Jökull - 01.12.1999, Page 18

event and the Preboreal oscillation, at times when
there is evidence for readvances of the Icelandic ice-
sheet margin in various parts of the island (Ingólfsson
and Norddahl, 1994), suggests that these changes in
relative sea level were caused by increased loading by
glacial ice (Rundgren et al., 1997), and the sea-level
reconstruction for northernmost Skagi supports the
very high uplift rates during the last deglaciation in
southwestern Iceland calculated by Ingólfsson et al.
(1995). Furthermore, the very high isostatic rebound
rates recorded combined with rapid shifts between
transgressions ánd regressions imply that the Ice-
landic lithosphere is extremely sensitive to changes in
volume of glacial ice, indicating low asthenospheric
viscosities below Iceland.
CONCLUSIONS
Considerable changes of vegetation, climate and
sea level took place on northemmost Skagi during the
Late Weichselian and Early Holocene. The deglacial
warming trend was interrupted by two cooling
episodes, the Younger Dryas cold event (c. 10,600-
9900 BP) and the Preboreal oscillation (c. 9800-9700
BP), that affected both terrestrial and limnic environ-
ments. Dwarf-shrubs were replaced by grasses and
herbs, and limnic productivity decreased. Moreover,
these setbacks caused glacier growth and increased
glacial loading that resulted in transgressions on north-
emmost Skagi. The palaeoclimatic reconstmction pre-
sented from northemmost Skagi is in accordance with
Late Weichselian and early Holocene palaeoclimatic
data from other parts of Iceland, and the good corre-
spondence with terrestrial, marine and ice-core records
from the North Atlantic region shows that there is a
close connection between deglacial changes in ocean
circulation and climatic conditions on Iceland.
ACKN O WLEDGEMENTS
This investigation, which was carried out as a
Ph.D. project within the Department of Quaternary
Geology, Lund University, would not have been possi-
ble without the constant support from my supervisors
Svante Björck and Olafur Ingólfsson. All the diatom
analyses were carried out by Hui Jiang, Department of
Quaternary Geology, Lund University, and Hafliði
Hafliðason, Geological Institute, University of
Bergen, performed the tephra analyses included in this
study. I am also very grateful for the help I have re-
ceived from Margrét Hallsdóttir (Reykjavík), Hörður
Kristinsson (Akureyri), Ami Hjartarson (Reykjavík),
the families at Hraun and Tjörn on Skagi, and Inga
Svala Jónsdóttir (Göteborg).
This investigation has been financed by a faculty re-
search 'scholarship and a graduate position at Lund Uni-
versity. Field work was financially supported by Kung-
liga Fysiografiska Sallskapet, the Swedish Natural Sci-
ence Research Council (grants to Björck and Ing-
ólfsson), Lunds Geologiska Faltklubb and the Ice-
landic Science Foundation (grant to Ingólfsson),
which is gratefully acknowledged.
Ágrip
Samantekt á sögu umhverfisbreytinga á
Skaga, Norðurlandi, 11.300-7800 BP
Niðurstöður rannsókna á seti fimm stöðuvatna á
Skaga á Norðurlandi (Torfadalsvatni, Hraunsvatni,
Geitakarlsvötnum, Kollusátursvatni og Neðstavatni),
sýna að umtalsverðar breytingar áttu sér stað á gróð-
urfari, loftslagi og sjávarstöðu á síðari hluta Weichsel
og fyrri hluta nútíma (mynd 2). I ljósi frjógreininga og
rannsókna á setsýnum úr stöðuvatnasetinu einkennd-
ist tímabilið frá 11.300 - 10.900 BP af graslendi
(grass-tundra) og lágri lífrænni virkni stöðuvatnanna,
en það bendir til kalds veðurfars. Mildara veðurfars-
ástand eftir 10.900 BP (mynd 4) endurspeglast í út-
breiðslu á lyngmóa og aukinni lífrænni virkni í stöðu-
vötnunum. Merki um upphaf yngra dryas kulda-
kaflans er að finna í kringum 10.600 BP, en þá varð
snögg kólnun sem leiddi til afturhvarfs til graslendis
og lágrar lífrænnar virkni í stöðuvötnunum. Kortlagn-
ing á menjum um sjávarstöðubreytingar sýnir að á
síðari hluta þessa kuldakafla varð hækkun á sjávar-
stöðu (mynd 5). I upphafi nútíma, eða um 9900 BP,
varð síðan snögg hlýnun sem varð til þess að gróður-
far færðist í átt að heiðagróðri með ýmsum blómjurt-
um (herb-tundra), auk þess sem lífræn virkni jókst á
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JÖKULL, No. 47, 1999