| 1. |
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| 2. |
- Fu, Ping, et al.
(författare)
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Glacial geomorphology of the Haizi Shan area, SE Tibetan Plateau
- 2009
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Konferensbidrag (refereegranskat)abstract
- The Haizi Shan area on the SE Tibetan Plateau is characterized by an elliptical relatively low relief plateau surrounded by steeper fluvial valleys. Glacial deposits and erosive imprints are widely distributed indicating former glacier expansions of varying extents in a presently ice-free area. We have initiated a project on the glacial history of the Haizi Shan area and we here present some initial mapping results. Glacial landforms have been mapped based on remote sensing (SRTM digital elevation model, Landsat ETM+ satellite imagery, and Google Earth) and one short reconnaissance field season. Well-preserved moraines from different stages and distinctive U-shaped glacial valleys are abundant (Fig. 1). In the Daocheng Valley southwest of the Haizi Shan Plateau we have mapped glacial deposits in the form of discontinued moraine ridges at Sangdui village. This line, which might be the maximum Quaternary glacial extent, can be traced for several kilometers along the western side of the valley as dispersed erratic boulders. This implies that during the maximum glaciation, ice from the Haizi Shan Plateau crossed the valley and reached up to the piedmont of the opposite mountain. Smaller in extent than the former, numerous large moraine ridges reach down towards valley floors along the edges of the Haizi Shan Plateau. In several locations these valleys lack cirque heads indicating former outlet glaciers emanating from a Haizi Shan ice cap. We will use TCN and OSL dates of samples collected from numerous ice marginal moraines of the Haizi Shan Plateau to determine a glacial chronology. Hence, using remote sensing, field investigations and numerical dating techniques for the Haizi Shan we aim to advance our knowledge on Quaternary glaciations of the SE Tibetan Plateau.
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| 3. |
- Heyman, Jakob, 1979-, et al.
(författare)
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A paleoglaciological reconstruction for Bayan Har Shan, NE Tibetan Plateau
- 2009
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 4. |
- Heyman, Jakob, 1979-, et al.
(författare)
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An evaluation of multiple working hypotheses to explain cosmogenic exposure age data from glacial deposits in the Bayan Har Shan, NE Tibetan Plateau
- 2009
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Ingår i: Proceedings.
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 5. |
- Heyman, Jakob, 1979-, et al.
(författare)
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Cosmogenic exposure ages of glacial boulders from the Tibetan Plateau - Age distributions support boulder exhumation/erosion and indicate old glacial deposits.
- 2009
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Ingår i: Geophysical Research Abstracts.
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Konferensbidrag (refereegranskat)abstract
- Terrestrial cosmogenic nuclide (TCN) exposure dating has become the most dominant technique for constraining glacial chronologies. This is particularly true for the Tibetan Plateau because of its low frequency of organic material (limiting the possibilities to use radiocarbon dating) and high altitude (favouring TCN dating with high cosmogenic nuclide production rates), with, consequently, a large number of TCN samples processed. However, multiple samples from one glacial deposit commonly yield a wide range of TCN ages which complicates their interpretation. Two principal possibilities can cause a wide range of ages to result from one glacial deposit. First, TCN ages may exceed the true age by a varying number of years as a result of pre-depositional cosmogenic nuclide inheritance. Second, TCN ages may underestimate the true age by a varying number of years as a result of post-depositional exhumation and/or erosion. By analysing trends within a large set of TCN ages we can evaluate whether inheritance (too old TCN ages) or exhumation/erosion (too young TCN ages) has best explanatory power. We have thus analysed 794 Be-10 TCN ages from 211 individual groups of glacial boulders collected from 30 different areas on the Tibetan Plateau. Analysis of the 211 sample group age distributions and the relationships with their maximum and minimum ages clearly reveals that older sample groups have wider age spread. This fact indicates that if inheritance is the cause of the wide age spread, older deposits have higher cosmogenic inheritance. However, the wide age spread and distinct age spread/deposition age-trend argue against this explanation. Furthermore, there is no significant inheritance in boulders from young (late Holocene) glacial deposits of the Tibetan Plateau. Exhumation/erosion of boulders, on the other hand, may explain the age distribution as a result of post-depositional shielding of samples. With degrading moraine ridges exhuming boulders and erosion of the boulder surfaces, previous shielding of the collected samples will result in TCN ages underestimating the true age to a varying degree depending on the rate and timing of exhumation/erosion. If exhumation/erosion is a continuous process, older deposits will have wider age spread due to the longer time (higher probability) of exhumation/erosion. Thus, the age distribution within groups of boulder TCN ages from the Tibetan Plateau indicates that cosmogenic inheritance is probably not an overarching problem, and that the spread in ages in glacial deposits is generally caused by boulder exhumation and/or erosion. By inference, the oldest boulder of each sample group most reliably constrains the minimum age of glacial deposition. Because the average of the 211 maximum ages is 61 ka and half of them are older than 25 ka, an important conclusion of our trend analysis is that the glacial geological record of the Tibetan Plateau to a large extent corresponds to glaciations pre-dating the global Last Glacial Maximum. Hence, the Tibetan Plateau offers a window into glaciations significantly older than is normally found in the northern hemisphere.
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| 6. |
- Heyman, Jakob, 1979-, et al.
(författare)
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Glacial boulder exposure ages from the Tibetan Plateau - old deposits and postglacial shielding
- 2009
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Konferensbidrag (refereegranskat)abstract
- Terrestrial cosmogenic nuclide (TCN) exposure dating is an important chronological tool in Quaternary glacial geology. For the Tibetan Plateau, with its lack of organic material (hindering radiocarbon dating) and high altitude (yielding high cosmogenic isotope production rates), TCN dating has been widely used over the last 10 years to provide evidence for limited glacial expansion during the last glacial cycle. However, for a large number of TCN samples, apparent exposure ages deviate from depositional ages as shown by wide age spreads from multiple samples. There are two principal geological explanations for the presence of incorrect and varying exposure ages; 1) pre-glacial exposure and 2) post-glacial shielding. While pre-glacial exposure results in inherited cosmogenic isotope concentrations (yielding too old exposure ages), post-glacial shielding results in reduced cosmogenic isotope concentrations (yielding too young exposure ages). To evaluate the likelihood of each explanation, and to provide guidance on how to interpret the often complex TCN exposure assemblages, we have compiled a large data set of 945 10Be TCN ages from glacial boulders on the Tibetan Plateau and 578 10Be TCN ages from glacial boulders displaced by Laurentide and European ice sheets. TCN ages from the Tibetan Plateau derive from 237 groups with multiple boulders. The grouping of boulders allows us to evaluate the age spread for locations of the same age. All TCN ages have been recalculated (from original publications) using the CRONUS-Earth online calculator version 2.2 (http://hess.ess.washington.edu/) which standardizes measurements using different 10Be standards (thus allowing comparison of multiple TCN age studies) and applies a new 10Be half-life of 1.36 Ma. TCN apparent exposure ages range from 0 to 450 ka and reveal a clear trend with wider age spread (higher uncertainty) with increasing age (valid for both minimum and maximum ages). This characteristic may be explained by shielding during post-glacial time, or, alternatively, would require very high and increasing inheritance with age if explained by pre-glacial exposure. To further evaluate these two explanatory models, we have employed two simple numerical models simulating inheritance and postglacial shielding. We have also compared the Tibetan age spreads with glacial boulder 10Be TCN ages for the Laurentide and European ice sheets, for which we have a relatively good idea of the glacial chronology. The outcome of our analysis is that, although we can not rule out inheritance for individual boulders, post-glacial shielding is a far more poweful explanatory model to explain the increasingly wide age spreads. By inference, the glacial boulder TCN record of the Tibetan Plateau reveals a paleoglaciological record which is significantly older than normally found in the Northern Hemisphere; with discernable glaciations up to several hundred thousand years old.
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| 7. |
- Heyman, Jakob, 1979-
(författare)
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Glacial geology of Bayan Har Shan, northeastern Tibetan Plateau
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The paleoglaciology of the Tibetan Plateau is still largely unexplored, despite its importance for regional and global climate reconstructions. In this thesis a comprehensive glacial geological record is presented from an extensive part of the northeastern Tibetan Plateau centred on the Bayan Har Shan. Glacial reconstructions for this region range from restricted mountain glaciers through the intermediate-size regional-scale Huang He ice sheet to a plateau-scale Tibetan ice sheet. To provide a robust basis for glacial reconstructions, this thesis provides conclusions based on two principle methods, remote sensing and field studies. The remote sensing of a 90 m resolution digital elevation model and 15- and 30 m resolution satellite imagery renders a detailed data set with complete spatial coverage of large- and medium-scale glacial landforms, and large-scale plateau geomorphology. Observations from fieldwork campaigns add detailed point information for the distribution of glacial deposits. Geomorphological glacial traces such as glacial valleys, glacial lineations, marginal moraines, meltwater channels, and hummocky terrain occur frequently in elevated mountain areas, indicating former alpine-style glaciations. Glacial deposits in the form of till, erratic boulders, and glaciofluvial sediments are common in areas with mapped glacial landforms, but also beyond, in areas lacking large-scale glacial landforms. For extensive plateau areas in-between formerly glaciated mountain blocks, there is a striking absence of glacial landforms and sediments, indicating that these areas, perhaps, never were ice covered. Interestingly, glacial deposits occur further away from the mountain blocks than the large- and medium-scale glacial landforms, indicating insignificant erosion beneath the maximum ice covers close to their margins. The large-scale geomorphology of the northeastern Tibetan Plateau is characterised by a low-relief plateau surface with glacial valleys in elevated mountain blocks and marginal steep V-shaped valleys. This geographical distribution indicates a dominance of glacial erosion in the elevated mountain areas and a dominance of fluvial erosion along the steep plateau margins, dissecting a relict plateau surface. The outline of the relict plateau surface mimics the proposed outline of the Huang He ice sheet, suggesting that the inferred ice sheet may represent a misinterpreted relict surface with scattered glacial traces. In conclusion, the glacial geology examined in the Bayan Har Shan region is consistent with paleo-glaciers of varying extent restricted to elevated mountain areas. Even though extensive icefields/ice caps were centred on discrete mountain areas, there is no indication that these ice masses merged but rather that they were separated from each other by unglaciated plateau areas. The presented glacial geological record will be used in further studies towards a robust paleoglaciological reconstruction for the northeastern Tibetan Plateau.
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| 8. |
- Heyman, Jakob, et al.
(författare)
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Glacial geomorphology of the Bayan Har sector of the NE Tibetan Plateau
- 2008
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Ingår i: Journal of Maps. - : Informa UK Limited. - 1744-5647. ; 2008, s. 42-62
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Tidskriftsartikel (refereegranskat)abstract
- We here present a detailed glacial geomorphological map covering 136,500 km2 of the Bayan Har sector of the northeastern Tibetan Plateau - an area previously suggested to have nourished the most extensive Quaternary glaciers of the Tibetan Plateau. The map, presented at a scale of 1:650,000, is based on remote sensing of a 90 m SRTM digital elevation model and 15/30 m Landsat ETM+ satellite imagery. Seven landform types have been mapped; glacial valleys, glacial troughs, glacial lineations,marginal moraines, marginal moraine remnants, meltwater channels and hummocky terrain. A large number of glacial landforms exist, concentrated around mountain blocks protruding above the surrounding plateau area, testifying to former glacial activity. In contrast, large plateau areas of lower altitude lack glacial landforms. The mapped glacial geomorphology indicates multiple former glacial advances primarily by valley and piedmont glaciers, but lends no support to the hypothesis of ice sheet scale glaciation in the area. The presented glacial geomorphological map demonstrates the usefulness of remote sensing techniques for mapping the glacial geomorphology of the Tibetan Plateau, and it will be used for reconstructing the paleoglaciology of the Bayan Har sector of the northeastern Tibetan Plateau.
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| 9. |
- Heyman, Jakob, 1979-, et al.
(författare)
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Palaeoglaciation of Bayan Har Shan, northeastern Tibetan Plateau: glacial geology indicates maximum extents limited to ice cap and ice field scales
- 2009
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Ingår i: Journal of Quaternary Science. - : John Wiley & Sons. - 0267-8179 .- 1099-1417. ; 24:7, s. 710-727
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
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