| 1. |
- 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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| 2. |
- 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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| 3. |
- Heyman, Jakob, 1979-, et al.
(författare)
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Constraining the glacial chronology of Bayan Har Shan, NE Tibetan Plateau – Cosmogenic exposure dating of boulders, surface pebbles/cobbles and sediment depth profiles
- 2009
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Ingår i: Geophysical Research Abstracts.
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 4. |
- Heyman, Jakob, et al.
(författare)
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Glacial landforms and deposits of the Bayan Har Shan, NE Tibetan plateau – a dataset for reconstructing the extent of former glaciations
- 2008
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 5. |
- 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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| 6. |
- Heyman, Jakob, et al.
(författare)
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Reconstructing former glacial extent of the NE Tibetan Plateau – combining remote sensing and field data of glacial geology
- 2008
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Ingår i: Geophysical Research Abstracts.
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 7. |
- Li, Yingkui, et al.
(författare)
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Unraveling complex exposure-burial histories of bedrock surfaces under ice sheets by integrating cosmogenic nuclide concentrations with climate proxy records
- 2008
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Ingår i: Geomorphology. ; 99:1-4, s. 139-149
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Tidskriftsartikel (refereegranskat)abstract
- The production, accumulation, and decay of cosmogenic radionuclides in rock surfaces subjected to episodes of exposure and burial by ice results in nuclide concentrations in present day rock surfaces that can be used to address a variety of questions in glacial geomorphology and Quaternary geology. Of particular importance is the fact that these nuclide concentrations reflect both the timing of initial exposure of the rock surface and the chronology of subsequent exposure, burial, and erosion episodes. For landscapes where geomorphic evidence indicates that little/no erosion occurred, constraining the timing of initial exposure and the number of phases of exposure and burial that a rock surface has been subjected to is possible using multiple cosmogenic radionuclide concentrations combined with proxies for the timing and duration of periods of ice cover, such as ice core or marine isotope records. However, interpretations based on this approach require determination of an appropriate cutoff value to separate the proxy record into ice-free and ice-covered conditions and assessment of the sensitivity of the results to different cutoff values.We have developed a numerical model to evaluate variations in total exposure and burial durations as a function of different proxy records and cutoff values. This program is available at http://www.missouri.edu/~liyk/ClimateProxyCurve.zip. Initial results for sites in West Antarctica and northern Sweden show that the method provides a quick and robust way to derive best-fit cutoff values and chronologies of burial and exposure, and small changes in cutoff values can result in significant shifts in results. The method described here provides new insight into the interpretation and reliability of multiple nuclide samples. This approach also has the potential to provide improved constraints for ice sheet dynamics and landscape evolution, and a means to assess the sensitivity of calculated initial exposure dates to assumptions about ice sheet history.
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| 8. |
- Napieralski, Jacob, et al.
(författare)
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Towards a GIS assessment of numerical ice sheet model performance using geomorphological data
- 2007
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Ingår i: Journal of Glaciology. ; 53:180, s. 71-83
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Tidskriftsartikel (refereegranskat)abstract
- A major difficulty in assimilating geomorphological information with ice-sheet models is the lack of a consistent methodology to systematically compare model output and field data. As an initial step in establishing a quantitative comparison methodology, automated proximity and conformity analysis (APCA) and automated flow direction analysis (AFDA) have been developed to assess the level of correspondence between modelled ice extent and ice-marginal features such as end moraines, as well as between modelled basal flow directions and palaeo-flow direction indicators, such as glacial lineations. To illustrate the potential of such an approach, an ensemble suite of 40 numerical simulations of the Fennoscandian ice sheet were compared to end moraines of the Last Glacial Maximum and the Younger Dryas and to glacial lineations in northern Sweden using APCA and AFDA. Model experiments evaluated in this manner were ranked according to level of correspondence. Such an approach holds considerable promise for optimizing the parameter space and coherence of ice-flow models by automated, quantitative assessment of multiple ensemble experiments against a database of geological or glaciological evidence.
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| 9. |
- Stroeven, Arjen, et al.
(författare)
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Landscape analysis of the Huang He headwaters, NE Tibetan Plateau — Patterns of glacial and fluvial erosion
- 2009
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Ingår i: Geomorphology. - : Elsevier. - 0169-555X .- 1872-695X. ; 103:2, s. 212-226
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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