Fróðskaparrit - 01.01.1987, Blaðsíða 49

WHEN, HOW, AND WHENCE?
53
similar to the Late Weichselian flora fur-
ther to the south (in the British Isles, Jó-
hansen 1982). For instance, Betula nana L.
immigrated to Scotland around 10400 BP,
to the Faroes around 9500 BP, and to
eastern Greenland around 8000 BP. An
immigration of B. nana to the Faroes from
Scotland (or possibly Scandinavia) there-
fore seems likely (Jóhansen 1982). It is also
notable that the first immigration to the
Faroes occurred shortly after the polar
front had reached Greenland, following its
southward advance in the Younger Dryas.
That the earliest faunas in Iceland and
the Faroes were apparently cool temperate
rather than arctic is interesting when com-
pared with the Late Pleistocene climatic re-
cord. Coope (1969, 1979) has discussed the
possibility of taxa emigrating from the Brit-
ish Isles to Scandinavia during the retreat
northwards of the polar front at about
13500 BP. In view of the fact that the front
advanced southwards a few thousand years
later, and that Midland England immedi-
ately south of the ice front appears to have
been devoid of invertebrates between
about 18000 BP and 14500 BP (Ruddiman
and Mclntyre 1981), it is difficult to per-
ceive of a dispersal to the Faroes during the
breakup of the Weichselian ice sheets at a-
round 13500 BP. The climatic tolerance of
the known pre-Landnám fossil insects in
Iceland and the Faroes (Jessen and Ras-
mussen 1922, Buckland 1988) seems rather
to indicate an immigration during the sec-
ond retreat northwards of the polar front
(at about 9500 BP in the case of the
Faroes), or, alternatively, that species that
had invaded earlier went extinct during the
Younger Dryas and left no (up to now re-
covered) traces in the fossil record.
Apparently, then, there were two (geo-
logically) sudden changes in climate, both
possible to explain by the retreat of the
polar front northwards. Summer tempera-
tures in the sea north of the polar front
were probably not above 6°C and a
temperature gradient across the front in
summer in Late Glacial times of about 7°C
has been proposed (Ruddiman et al. 1977).
This may perhaps explain the sudden
terrestrial faunal changes in the British
Isles, which independently suggest an in-
crease in July temperatures of about 7°C
(Coope and Brophy 1972).
A rapid warming occurred in the British
Isles around 10000 BP (indicating a similar
warming in the Faroes at about 9500 BP
when the polar front had passed still furth-
er northwards); this is evidenced both by
fossil insect remains and deep-sea core data
(Ruddiman et al. 1977, Coope 1979, Os-
borne 1980). Apparently deglaciation was
rapid (Andersen 1980). At about 9500 BP
a temperate insect fauna had been estab-
lished in SW Scotland and replaced the
previous aboreal fauna (Ashworth 1973).
Such an amelioration of climate had
previously been suggested by paleobotani-
cal data (Iversen 1954).
Even if arctic species immigrated to the
northern islands during the first retreat
northwards of the polar front, as suggested
by Coope (1979), the rapid response of the
fauna to climatic change would have resul-
ted in an extinction of the arctic elements
(data from Midland England indicate such
an extinction; see above; Ruddiman and
Mclntyre 1981). Ashworth (1973) gives
similar examples of rapid faunal changes
with changing climate.