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- Amman, B., et al.
(författare)
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Quantification of biotic responses to rapid climatic changes around the Younger Dryas – a synthesis.
- 2000
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Ingår i: Palaeogeography, Palaeoclimatology, Palaeoecology. - 0031-0182 .- 1872-616X. ; 159:3-4, s. 313-347
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Tidskriftsartikel (refereegranskat)abstract
- To assess the presence or absence of lags in biotic responses to rapid climatic changes, we: (1) assume that the delta(18)O in biogenically precipitated carbonates record global or hemispheric climatic change at the beginning and at the end of the Younger Dryas without any lag at our two study sites of Gerzensee and Leysin, Switzerland; (2) derive a time scale by correlating the delta(18)O record from these two sites with the delta(18)O record of the GRIP ice core; (3) measure delta(18)O records in ostracods and molluscs to check the record in the bulk samples and to detect possible hydrological changes; (4) analyse at Gerzensee and Leysin as well as at two additional sites (that lack carbonates and hence a delta(18)O record) pollen, plant macrofossils, chironomids, beetles and other insects, and Cladocera; (5) estimate our sampling resolution using the GRIP time scale for the isotope stratigraphies and the biostratigraphies; and (6) summarise the major patterns of compositional change in the biostratigraphies by principal component analysis or correspondence analysis. We conclude that, at the major climatic shifts at the beginning and end of the Younger Dryas, hardly any biotic lags occur (within the sampling resolution of 8-30 years) and that upland vegetation responded as fast as aquatic invertebrates. We suggest that the minor climatic changes associated with the Gerzensee and Preboreal oscillations were weakly recorded in the biostratigraphies at the lowland site, but were more distinct at higher altitudes. Individualistic responses of plant and animal species to climatic change may reflect processes in individuals (e.g. productivity and phenology), in populations (e.g. population dynamics), in spatial distributions (e.g. migrations), and in ecosystems (e.g. trophic state). We suggest that biotic responses may be telescoped together into relatively short periods (50 to 150 years), perhaps disrupting functional interactions among species and thus destabilising ecosystems.
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| 2. |
- Birks, H.H., et al.
(författare)
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The development of the aquatic ecosytem at Kråkenes Lake, western Norway, during the late glacial and early Holocene - a synthesis.
- 2000
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Ingår i: Journal of Paleolimnology. - 0921-2728 .- 1573-0417. ; 23:1, s. 91-114
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Tidskriftsartikel (refereegranskat)abstract
- This paper synthesises the palaeoecological reconstructions, including palaeoclimatic inferences, based on the available fossil record of plants (pollen, macrofossils, mosses, diatoms) and animals (chironomids, Cladocera, Coleoptera, Trichoptera, oribatid mites) in the late-glacial and early-Holocene sediments of Kråkenes Lake, western Norway, with special emphasis on changes in the aquatic ecosystem. New percentage and influx pollen diagrams for selected taxa provide insights into the terrestrial setting. The information from all the proxies is collated in a stratigraphical chart, and the inferred changes in the lake and its catchment are discussed. The individual fossil sequences are summarised by detrended correspondence analysis (DCA), and sample scores on the first DCA axes are plotted against an estimated calendar-year timescale for comparison of the timing and magnitude of changes in assemblage composition. The DCA plots show that the large late-glacial biotic changes were synchronous, and were driven by the overriding forcing factor of temperature. During the early Holocene, however, the changes in different groups were more gradual and were independent of each other, showing that other factors were important and interactive, such as the inwash of dissolved and particulate material from the catchment, the base and nutrient status of the lake-water, and the internal processes of ecosystem succession and sediment accumulation. This multi-disciplinary study, with proxies for changes in the lake and in the catchment, highlights the dependence of lake biota and processes not only on regional climatic changes but also on changes in the lake catchment and on internal processes within the lake. Rates of change for each group are also estimated and compared. The reaction times to the sharp temperature changes at the start and end of the Younger Dryas were very rapid and occurred within a decade of the temperature change. Aquatic organisms tracked the temperature and environmental changes very closely, and are probably the best recorders of late-glacial climatic change in the fossil record.
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| 6. |
- Hajdas, I., et al.
(författare)
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Radiocarbon and absolute chronology of the Late-Glacial record from Hauterive/Rouges-Terres, Lake Neuchdtel (CH)
- 2004
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Ingår i: Nuclear Instruments and Methods in Physics Research Section B. - : Elsevier BV. - 0168-583X .- 1872-9584. ; 223, s. 308-312
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Tidskriftsartikel (refereegranskat)abstract
- Radiocarbon ages of 87 samples made up of terrestrial macrofossils (mainly twigs of Salix sp.) were washed out from sediments sampled at Hauterive/Rouges-Terres, Lake Neuchatel, Switzerland, and measured using the AMS dating technique. These radiocarbon ages, which range from 8000 to 14,500 BP, were used to construct an absolute chronology of the record. A model of a sequence with known calendar ages for the period between 9000 to 17,500 cal BP was used to calibrate the 14 C ages. Three known calendar ages were set by pollen based dating (biozones) of palaeoclimatic events at 11,450 cal BP (Younger Dryas/Preboreal), 13,450 cal BP (Gerzensee Oscillation) and 14,650 cal BP (Oldest Dryas/ Bolling; see figure). This study shows that high resolution radiocarbon dating combined with such a calibration approach provides a powerful tool for dating stratigraphic sections by minimizing effects such as inversions in calendar ages and large errors caused by wiggles and plateaus in the radiocarbon time-scale.
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| 9. |
- Lemdahl, Geoffrey
(författare)
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Late-glacial and early-Holocene Coleoptera assemblages as indicators of local environment and climate at Kråkenes Lake, western Norway.
- 2000
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Ingår i: Journal of Paleolimnology. - 0921-2728 .- 1573-0417. ; 23:1, s. 57-66
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Tidskriftsartikel (refereegranskat)abstract
- Thirty-six Coleoptera (beetle) taxa and other insects were identified from the late-glacial and early-Holocene sediments at Krakenes Lake. Compared with other Scandinavian late-glacial sites, this is a rather sparse record. The water beetles found in the Allerod are characteristic of a poorly vegetated clear-water lake. The terrestrial fauna is indicative of dwarf-shrub and moss vegetation. A marked decline in the number of species at the start of the Younger Dryas was rather rapid, probably over less than 80 calendar yrs. No obligate tundra species replaced the Allerod fauna. Most of the Younger Dryas is virtually devoid of beetles. The increase in numbers and diversity of both aquatic and terrestrial species at the Younger Dryas/Holocene transition is very rapid. After an initial pioneer stage, beetles associated with dwarf-shrub heath and willow scrub appeared, but no obligate tree or forest taxa were recorded. Mutual Climatic Range (MCR) temperature reconstructions suggest that the Allerod was colder and more continental than present. The near absence of beetles in the Younger Dryas probably reflects very cold conditions. A rapid temperature rise at the start of the Holocene resulted in a warmer and more continental climate than present.
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| 10. |
- Lemdahl, Geoffrey
(författare)
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Lateglacial and Early Holocene insect assemblages from sites at different altitudes in the Swiss Alps - implications on climate and environment.
- 2000
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Ingår i: Palaeogeography, Palaeoclimatology, Palaeoecology. - 0031-0182 .- 1872-616X. ; 159:3-4, s. 293-312
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Tidskriftsartikel (refereegranskat)abstract
- Insect analyses from four sites at differentaltitudes in the SwissAlps are presented. The dominant insect group considered here is Coleoptera (beetles), though other insect orders are also recorded. The study includes the first detailed insect records dating to the Younger Dryas in Switzerland. The quantitative climatic reconstructions based on coleopteran assemblages were made using the MCR method. Mean temperatures of the warmest month similar to modern intensities are indicated at the end of Allerød interstadial (AL) and at the beginning of the Holocene (PB). The winters during the AL were rather colder, implying more continental conditions than at present. During the Younger Dryas stadial (YD), the mean temperatures of the warmest month were 5–8°C colder than those of the present day, and the mean temperatures of the coldest month were about 10–12°C colder than today. A rapid change in the faunal composition was found at the Gerzensee and Zeneggen sites during the transition YD/PB. The change is characterized by the disappearance of cold-adapted species between two contiguous samples (i.e. within 1.0 cm of sediment sampled). At Gerzensee, the cold-adapted species assemblage was immediately replaced by temperate species. The Coleoptera thus indicate extremely rapid and intense climatic oscillations at the Lateglacial–Holocene transition.
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