| 1. |
- Aradottir, N., et al.
(författare)
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Drumlin formation within the Bustarfell drumlin field, northeast Iceland: integrating sedimentological and ground-penetrating radar data
- 2022
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Ingår i: Journal of Quaternary Science. - : Wiley. - 0267-8179 .- 1099-1417. ; 38:3, s. 386-402
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Tidskriftsartikel (refereegranskat)abstract
- Drumlins are important bedforms of former glaciated landscapes as they demonstrate past ice-flow directions and elucidate processes that operated at the ice/bed interface. Recently mapped drumlins and other streamlined subglacial bedforms in northeast Iceland reveal the flow-sets of cross-cutting palaeo-ice streams that were active within the Iceland Ice Sheet (IIS) during and following the Last Glacial Maximum. Here we study the Bustarfell drumlin field within the Vopnafjorour-Jokuldalsheioi flow-set. The internal architecture of two drumlins was investigated using sedimentological analysis and ground-penetrating radar (GPR, 50 and 100 MHz) to illuminate subglacial processes that contributed to drumlin formation, as well as the history and dynamics of the IIS. On the stoss side of one of the drumlins, two subglacial traction till units were identified, separated by a thick unit of deformed glaciofluvial sand and gravel. The core of glaciofluvial material suggests that the drumlin formed around well-drained patches (sticky spots) in the subglacial bed that retarded the ice flow locally through increased basal drag and encouraged till deposition. Furthermore, our GPR data indicate a combination of erosional and depositional processes. We suggest that the glaciofluvial sediments were deposited as small ice-marginal fans on the Bustarfell plateau, possibly during the Bolling-Allerod interstadial, and that the drumlins were formed around these fans during a subsequent readvance during the Younger Dryas.
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| 2. |
- Ingolfsson, O, et al.
(författare)
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Glacial and climate history of the Antarctic Peninsula since the Last Glacial Maximum
- 2003
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Ingår i: Arctic, Antarctic and Alpine Research. - 1938-4246. ; 35:2, s. 175-186
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Forskningsöversikt (refereegranskat)abstract
- During the Last Glacial Maximum (LGM), ice thickened considerably and expanded toward the outer continental shelf around the Antarctic Peninsula. Deglaciation occurred between >14 ka BP and ca. 6 ka BP, when interglacial climate was established in the region. Deglaciation of some local sites was as recent as 4-3 ka BP. After a climate optimum, peaking ca. 4-3 ka BP, a distinct climate cooling occurred. It is characterized at a number of sites by expanding glaciers and ice shelves. Rapid warming during the past 50 yr may be causing instability of some Antarctic Peninsula ice shelves. Detailed reconstructions of the glacial and climatic history of the Antarctic Peninsula since LGM are hampered by scarcity of available archives, low resolution of many datasets, and problems in dating samples. Consequently, the configuration of LGM ice sheets, pattern of subsequent deglaciation, and environmental changes are poorly constrained both temporally and spatially.
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| 3. |
- McCracken, R. G., et al.
(författare)
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Origin of the active drumlin field at Mulajokull, Iceland: New insights from till shear and consolidation patterns
- 2016
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791. ; 148, s. 243-260
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Tidskriftsartikel (refereegranskat)abstract
- Stratigraphic and morphologic data previously collected from the forefleld of Millajokull, Iceland, suggest that its recent surge cycles are responsible for the formation of drumlins there and that their relief reflects both deposition on drumlins and erosion between them. We have tested these ideas and aspects of leading models of drumlin formation by studying past patterns of bed deformation and effective stress in basal tills of the glacier's forefield. Patterns of till strain indicated by the anisotropy of magnetic susceptibility (AMS) of similar to 2300 intact till samples indicate that till was deposited during shear deformation, with shearing azimuths and planes that conform to the drumlin morphology. Thus, till deposition occurred as drumlins grew, in agreement with LiDAR data indicating that the degree of aggradation of the glacier forefleld is largest in areas subjected to the most surges. Previously described unconformities on the drumlin flanks, however, indicate that drumlin relief at Mulajokull has resulted, in part, from erosion. Given that the last surge deposited a till layer both on and between drumlins, a reasonable hypothesis is that erosion between drumlins occurred during normal (quiescent) flow of the glacier between surges. Densities of till samples, analyzed in conjunction with laboratory consolidation tests, indicate that effective stresses on the bed during such periods were on the order of 100 kPa larger between drumlins than within them, an observation consistent with subglacial channels at low water pressure occupying interdrumlin areas. Transport of sediment by turbulent flow in these channels or high effective stress adjacent to them causing enhanced till entrainment in ice or increased depths of bed deformation would promote the sediment flux divergence necessary to erode areas between drumlins. The observation that effective stresses were higher between drumlins than within them is the opposite of that presumed in leading models of drumlin formation. Moreover, the lack of AMS-fabric evidence of longitudinal compression in drumlin tills does not support some models of drumlin formation that invoke negative till-flux gradients in a deforming bed.
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| 4. |
- Möller, Per, et al.
(författare)
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Severnaya Zemlya, Arctic Russia: a nucleation area for Kara Sea ice sheets during the Middle to Late Quaternary
- 2006
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791. ; 25:21-22, s. 2894-2936
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Tidskriftsartikel (refereegranskat)abstract
- Quaternary glacial stratigraphy and relative sea-level changes reveal at least four expansions of the Kara Sea ice sheet over the Severnaya Zemlya Archipelago at 79 degrees N in the Russian Arctic, as indicated from tills interbedded with marine sediments, exposed in stratigraphic superposition, and from raised-beach sequences that occur at altitudes up to 140 m a.s.l. Chronologic control is provided by AMS C-14, electron-spin resonance, green-stimulated luminescence, and aspartic-acid geochronology. Major glaciations followed by deglaciation and marine inundation occurred during MIS 10-9, MIS 8-7, MIS 6-5e and MIS 5d-3. The MIS 6-5e event, associated with the high marine limit, implies ice-sheet thickness of > 2000m only 200km from the deep Arctic Ocean, consistent with published evidence of ice grounding at similar to 1000m water depth in the central Arctic Ocean. Till fabrics and glacial tectonics record repeated expansions of local ice caps exclusively, suggesting wet-based ice cap advance followed by cold-based regional ice-sheet expansion. Local ice caps over highland sites along the perimeter of the shallow Kara Sea, including the Byrranga Mountains, appear to have repeatedly fostered initiation of a large Kara Sea ice sheet, with exception of the Last Glacial Maximum (MIS 2), when Kara Sea ice did not impact Severnaya Zemlya and barely graced northernmost Taymyr Peninsula.
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| 5. |
- Svendsen, JI, et al.
(författare)
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Late quaternary ice sheet history of northern Eurasia
- 2004
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791. ; 23:11-13, s. 1229-1271
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Forskningsöversikt (refereegranskat)abstract
- The maximum limits of the Eurasian ice sheets during four glaciations have been reconstructed: (1) the Late Saalian (> 140 ka), (2) the Early Weichselian (100-80 ka), (3) the Middle Weichselian (60-50 ka) and (4) the Late Weichselian (25-15 ka). The reconstructed ice limits are based on satellite data and aerial photographs combined with geological field investigations in Russia and Siberia, and with marine seismic- and sediment core data. The Barents-Kara Ice Sheet got progressively smaller during each glaciation, whereas the dimensions of the Scandinavian Ice Sheet increased. During the last Ice Age the Barents-Kara Ice Sheet attained its maximum size as early as 90-80,000 years ago when the ice front reached far onto the continent. A regrowth of the ice sheets occurred during the early Middle Weichselian, culminating about 60-50,000 years ago. During the Late Weichselian the Barents-Kara Ice Sheet did not reach the mainland east of the Kanin Peninsula, with the exception of the NW fringe of Taimyr. A numerical ice-sheet model, forced by global sea level and solar changes, was run through the full Weichselian glacial cycle. The modeling results are roughly compatible with the geological record of ice growth, but the model underpredicts the glaciations in the Eurasian Arctic during the Early and Middle Weichselian. One reason for this is that the climate in the Eurasian Arctic was not as dry then as during the Late Weichselian glacial maximum.
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| 6. |
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| 7. |
- 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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| 8. |
- Cornell, David H., 1948, et al.
(författare)
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Zircon dating reveals an Archaean crust beneath the Kalahari sand.
- 2008
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Ingår i: Annual Meeting Mineralogical Network, Natural History Museum, University of Oslo, June 2008. ; abstracts
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Continents are subdivided into structural provinces based on the age of the last orogeny or major magmatic event which imprinted the structural fabric on the bedrock. These first-order subdivisions are important in understanding how the continents were assembled. The continuity and boundaries of provinces such as the Namaqua-Natal Province in Southern Africa have been established, however adjoining Kheis province which lies almost entirely under the Tertiary to Recent Kalahari sands, has thus far defied attempts to understand its age and geological history. A foreland thrust complex developed in 1.92 Ga redbeds at Olifantshoek (Fig. 1a) was postulated by Cornell et al. 1998. However the much younger 1.2 Ga to 1.1 Ga Sinclair Sequence volcanic and plutonic rocks exposed in the same area suggest that the Kheis Province could have experienced an Andean-type orogeny at ~1.2 Ga. Thus Moen (1999) suggested that much of the Kheis Province was coeval with and belonged to the ~1.2 Ga Namaqua-Natal Province. At Rietfontein on the eastern margin of the Kheis, tillites of the Permian Dwyka Group, were found to contain abundant cobble-sized clasts of crystalline bedrock. Bearing in mind the generally westward transport directions for the Dwyka Ice sheet (Fig. 1a), these exposures were investigated to see if they contained information about what lies under the Kalahari. These granotoid cobbles are badly weathered, but we succeeded in dating them using an ion probe and laser ablation ICPMS, by the U-Pb method. The data is largely discordant, reflecting ancient lead loss at the time of glaciation. Nevertheless we find compelling evidence that Archean and 2.0 Ga (Bushveld Complex age) granitic crust exists beneath the Kalahari. We also failed to find any evidence for the existence of younger granitic crust or high-grade metamorphic terranes. The published concepts of the Kheis Province as either a 1900 - 1750 Ma collisional orogen, or an extension of the 1200 Ma Namaqua Province find no support in our data. The possibility of Archean and Bushveld-aged rocks occuring as basement beneath the Kalahari has important implications for diamond, base and precious metal prospecting in the western Kalahari in South Africa, Botswana and Namibia. These possibilities should be tested by drilling with geophysical control, the last such campaign having been completed in Botswana in 1984.
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| 9. |
- Hjort, Christian, et al.
(författare)
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Holocene deglaciation and climate history of the northern Antarctic Peninsula region : a discussion of correlations between the Southern and Northern Hemispheres
- 1998
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Ingår i: ; , s. 110-112
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Konferensbidrag (refereegranskat)abstract
- The chronology of post-last Glacial Maximum dceglaciation in the northern Antarctic Peninsula region is discussed. It is concluded that, contrary to what was earlier believed, the deglaciation process here was largely out-of-phase with that in the Northern Hemisphere. Although, for global eustatic reasons, the marine-based glaciers may have retreated simultaneously with ice-melting in the Northern Hemisphere, the land-based glaciers retreated only slowly during the first half of the Hulocene, about 9000-5000 BP. This may have been due either to increased precipitation counterweighing ablation or to delayed warming: A distinct but rather brief glacial readvance took place. around 5000 BP, probably caused by a period of renewed cooling It was followed by the Holocene climatic optimum, about 4000-3000 Be This warm "hypsithermal" period thus came much later than its equivalent in the Northern Hemisphere, but it roughly coincided with the Milankovitchean Holocene insolation maximum for these southern latitudes.
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| 10. |
- Ingolfsson, O., et al.
(författare)
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Antarctic glacial history since the Last Glacial Maximum: an overview of the record on land
- 1998
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Ingår i: Antarctic Science. - 1365-2079. ; 10:3, s. 326-344
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Tidskriftsartikel (refereegranskat)abstract
- This overview examines available circum-Antarctic glacial history archives on land, related to developments after the Last Glacial Maximum (LGM). It considers the glacial-stratigraphic and morphologic records and also biostratigraphical information from moss banks, lake sediments and penguin rookeries, with some reference to relevant glacial marine records. It is concluded that Holocene environmental development in Antarctica differed from that in the Northern Hemisphere. The initial deglaciation of the shelf areas surrounding Antarctica took place before 10 000 14C yrs before present (BP), and was controlled by rising global sea level. This was followed by the deglaciation of some presently ice-free inner shelf and land areas between 10 000 and 8000 yr BP. Continued deglaciation occurred gradually between 8000 yr BP and 5000 yr BP. Mid-Holocene glacial readvances are recorded from various sites around Antarctica. There are strong indications of a circum-Antarctic climate warmer than today 4700-2000 yr BP. The best dated records from the Antarctic Peninsula and coastal Victoria Land suggest climatic optimums there from 4000-3000 yr BP and 3600-2600 yr BP, respectively. Thereafter Neoglacial readvances are recorded. Relatively limited glacial expansions in Antarctica during the past few hundred years correlate with the Little Ice Age in the Northern Hemisphere.
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