| 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, et al.
(författare)
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Landscape evolution of the northeastern Tibetan plateau - relict surfaces and fluvial margins
- 2007
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Ingår i: Geophysical Research Abstracts.
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 4. |
- Li, Yingkui, et al.
(författare)
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Patterns of landscape evolution on the central and northern Tibetan Plateau investigated using in-situ produced Be-10 concentrations from river Sediments
- 2014
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Ingår i: Earth and Planetary Science Letters. - : Elsevier BV. - 0012-821X .- 1385-013X. ; 398, s. 77-89
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Tidskriftsartikel (refereegranskat)abstract
- Quantifying long-term erosion rates across the Tibetan Plateau and its bordering mountains is of critical importance to an understanding of the interaction between climate, tectonic movement, and landscape evolution. We present a new dataset of basin-wide erosion rates from the central and northern Tibetan Plateau derived using in-situ produced Be-10 concentrations of river sediments. Basin-wide erosion rates from the central plateau range from 10.1 +/- 0.9 to 36.8 +/- 3.2 mm/kyr, slightly higher than published local erosion rates measured from bedrock surfaces. These values indicate that long-term downwearing of plateau surfaces proceeds at low rates and that the landscape is demonstrably stable in the central plateau. In contrast, basin-wide erosion rates from the Kunlun Shan on the northern Tibetan Plateau range from 19.9 +/- 1.7 to 163.2 +/- 15.9 mm/kyr. Although the erosion rates of many of these basins are much higher than the rates from the central plateau, they are lower than published basin-wide erosion rates from other mountains fringing the Tibetan Plateau, probably because the basins in the Kunlun Shan include both areas of low-relief plateau surface and high-relief mountain catchments and may also result from retarded fluvial sediment transport in an arid climate. Significantly higher basin-wide erosion rates derived from the Tibetan Plateau margin, compared to the central plateau, reflect a relatively stable plateau surface that is being dissected at its margins by active fluvial erosion.
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| 5. |
- Li, Yingkui, et al.
(författare)
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Timing and extent of Quaternary glaciations in the Tianger Range, eastern Tian Shan, China, investigated using Be-10 surface exposure dating
- 2014
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 98, s. 7-23
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Tidskriftsartikel (refereegranskat)abstract
- Reconstructing glacial chronologies with consistent methods is critical for efforts to examine the timing and pattern of past climate change. Cosmogenic Be-10 surface exposure dating has been widely used to constrain the timing of glacial events on the Tibetan Plateau and in Central Asia. However, few such studies have been conducted in the Chinese Tian Shan and available Be-10 ages from this region have only provided evidence for glacial events during the global Last Glacial Maximum (gLGM) and Lateglacial. Here, we present 45 Be-10 surface exposure ages from glacial landforms in the Ala and Daxi valleys, two formerly glaciated valleys draining the Tianger Range, eastern Tian Shan. Combined with previously published Be-10 surface exposure ages from the Daxi Valley in the source area of the Urumqi River, the new ages record five major glacial events during Marine Oxygen Isotope Stages (MIS) 6 or older, 4, 3, 2, and 1 (during the Little Ice Age, LIA). Landforms from glacial events since MIS 2 are found on the northern slope of the Tianger Range (Daxi Valley), whereas evidence for the older glacial events is only preserved on its southern slope (Ala Valley). This disparity may be caused by different preservation- and micro-climatic conditions on the northern and southern slopes of this mountain range, due to differences in gradient and aspect. The LIA glacial advances are apparently the only Holocene glacial event recorded in this area. Earlier Holocene glacial events were probably so restricted in extent that they were destroyed by subsequent LIA advances.
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| 6. |
- 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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| 7. |
- Stroeven, Arjen, et al.
(författare)
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Paleoglaciology of the Bayan Har Mountain area, eastern Tibetan Plateau
- 2006
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Ingår i: Asian Conference on Permafrost.
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Konferensbidrag (refereegranskat)abstract
- The glacial history of the Tibetan Plateau is a topic of considerable interest because of its significance for regional and global environmental reconstructions, and its interaction with variations in monsoon strength and plateau uplift. Published glacial reconstructions for the last glaciation range from a large ice sheet covering the entire Tibetan Plateau to extended valley glaciation forming discrete glaciated mountain blocks. Although current chronologies appear to underpin the restricted glaciation model, there appears to be enough regional variation to motivate further study, especially the glacial history predating the last glaciation. We therefore study the glacial history of a large upland section of the eastern Tibetan Plateau centered on the currently unglaciated Bayan Har Mountains (BHM), partly because chronological constraints are entirely absent, and partly because the area may once have been covered by an ice sheet of intermediate proportions. The BHM area, which houses the headwaters of the Huang He (Yellow River), contains a wide array of glacial deposits and morphologies. Moreover, it appears that superseding glaciations were ever limiting in extent and the area therefore presents optimal conditions to investigate glaciations over long time periods.We report from an on-going investigation into the extent and chronology of Quaternary glaciers in this region, manifested in glacial deposits and landforms (e.g., erratics, end moraines, tills and trough valleys). Previous studies have indicated the occurrence of two phases of mountain glaciation during the last glaciation (OIS 2-4), with mountain glaciers distributed around the highest summits, and two prior glaciations of ice sheet glaciation character (the penultimate glaciation, OIS 6, and the Huang He ice sheet, OIS 12).We mapped the glacial morphology of the area using satellite images and a DEM of 90 m resolution. Large-scale glacial landforms such as cirques, glacial troughs and U-shaped valleys indicate repeated glaciations, and so do series of moraine ridges and meltwater channels. The abundance of glacial traces detectable through remote sensing techniques diminish with decreasing elevation, and it appears that evidence for former ice sheets are based mainly on sedimentary evidence.In an introductory field work in 2005, surface boulders (including erratics) and boulders in till profiles have been sampled for dating using terrestrial cosmogenic nuclide (TCN) concentrations in quartz. Sampling was carried out along a 300 km stretch of the Qingkang highway, crossing the 80,000 km2 area of the Huang He ice sheet. We intend to present these first TCN results at the meeting.Our study will present new data for the paleoglaciology of the eastern Tibetan Plateau, and will contribute to the resolution of questions such as:• What glacial fluctuations occurred in the BHM area throughout the last glaciation?• When did glaciation pre-dating the last glaciation occur?• Was the area ever covered by an ice sheet?• What is the relation Tibetan glaciation – uplift – climate variations?These are questions of special significance also for former periglacial conditions, as reconstructed glaciers and ice sheets had a fundamental effect on regional paleoenvironmental conditions.
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