| 1. |
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| 2. |
- Adolphi, Florian, et al.
(författare)
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Persistent link between solar activity and Greenland climate during the Last Glacial Maximum
- 2014
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Ingår i: Nature Geoscience. - 1752-0908 .- 1752-0894. ; 7:9, s. 662-666
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Tidskriftsartikel (refereegranskat)abstract
- Changes in solar activity have previously been proposed to cause decadal- to millennial-scale fluctuations in both the modern and Holocene climates(1). Direct observational records of solar activity, such as sunspot numbers, exist for only the past few hundred years, so solar variability for earlier periods is typically reconstructed from measurements of cosmogenic radionuclides such as Be-10 and C-14 from ice cores and tree rings(2,3). Here we present a high-resolution Be-10 record from the ice core collected from central Greenland by the Greenland Ice Core Project (GRIP). The record spans from 22,500 to 10,000 years ago, and is based on new and compiled data(4-6). Using C-14 records(7,8) to control for climate-related influences on Be-10 deposition, we reconstruct centennial changes in solar activity. We find that during the Last Glacial Maximum, solar minima correlate with more negative delta O-18 values of ice and are accompanied by increased snow accumulation and sea-salt input over central Greenland. We suggest that solar minima could have induced changes in the stratosphere that favour the development of high-pressure blocking systems located to the south of Greenland, as has been found in observations and model simulations for recent climate(9,10). We conclude that the mechanism behind solar forcing of regional climate change may have been similar under both modern and Last Glacial Maximum climate conditions.
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| 3. |
- Andresen, Camilla Snowman, et al.
(författare)
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Early Holocene terrestrial climatic variability along a North Atlantic Island transect: palaeoceanographic implications
- 2007
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791. ; 26:15-16, s. 1989-1998
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Tidskriftsartikel (refereegranskat)abstract
- A synthesis of the early Holocene climatic development in the North Atlantic region is presented, based on three previously published lake records from southern Greenland (Lake N14), Iceland (Lake Torfadalsvatn) and the Faroe Islands (Lake Lykkjuvotn). The interval 11500-8500 cal BP has been divided into five phases with respect to the inferred strength of the North Atlantic Current (NAC) and Irminger Currents (IC). Phase 1 (11 500-10 750 cal BP) was characterised by the first establishment of the NAC and IC in the vicinity of the studied sites, interrupted by the Preboreal Oscillation around 11 200cal BP. Phase 2 (10 750-10 100 cal BP) was marked by a further warming step in southern Greenland rather concordant with a change into colder and more variable winters on the Faroe Islands. It is proposed that this could partly be related to a series of melt water outbursts disturbing the thermohaline circulation in the eastern Atlantic Ocean, resulting in a warming trend in the western region. During Phase 3 (10 100-9400cal BP) the strength of the IC reaching northwestern Iceland intensified. A more stable regime in surface circulation was established at the onset of Phase 4 (9400-8900 cal BP) in southern Greenland and was followed by a change towards further warm conditions on Iceland at the onset of Phase 5 (8900-8500 cal BP). (C) 2007 Elsevier Ltd. All rights reserved.
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| 4. |
- Andresen, Camilla Snowman, et al.
(författare)
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Holocene climate changes in southern Greenland: evidence from lake sediments
- 2004
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Ingår i: Journal of Quaternary Science. - : Wiley. - 1099-1417 .- 0267-8179. ; 19:8, s. 783-795
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Tidskriftsartikel (refereegranskat)abstract
- A Holocene lake sediment record is presented from Lake N14 situated on Angissoq island 15 km off the main coast of southern Greenland. The palaeoclimatic development has been interpreted on the basis of flux and percentage content of biogenic silica, clastic material, organic material and sulphur as well as sedimentation rate, moss content and magnetic susceptibility. A total of 43 radiocarbon dates has ensured a reliable chronology. It is argued that varying sediment composition mainly reflects changing precipitation. By analogy with the present meteorological conditions in southern Greenland, Holocene climate development is inferred. Between 11550 and 9300 cal. yr BP temperature and precipitation increase markedly, but this period is climatically unstable. From 9300 yr BP conditions become more stable and a Holocene climatic optimum, characterised by warm and humid conditions, is observed from 8000 to 5000 cal. yr BP. From 4700 cal. yr BP the first signs of a climatic deterioration are observed, and from 3700 cal. yr BP the climate has become more dry and cold. Superimposed on the climatic long-term trend is climate variability on a centennial time-scale that increases in amplitude after 3700 cal. yr BP. A climatic scenario related to the strength and position of the Greenland high-pressure cell and the Iceland low-pressure cell is proposed to explain the Holocene centennial climate variability. A comparison of the Lake N14 record with a terrestrial as well as a marine record from the eastern North Atlantic Ocean suggests that the centennial climate variability was uniform over large areas at certain times.
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| 5. |
- Andresen, Camilla Snowman, et al.
(författare)
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Holocene climate variability at multidecadal time scales detected by sedimentological indicators in a shelf core NW off Iceland
- 2005
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Ingår i: Marine Geology. - : Elsevier BV. - 0025-3227. ; 214:4, s. 323-338
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Tidskriftsartikel (refereegranskat)abstract
- A Holocene sediment record is presented from the Djupall trough, situated on the inner shelf northwest of Iceland. The paleoclimatic development has been interpreted on the basis of mass accumulation rate, carbonate content, mean grain size, sediment petrology and 29 radiocarbon dates. The results demonstrate in the early Holocene (10,000-8000 cal year BP) high sediment accumulation rates attributable to the effect of enhanced sediment mobilisation under influence of a relatively low sea level and remobilisation of glacial sediments on the sparsely vegetated Vestfirdir peninsula. The data suggest that a general southward displacement of the Polar Front commenced around 5000-4000 cal year BP. A new proxy for Holocene climatic variability is presented by the basalt/plagioclase ratio in the 63-100 mum fraction. High basalt/plagioclase values are primarily related to periods of increased storminess and bottom current energy, which enhanced the transport of basaltic sediment from the coastal zone towards the outer shelf. Advection of polar waters containing basalt-rich IRD from the eastern Greenland Blosseville Kyst basalt province may also have contributed to increased basalt/plagioclase ratios. The correlation between basalt/plagioclase ratios and proxies of solar activity (C-14 production and Be-10 flux) was explored and suggests that some of the centennial-scale peaks in colder climate could be related to increased nuclide production in the upper atmosphere. In addition, it was found that the Medieval Warm Period (c. 1100-700 cal year BP) was characterised by strong cyclone activity over the Iceland region. Intense atmospheric circulation during this period has been confirmed also by other studies in the northern North Atlantic region.
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| 6. |
- Andresen, Camilla Snowman, et al.
(författare)
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Rapid Holocene climate changes in the North Atlantic: evidence from lake sediments from the Faroe Islands
- 2006
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Ingår i: Boreas. - : Wiley. - 1502-3885 .- 0300-9483. ; 35:1, s. 23-34
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Tidskriftsartikel (refereegranskat)abstract
- Holocene records from two lakes on the Faroe Islands were investigated to determine regional climatic variability: the fairly wind-exposed Lake Starvatn on Streymoy and the more sheltered Lake Lykkjuvotn on Sandoy. Sediment cores were analysed for content of biogenic silica, organic carbon and clastic material, and magnetic susceptibility. In addition, a new qualitative proxy for past lake ice cover and wind activity was developed using the flux of clastic grains that are larger than 255 mu m. Both long-term and short-term climatic developments were similar between the two lakes, suggesting a response to a regional climate signal. The long-term climate development is characterized by early Holocene rapid warming followed by Holocene climatic optimum conditions ending around 8300 cal. yr BP. A more open landscape as evidenced from increased sand grain influx in the period 8300-7200 cal. yr BP could reflect the aftermath of the 8200 cal. yr BP event, although the event itself is not recognized in either of the two lake records. From around 7200 cal. yr BP the mid-Holocene climate deterioration is observed and from 4200 cal. yr BP the climate deteriorated further with increased amplitude of centennial cooling episodes.
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| 7. |
- Andresen, Camilla Snowman, et al.
(författare)
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Sholocene climate variability in the Denmark Strait region - A land-sea correlation of new and existing climate proxy records
- 2005
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Ingår i: Geografiska Annaler. Series A. Physical Geography. - : Informa UK Limited. - 0435-3676 .- 1468-0459. ; 87:1, s. 159-174
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Tidskriftsartikel (refereegranskat)abstract
- Two well dated Holocene sediment records bordering the Denmark Strait region have been used to reconstruct past climate variability. The content of biogenic silica, clastic and organic material and moss in a lacustrine record from Lake N14 has been used to infer past variability in precipitation and temperature in southern Greenland. Sedimentologic and petrologic composition of sand in a shelf sediment record from the Djupall trough is used to infer past variability in the northwestern storm activity on northwestern Iceland, which probably also affected the inflow of polar waters from the East Greenland Current. Our evaluation of these records with a number of previous studies from the region documents Holocene climatic optimum conditions peaking between 8000 and 6500 calendar years before present (cal yr BP). Mid-Holocene climate deterioration set in around 5000 cal yr BP followed by a further marked setback around 3500 cal yr Bp. A stacking of climate variability on a centennial timescale from previous studies in the area shows a fairly good correspondence to the timing of marked cold and warm events as evidenced from the Lake N14 and the Djupull trough records. Cooler periods are explained as the response to marked incursions of ice-laden polar water from the Arctic Ocean to the Denmark Strait region. Cool northerly and northwesterly winds along the East Greenland coast in relation to frequent strong atmospheric low pressure in the Barents Sea, coupled with strong high pressure over Greenland, would have favoured southward export of polar waters. A comparison with the proxy records of nuclide production (C-14 and Be-10) suggests that solar activity may have had some influence on the atmospheric pressure distribution in the Denmark Strait region.
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| 8. |
- Anjar, Johanna, et al.
(författare)
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MIS 3 marine and lacustrine sediments at Kriegers Flak, southwestern Baltic Sea
- 2010
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Ingår i: Boreas. - : Wiley. - 1502-3885 .- 0300-9483. ; 39:2, s. 360-366
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Tidskriftsartikel (refereegranskat)abstract
- Sediment cores from the Kriegers Flak area in the southwestern Baltic Sea show a distinct lithological succession, starting with a lower diamict that is overlain by a c. 10 m thick clay unit that contains peat, gyttja and other organic remains. On top follows an upper diamict that is inter-layered with sorted sediments and overlain by an upward-coarsening sequence with molluscs. In this paper we focus on the clay unit, which has been subdivided into three subunits: (A) lower clay with benthic foraminifera and with diamict beds in the lower part; (B) thin beds of gyttja and peat, which have been radiocarbon-dated to 31-35 14C kyr BP (c. 36-41 cal. kyr BP); and (C) upper clay unit. Based on the preliminary results we suggest the following depositional model: fine-grained sediments interbedded with diamict in the lower part (subunit A) were deposited in a brackish basin during a retreat of the Scandinavian Ice Sheet, probably during the Middle Weichselian. Around 40 kyr BP the area turned into a wetland with small ponds (subunit B). A transgression, possibly caused by the damming of the Baltic Basin during the Kattegat advance at 29 kyr BP, led to the deposition of massive clay (subunit C). The data presented here provide new information about the paleoenvironmental changes occurring in the Baltic Basin following the Middle Weichselian glaciation.
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| 9. |
- Anjar, Johanna, et al.
(författare)
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Palaeoenvironments in the southern Baltic Sea Basin during Marine Isotope Stage 3 : a multi-proxy reconstruction
- 2012
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791. ; 34, s. 81-92
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Tidskriftsartikel (refereegranskat)abstract
- Sediment cores from Kriegers Flak in the southwestern Baltic Sea provide a unique possibility to study the Middle Weichselian history of the Baltic Basin. Three Weichselian interstadial units have been identified and are attributed to Marine Isotope Stage 3 (MIS 3, 60-25 ka). The oldest unit A is characterized by a deglaciation sequence, gradually turning into brackish-water clay. The low-diversity benthic foraminiferal fauna, stable oxygen isotope values and Mg/Ca ratios indicate low temperature and salinity for unit A. A hiatus separates unit A from the overlying unit B, which was deposited in wetlands and shallow lakes between 42 and 36 cal. ka BP. Macrofossil and pollen analyses indicate deposition of unit B in an area dominated by a tree-less open tundra environment, possibly with some birch and pine in sheltered positions. The uppermost unit C is characterized by clay deposition, including redeposition of material from older sediments (unit B). Optically stimulated luminescence (OSL) ages indicate deposition of unit C between 26 and 28.5 ka. Radiocarbon ages of 39 and 40 cal. ka BP for unit C are considered to be too old. The following depositional model is suggested: (I) Isostatic depression of the region after an advance of the Scandinavian Ice Sheet (SIS), most likely the Ristinge ice advance, previously dated to c. 55-50 ka, enabled an inflow of marine waters into the Baltic Basin during the deposition of unit A. (II) Isostatic rebound caused a regression at Kriegers Flak, and a hiatus between units A and B. (III) Wetlands and smaller lakes formed in the uplifted area between 42 and 36 cal. ka BP (unit B). (IV) Deposition of glaciolacustrine clays at Kriegers Flak demonstrates that a growing SIS dammed Kattegat and the Baltic Basin c. 28.5 to 26 ka. The new evidence from Kriegers Flak provides a solid framework for future MIS 3 palaeoenvironmental reconstructions in the circum-Baltic area.
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| 10. |
- Bennike, O, et al.
(författare)
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Chronology of the last recession of the Greenland Ice Sheet
- 2002
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Ingår i: Journal of Quaternary Science. - : Wiley. - 1099-1417 .- 0267-8179. ; 17:3, s. 211-219
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Tidskriftsartikel (refereegranskat)abstract
- A new deglaciation chronology for the ice-free parts of Greenland, the continental shelf and eastern Ellesmere Island (Canada) is proposed. The chronology is based on a new compilation of all published radiocarbon dates from Greenland, and includes crucial new material from southern, northeastern and northwestern Greenland. Although each date provides only a minimum age for the local deglaciation, some of the dates come from species that indicate ice-proximal glaciomarine conditions, and thus may be connected with the actual ice recession. In addition to shell dates, dates from marine algae, lake sediments, peat, terrestrial plants and driftwood also are included. Only offshore and in the far south have secure late-glacial sediments been found. Other previous reports of late-glacial sediments (older than 11.5 cal. kyr BP) from onshore parts of Greenland need to be confirmed. Most of the present ice-free parts of Greenland and Nares Strait between Greenland and Ellesmere Island were not deglaciated until the early Holocene. Copyright (C) 2002 John Wiley Sons, Ltd.
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| 11. |
- Bennike, O, et al.
(författare)
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Estimates of South Greenland late-glacial ice limits from a new relative sea level curve
- 2002
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Ingår i: Earth and Planetary Science Letters. - 1385-013X. ; 197:3-4, s. 171-186
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Tidskriftsartikel (refereegranskat)abstract
- Marine-lacustrine isolation contacts from seven basins in the Nanortalik area, South Greenland have been analysed and dated. The basins were isolated from the sea as a consequence of isostatic rebound following deglaciation. The isolation contacts were identified with litho- and biostratigraphical analyses, especially sedimentary changes, grey scale analyses and analyses of macroscopical remains of plants and animals. Dating was performed by analytical mass spectroscopy radiocarbon dating of macrofossils and bulk sediment samples. A slow initial relative sea level fall that begins at 13.8 cal ka BP changes to a rapid relative sea level fall before the sea level fell below the present-day sea level just prior to 10 cal ka BP. The emergence curve goes further back in time than any previous emergence curve constructed from Greenland, which reflects the early deglaciation of the studied region. The glacio-isostatic crustal rebound following deglaciation was around 110 in. The sea level history indicates that the margin of the Greenland ice sheet probably extended out to the shelf margin during the Last Glacial Maximum, and that the ice thickness must have been at least 1500 in over the outer coast. Thus the highest coastal mountains would have been ice-covered, which is surprising given their alpine character. In addition, the major part of the recession of the ice must have occurred relatively late and quickly, maybe from 14 to 12 cal ka BP. The late Holocene transgression may, at least in part, be due to increased isostatic loading as a consequence of advancing glaciers during the Neoglaciation.
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| 12. |
- Bennike, Ole, et al.
(författare)
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Strandforskydning i Grönland
- 2006
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Ingår i: Groenland (Charlottenlund). - 0017-4556. ; 5, s. 188-202
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Tidskriftsartikel (populärvet., debatt m.m.)
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| 13. |
- Bennike, Ole, et al.
(författare)
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Tristan da Cunha
- 2005
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Ingår i: Naturens Verden. - 0028-0895. ; 88:7/8, s. 10-23
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Tidskriftsartikel (populärvet., debatt m.m.)
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| 14. |
- Berglund, Björn E., et al.
(författare)
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Late Weichselian environmental change in southern Sweden and Denmark
- 1994
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Ingår i: Journal of Quaternary Science. - : Wiley. - 0267-8179 .- 1099-1417. ; 9:2, s. 127-132
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Tidskriftsartikel (refereegranskat)abstract
- A synthesis is presented of the envronmental and climatic variations that are inferred to have occurred in southern Sweden (up to latitude 59°N) and Denmark during the Weichselian Late-glacial (14-9 ka BP). The chronology and characteristics of the main phases of deglaciation, sea-level change, periglacial activity, soil development, vegetation cover and climate change are summarised. A curve representing the main changes in temperature, including quantitative estimates based upon beetle data and using the ‘mutual climatic range’ method, is presented.
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| 15. |
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| 16. |
- Bernhardson, Martin, et al.
(författare)
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Sand drift events and surface winds in south-central Sweden : From the deglaciation to the present
- 2019
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791. ; 209, s. 13-22
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Tidskriftsartikel (refereegranskat)abstract
- During the transition from the last glacial into the Holocene interglacial, the atmospheric circulation in the North Atlantic area changed from a glacial mode, influenced by the presence of large ice sheets, into a system more similar to that of the present day. This change had implications for wind, temperature and precipitation patterns that can be traced in geological archives. Here, we focus on reconstructing past wind directions over southern Scandinavia, as inferred from the net sediment transport direction of inland dunes in south-central Sweden. We analysed single dunes and dune fields along a 540-km-long S-N transect with LiDAR (Light detection and ranging) derived DEMs (digital elevation models) to determine their geomorphological type and the directional mode of the wind that led to their formation. Luminescence ages of sand drift and dune formation were compiled from previous studies, allowing us to build a chronology of sand drift after the deglaciation of south-central Sweden. The majority of the dunes appear to have been formed by westerly or northwesterly winds and stabilised ∼10.5–9.0 ka. Although this is rather close in time to the local deglaciation for the northern sites, it is long after the disappearance of ice from the more southern sites. The coeval dune stabilisation over such a large geographical area suggests that regional environmental conditions played an important role. It is likely that an unstable climate in conjunction with low groundwater tables during the early Holocene locally delayed vegetation from stabilising the dunes.
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| 17. |
- Bjork, Anders Anker, et al.
(författare)
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Possible Late Pleistocene volcanic activity on Nightingale Island, South Atlantic Ocean, based on geoelectrical resistivity measurements, sediment corings and (14)C dating
- 2011
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Ingår i: GFF. - : Informa UK Limited. - 2000-0863 .- 1103-5897. ; 133:3-4, s. 141-147
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Tidskriftsartikel (refereegranskat)abstract
- Tristan da Cunha is a volcanic island group situated in the central South Atlantic. The oldest of these islands, Nightingale Island, has an age of about 18Ma. In the interior of the island, there are several wetlands situated in topographic depressions. The ages of these basins have been unknown, and their genesis has been debated. Aiming towards the reconstruction of the geomorphological history of these basins, we conducted geoelectrical resistivity measurements to map the subsurface topography, extracted peat and sediment cores and dated the onset of sedimentation applying the radiocarbon method. The irregular shapes of the basins and the lack of clear erosional features indicate that they are not eruption craters and were not formed by erosion. Instead, we regard them as morphological depressions formed between ridges of trachytic lava flows and domes at a late stage of the formation of the volcanic edifice. The onset of sedimentation within these basins appears to have occurred between 24 and 37 ka with the highest situated wetland yielding the highest ages. These ages are very young compared to the timing of the main phase of the formation of the island, implying volcanic activity on the island during the Late Pleistocene.
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| 18. |
- Björck, Svante, et al.
(författare)
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A Holocene lacustrine record in the central North Atlantic: proxies for volcanic activity, short-term NAO mode variability, and long-term precipitation changes
- 2006
-
Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791. ; 25:1-2, s. 9-32
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Tidskriftsartikel (refereegranskat)abstract
- Lake and peat corings on three Azores islands in the central North Atlantic, resulted in the discovery of a 6000 year long lacustrine sequence in a small crater lake, Lake Caveiro, on the island of Pico. This island is dominated by Pico mountain (2351 m), Portugal's highest mountain, and the lake site is situated at 903 in asl. Two sediment profiles, one central and one littoral, were sampled. Due to large facial shifts and disconformities in the littoral cores the analyses were concentrated on the central core; only the earliest 1000 years of the littoral core were studied to complement the central profile. We used sedimentology, geochemistry, diatom analyses, magnetic properties, and multivariate statistics, together with C-14 and Pb-210 dating techniques, to analyse the environmental history of the lake. Volcanic activity seems to have had a dominating impact on sediment changes and partly also on the diatom assemblages; a large number of tephras are found and seem to be connected with large (diatom) inferred pH variations. However, by a combination of methods, including multivariate techniques, we infer that precipitation changes can be detected through the volcanic noise. In the youngest part of the record (AD 1600-2000), with its decadal resolution, these humidity variations seem partly related to shifts in dominating NAO mode. The more long-term precipitation changes further back in time (350-5100 cal yr BP) roughly correspond to the well-known North Atlantic drift-ice variations as well as other North Atlantic records; low precipitation during drift-ice periods. We think these alterations were driven by changes in the thermolialine circulation as large-scale equivalences to the Great Salt Anomaly; low sea surface A temperatures and changes in circulation patterns of the central North Atlantic decreased the regional precipitation. Cooler/drier periods occurred 400-800, 1300-1800, 2600-3000, 3300-3400 and possibly also 4400-4600 cal yr BP,. while 300-400, 900-1000, 2000-2400, 3100-3200, 3800-4000 and 4700-5000 cal yr BP seem to have been more humid phases on the Azores. (c) 2005 Elsevier Ltd. All rights reserved.
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| 19. |
- Björck, Svante, et al.
(författare)
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A magnetostratigraphic comparison between 14C years and varve years during the late Weichselian, indicating significant differences between the time-scales
- 1987
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Ingår i: Journal of Quaternary Science. - : Wiley. - 1099-1417 .- 0267-8179. ; 2:2, s. 133-140
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Tidskriftsartikel (refereegranskat)abstract
- A 14C-dated magnetostratigraphy of absolute declination and inclination between 12500 and 10000 14C yr BP was recently developed for southern Sweden. Recently also the Swedish geochronological time-scale, based on c. 11 500 annually deposited clay-varves, was connected with the present. It should therefore be possible to compare the two chronologies with a reliable magnetostratigraphic record in an appropriate clay-varve section. We have found such a site within the Middle Swedish end-moraine zone. Statistical correlations between the two independently dated time-scales suggest that at 10500–10200 14Cy r BP the varve chronology exceeds the 14C chronology by the order of 500-600 varve yr. Other correlations indicate that the difference between the two chronologies was less at 11000 14C yr BP, and further correlations between the time-scales at 12000 14C yr BP suggest that the difference between the chronologies increased steadily from 12000 to 10000 14C yr BP. If these correlations are correct they imply that the 14C production rate increased steadily during the Late Weichselian.
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| 20. |
- Björck, Svante, et al.
(författare)
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A South Atlantic island record uncovers shifts in westerlies and hydroclimate during the last glacial
- 2019
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Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 15:6, s. 1939-1958
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Tidskriftsartikel (refereegranskat)abstract
- Changes in the latitudinal position and strength of the Southern Hemisphere westerlies (SHW) are thought to be tightly coupled to important climate processes, such as cross-equatorial heat fluxes, Atlantic Meridional Overturning Circulation (AMOC), the bipolar seesaw, Southern Ocean ventilation and atmospheric CO2 levels. However, many uncertainties regarding magnitude, direction, and causes and effects of past SHW shifts still exist due to lack of suitable sites and scarcity of information on SHW dynamics, especially from the last glacial. Here we present a detailed hydroclimate multiproxy record from a 36.4-18.6 kyr old lake sediment sequence on Nightingale Island (NI). It is strategically located at 37ĝF S in the central South Atlantic (SA) within the SHW belt and situated just north of the marine Subtropical Front (SF). This has enabled us to assess hydroclimate changes and their link to the regional climate development as well as to large-scale climate events in polar ice cores. The NI record exhibits a continuous impact of the SHW, recording shifts in both position and strength, and between 36 and 31 ka the westerlies show high latitudinal and strength-wise variability possibly linked to the bipolar seesaw. This was followed by 4 kyr of slightly falling temperatures, decreasing humidity and fairly southerly westerlies. After 27 ka temperatures decreased 3-4 ĝC, marking the largest hydroclimate change with drier conditions and a variable SHW position. We note that periods with more intense and southerly-positioned SHW seem to be related to periods of increased CO2 outgassing from the ocean, while changes in the cross-equatorial gradient during large northern temperature changes appear as the driving mechanism for the SHW shifts. Together with coeval shifts of the South Pacific westerlies, our results show that most of the Southern Hemisphere experienced simultaneous atmospheric circulation changes during the latter part of the last glacial. Finally we can conclude that multiproxy lake records from oceanic islands have the potential to record atmospheric variability coupled to large-scale climate shifts over vast oceanic areas..
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| 21. |
- Björck, Svante, et al.
(författare)
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Aeolian sediment in raised bog deposits, Halland, SW Sweden: a new proxy record of Holocene winter storminess variation in southern Scandinavia?
- 2004
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Ingår i: The Holocene. - : SAGE Publications. - 0959-6836 .- 1477-0911. ; 14:5, s. 677-688
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Tidskriftsartikel (refereegranskat)abstract
- Cores of peat taken from two raised bogs in the near-coastal part of Halland, SW Sweden, were examined for their content of wind-transported clastic material. The Boarps Mosse core has a length of 262 cm and covers the last 2850 years (age model based on three radiocarbon dates), while the Hyltemossen core has a length of 347 cm and records the last 7000 years (age model based on eight radiocarbon dates). The peat contains aeolian quartz silt and sand (from trace amount to more than 10 weight % of the dry peat sample), and the grain-size composition of the minerogenic sediment indicates that the material is composed of both far-travelled dust and local wind-eroded silt and sand. Grains larger than 0.2 mm are present in many intervals and probably record niveo-aeolian transport in connection with severe snowstorms. A systematic count of quartz grains between 0.2-0.35 mm and larger than 0.35 mm was carried out to determine variations in aeolian sand influx (ASI) and winter wind climate with time. The Boarps Mosse site is situated in a hilly terrain and is primarily open to southerly and northerly winds, while the Hyltemossen site is situated in a more. at landscape open to winds from most directions. Furthermore, at Boarps Mosse dense forest stands seem to have protected the site from significant aeolian sand influx until about AD 1000, while aeolian sand influx apparently was less influenced by local forest development at Hyltemossen. However, using periods with low ASI values as marker horizons it is possible to identify a number of periods of high ASI values during the last 2500 years. These periods, which probably record high winter storminess, are dated to 130, 300, 400, 475, (700), 900, (1000), (1100), (1150), (1350), (1450), 1675, (1800), 1875, 1950-2000, (2050), 2150, 2225 and (2300) cal. yr BP. In addition, the Hyltemossen site documents peak storminess at 2450, 2725, 2850-3050, 3275, 3675, 4100-4200, 4550, 4900, 5100-5200 and 5450-5700 cal. yr BP. Most of the aeolian activity phases in Halland correlate with well-known cold periods in northwest Europe, suggesting a link between winter storminess in southern Scandinavia and expansion of the polar vortex.
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| 22. |
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| 23. |
- Björck, Svante, et al.
(författare)
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Anomalously mild Younger Dryas summer conditions in southern Greenland
- 2002
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 30:5, s. 427-430
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Tidskriftsartikel (refereegranskat)abstract
- The first late-glacial lake sediments found in Greenland were analyzed with respect to a variety of environmental variables. The analyzed sequence covers the time span between 14400 and 10500 calendar yr B.P., and the data imply that the conditions in southernmost Greenland during the Younger Dryas stadial, 12800-11550 calendar yr B.P., were characterized by an arid climate with cold winters and mild summers, preceded by humid conditions with cooler summers. Climate models imply that such an anomaly may be explained by local climatic phenomenon caused by high insolation and Fohn effects. It shows that regional and local variations of Younger Dryas summer conditions in the North Atlantic region may have been larger than previously found from proxy data and modeling experiments.
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- Björck, Svante, et al.
(författare)
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Consistently large marine reservoir ages in the Norwegian Sea during the Last Deglaciation
- 2003
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Ingår i: Quaternary Science Reviews. - 0277-3791. ; 22:5-7, s. 429-435
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Tidskriftsartikel (refereegranskat)abstract
- With the exception of the GS-1/Younger Dryas cold period marine reservoir ages for the Last Deglaciation in the North Atlantic-Norwegian Sea are generally assumed to have been around 400-500 radiocarbon years in magnitude (Earth Planet. Sci. Lett. 126 (1994) 275; Radiocarbon 37 (1995) 53; Quat. Res. 52 (1999) 104; Nature 412 (2001) 724). By comparing the climate records obtained from the GRIP ice-core (Nature 359 (1992) 311; J. Quat. Sci. 13(4) (1998) 283) and from North Atlantic/Norwegian Sea cores (Quat. Res. 52 (1999) 104; Geology 23 (12) (1995) 1059; Nature 356 (1991) 757; Nature 356 (1992) 757; Paleoceanography 3(1) (1988) 1; Nature 343 (1990) 612; Earth Planet. Sci. Lett. 126 (1994) 275), with radiocarbon-dated European continental records, we show that marine reservoir ages in the Norwegian Sea were of the order of 1000 C-14 yr, including large uncertainties. This approach rests on the reasonable assumption that climate changes throughout the NE Atlantic and Europe were more or less synchronous at the centennial scale. Fairly large variations in reservoir ages over time may have been caused by changing atmospheric C-14 content. The results indicate that detailed land-sea correlations for the North Atlantic during the Last Deglaciation are not feasible using radiocarbon dating alone. (C) 2003 Elsevier Science Ltd. All rights reserved.
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