliocene and Quaternary
deposits on Tjörnes, North Iceland, con-
tain a wealth of marine fossils. Mollusc
assemblages form the basis of biostrati-
graphic zonation of the Tjörnes beds.
The lowest biozone, the Tapes Zone, con-
tains extinct species, and the fauna as
well as oxygen isotope data indicates sea
temperature about or above 15°C. The
second biozone (Mactra Zone) indicates
temperature fluctuations, falling to 7–8°C
towards the top, but the uppermost,
Serripes Zone, shows a general cooling
trend down to about 12°C towards the
top. A dramatic increase of Pacific mol-
luscs is observed at the base of the
Serripes Zone, coeval with oceanographic
changes in the Pacific and the closing of
the Panama isthmus. The Pliocene de-
posits also contain marine microfossils,
such as ostracods and foraminifera, as
well as dinoflagellates and pollen.
Terrestrial lignite horizons in the se-
quence contain plant fossils. Quaternary
interglacial and deglacial deposits on
Tjörnes also contain marine molluscs
and foraminifera showing environmen-
tal changes consistent with the lithologi-
cal evidence of glacial-interglacial cyclic-
ity. Other Tertiary and early Quaternary
localities containing marine fossils in
Iceland include Mýrdalur in South
Iceland and Snæfellsnes, West Iceland.
Lateglacial and Holocene fossilifer-
ous sediments in Iceland are abundant
and generally associated with sea-level
changes due to isostatic adjustments to
ice cover and eustatic changes due to
variable terrestrial ice volumes. In addi-
tion to traditional lateglacial and early
Holocene localities, increasing effort
has been directed to the study of lake
deposits and shelf sediments, applying
tephrochronology to the dating of high
resolution climate archives. Microfossils
such as diatoms, chironomids, coccol-
iths, dinoflagellates and foraminifera
play an important part of these recent
and ongoing studies.
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