Náttúrufræðingurinn - 2007, Blaðsíða 36

Náttúrufræðingurinn
gerir í dag. Núverandi útbreiðsla
virðist tilkomin eftir að síðasta jök-
ulskeiði ísaldar lauk.
SUMMARY
Late Langhian to early Serravallian
floras of Iceland
The oldest Miocene floras known today
are found on the Northwestern
Peninsula, in the 15 Ma (late Langhian)
sediments on Mount Þórishlíðarfjall, Sel-
árdalur, and in the Botn sediments, Súg-
andafjörður. The sediments are referred
to as the Selárdalur-Botn Formation. The
second oldest flora is found in the 13.5
Ma (early Serravallian) sediments on
Mount Tafla close to the Ketilseyri farm
and in Lambadalur, referred to as the
Dufansdalur-Ketilseyri Formation.
The most characteristic feature of the
15 Ma Selárdalur flora is the dominance
of Fagus (> 90% of macrofossils).
Components of the Selárdalur flora are
typical representatives of hardwood for-
ests with a humid warm temperate app-
earance as found today in eastem North
America (Appalachians), western
Eurasia (northem and eastem Black Sea,
southem Caspian Sea), and East Asia
(Japan, central and eastern China).
Typical taxa are, among others, Tilia and
Aesculus. The Selárdalur flora represents
broadleaved deciduous and evergreen
(beech) forests found mainly on well-
drained slopes. Only few taxa have been
recorded from the Botn sediments. The
most prominent ones are Glyptostrobus
and Sequoia, which are represented by
vegetative and fruiting twigs, whereas
Fagus is mostly represented there by
cupules and nuts and only very few frag-
mentary leaves. It is likely that
Glyptostrobus and partly Sequoia where el-
ements of floodplains adjacent to lakes.
While Glyptostrobus tolerates high
ground water table, Sequoia may have
grown in slightly more elevated areas
(hummocks), intermixed with some
other hardwood taxa, such as Fagus. This
lowland type of vegetation is likely to
have merged into a hardwood forest sim-
ilar to the one known from Selárdalur.
The pollen and macrofossil data from
the Selárdalur-Botn Formation (15 Ma)
give the impression of a broadleaved
deciduous and evergreen forest with an
admixture of conifers that covered
mountain slopes and canyons. These for-
ests were dominated by Fagus friedrichii,
Tilia selardalense, Aesculus sp., Ulmus sp.,
Cercidipyllum sp., Platanus leucophylla,
Magnolia sp., Rhododendron sp., and Lon-
icera sp. At the bottoms of valleys elem-
ents such as Alnus sp. and Salix sp.
became more prominent as the ground-
water level rose. In areas with high
groundwater table, in valleys, around
lakes and rivers, on floodplains, and in
delta regions, conifers, mostly Glypto-
strobus europaeus, and Alnus sp., dom-
inated the floral assemblages. On humm-
ocks, alluvial plains and well-drained
lowland sites Sequoia abietina may have
been more prominent. The pollen and
macrofossil data from the Dufansdalur-
Ketilseyri Formation (13.5 Ma) do not
indicate significant changes in the broad-
leaved forests except for the sudden decr-
ease of Tilia type pollen that was so con-
spicuous in the older (15 Ma) formation.
A more prominent shift is seen in the
conifers where the amount of Taxodi-
aceae pollen decreases considerably and
Picea pollen become prominent.
Most of the taxa recognized from the
15 Ma formation have diaspores that are
dispersed over short distances by wind
(Coniferales, Cercidiphyllum, Platanus,
Ulmus), while only few may be tran-
sported over long distances (Betula and
Rhododendron). Diaspores of the remain-
ing taxa are dispersed by animals over
short distances (Fagus and Aesculus by
mammals), or long distances in various
ways (Magnolia and Lonicera by birds,
endozoochory; Platanus by mammals or
birds, exozoochory). Terrestrial mam-
mals from Icelandic sediments are
extremely rare and only a few bones
belonging to a small deer have been
recovered from Pliocene sediments
(3.5-3.0 Ma). Plant fossils indicate the
presence of terrestrial vertebrates long
before that time and it seems that the lack
of terrestrial vertebrate fossils is mainly
due to the unfavourable conditions for
calcareous bones to be preserved.
The dispersal mechanisms encounter-
ed in late Langhian (15 Ma) plants from
Iceland strongly indicate that colon-
ization took place via land or over short
seaways. At least Fagus and Aesculus
have no potential for long-distance
dispersal and taxa such as UIiuus, Fraxin-
us, and Tilia have a restricted dispersal
radius. Only Betula and Rhododendron
would have had the possibility to reach
Iceland crossing extensive water masses.
This indicates the presence of an almost
continuous land connection to either
Greenland/North America or the Faeroe
Islands/Europe when these taxa colon-
ized proto-Iceland. Most of the elements
of the early floras from Iceland do not
indicate a particular source area (North
America/Greenland versus Faeroe Is-
lands/Europe), but belong to a widespr-
ead Northem Flemispheric element (all
taxodiaceous taxa, Cercidiphyllum,
Magnolia, Platanus, Ulmus, Lonicera,
Rhododendron) and could have reached
proto-Iceland both from the east and
west. Interestingly, a considerable num-
ber of higher taxa (Glyptostrobus, Cercidip-
hyllum, Aesculus, Platanus, Ulmus,
Magnolia etc.) were also part of the
Brito-Arctic Igneous Province (BIP) flo-
ras, although these floras are at least 20
million years older. All species found in
Icelandic Miocene sediments differ from
the ones in the BIP floras, with the
exception of Ghyptostrobus europaeus.
Palaeobotanical data from Icelandic
sediments indicate migration of plants
over the proto-Iceland region long after
the accumulation of the BIP floras. It is a
matter of speculation, however, how
long these migration routes were open
during the Neogene. Considering a suba-
erial Greenland-Scotland Transverse
Ridge in this region long before 16 Ma
(the oldest rocks in Iceland) it cannot be
ruled out that at least parts of this flora
arrived much earlier in proto-Iceland
and persisted until the accumulation of
the 15 Ma Selárdalur and Botn sedi-
ments.
Comparison of fossils to modem liv-
ing taxa indicates that the mean annual
temperature in Iceland 15-13.5 Ma was
between 9.3°C and 10.5°C.
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