Endre søk
Begrens søket
1 - 32 of 32
RefereraExporteraLink til resultatlisten
Permanent link
Referera
Referensformat
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annet format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annet språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf
Treff pr side
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sortering
  • Standard (Relevans)
  • Forfatter A-Ø
  • Forfatter Ø-A
  • Tittel A-Ø
  • Tittel Ø-A
  • Type publikasjon A-Ø
  • Type publikasjon Ø-A
  • Eldste først
  • Nyeste først
  • Skapad (Eldste først)
  • Skapad (Nyeste først)
  • Senast uppdaterad (Eldste først)
  • Senast uppdaterad (Nyeste først)
  • Disputationsdatum (tidligste først)
  • Disputationsdatum (siste først)
  • Standard (Relevans)
  • Forfatter A-Ø
  • Forfatter Ø-A
  • Tittel A-Ø
  • Tittel Ø-A
  • Type publikasjon A-Ø
  • Type publikasjon Ø-A
  • Eldste først
  • Nyeste først
  • Skapad (Eldste først)
  • Skapad (Nyeste først)
  • Senast uppdaterad (Eldste først)
  • Senast uppdaterad (Nyeste først)
  • Disputationsdatum (tidligste først)
  • Disputationsdatum (siste først)
Merk
Maxantalet träffar du kan exportera från sökgränssnittet är 250. Vid större uttag använd dig av utsökningar.
  • 1.
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Brattforsheden: Vad sanden kan berätta om dess historia2007Annet (Annet (populærvitenskap, debatt, mm))
  • 2.
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Optiskt stimulerad luminiscensdatering (OSL) av nedisningshistorien i södra och mellersta Sverige2008Inngår i: SGU-rapport 2008-9: FoU-seminarium vid SGU 4 mars 2008 Dokumentation, 2008, s. 40-Konferansepaper (Annet vitenskapelig)
  • 3.
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi.
    Residual OSL signals from modern Greenlandic river sediments2007Inngår i: Geochronometria, ISSN 1897-1695, Vol. 26, s. 1-9Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Ripple-laminated sandy deposits at shallow water depths in four rivers on Jameson Land, East Greenland were sampled for optically stimulated luminescence (OSL) dating. Silt-sized grains have significantly higher equivalent doses (~1.1 Gy) than sand grains (~0.1 Gy). This suggests that coarse grain-size fractions are better bleached and more suitable than fine grains for OSL dating of glaciofluvial/fluvial sediments. A sample from a sidebar deposited during the spring flood yielded 1.0 Gy (~500 years) while a subaerial deposit was completely zeroed. The spring flood deposit is considered to be most similar to deglacial conditions and incomplete bleaching of this amount (1 Gy) is generally not a significant source of error for sediments of Pleistocene age. Most samples have rather poor luminescence characteristics and are affected by thermal transfer if preheat temperatures at or above 260°C are used.

  • 4.
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Sub-till glaciofluvial sediments at Hultsfred, South Swedish Upland2010Inngår i: GFF, ISSN 1103-5897, E-ISSN 2000-0863, Vol. 132, nr 3&4, s. 153-159Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In a gravel pit near Hultsfred, at the eastern margin of the South Swedish Upland, gravelly, sandy and silty-sandy beds covered by a thick, strati?ed gravelly diamicton are exposed. Large folds and shear-planes indicate post-depositional deformation of the sorted sediments, which according to OSL dating have a maximum Middle Weichselian (,60 ka) age. The sand and gravel are interpreted as glacio?uvial sediments that were deposited laterally during a Middle Weichselian deglaciation, or possibly subglacially during the Late Weichselian. The large-scale deformation was due to glaciotectonics, caused by an overriding wet-based ice sheet during the last deglaciation. The subglacial hydrostatical pressure was high and led to the formation of downward-injected clastic dykes. A basal till, which forms the cover moraine in the area, caps the succession. The fairly frequent occurrence of sub-till sediments on the South Swedish Upland demonstrates the limited effect of glacial erosion in parts of this area. During the Last Glacial Maximum, the South Swedish Upland was an ‘island’ of slow-moving, polythermal and at least partly non-erosive ice, surrounded by faster ice-streams in the Baltic and in the Skagerrak. The ice sheet may have left its main impact on the landscape during short periods during advance and deglaciation only.

  • 5.
    Alexanderson, Helena
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Eskola, Kari O
    Helmens, Karin F
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Optical dating of a Late Quaternary sediment sequence from Sokli, northern Finland2008Inngår i: Geochronometria, Vol. 32, s. 51-59Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Interstadial and non-glacial stadial sediments collected in boreholes from Sokli in northeastern Finland have been dated by optically stimulated luminescence on quartz and feldspar grains. The quartz OSL ages follow stratigraphic order, with one exception, and support the litho- and biostratigraphical correlation with the NW European mainland climate-stratigraphy and the marine oxygen-isotope stages. Feldspar IRSL dates generally overestimate the age, probably due to incomplete bleaching.

    The data show that during the last glacial cycle northeastern Finland was not glaciated until MIS 5b, around 90 ka. Interstadial conditions occurred around ~94 ka (MIS 5c), ~74-80 ka (MIS 5a) and 42-54 ka (MIS 3).

    The OSL ages have large standard errors mainly due to small sample sizes, relatively poor luminescence characteristics and uncertainties in dose-rate determinations.

  • 6.
    Alexanderson, Helena
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Henriksen, Mona
    Landvik, Jon Y
    Ryen, Heidi T
    Chronologies of the glacial history of Svalbard – a review2010Inngår i: Arctic Palaeoclimate and its Extremes (APEX) abstract volume, 2010, s. 21-21Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    The basis of the glacial history of Svalbard lies in stratigraphic studies of terrestrial and marine geological records. Classic litho- and biostratigraphy give us local relative chronologies of events but to compare with other areas and records we are dependent on absolute age control.

     

    This presentation will focus on the Middle to Late Quaternary terrestrial record to which a range of different absolute dating techniques have been applied; these techniques all have their pros and cons regarding materials, resolution, age range, etc. Amino acid chronology, although not a strictly absolute dating method, may provide some age inferences. Radiocarbon dating of organic material such as shell and bone is still the most common technique and one that is generally considered reliable. It is however limited by its age range to the last ~50 ka, and our interest goes further back than so. Various versions of luminescence dating, particularly optically stimulated luminescence (OSL), have therefore become strong contenders during the recent decade, mainly due to their longer age range and applicability to a wider range of deposits than radiocarbon dating. Luminescence dating nevertheless has its drawbacks too, for Svalbard these are mainly related to the risk of incomplete bleaching (leading to age overestimation) and poor resolution (leading to large uncertainties for final ages). Other techniques that have been used on fossil material include electron spin resonance (ESR) and uranium-thorium (U/Th) dating. In common for most techniques is that we cannot date glacial events, but the intervening non-glacial (interstadial, interglacial) episodes. Cosmogenic exposure dating is an exception, which is also able to provide information on the thickness of ice sheets through time.

     

    From a geologist’s point of view we will discuss these different techniques, their usefulness and what we can expect of them. Do the dating methods meet our wishes regarding resolution? Are results from different methods comparable? The discussion will be based on our experience from the ongoing SciencePub project (www.ngu.no/sciencepub) and on literature and we will show examples of method applications and comparisons.

  • 7.
    Alexanderson, Helena
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Håkansson, Lena
    Dating the glacial history of northeast Greenland2007Inngår i: First Conference on Arctic Palaeoclimate and its Extremes, 2007Konferansepaper (Annet (populærvitenskap, debatt, mm))
  • 8.
    Alexanderson, Helena
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Johnsen, Timothy
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Murray, Andrew S.
    Nordic Laboratory for Luminescence Dating, Department of Earth Sciences, Aarhus University Risø DTU, DK-4000 Roskilde, Denmark .
    Re-dating the Pilgrimstad Interstadial with OSL: a warmer climate and a smaller ice sheet during the Swedish Middle Weichselian (MIS 3)?2010Inngår i: Boreas, ISSN 0300-9483, E-ISSN 1502-3885, Vol. 39, nr 2, s. 367-376Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Pilgrimstad in central Sweden is an important locality for reconstructing environmental changes during the last glacial period (the Weichselian). Its central location has implications for the Scandinavian Ice Sheet as a whole. The site has been assigned an Early Weichselian age (marine isotope stage (MIS) 5 a/c; >74 ka), based on pollen stratigraphic correlations with type sections in continental Europe, but the few absolute dating attempts so far have given uncertain results. We re-excavated the site and collected 10 samples for optically stimulated luminescence (OSL) dating from mineral- and organic-rich sediments within the new Pilgrimstad section. Single aliquots of quartz were analysed using a post-IR blue single aliquot regenerative-dose (SAR) protocol. Dose recovery tests were satisfactory and OSL ages are internally consistent. All, except one from an underlying unit that is older, lie in the range 52–36 ka, which places the interstadial sediments in the Middle Weichselian (MIS 3); this is compatible with existing radiocarbon ages, including two measured with accelerator mass spectrometry (AMS). The mean of the OSL ages is 44±6 ka (n=9). The OSL ages cannot be assigned to the Early Weichselian for all reasonable adjustments to water content estimates and other parameters. The new ages suggest that climate was relatively mild and that the Scandinavian Ice Sheet was absent or restricted to the mountains for at least parts of MIS 3. These results are supported by other recent studies completed in Fennoscandia.

  • 9.
    Alexanderson, Helena
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Landvik, Jon Y.
    Ryen, Heidi T.
    Chronology and styles of glaciation in an inter-fjord setting, northwestern Svalbard2011Inngår i: Boreas, ISSN 0300-9483, E-ISSN 1502-3885, Vol. 40, nr 1, s. 175-197Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A 30-m-thick sedimentary succession at Leinstranda on the southwestern coast of Brøggerhalvøya, northwestern Svalbard, spans the two last glacial–interglacial cycles and reveals information on glacial dynamics, sea-level changes and the timing of these events. We investigated the deposits using standard stratigraphical and sedimentological techniques, together with ground-penetrating radar, and established an absolute chronology based mainly on optically stimulated luminescence dating. We identi?ed facies associations that represent depositional settings related to advancing, overriding and retreating glaciers, marine and littoral conditions and periglacial surfaces. The environmental changes show an approximate cyclicity and reflect glaciations followed by high sea levels and later regression. The luminescence chronology places sea-level highstands at 185±8 ka, 129±10 ka,99±8 ka and 36±3 ka. These ages constrain the timing of recorded glaciations at Leinstranda to prior to c. 190 ka, between c. 170 and c. 140 ka (Late Saalian) and between c. 120 ka and c. 110 ka (Early Weichselian). The glaciations include phases with glaciers from three different source areas. There is no positive evidence for either Middle or Late Weichselian glaciations covering the site, but there are hiatuses at those stratigraphic levels. A high bedrock ridge separates Leinstranda from the palaeo-ice stream in Kongsfjorden, and the deposits at Leinstranda reflect ice-dynamic conditions related to ice-sheet evolution in an inter-fjord area. The environmental information and theabsolute chronology derived from our data allow for an improved correlation with the marine record, and for inferences to be made about the interaction between land, ocean and ice during the last glacial–interglacial cycles.

  • 10.
    Alexanderson, Helena
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Murray, Andrew
    Problems and potential of OSL-dating deglacial and interstadial sediments in Sweden2009Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    Several recent OSL-ages from Sweden have been inconsistent with geological interpretation, commonly due to age overestimation, and have been met with some scepticism. We explore the problems and potential of OSL-dating in Sweden by analysing quartz OSL-ages from several known-age deglacial sites in southern and central Sweden and by investigating their luminescence properties. Two problems for obtaining good and reliable ages are incomplete bleaching and low-sensitivity quartz. Incomplete bleaching is partly, but not entirely, the cause of age overestimation and can be avoided by selecting suitable sediment facies. Low sensitivity leads to practical problems in measurements and larger uncertainties in dose estimates. On the other hand, potential for good OSL-dating is shown by high-sensitivity quartz from sites that contain both Dala sandstone clasts and sediments that may have had extensive reworking during the Quaternary.

    This knowledge is useful for interpreting OSL-results from unknown-age interstadial sites. Most interstadial sites in Sweden contain only fragmentary records and represent single events and OSL-ages from such sites lack a context to which the results can be related. It is for example rarely possible to get OSL-samples from a stratigraphic succession that spans a significantly longer time than the resolution of the method or find sites where dates from more than one stratigraphically overlying interglacial, interstadial or deglacial bed can be compared. As interstadial OSL-ages thus largely have to stand on their own, multiple samples and detailed data analyses are necessary to give credibility to the results given the Swedish record of OSL-difficulties. From a luminescence point of view, sites fulfilling the positive criteria identified for deglacial deposits should be selected, but the setting and nature of the most geologically interesting interstadial sites are not always obliging.

  • 11.
    Alexanderson, Helena
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Murray, Andrew S
    Problems and potential of OSL dating Weichselian and Holocene sediments in Sweden2010Inngår i: Abstracts and Proceedings of the Geological Society of Norway, NGF Number 1, 2010, 2010, s. 3-4Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    Recent OSL dating of various Late Quaternary deposits in Sweden has resulted in several sets of ages that appear inconsistent with geological interpretation; they usually overestimate the expected age. We explore the problems and potential of OSL dating in Sweden by analysing quartz OSL-ages from several known-age sites in Sweden and by investigating their luminescence properties. Two difficulties in obtaining accurate and precise ages are incomplete bleaching and low-sensitivity quartz. Incomplete bleaching can give rise to age overestimation, but the importance of this effect can be minimised by selecting suitable sediment facies, although for some settings unexplained overestimates still remain. Low quartz OSL sensitivity leads to practical problems in measurements and larger uncertainties in dose estimates, but not necessarily to systematic age overestimation. Sites that contain Dala sandstone clasts and/or sediments that may have had extensive reworking during the Quaternary seem to provide high-sensitivity quartz. We consider such sites to show the most potential for accurate and precise OSL-dating in Sweden.

    For typical Swedish material, the scatter in doses makes OSL-ages less precise and the method cannot usually compete with radiocarbon dating for young (Late Weichselian-Holocene) sediments unless the deposits simply lack organic material. The lowest overall uncertainty for the samples in this study is 4% of the age (average is 7%) and we expect that OSL ages from Sweden cannot be much better known than this. A 5-10% precision in age is, however, usually not a problem for older sediments and OSL can advantageously be used to date for example interstadial deposits. Nevertheless, given the Swedish record of OSL-difficulties it is important to work with multiple samples and adapt analytical protocols to each site or sample to give credibility to the results.

  • 12.
    Alexanderson, Helena
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi.
    Murray, Andrew S.
    Was southern Sweden ice free at 19–25 ka, or were the post LGM glacifluvial sediments incompletely bleached?2007Inngår i: Quaternary Geochronology, ISSN 1871-1014, Vol. 2, nr 1-4, s. 229-236Artikkel i tidsskrift (Annet (populærvitenskap, debatt, mm))
    Abstract [en]

    Glacifluvial deposits along an ice-marginal zone in Småland, southern Sweden, have been dated using post-IR blue OSL. To test for incomplete bleaching, we adopted two strategies: analysis of modern analogues and small-aliquot dose distributions. Samples of modern fluvial sediments show no significant incomplete bleaching; they yield equivalent doses of only 0.5-2 Gy (~0.25-4% of our glacifluvial sediment doses). Small-aliquot dose distributions do not provide any evidence for incomplete bleaching. The sediments are believed to have been deposited during deglaciation and appear to fall into two age groups: 19-25 ka (mainly sandur sediments) and 33-73 ka (mainly deltaic sediments). Compared to the expected ages (13-15 ka), even the younger glacifluvial OSL ages appear up to 10 000 years (~25 Gy) too old. The ages are nevertheless stratigraphically consistent and correspond between sites; we deduce that the 19-25 ka ages are true deposition ages. For glacifluvial sedimentation to take place on the South Swedish Upland at this time either a very early deglaciation is required, or alternatively ice-free conditions just prior to the LGM. The deltaic sediments (33-73 ka) were most likely not significantly bleached during deposition and thus they date events prior to the latest ice advance.

  • 13. Curry, B. Brandon
    et al.
    Konen, Michael E.
    Larson, Timothy H.
    Yansa, Catherine H.
    Hackley, Keith C.
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Lowell, Thomas V.
    The DeKalb mounds of northeastern Illinois as archives of deglacial history and postglacial environments2010Inngår i: Quaternary Research, ISSN 0033-5894, E-ISSN 1096-0287, Vol. 74, nr 1, s. 82-90Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The ""type"" DeKalb mounds of northeastern Illinois, USA (42.0 degrees N, -88.7 degrees W), are formed of basal sand and gravel overlain by rhythmically bedded fines, and weathered sand and gravel. Generally from 2 to 7 m thick, the fines include abundant fossils of ostracodes and uncommon leaves and stems of tundra plants. Rare chironomid head capsules, pillclam shells, and aquatic plant macrofossils also have been observed. Radiocarbon ages on the tundra plant fossils from the ""type"" region range from 20,420 to 18,560 cal yr BP. Comparison of radiocarbon ages of terrestrial plants from type area ice-walled lake plains and adjacent kettle basins indicate that the topographic inversion to ice-free conditions occurred from 18,560 and 16,650 cal yr BP. Outside the ""type"" area, the oldest reliable age of tundra plant fossils in DeKalb mound sediment is 21,680 cal yr BP; the mound occurs on the northern arm of the Ransom Moraine (-88.5436 degrees W, 41.5028 degrees N). The youngest age, 16,250 cal yr BP, is associated with a mound on the Deerfield Moraine (-87.9102 degrees W, 42.4260 degrees N) located about 9 km east of Lake Michigan. The chronology of individual successions indicates the lakes persisted on the periglacial landscape for about 300 to 1500 yr.

  • 14.
    Helmens, Karin
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi.
    Johansson, Peter W
    Räsänen, Matti E
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi.
    Eskola, Kari O
    Ice-free intervals continuing into Marine Isotope Stage 3 at Sokli in the central area of the Fennoscandian glaciations2007Inngår i: Bulletin of the Geological Society of Finland, Vol. 79, s. 17-39Artikkel i tidsskrift (Fagfellevurdert)
  • 15.
    Heyman, Jakob
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Stroeven, Arjen
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Caffee, Marc W
    Department of Physics/Purdue Rare Isotope Measurement Laboratory, Purdue University, USA.
    Li, Yingkui
    Department of Geography, University of Missouri-Columbia, USA.
    Harbor, Jon
    Department of Earth and Atmospheric Sciences, Purdue University, USA.
    Hättestrand, Clas
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Zhou, Liping
    Department of Geography, Peking University, China.
    Constraining the glacial chronology of Bayan Har Shan, NE Tibetan Plateau – Cosmogenic exposure dating of boulders, surface pebbles/cobbles and sediment depth profiles2009Inngår i: Geophysical Research Abstracts: Vol. 11, EGU2009-12053, 2009, 2009Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The paleoglaciology of the Tibetan Plateau remains elusive, with important hiata regarding the style, extent, and timing of glaciations. Bayan Har Shan is a mountain region on the northeastern Tibetan Plateau, in a transition zone from the dry interior of the plateau in the west to the wetter eastern margin affected by the Asian monsoon. Bayan Har Shan hosts an ample record of glacial landforms and deposits indicating paleo-glaciers ranging from cirque and valley glaciers to ice-fields and ice caps. These glaciers, it has been suggested, also nourished a regional ice sheet. In an attempt to constrain the timing of glaciations in Bayan Har Shan, we have performed terrestrial cosmogenic nuclide (TCN) exposure dating on surface boulders and pebbles/cobbles from glacial deposits, and on pebbles in sediment depth profiles. The aim has been two-fold: to constrain the glacial chronology and to compare and evaluate the TCN ages of the three different TCN sample types.

    We present the result of 67 Be-10 measurements from 15 sites in central Bayan Har Shan (40 boulder samples, 12 surface pebbles/cobbles samples and 15 depth profile samples from four depth profiles). The obtained TCN apparent exposure ages of boulders and surface pebbles/cobbles range from 3 ka to 145 ka with wide age spreads within groups of samples collected from one glacial deposit. Our TCN results of three different sample types (boulders, surface pebbles/cobbles and depth profile pebbles) from the northeastern Tibetan Plateau form an intriguing data set that may yield different age estimates with different interpretation strategies. However, they permit the following conclusions to be advanced:

    • Pebbles/cobbles ages are broadly in agreement with boulder ages.

    • Three depth profiles yield exponential curves for Be-10 concentrations with depth, in agreement with theoretical TCN depth profiles; ages are in broad agreement with boulder and surface pebbles/cobbles samples.

    • Maximum ages (adopting an approach where the maximum ages constrain the minimum age of formation) of multiple sample sites are all c. 50 ka or older. This is underlined by the maximum ages around 50 ka from three moraines formed by glaciers just a few kilometres long, indicating that there has been no significant glaciation of central Bayan Har Shan over the last 50 ka.

  • 16.
    Heyman, Jakob
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Stroeven, Arjen
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Hättestrand, Clas
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Li, Yingkui
    Harbor, Jon
    Caffee, Marc
    Zhou, Liping
    Veres, Daniel
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Glacial landforms and deposits of the Bayan Har Shan, NE Tibetan plateau – a dataset for reconstructing the extent of former glaciations2008Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Glacial reconstructions of the Tibetan plateau range from a plateau-scale ice sheet to restricted valley glaciers and ice caps. However, the Tibetan glacial landforms and sediments – although forming a crucial tool for paleoglaciological reconstructions – have rarely been mapped for larger areas. We here present data on the glacial landforms and deposits in the Bayan Har Shan area on the northeastern Tibetan plateau, previously suggested to have nourished the most extensive Quaternary Tibetan ice mass. Detailed geomorphological mapping based on remote sensing and extensive field studies reveal a generous array of glacial landforms and deposits, indicating former glaciers of varying extent. Large scale glacial landforms mapped from a digital elevation model and satellite imagery are abundant in elevated mountain blocks. The mapped landforms testify of alpine style glaciation but lend no support to the existence of any ice sheet. Field observations of glacial, and non-glacial, deposits further enhance the dataset concerning former glacial extent. Tills and erratic boulders are present within the glacial landscape based on remote sensing, but in several localities they also exist further down some distance outside mapped glacial landforms. There is a notable absence of glacial deposits around the Huang He valley and in the northern part of the study area, where they have previously been reported as evidence of a paleo-ice sheet. We argue for a non-glacial origin of deposits in these areas, because we have not found any indications of a glacial origin. The mapped landforms and deposits display an interesting dataset for paleoglaciological reconstructions. While the glacial landforms from remote sensing – by virtue of completeness covering extensive areas – present a good image of the more restricted glaciations, the identified most extensive glaciation is so far only recorded as point data in the form of glacial deposits.

  • 17.
    Heyman, Jakob
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Stroeven, Arjen
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Hättestrand, Clas
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Li, Yingkui
    Harbor, Jon
    Caffee, Marc
    Zhou, Liping
    Veres, Daniel
    Reconstructing former glacial extent of the NE Tibetan Plateau – combining remote sensing and field data of glacial geology2008Inngår i: Geophysical Research Abstracts, 2008Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Glacial reconstructions of the Tibetan Plateau range from a plateau-scale ice sheet to restricted valley glaciers and ice caps. However, glacial landforms and sediments – although forming a crucial fundament for paleoglaciological reconstructions – have rarely been mapped for extensive areas of the Tibetan Plateau. The NE Tibetan Plateau hosts a wide array of glacial landforms and deposits, and the area has been suggested to have nourished an extensive Quaternary ice mass on the Tibetan Plateau – the Huang He ice sheet. We here present data on the glacial geology of the Bayan Har Shan area, NE Tibetan Plateau, based on two diverse methods: remote sensing and field observations. Using the SRTM 90 m resolution digital elevation model, Landsat ETM+ satellite images and Google EarthTM imagery, a detailed mapping of the glacial geomorphology for a 135.000 km2 area has been performed. Mapped landforms include glacial valleys/troughs, marginal moraines, glacial lineations, meltwater channels and hummocky terrain. During 2005-2007 field work we have gathered data on glacial and non-glacial deposits. Deposits affirmative of glacial action occur in the form of till, glaciofluvial sediments and erratic boulders. Using a simple identification scheme, based on the abundance of erratic boulders, striated clasts and presence of diamictic sediments, we have mapped the occurrence of glacial deposits.

    The remote sensing and field data in general strongly support the presence of former glaciers centred on mountain blocks, and offers no support for the former existence of an ice sheet. However, there is a discrepancy between the glacial geomorphology mapped by remote sensing and the distribution of glacial deposits as mapped in the field. Glacial landforms mapped by remote sensing indicate former glaciers of varying extent, ranging from cirque glaciers to extended valley glacier networks, with glacial U-shaped valleys up to 60 km long. Whereas glacial deposits occur most frequently in the areas of mapped glacial landforms, they also occur up to 25 km outside mapped glacial landforms and indicate ice cap/ice field glaciation, presumably predating more restricted glaciations marked by marginal moraines and meltwater channels. The presence of glacial deposits in the absence of glacial morphology has implications for the large-scale glacial imprint, as glacial landforms of the most extensive glaciation(s) have either been eroded/degraded, or been buried by subsequent deposits, or else were never been formed. On the basis of an absence of erosional morphology, we conclude that erosion by such an enlarged ice cap/ice field beyond the mountains has been negligible.

  • 18.
    Heyman, Jakob
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Stroeven, Arjen
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Hättestrand, Clas
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Harbor, Jon
    Zhou, Liping
    Dong, Jianyi
    Li, Yinkui
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Caffee, Marc
    Ma, Haizhou
    Liu, Gengnian
    Landscape evolution of the northeastern Tibetan plateau - relict surfaces and fluvial margins2007Inngår i: Geophysical Research Abstracts, 2007Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The actively uplifting Tibetan plateau has a profound impact on climate and displays a landscape marked by geomorphological action. This is because the uplift is counteracted by intense fluvial incision of some of the world’s largest rivers and their tributaries that drain the plateau. Glaciers and glacial landforms occur predominantly in and around the highest elevation areas. By investigating the imprints of glacial and fluvial erosion we can enhance our understanding of the long-term landscape evolution, as well as illuminate the paleoglaciology of the Tibetan plateau. We here present an investigation of the large-scale geomorphology of the northeastern Tibetan plateau and its implication for landscape evolution and paleoenvironmental reconstructions.

    The northeastern part of the Tibetan plateau is characterized by a plateau surface at c. 4300 m asl with higher mountain groups reaching up to 1500 m above the surrounding plateau surface. We used SRTM 90 m digital elevation model, satellite images and Google Earth imagery to map the large-scale geomorphology for an area of c. 135.000 km2 centered around the Bayan Har mountains. Our mapping reveals a clear pattern of substantial glacial erosion on the highest, central parts of the mountain areas and decreasing amounts of glacial erosion with decreasing elevation and increasing distance away from these centers of glaciation. Beyond the areas of glacial erosion, there is a low-relief fluvial landscape that typifies the rest of the plateau surface. The plateau margins are formed by steep fluvial valleys which cut backwards into the gentle sloping relict plateau surface. Thus, the overall landscape may be divided into three classes; (i) glacially eroded surfaces in the highest areas, (ii) a relict, low-relief plateau surface, and (iii) a steep, fluvial landscape juxtaposing the former two classes.

    The distribution of the different landscapes indicates the following temporal evolution of the landscape. The glacial landforms indicate a repeated glaciation of the mountain areas. The steep fluvial valleys consuming the relict plateau surface represent an ongoing adjustment of the river channels to the actively uplifting plateau margin. The pattern of abandoned fluvial erosion of the northern part of the study area supports the notion of a stepwise uplift. This is because progressively younger uplift of the northern parts of the area induced a piracy of originally N-flowing rivers to currently ESE-flowing rivers along major faults (such as we infer for the Huang He river). It is noteworthy that the outline of the relict landscape, that is the pronounced break in slope between the low-relief relict landscape and the young fluvial landscape, coincides almost completely with the outline of a hypothesized former ice sheet, the Huang He ice sheet. We have not been able to confirm the presence of geomorphology or stratigraphy that would support this reconstruction. If true, however, our notion of outline conformance could indicate that the Huang He ice sheet may actually have been larger than suggested and that glacial traces are being consumed by the fluvial incision triggered by plateau uplift.

  • 19.
    Heyman, Jakob
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Stroeven, Arjen P
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Hättestrand, Clas
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Harbor, Jon
    Department of Earth and Atmospheric Sciences, Purdue University, USA.
    Li, Yingkui
    Department of Geography, University of Tennessee, USA.
    Caffee, Marc W
    Department of Physics, Purdue Rare Isotope Measurement Laboratory, Purdue University, USA.
    Zhou, Liping
    Department of Geography, Peking University, China.
    Veres, Daniel
    'Emil Racovita' Institute of Speleology, Romania.
    Liu, Feng
    Department of Geography, Peking University, China.
    Machiedo, Martin
    Department of Geology, University Centre in Svalbard (UNIS), Norway.
    Palaeoglaciation of Bayan Har Shan, northeastern Tibetan Plateau: glacial geology indicates maximum extents limited to ice cap and ice field scales2009Inngår i: Journal of Quaternary Science, ISSN 0267-8179, E-ISSN 1099-1417, Vol. 24, nr 7, s. 710-727Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Key locations within an extensive area of the northeastern Tibetan Plateau, centred on Bayan Har Shan, have been mapped to distinguish glacial from non-glacial deposits. Prior work suggests palaeo-glaciers ranging from valley glaciers and local ice caps in the highest mountains to a regional or even plateau-scale ice sheet. New field data show that glacial deposits are abundant in high mountain areas in association with large-scale glacial landforms. In addition, glacial deposits are present in several locations outside areas with distinct glacial erosional landforms, indicating that the most extensive palaeo-glaciers had little geomorphological impact on the landscape towards their margins. The glacial geological record does indicate extensive maximum glaciation, with local ice caps covering entire elevated mountain areas. However, absence of glacial traces in intervening lower-lying plateau areas suggests that local ice caps did not merge to form a regional ice sheet on the northeastern Tibetan Plateau around Bayan Har Shan. No evidence exists for past ice sheet glaciation.

  • 20.
    Heyman, Jakob
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Stroeven, Arjen P.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Caffee, Marc W.
    Hattestrand, Clas
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Harbor, Jonathan M.
    Li, Yingkui
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Zhou, Liping
    Hubbard, Alun
    Palaeoglaciology of Bayan Har Shan, NE Tibetan Plateau: exposure ages reveal a missing LGM expansion2011Inngår i: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 30, nr 15-16, s. 1988-2001Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The Bayan Har Shan, a prominent upland area in the northeastern sector of the Tibetan Plateau, hosts an extensive glacial geological record. To reconstruct its palaeoglaciology we have determined (10)Be exposure ages based on 67 samples from boulders, surface pebbles, and sediment sections in conjunction with studies of the glacial geology (remote sensing and field studies) and numerical glacier modelling. Exposure ages from moraines and glacial sediments in Bayan Har Shan range from 3 ka to 129 ka, with a large disparity in exposure ages for individual sites and within the recognised four morphostratigraphical groups. The exposure age disparity cannot be explained by differences in inheritance without using unrealistic assumptions but it can be explained by differences in post-depositional shielding which produces exposure ages younger than the deglaciation age. We present a palaeoglaciological time-slice reconstruction in which the most restricted glaciation, with glaciers less than 10 km long, occurred before 40-65 ka. More extensive glaciations occurred before 60-100 ka and 95-165 ka. Maximum glaciation is poorly constrained but probably even older. The Bayan Hat Shan exposure age dataset indicates that glaciers on the northeastern Tibetan Plateau have remained surprisingly restricted for at least 40 ka, including the global last glacial maximum (LGM). This case of a missing LGM is further supported by high-resolution glacier modelling experiments.

  • 21.
    Heyman, Jakob
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Stroeven, Arjen P
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Caffee, Marc W
    Department of Physics, Purdue Rare Isotope Measurement Laboratory, Purdue University.
    Hättestrand, Clas
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Harbor, Jon
    Department of Earth and Atmospheric Sciences, Purdue University.
    Li, Yingkui
    Department of Geography, University of Tennessee, Knoxville.
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Zhou, Liping
    Department of Geography, Peking University.
    Hubbard, Alun
    Institute of Geography and Earth Sciences, Aberystwyth University.
    Palaeoglaciology of Bayan Har Shan, NE Tibetan Plateau: the case of a missing LGM expansionManuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    The Bayan Har Shan, a prominent upland area in the northeastern sector of the Tibetan Plateau, hosts an extensive glacial geological record. To reconstruct its palaeoglaciology we have determined 10Be apparent exposure ages based on 67 samples from boulders, surface pebbles, and sediment sections in conjunction with studies of the glacial geology (remote sensing and field studies) and numerical glacier modelling. Apparent exposure ages from moraines and glacial sediments in Bayan Har Shan range from 3 ka to 129 ka, with a large disparity in ages for individual sites and within the recognised four morphostratigraphical groups. The age disparity is inexplicable as arising from differences in inheritance without the application of unrealistic assumptions but it can be explained as arising from differences in post-glacial shielding, yielding exposure ages younger than the deglaciation age. We present a palaeoglaciological time-slice reconstruction in which the most restricted glaciation, with glaciers less than 10 km long, occurred before 40-65 ka. More extensive glaciations occurred before 60-100 ka and 95-165 ka. Maximum glaciation is poorly constrained but probably even older. The Bayan Har Shan exposure age dataset indicates that glaciers on the northeastern Tibetan Plateau have remained surprisingly restricted for at least 40 ka, including the global last glacial maximum (LGM). This case of a missing LGM is supported by high-resolution glacier modelling experiments.

  • 22.
    Heyman, Jakob
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Stroeven, Arjen P
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Caffee, Marc W
    Department of Physics/Purdue Rare Isotope Measurement Laboratory, Purdue University, USA.
    Li, Yingkui
    Department of Geography, University of Tennessee, USA.
    Zhou, Liping
    Department of Urban and Environmental Sciences, Peking University, China.
    Liu, Gengnian
    Department of Urban and Environmental Sciences, Peking University, China.
    Hättestrand, Clas
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Fu, Ping
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Harbor, Jon
    Department of Earth and Atmospheric Sciences, Purdue University, USA.
    An evaluation of multiple working hypotheses to explain cosmogenic exposure age data from glacial deposits in the Bayan Har Shan, NE Tibetan Plateau2009Inngår i: Proceedings, 2009Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Many questions remain unanswered regarding the Quaternary glaciations of the Tibetan Plateau. We have used terrestrial cosmogenic nuclide (TCN) exposure age dating of glacial deposits to examine the style, extent, and timing of past glaciations of the Bayan Har Shan, a mountain region on the northeastern Tibetan Plateau. This area lies within a transition zone between the dry interior of the Tibetan Plateau and the wetter eastern margin affected by the Asian monsoon. Bayan Har Shan has many glacial landforms and deposits that provide evidence for former glaciation ranging from cirque and valley glaciers to ice-fields and ice caps.

    In an attempt to constrain the timing of glaciations in Bayan Har Shan, we have performed TCN exposure dating on 65 samples in central Bayan Har Shan from glacial deposits. boulders (39 samples), on surface pebbles/cobbles (12 samples), and on pebbles in sediment depth profiles (14 samples from four profiles) allow us to examine the timing and extent of glaciations in this area. As is often the case, there are some challenges in interpreting the range of TCN apparent exposure ages that is found in data from several samples and sample types on a single deposit and from samples taken at various sites. Thus we evaluate multiple working hypotheses to explain apparent exposure ages on glacial deposits, which in this case range from 3 ka to 129 ka. We consider three different hypotheses; 1) some samples have erroneously old exposure ages due to inheritance, 2) samples have been preserved under cold-based, non-erosive ice, and 3) samples have experienced only post-glacial shielding. Only when we adopt a hypothesis that assumes no prior exposure, and thus that maximum apparent exposure ages constrain the minimum age of formation of a feature (working hypotheses 3), do we find broad consistency between apparent exposure ages from different sample types (erratic boulders, surface pebbles/cobbles and pebbles from depth profiles). This leads to the conclusion that all of the sites of former glaciations we examined are at least 50ka in age, and that there has been no large-scale expansion of glaciers in the central Bayan Har Shan over the last 50ka.

  • 23.
    Heyman, Jakob
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Stroeven, Arjen P
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Hättestrand, Clas
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Caffee, Marc W
    Department of Physics, PRIME Lab, Purdue University.
    Fu, Ping
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Harbor, Jon
    Department of Earth And Atmospheric Sciences, Purdue University.
    Hubbard, Alun
    Institute of Geography and Earth Sciences, Aberystwyth University.
    Li, Yingkui
    Department of Geography, University of Tennessee.
    Zhou, Liping
    Department of Geography, Peking University.
    LGM Tibetan Plateau glaciers were not much larger than today2010Inngår i: Geophysical Research Abstracts, 2010Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The Tibetan Plateau is the largest and highest elevated area on Earth with consequential impacts on regional (monsoon development) and global (CO2 sequestering) climate patterns and evolution, and with its glaciers providing meltwater for some of the largest rivers of the world. The glacial history of the Tibetan Plateau is dominantly characterized by glaciers and ice caps centered on elevated mountain regions of the plateau, as evidenced by an extensive glacial geological record. Here we present the outcome of a five year project aiming towards a palaeoglaciological reconstruction for the Bayan Har Shan region of the northeastern Tibetan Plateau. We have used remote sensing, field studies and 10Be exposure ages towards a robust reconstruction of former glaciation. Glacial landforms and sediments in Bayan Har Shan, distributed around elevated mountain areas, indicate a maximum Quaternary glaciation significantly larger than today. We have dated 40 boulders, 12 surface pebbles samples, and 15 depth profile samples (in 4 depth profiles) from 15 sites (mainly moraine ridges) using 10Be exposure dating. Our boulder and pebble exposure ages range from 3 ka to 128 ka with large age spreads within populations of individual sites. Based on the premise that cosmogenic age spreads within populations are caused by post-depositional shielding which yields exposure ages younger than deglaciation ages (see Heyman et al. Abstract/Poster in session CL4.7/GM2.4/SSP2.5/SSP3.9: EGU2010-14159-1) and based on the exposure ages of the multiple sample types, all dated glacial deposits pre-date the global Last Glacial Maximum (LGM). Our results further indicate that even the innermost and highest of the dated moraines, formed by glaciers <10 km long, have minimum deglaciation ages of 45 ka. These results agree well with those sites on the Tibetan Plateau where samples close outside present-day glacier margins have yielded exposure ages significantly older than the LGM. In fact, for sites where exposure age studies have been performed on the Tibetan Plateau, it is a rule rather than an exception with pre-LGM exposure ages close outside present-day glacier margins. This indicates that during the LGM, when large ice sheets covered North America and northern Europe, glaciers on the northeastern Tibetan Plateau, and perhaps the plateau at large, did not grow much larger than today.

    To explore the climate implications of restricted Tibetan Plateau LGM glaciers, we employ a high-resolution 3D glacier model forced with static climate perturbations of the present-day climate (WorldClim data:http://www.worldclim.org/). Allowing glaciers to grow and expand to but not exceed well-dated moraines enables us to derive and present climate constraints for the Tibetan Plateau during the LGM.

  • 24.
    Heyman, Jakob
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Stroeven, Arjen P
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Hättestrand, Clas
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Harbor, Jon
    Department of Earth and Atmospheric Sciences, Purdue University, USA.
    Caffee, Marc W
    Department of Physics/Purdue Rare Isotope Measurement Laboratory, Purdue University, USA.
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Li, Yingkui
    Department of Geography, University of Tennessee, USA.
    Zhou, Liping
    Department of Urban and Environmental Sciences, Peking University, China.
    Liu, Gengnian
    Department of Urban and Environmental Sciences, Peking University, China.
    Fu, Ping
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    A paleoglaciological reconstruction for Bayan Har Shan, NE Tibetan Plateau2009Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The paleoglaciology of the Tibetan Plateau has remained elusive because extensive areas still lack detailed scrutiny. We here present a paleoglaciological reconstruction for the Bayan Har Shan region, NE Tibetan Plateau, which could serve as a working model to investigate other poorly investigated regions. The reconstruction is primarily based on three methods for revealing the glacial history; 1) remote sensing (geomorphology), 2) field studies (stratigraphy), and 3) numerical dating techniques. Remote sensing (SRTM elevation data, Landsat ETM+ satellite imagery and Google Earth) of a 136 500 km2 area reveals an abundance of glacial landforms in the highest mountain areas and an absence of glacial landforms on intervening plateau surfaces. Stratigraphical data collected during three field seasons supplement the picture emerging from remote sensing. Glacial deposits (including erratic boulders and till) occur in the elevated mountain areas but are absent on the intervening plateau areas. Marginal moraines in central Bayan Har can be grouped to represent at least three separate glacial extents and scattered observations of glacial deposits indicate the presence of a fourth (and maximum) glacial extent. To tie the glacial geological record to a chronology we have employed terrestrial cosmogenic nuclide (TCN) exposure and optically stimulated luminescence (OSL) dating. Beryllium apparent exposure ages of 65 glacial boulders, surface cobbles/pebbles and depth profile samples yield minimum ages for the three youngest glacial extents of 40-65 ka, 60-100 ka, and 95-165 ka (with the wide age ranges due to TCN dating uncertainties). A preliminary OSL age of c. 160 ka from glacial sediments of the oldest of these glacial extents supports our interpretation based on TCN dating.

    The glacial extent presented here is more restricted than most previous reconstructions, most notably with very restricted glaciers over at least the last 40-65 ka. These results indicate that while continental-scale ice sheets evolved and disappeared in North America and Eurasia over the last half of the last glacial cycle, the NE corner of the Tibetan Plateau experienced relatively minor glacial fluctuations.

  • 25. Håkansson, Lena
    et al.
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Aldahan, Ala
    Briner, Jason
    Hjort, Christian
    Possnert, Göran
    Cosmogenic Exposure Ages From Northeast Greenland Constrain the Extent of the Greenland Ice Sheet During the Last Glacial Maximum2007Inngår i: AGU Fall Meeting, 2007Konferansepaper (Annet (populærvitenskap, debatt, mm))
  • 26. Håkansson, Lena
    et al.
    Briner, Jason
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Aldahan, Ala
    Possnert, Goran
    Be-10 ages from central east Greenland constrain the extent of the Greenland ice sheet during the Last Glacial Maximum2007Inngår i: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 26, nr 19-21, s. 2316-2321Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Traditional ice sheet reconstructions have suggested two distinctly different ice sheet regimes along the East Greenland continental margin during the Last Glacial Maximum (LGM): ice to the shelf break south of Scoresby Sund and ice extending no further than to the inner shelf at and north of Scoresby Sund. We report new Be-10 ages from erratic boulders perched at 250 m a.s.l. on the Kap Brewster peninsula at the mouth of Scoresby Sund. The average Be-10 ages, calculated with an assumed maximum erosion rate of 1 cm/ka and no erosion (respectively, 17.3 +/- 2.3 ka and 15.1 +/- 1.7 ka) overlap with a period of increased sediment input to the Scoresby Sund fan (19-15 ka). The results presented here suggest that ice reached at least 250 m a.s.l. at the mouth of Scoresby Sund during the LGM and add to a growing body of evidence indicating that LGM ice extended onto the outer shelf in northeast Greenland.

  • 27. Håkansson, Lena
    et al.
    Briner, Jason
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi.
    Aldahan, Ala
    Possnert, Göran
    10Be ages from central east Greenland constrain the extent of the Greenland ice sheet during the Last Glacial Maximum2007Inngår i: Quaternary Science Reviews, ISSN 0277-3791, Vol. 26, s. 2316–2321-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Traditional ice sheet reconstructions have suggested two distinctly different ice sheet regimes along the East Greenland continental margin during the Last Glacial Maximum (LGM): ice to the shelf break south of Scoresby Sund and ice extending no further than to the inner shelf at and north of Scoresby Sund. We report new 10Be ages from erratic boulders perched at 250 m a.s.l. on the Kap Brewster peninsula at the mouth of Scoresby Sund. The average 10Be ages, calculated with an assumed maximum erosion rate of 1 cm/ka and no erosion (respectively, 17.3±2.3 ka and 15.1±1.7 ka) overlap with a period of increased sediment input to the Scoresby Sund fan (19–15 ka). The results presented here suggest that ice reached at least 250 m a.s.l. at the mouth of Scoresby Sund during the LGM and add to a growing body of evidence indicating that LGM ice extended onto the outer shelf in northeast Greenland.

  • 28.
    Johnsen, Timothy
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Murray, Andrew S
    Re-dating the Pilgrimstad interstadial: a warmer climate and a smaller ice sheet during the Swedish Middle Weichselian (MIS 3)?2008Inngår i: 12th International Conference on Luminescence and Electron Spin Resonance Dating, LED2008,: Beijing, China, 18-22 September 2008, 2008, s. 199-Konferansepaper (Annet vitenskapelig)
  • 29.
    Johnsen, Timothy F.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Fabel, Derek
    Freeman, Stewart PHT
    New 10Be cosmogenic ages from the Vimmerby moraine confirm the timing of Scandinavian Ice Sheet deglaciation in southern Sweden2009Inngår i: Geografiska Annaler. Series A, Physical Geography, ISSN 0435-3676, E-ISSN 1468-0459, Vol. 91A, nr 2, s. 113-120Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The overall pattern of deglaciation of the southern part of the Scandinavian Ice Sheet has been considered established, although details of the chronology and ice sheet dynamics are less well known. Even less is known for the south Swedish Upland because the area was deglaciated mostly by stagnation. Within this area lies the conspicuous Vimmerby moraine, for which we have used the terrestrial cosmogenic nuclide (10Be) exposure dating technique to derive the exposure age of six glacially transported boulders. The six 10Be cosmogenic ages are internally consistent, ranging from 14.9 ± 1.5 to 12.4 ± 1.3 ka with a mean of 13.6 ±0.9 ka. Adjusting for the effects of surface erosion, snow burial and glacio-isostatic rebound causes the mean age to increase only by c. 6% to c. 14.4± 0.9 ka. The 10Be derived age for the Vimmerby moraine is in agreement with previous estimates forthe timing of deglaciation based on radiocarbon dating and varve chronology. This result shows promise for further terrestrial cosmogenic nuclide exposure studies in southern Sweden.

  • 30. Malmberg Persson, Kärstin
    et al.
    Persson, Magnus
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi.
    Lindén, Anders
    När iskanten tog paus i Vimmerby2008Inngår i: Geologiskt forum, Vol. 15, nr 60, s. 12-15Artikkel i tidsskrift (Annet (populærvitenskap, debatt, mm))
  • 31. Näslund, Jens-Ove
    et al.
    Wohlfarth, Barbara
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för geologi och geokemi.
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Helmens, Karin
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Hättestrand, Martina
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Jansson, Peter
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Kleman, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Lundqvist, Jan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Brandefelt, Jenny
    Houmark-Nielsen, Michael
    Kjellström, Erik
    Strandberg, Gustav
    Knudsen, Karen-Luise
    Krog Larsen, Nikolai
    Ukkonen, Pirkko
    Mangerud, Jan
    Fennoscandian paleo-environment and ice sheet dynamics during Marine Isotope Stage (MIS) 3: Report of a workshop held September 20–21, 2007 in Stockholm, Sweden2008Rapport (Annet vitenskapelig)
  • 32.
    Stroeven, Arjen
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Hättestrand, Clas
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Heyman, Jakob
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Harbor, Jon
    Li, Yingkui
    Zhou, Liping
    Caffee, Marc
    Alexanderson, Helena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Kleman, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för naturgeografi och kvartärgeologi (INK).
    Ma, Haizhou
    Liu, Gengnian
    Landscape analysis of the Huang He headwaters, NE Tibetan Plateau — Patterns of glacial and fluvial erosion2009Inngår i: Geomorphology, ISSN 0169-555X, E-ISSN 1872-695X, Vol. 103, nr 2, s. 212-226Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The large-scale geomorphology of the Huang He (Yellow River) headwaters, centered around the Bayan Har Shan (5267 m asl) in the northeastern part of the Tibetan Plateau, is dominated by an uplifted remnant of a low-relief relict plateau with several mountain ranges. We have performed geomorphological mapping using SRTM topographic data and Landsat 7 ETM+ satellite imagery to evaluate landscape characteristics and patterns, and to investigate the relative importance of different erosional processes in the dissection of this plateau remnant. The distribution of valley morphologies indicates that the eastern and southern margins of the plateau remnant have been extensively dissected by the Huang He and Chang Jiang (Yangtze) rivers and associated tributaries, while the mountain ranges have valley morphologies with U-shaped cross-sections that indicate large impacts from glacial erosion during Quaternary glaciations.

    An east-west decrease in the abundance of glacial valleys in mountains above 4800 m asl suggests that the diminishing size of the mountain blocks, coupled with increased continentality, resulted in more restricted glaciations to the west. Glacial valleys in mountain blocks on the plateau remnant are wider and deeper than adjacent fluvial valleys. This indicates that, integrated over time, the glacial system has been more effective in eroding the mountains of the relict upland surface than the fluvial system. This erosion relationship is reversed, however, on the plateau margin where dramatic fluvial rejuvenation in valleys that are part of the Huang He and Chang Jiang watersheds has consumed whatever glacial morphology existed. A remarkable correspondence exists between the outline of the relict plateau remnant and the outline that has been proposed for the Huang He Ice Sheet. This coincidence could mean that the Huang He Ice Sheet was larger than originally proposed, but that evidence for this has been consumed by fluvial incision at the plateau margin. Alternatively, this coincidence could indicate that what has been described as an ice sheet border is merely the outline of a relict plateau landscape.

    In apparent support of the latter, the absence of large-scale glacial geomorphological evidence on the plains of the relict plateau surface is not consistent with the hypothesis of a Huang He Ice Sheet.

1 - 32 of 32
RefereraExporteraLink til resultatlisten
Permanent link
Referera
Referensformat
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annet format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annet språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf