| 1. |
- Björck, S., et al.
(författare)
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The Preboreal oscillation around the Nordic Seas : Terrestrial and lacustrine responses
- 1997
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Ingår i: Journal of Quaternary Science. - 0267-8179. ; 12:6, s. 455-465
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Tidskriftsartikel (refereegranskat)abstract
- The occurrence of an early Preboreal climatic cooling/oscillation (PBO) in lacustrine and glacial records from northwest Europe, Iceland and Greenland is reviewed and documented. The often subtle response of the proxy records to this oscillation, in combination with its short duration, make it difficult to detect. Owing to its chronostratigraphic position between the 10000-9900 and 9600-9500 14C plateaux (c. 11 300-11 150 calendar yr BP) it is also difficult to 14C date with precision. We find that the vegetation response to the PBO varies between sites and regions. In contrast to the pioneer vegetation in Iceland and southern Sweden, the expanding birch-pine forest in Germany-Denmark was more susceptible to deteriorating growing conditions. The combined lacustrine, tree-ring and glacial records imply that the PBO was characterised by cool and humid conditions throughout northwestern and central Europe. This is documented by vegetation changes, decreased aquatic production, increased soil erosion, increased 2H and 13C content in tree-rings, readvances or stillstands of the ice sheet in Norway and Finland, and ingression of brackish water into the Baltic. Icelandic proxy records from lake sediments and glacial moraines imply cooler conditions than during the previous Preboreal period, but not as extreme as during the Younger Dryas. Greenland records suggest that the early Preboreal was characterised by ice readvances, as an effect of cool climate and increased precipitation (in relation to the Younger Dryas). It was not until the end of the PBO that climate was warm enough to melt the land-based ice sheet. This Preboreal oscillation, found on both sides of the Nordic Seas, is interpreted as an effect of increased freshwater forcing on the thermohaline circulation in the Nordic Seas, which is implied by a simultaneous and distinct rise in the atmospheric 14C/12C ratio. A slow-down of the thermohaline circulation may temporarily have pushed the Polar Front further south.
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| 2. |
- Rundgren, Mats
(författare)
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Biostratigraphic Evidence of the Allerød-Younger Dryas-Preboreal Oscillation in Northern Iceland
- 1995
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Ingår i: Quaternary Research. - : Cambridge University Press (CUP). - 0033-5894. ; 44:3, s. 405-416
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Tidskriftsartikel (refereegranskat)abstract
- Basal sediments of Lake Torfadalsvatn, northern Iceland, record changes in terrestrial and limnic environments in the period 11,300-9000 14C yr B.P. These changes were probably forced by climate and connected with displacements of the marine polar front and sea-ice margin. Pollen, spores, green algae (Pediastrum), saturation isothermal remanent magnetization, and carbon content of the basal sediments provide the first detailed biostratigraphic record of the last glacial-interglacial transition in Iceland. During the first pioneer phase, beginning at ca. 11,300 14C yr B.P., grasses and fell-field herbs became established, and lake productivity was very low. At ca. 10,900 14C yr B.P., climatic and soil conditions became favorable for shrubs and dwarf shrubs. This change, together with increased limnic productivity, clearly indicates long seasons without ice-cover in the sea immediately north of Iceland. A return to a colder climate (Younger Dryas), probably in connection with a southward displacement of the marine polar front, occurred by 10,600 14C yr B.P. Shrub and dwarf-shrub vegetation disappeared, and limnic productivity diminished. A second pioneer vegetation phase, dominated by Oxyria/Rumex and grasses, was initiated by a change to longer seasons without sea ice at ca. 9900 14C yr B.P. This warming is also evident as a contemporaneous increase in lake productivity. After ca. 9400 14C yr B.P. the reestablishment of dwarf-shrub heaths and very high limnic productivity indicate further warming.
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| 3. |
- Rundgren, Mats, et al.
(författare)
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Dynamic sea-level change during the last deglaciation of northern Iceland
- 1997
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Ingår i: Boreas. - : Wiley. - 0300-9483 .- 1502-3885. ; 26:3, s. 201-215
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Tidskriftsartikel (refereegranskat)abstract
- A detailed reconstruction of deglacial relative sea-level changes at the northern coast of Iceland, based on the litho- and biostratigraphy of lake basins, indicates an overall fall in relative sea level of about 45 m between 11 300 and 9100 BP, corresponding to an isostatic rebound of 77 m. The overall regression was interrupted by two minor transgressions during the late Younger Dryas and in early Preboreal, and these were probably caused by a combination of expansions of local ice caps and readvances of the Icelandic inland ice-sheet margin. Maximum absolute uplift rates are recorded during the regressional phase between the two transgressions (10 000-9850 BP), with a mean value of c. 15 cm ·14C yr-1 or 11-12 cm ·cal. yr-1. Mean absolute uplift during the regressional phase following the second transgression (9700-9100 BP) was around 6 cm ·14C yr-1, corresponding to c. 3 cm · cal. yr-1, and relative sea level dropped below present-day sea level at 9000 BP.
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| 4. |
- Rundgren, Mats
(författare)
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Early-holocene vegetation of northern Iceland : Pollen and plant macrofossil evidence from the Skagi peninsula
- 1998
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Ingår i: Holocene. - : SAGE Publications. - 0959-6836. ; 8:5, s. 553-564
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Tidskriftsartikel (refereegranskat)abstract
- Pollen and plant macrofossil records from two lakes on northernmost Skagi peninsula, northern Iceland, reflect a progressive closing of the vegetation cover during the early Holocene. This development was connected with the succession from an initial herb-tundra phase characterized by Oxyria digyna, Poaceae and Caryophyllaceae, through an intermediate dwarf-shrub phase dominated by Salix and Empetrum nigrum, to a shrub and dwarf-shrub phase with Juniperus communis, Betula nana, Salix and Poaceae. This development is similar to that recorded at other sites in northern Iceland, both with respect to the taxa involved and the timing of transitions between successional phases, which suggests that early-Holocene vegetational development in northern Iceland was forced by broad-scale climatic changes. The palaeobotanical data suggest a gradual early-Holocene warming in northern Iceland, which probably was connected with northward displacements of Atlantic waters and North Atlantic air masses. The deposition of the Saksunarvatn ash at 9000 BP caused a transitory phase of vegetational succession on northernmost Skagi.
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| 5. |
- Rundgren, Mats, et al.
(författare)
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Plant survival in Iceland during periods of glaciation?
- 1999
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Ingår i: Journal of Biogeography. - : Wiley. - 0305-0270 .- 1365-2699. ; 26:2, s. 387-396
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Tidskriftsartikel (refereegranskat)abstract
- Aim: The paper addresses the classical question of possible plant survival in Iceland during the last glacial period in the light of a palaeobotanical record from northern Iceland, spanning the period 11,300-9000 BP, including the Younger Dryas stadial. We review the Late Cenozoic fossil plant record, the past debate on glacial plant refugia in Iceland, and the evidence for ice-free areas during the Weichselian. Location: The investigated lake sediment record comes from Lake Torfadalsvatn, which is situated in the northwestern part of the Skagi peninsula in northern Iceland. Methods: The sediment chronology was constructed from the occurrence of the Vedde Ash and the Saksunarvatn ash, two well-dated Icelandic tephras, together with the results from five AMS and conventional radiocarbon dates performed on bulk sediment samples. The vegetational reconstruction was based on detailed pollen analysis of the sediment sequence. Results: The pollen analysis revealed that many of the taxa present in the area prior to the Younger Dryas stadial continued to produce pollen during that cold event. The more or less immediate reappearance of a few other pollen taxa at the Younger Dryas-Preboreal boundary suggests that these plants also survived, even if they did not produce sufficient pollen to be recorded during the Younger Dryas stadial. Main conclusions: We conclude that the relatively high plant diversity found in high Arctic areas and present-day nunataks in Iceland and Greenland, together with the fact that many plant species were able to survive the Younger Dryas stadial on the Skagi peninsula, suggest that species with high tolerance for climate fluctuations also survived the whole Weichselian in Iceland. This conclusion is supported by recent palaeoclimatic data from ice-cores and deep-sea sediments, indicating that Icelandic climate during the last glacial was only occasionally slightly colder than during the Younger Dryas stadial.
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| 6. |
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| 7. |
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| 9. |
- Elgered, Gunnar, 1955, et al.
(författare)
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Ground-based microwave radiometry and long-term observations of atmospheric water vapor
- 1998
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Ingår i: Radio Science. - 0048-6604 .- 1944-799X. ; 33:3, s. 707-717
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Tidskriftsartikel (refereegranskat)abstract
- Microwave radiometer data and radiosonde data from the time period 1981-1995 have been used to study long-term trends in the integrated precipitable water vapor (IPWV). The two instruments have operated 37 km apart on the Swedish west coast. Model parameters are estimated for the entire data sets as well as for subsets of the data. The IPWV model parameters are a mean value, a linear drift with time, and the amplitude and phase of an annual component. The radiosonde data, which are uniformly sampled in time, show an increase in the IPWV of 0.03 mm/yr with a statistical standard deviation of 0.01 mm. The microwave radiometer data, which are not at all uniformly sampled in time, show -0.02+/-0.01 mm/yr. We show that the disagreement is caused by the different sampling of the data for the two instruments. When the two data sets are reduced to include only data that are sampled simultaneously, we find an agreement between all estimated model parameters, given their statistical uncertainties. This suggests that if the microwave radiometer had also been operating continuously over the 15-year period, its data would have implied a linear trend similar to the result obtained from the radiosonde data. The general quality of the data, in terms of the short time scatter, has been improved over the time period. The root mean square (RMS) difference between the IPWV measured by the radiometer and by the radiosondes was 2.1 mm during the first 5 years and was reduced to 1.6 mm during the last 4 years. These values include the real difference in the IPWV between the two sites. The bias, radiometer-radiosonde, was 0.1 mm for the whole data set and varied between -0.2 and 0.9 mm for smaller data sets of a few years.
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| 10. |
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