| 1. |
- Dahl-Jensen, D., et al.
(författare)
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Eemian interglacial reconstructed from a Greenland folded ice core
- 2013
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 493:7433, s. 489-494
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Tidskriftsartikel (refereegranskat)abstract
- Efforts to extract a Greenland ice core with a complete record of the Eemian interglacial (130,000 to 115,000 years ago) have until now been unsuccessful. The response of the Greenland ice sheet to the warmer-than-present climate of the Eemian has thus remained unclear. Here we present the new North Greenland Eemian Ice Drilling ('NEEM') ice core and show only a modest ice-sheet response to the strong warming in the early Eemian. We reconstructed the Eemian record from folded ice using globally homogeneous parameters known from dated Greenland and Antarctic ice-core records. On the basis of water stable isotopes, NEEM surface temperatures after the onset of the Eemian (126,000 years ago) peaked at 8 +/- 4 degrees Celsius above the mean of the past millennium, followed by a gradual cooling that was probably driven by the decreasing summer insolation. Between 128,000 and 122,000 years ago, the thickness of the northwest Greenland ice sheet decreased by 400 +/- 250 metres, reaching surface elevations 122,000 years ago of 130 +/- 300 metres lower than the present. Extensive surface melt occurred at the NEEM site during the Eemian, a phenomenon witnessed when melt layers formed again at NEEM during the exceptional heat of July 2012. With additional warming, surface melt might become more common in the future.
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| 2. |
- Hammarlund, Dan, et al.
(författare)
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Climate and environment during the Younger Dryas (GS-1) as reflected by composite stable isotope records of lacustrine carbonates at Torreberga, southern Sweden
- 1999
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Ingår i: Journal of Quaternary Science. - 1099-1417. ; 14:1, s. 17-28
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Tidskriftsartikel (refereegranskat)abstract
- Climatic and environmental changes during the Younger Dryas stadial (GS-1) and preceding and following transitions are inferred from stable carbon and oxygen isotope records obtained from the sediments of ancient Lake Torreberga, southern Sweden. Event GS-1 is represented in the sediment sequence by 3.5 m of clay containing lacustrine carbonates of various origins. Comparison of isotopic records obtained on mollusc shells, ostracod valves, and Chara encrustations precipitated during specific seasons of the year supports estimates of relative changes in both lake water and mean annual air temperatures. Variations in soil erosion rates can also be estimated from a simple isotope-mass-balance model to separate allochthonous and autochthonous carbonate contributions to the bulk carbonate content of the sediments. The well-known, rapid climatic shifts characterising the Last Termination in the North Atlantic region are clearly reflected in the isotopic data, as well as longer-term changes within GS-1. Following maximum cooling shortly after the Allerod-Younger Dryas (GI-1-GS-1) transition, a progressive warming and a slight increase in aquatic productivity is indicated. At the Younger Dryas-Preboreal (GS-1-PB) transition mean annual air temperature rapidly increased by more than 5 degrees C and summer lake-water temperature increased by ca. 12 degrees C. The subsequent Preboreal oscillation is characterised by an increase in soil erosion and a slight decrease in mean annual air temperature. These results are in harmony with recent findings about large-scale climate dynamics during the Last Termination. Copyright (C) 1999 John Wiley & Sons, Ltd.
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| 3. |
- Hammarlund, Dan, et al.
(författare)
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Holocene changes in atmospheric circulation recorded in the oxygen-isotope stratigraphy of lacustrine carbonates from northern Sweden
- 2002
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Ingår i: The Holocene. - : SAGE Publications. - 0959-6836 .- 1477-0911. ; 12:3, s. 339-351
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Tidskriftsartikel (refereegranskat)abstract
- The oxygen-isotope composition of local precipitation (delta(18)O(P)) is reconstructed from carbonate lake-sediment components in a sediment core covering the last 10000 calendar years from Lake Tibetanus, a small, hydrologically open, groundwater-fed take in the Abisko area, northern Sweden. Comparison of the delta(18)O(P) history with a pollen-based palaeotemperature record from the same core clearly reveals pronounced deviations from the normally expected temporal delta(18)O(P)-temperature relation (so-called 'Dansgaard relation') that may be a function of changing oceanicity. The transition from relatively moist, maritime conditions in the early Holocene to a much drier climate after 6500 cal. BP is reflected by major changes in forest extent and composition as recorded by pollen and plant macrofossil data. At the time of maximum influence of westerly air-mass circulation (high zonal index) c. 9500 cal. BP, brought about by high summer insolation and enhanced meridional pressure gradients. delta(18)O(P) at Lake Tibetanus was about 2%, higher than would be predicted by the modem isotope-temperature relation, The occurrence of long-term changes in delta(18)O(P)-temperature relations, which are more sensitive measures of palaeoclimate than either delta(18)O(P) or temperature alone, needs to be taken into account when extracting palaeoclimatic information from continental oxygen-isotope records.
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| 4. |
- Hammarlund, Dan, et al.
(författare)
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Limnic responses to increased effective humidity during the 8200 cal. Yr BP cooling event in southern Sweden
- 2005
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Ingår i: Journal of Paleolimnology. - : Springer Science and Business Media LLC. - 0921-2728 .- 1573-0417. ; 34:4, s. 471-480
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Tidskriftsartikel (refereegranskat)abstract
- Highly resolved sediment stratigraphic and stable isotope records from a lake in south-central Sweden give evidence of relatively humid summers during the wide-spread cold event at c. 8200 cal. BP. A transient lake-level rise led to increased catchment erosion followed by enhanced phytoplankton production and disturbance of the Chara-dominated algal community. An associated increase in the deposition of organic material resulted in hypolimnetic oxygen deficit and methane formation as inferred from elevated sulphur content of the sediments and strong depletion in C-13 of bulk organic material. In combination with dry and cold winter conditions as revealed by other proxy records, these data suggest the development of a strongly seasonal climate in northwestern Europe in response to altered atmospheric circulation at this stage. Enhanced zonal circulation with frequent cyclonic activity and increased effective humidity probably characterized the summer climate of southern and central Scandinavia, consistent with conditions inferred for continental Europe. In contrast, the winter climate was dominated by blocking high-pressure circulation, giving rise to cold and dry conditions.
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| 5. |
- Hammarlund, Dan, et al.
(författare)
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Multi-component carbon isotope evidence of early Holocene environmental change and carbon-flow pathways from a hard-water lake in northern Sweden
- 1997
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Ingår i: Journal of Paleolimnology. - 0921-2728. ; 18:3, s. 219-233
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Tidskriftsartikel (refereegranskat)abstract
- A 9000-year carbonate-rich sediment sequence from a small hard-water lake in northernmost Sweden was studied by means of multi-component stable carbon isotope analysis. Radiocarbon dating of different sediment fractions provides chronologic control and reveals a rather constant hard-water effect through time, suggesting lake has remained hydrologically open throughout the Holocene. Successive depletion of C-13 in fine-grained calcite and carbonate shells during the early Holocene correlate with a change in catchment vegetation from pioneer herb communities to boreal forest. The vegetational change and associated soil development likely gave rise to an increased supply of C-13-depleted carbon dioxide in groundwater recharging the lake. This process is therefore believed to be the main cause of decreasing values of delta(13)C in dissolved inorganic carbon of the lake and thereby in limnic carbonates. Strongly C-13-depleted sedimentary organic matter may be related to enhanced kinetic fractionation during photosynthetic assimilation by means of proton pumping in Characean algae. This interpretation is supported by a substantial offset between delta(13)C of DIC as recorded by mollusc shells and delta(13)C Of fine-grained calcite.
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| 6. |
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| 7. |
- Hammarlund, Dan, et al.
(författare)
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Rapid hydrological changes during the Holocene revealed by stable isotope records of lacustrine carbonates from Lake Igelsjon, southern Sweden
- 2003
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Ingår i: Quaternary Science Reviews. - 0277-3791. ; 22:2-4, s. 353-370
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Tidskriftsartikel (refereegranskat)abstract
- A Holocene sediment sequence from Lake Igelsjon, south central Sweden, was studied by stable oxygen- and carbon-isotope analyses of different carbonate components. The deposit, which covers the time-span from ca 11,500 cal BP to the present, was laid down in a small, kettle-hole lake, the hydrological balance of which is presently dominated by groundwater flow. Isotopic records obtained on bulk carbonates originating mainly from summer-produced, calcitic algal encrustations exhibit several rapid shifts of more than 2parts per thousand, likely reflecting pronounced hydrological variations. Corresponding isotopic data obtained on calcitic gastropod opercula from parts of the profile show subdued responses to major climatic shifts, probably related to an extended calcification season. The isotopic records were complemented by studies of modern isotope hydrology, and our interpretations are based on a simplistic climate-hydrology model in which variations in groundwater generation within the lake catchment produce changes in groundwater level and related adjustments of lake level and surface/volume ratio of the basin during the ice-free season. Assumed periods of decreased lake volume in a relatively dry climate (low lake level) are characterised by enrichment in O-18 and C-13 resulting from increased evaporation/inflow ratio and atmospheric equilibration, respectively. In clear contrast to this situation, intervals of more humid climatic conditions give rise to increased lake volume (high lake level), possibly surface over flow, and relatively depleted isotopic ratios. Relatively humid conditions, which may correlate to a wide-spread cooling event recorded by various proxies across the North Atlantic region, are indicated by distinct isotopic shifts at ca 8300 and 8000 cal BP, bracketing a period of O-18-depletion. The period between ca 8000 and 4000 cal BP was characterised by relatively dry and stable climatic conditions, whereas the subsequent part of the Holocene experienced a more humid and variable climate following marked and coherent depletions in O-18 and C-13 at ca 4000 cal BP. (C) 2002 Elsevier Science Ltd. All rights reserved.
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| 8. |
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| 9. |
- Hyrup, B., et al.
(författare)
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STRUCTURE-ACTIVITY STUDIES OF THE BINDING OF MODIFIED PEPTIDE NUCLEIC-ACIDS (PNAS) TO DNA
- 1994
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 116:18, s. 7964-7970
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Tidskriftsartikel (refereegranskat)abstract
- Peptide nucleic acid (PNA) oligomers where one of the repeating backbone units is extended with a methylene group to either N-(2-aminoethyl)-beta-alanine or N-(3-aminopropyl)glycine were prepared. Alternatively, the linker to the nucleobase was extended from methylenecarbonyl to ethylenecarbonyl. The thermal stability of the hybrids between these PNA oligomers and complementary DNA oligonucleotides was significantly lower than that of the corresponding complexes involving unmodified PNA. However, the sequence selectivity was retained. Thymidyl decamers with all N-(2-aminoethyl)-beta-alanine or N-(3-aminopropyl)glycine backbones were prepared and shown to be unable to hybridize to the complementary (dA)(10) oligonucleotides, whereas a PNA decamer containing only ethylenecarbonyl linkers between the nucleobases and the N-(2-aminoethyl)glycine backbone showed weak but sequence-specific affinity for complementary DNA. All hybrids involving homopyrimidine PNA oligomers exhibited (PNA)(2)/DNA stoichiometry, whereas mixed-sequence PNA oligomers formed PNA/DNA duplexes. The preferred binding direction between the modified PNA and DNA in the duplex motifs was antiparallel, as previously reported for complexes involving unmodified PNA.
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