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Glacial geology of Bayan Har Shan, northeastern Tibetan Plateau Jakob Heyman 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 paleoglaciers 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. Cover photo: Erratic granite boulder north of central Bayan Har Shan with a U-shaped cross-section of the parent glacial valley in the background. 1 This thesis consists of a summary, three papers and a map: Paper I, including a map on attached CD (P I) Heyman J, Hättestrand C, Stroeven AP. in press: Glacial geomorphology of the Bayan Har sector of the NE Tibetan plateau. Journal of Maps. Paper II (P II) Stroeven AP, Hättestrand C, Heyman J, Harbor J, Li YK, Zhou LP, Caffee MW, Alexanderson H, Kleman J, Ma HZ, Liu GN. in press: Landscape analysis of the Huang He headwaters, NE Tibetan Plateau – patterns of glacial and fluvial erosion. Geomorphology. Paper III (P III) Heyman J, Stroeven AP, Alexanderson H, Hättestrand C, Li YK, Harbor J, Caffee MW, Zhou LP, Veres D, Liu F, Machiedo M. manuscript: Field data for the glacial extent in Bayan Har Shan, NE Tibetan Plateau – paleo-glaciers restricted to separate mountain blocks. For P I, I performed the mapping, building on initial mapping by Clas Hättestrand and Arjen Stroeven, and I wrote most of the text and drew the figures, with significant input from my coauthors. For P II, Clas Hättestrand and Arjen Stroeven conducted the mapping and wrote most of the text. I contributed significantly with data analysis, writing and illustrations. For P III, I supervised and performed most of the data collection, and I was responsible for data analysis, writing and illustrations, with significant input from my co-authors. 2 Introduction areas, as exemplified by the Li et al. (1991) glacial reconstruction for the Tibetan Plateau (cf. Fig. 1). However, because there is an absence of glacial geological data for extensive parts of the plateau (c.f. P I), presented plateau-wide reconstructions are at best crude estimates. There is, consequently, a frequently broadcasted need for further detailed studies of the Tibetan glacial geology (e.g. Osmaston 1989; Derbyshire et al. 1991; Rutter 1995; Zheng and Rutter 1998; Benn and Owen 2002; Lehmkuhl and Owen 2005). One of the more intriguing corners of the Tibetan Plateau in this regard is the Bayan Har Shan (Shan means mountain) with the source area of Huang He (Yellow River) as it has been credited with variable paleo-ice extents. This is where the work contained in this thesis has its focus (Fig. 1). The Tibetan Plateau is a topographic feature of extraordinary dimension and, consequently, it has an important impact on regional and global climates. It has been proposed, for example, that the uplift of the plateau, in response to the Indian-Asian continental collision, caused the Asian monsoon dynamics and a global cooling trend which triggered the onsets of Quaternary climate oscillations (e.g. Molnar and England 1990; Raymo and Ruddiman 1992; An et al. 2001). However, the glacial history of the Tibetan Plateau, in spite of its proposed significance for climate change, is a research field still in its infancy. While research on Laurentide and Fennoscandian glaciations have at least partly attained a stage of detailed analysis, research on the Tibetan Plateau glacial history still deals with the basic questions of extent and timing. The upper reaches of Huang He receive their water from the northern slopes of Bayan Har Shan. In this area, a regional ice sheet has been proposed to have existed – the Huang He ice sheet. The Huang He ice sheet, with a peculiar triangular outline and of Icelandic size, was first described in the palaeoglaciological map by Li et al. (1991) as an “Estimated extent of paleoglaciation” (Fig. 1). This map unit contrasts with the apparently somewhat more certain “Extent of paleoglaciation” which marks all other formerly glaciated areas, including a more restricted glacier extent around Bayan Har Shan that falls within the Huang He ice sheet area (Fig. 1). Based mainly on field investigations, Zhou and Li (1998) presented a glacial reconstruction comprising four glacial stages in the Huang He ice sheet area. Two stages were identified where glaciers were restricted to the highest mountain areas and these were proposed to correspond to the early and late last glaciation. These stages were proposed to have been predated by a more extensive marine oxygen isotope stage (MIS) 6 glacial stage and a most extensive Huang He ice sheet MIS 12 glacial stage. However, the physical basis for this reconstruction is meagre, with only scattered observations of glacial landforms and deposits and an absence of chronological control. Alternative interpretations for the glacial history of the Bayan Har Shan exist (Lehmkuhl et al. 1998; Zheng and Rutter 1998), and these favour restricted glaciations based on an absence of unambiguous glacial traces and they question, therefore, the existence of regional-scale and plateau-scale ice sheets in this area. Likely triggered by the suggestion of a plateau-scale ice sheet (Kuhle 1985, 2004 and references there-in), the last 20 years have seen a significant increase in research on the Tibetan Plateau glacial history. Kuhle (2004 and references there-in), based on inferred glacial geological traces and reconstructed Equilibrium Line Altitudes (ELA), has suggested that almost the entire plateau was covered by an ice sheet of dimensions similar to the Greenland ice sheet during the global Last Glacial Maximum (LGM). This idea has been subjected to intense disapproval (e.g. Derbyshire et al. 1991; Rutter 1995; Zheng and Rutter 1998; Owen et al., 2005; Lehmkuhl and Owen 2005) and a large amount of data strongly indicates that at least during the last few hundred thousand years there has been no plateau-scale ice sheet on the Tibetan Plateau. Two circumstances stand out as particularly problematic for a plateau-scale LGM ice sheet concept. First, there is a remarkable absence of glacial indicators (such as glacial lineations, eskers, ribbed moraines, marginal moraines, till, and erratic boulders) for extensive areas of the plateau outside mountain areas. Second, terrestrial cosmogenic nuclide exposure dating and optically stimulated luminescence dating techniques, which are effective chronological tools for glacial geologists (e.g. Gosse and Philips 2001; Bierman 2007), have provided LGM ages from restricted mountain areas and a large number of ages on glacial landforms and deposits older than the late last glaciation (e.g., Owen et al. 2005). To summarize, the concept of a plateau-scale Tibetan ice sheet during the LGM is generally rejected based on available dating evidence, and the presence of a unified plateauscale ice sheet at any other time before the LGM faces the current uphill-struggle of an absence of landforms and deposits in its support. The Bayan Har Shan investigation area can be described as a low-relief plateau at c. 4300 metres above sea level (m a.s.l.) with elevated mountain areas generally reaching up to 1000 m higher, and including the significantly higher Anyemaqen Shan (6282 m a.s.l.) in its northeastern corner. Tributaries of Chang Jiang (Yangtze River), the major river bordering the study area towards the southwest, and Huang He in the east, are cutting deep V-shaped valleys into the plateau The paleoglaciology of the Tibetan Plateau is dominantly thought of as a collection of paleo-glaciers of varying extent but restricted to elevated mountain 3 Fig. 1. The Tibetan Plateau with the study area in the northeastern corner and the interpretation of the Quaternary glacial distribution map (Li et al. 1991). area. Mountain ranges are generally trending along fault lines in a NW-SE orientation. Remote sensing of the entire study area has been performed using two data sets: the Shuttle Radar Topography Mission (SRTM) 90 m resolution Digital Elevation Model (DEM) (Jarvis et al. 2006), and Landsat ETM+ 15 m resolution satellite imagery (acquired from Global Land Cover Facility, http://www.landcover.org, accessed 17 January 2008). For 3D visualization, Google EarthTM software has been frequently employed. The remote sensing methodology, by virtue of its complete coverage over extensive areas, provides a powerful tool to yield spatially-continuous records of glacial landforms over large areas and based on uniform information. Its predominant limitation is the spatial resolution of the data, where small-scale features, such as small moraine ridges and roche moutonnées, may fall outside the lower limit of recognition. Clearly, the paleoglaciology of the northeastern Tibetan plateau is not satisfactory researched, with several crucial questions still unresolved. In my PhD project I am aiming to provide a robust paleoglaciological reconstruction of the Bayan Har Shan area based on mapping, field evidence and numerical dating techniques. Ideally, a paleoglaciological reconstruction would provide a temporally continuous glacial development for the study area, including details on glacier outline, topography, thermal regime, and ice dynamics (cf. Kleman et al. 2002; Napieralski et al., 2007). However, due to the complexity and difficulties of the glacial inversion problem (Kleman et al. 2006), paleoglaciological reconstructions are more typically composed of two main parts; a spatial dimension giving the glacier outlines, and a temporal dimension giving the chronology, presenting time slice images of former glacial extents (e.g. Boulton and Clark 1990; Kleman et al. 1997; Svendsen et al. 2004; Ehlers and Gibbard 2007). In this classical way, my thesis deals with the spatial dimension of the northeastern Tibetan Plateau paleoglaciology. In particular, it presents the input data (in P I, P II, and P III) for a robust reconstruction of the former extent of glaciers. Three field campaigns were carried out during 2005, 2006 and 2007. We have investigated landforms and sediments with the aim to either associate features with the former presence of a glacier or to exclude glacial processes as the formative agent. The field campaigns yielded detailed information on glacial geology, however, somewhat curtailed by its restricted spatial coverage (along roads and walking tracks). Results Methodology The results of this thesis are graphically presented in Figure 2, displaying mapped glacial geomorphology (from P I) from remote sensing and the distribution of observed glacial deposits (from P III) from fieldwork, and are summarized below. Two general methods have been used to map the extent of former glaciers: remote sensing and field investigations. 4 Fig. 2. Glacial geology of Bayan Har Shan. P I: Heyman J, Hättestrand C, Stroeven AP. in press: Glacial geomorphology of the Bayan Har sector of the NE Tibetan plateau. Journal of Maps. P II: Stroeven AP, Hättestrand C, Heyman J, Harbor J, Li YK, Zhou LP, Caffee MW, Alexanderson H, Kleman J, Ma HZ, Liu GN. in press: Landscape analysis of the Huang He headwaters, NE Tibetan Plateau – patterns of glacial and fluvial erosion. Geomorphology. We here present the distribution of glacial landforms in the Bayan Har Shan study area, including glacial valleys and troughs, glacial lineations, marginal moraines and moraine remnants, meltwater channels, and hummocky terrain. However, there is a notable absence of landform assemblages that are typical for ice sheets elsewhere on Earth such as glacial lineation swarms, ribbed moraines, and eskers. Mapping procedures using remote sensing techniques are described and also include definitions of the mapped landforms. The glacial landforms are concentrated in and around well-defined elevated mountain areas, while there is a remarkable absence of glacial landforms for extensive lower-lying plateau areas. In summary, evidence displayed on the map that is part of this publication offers support for former alpine style glaciation but renders no support to the existence of larger-scale ice sheet glaciation. In this paper we present an analysis of the large-scale geomorphology of the Bayan Har Shan study area, and the relative importance of glacial and fluvial erosion in shaping the plateau area. Based on remote sensing, a tripartite classification system was presented, glacial landscapes (including U-shaped glacially eroded valleys), relict upland surfaces, and fluvial landscapes dominated by V-shaped valleys formed by fluvial incision of tributaries to the Chang Jiang and Huang He. In elevated mountain areas of the relict upland surface, the glacial valleys are wider and deeper than adjacent fluvial valleys, indicating that, integrated over time, the glacial system has been more erosive than the fluvial system. Along the plateau margin, however, this relationship is reversed with dramatic fluvial rejuvenation having consumed/consuming whatever 5 glacial morphologies existed. The outline of the relict upland surface closely mimics the outline of the proposed Huang He ice sheet. This circumstance could indicate one of two options with conflicting glacial implications: (1) The Huang He ice sheet was even more extensive because parts of the glacial record are being consumed by fluvial incision. (2) The proposed regional ice sheet is based on a mis-interpretation of a relict upland surface, whose outline is determined by marginal fluvial incision, with scattered glacial traces. In the absence of large-scale glacial landforms for extensive areas within the study area, the preferred interpretation is the latter of these. beyond that mapped for glacial geomorphologies, indicating an absence of large-scale glacial landforms in areas of most extensive glaciation. In addition, new data shows that previously proposed glacial deposits located in lower-lying plateau areas were probably formed by non-glacial processes. In summary, field evidence support the presence of extensive ice masses in the form of separate ice caps or icefields over elevated mountain areas but it leaves no support to the presence of any former ice sheet. P III: Heyman J, Stroeven AP, Alexanderson H, Hättestrand C, Li YK, Harbor J, Caffee M, Zhou LP, Veres D, Liu F, Machiedo M. Manuscript: Field data for the glacial extent in Bayan Har Shan, NE Tibetan Plateau – paleoglaciers restricted to separate mountain blocks. A preliminary and tentative reconstruction of the maximum glacial extent permitted by the mapped glacial geology (Fig. 2, P I, P II, P III) is presented in Figure 3. The reconstruction encompasses all glacial traces (Fig. 2) and includes a glacial altitude increase towards the interior of the plateau (Li et al. 1991; Lehmkuhl 1998; Benn and Owen 1998; P II). The reconstructed maximum glacial extent covers 68.000 km2, representing an extensive glaciation of approximately 50% of the 136.500 km2 study area. The dominant ice mass was elongated in a NW-SE orientation along Bayan Har Shan. It was, however, significantly smaller than the proposed Huang He ice sheet of estimated 95.500 km2 (Li et al. 1991) or 85.500 km2 (Zhou and Li 1998). Glacial extent In this paper we present field evidence for glacial extent in the Bayan Har Shan and compare glacial geological point observations to the pattern of glaciation inferred from remote sensing data (P I, P II). For the identification and mapping of glacial deposits we relied on glacial indices such as the presence of erratic boulders, of striated clasts, of diamictic sediments, and of clasts of multiple lithologies. Similar to the distribution of glacial geomorphology, glacial deposits are centred on and around elevated mountain areas. However, glacial deposits occur also in a zone Fig. 3. A preliminary and tentative reconstruction for the maximum glacial extent based on the glacial geological record (cf. Fig. 2). 6 Conclusions represented an icefield or ice cap-type of glaciation. Based on satellite data of the northeastern Tibetan Plateau and field observations during three consecutive years, the following conclusions can be drawn: Outlook The glacial geological record presented in this thesis is an ample record of former glaciations on the northeastern Tibetan Plateau. Still, to use this record for a paleoglaciological reconstruction is not a simple and straight-forward task. The glacial geological record varies considerably between different areas, potentially caused by varying glacial histories and glacier dynamics, but also due to the spatial coverage of our field studies. Generally, there is a great amount of subjectivity when transforming a glacial geological record into discrete glacial extents (e.g. Li et al. 1991; Zhou and Li 1998; Fig. 3), leaving the field open for speculative reconstructions of poor foundation. To get around this problem, I intend to perform the transformation of the glacial geology into glacial extents using a clear methodology based on ELA reconstructions. A four point procedure for this transformation is outlined below: • Glacial landforms occur frequently in elevated mountain areas on the relict plateau surrounding the Bayan Har Shan, indicating former glaciation strongly controlled by altitude. Distinct sets of marginal moraines and meltwater channels help distinguishing between discrete glacial extents ranging from cirque glaciers to extensive valley glacier networks. There is an absence of glacial landforms for lower-lying intervening plateau areas and ice sheet landform assemblages characteristic for northern Hemisphere mid-altitude ice sheets are lacking altogether. Thus, glacial geomorphology indicates only the former presence of alpine style glaciations. • Glacial deposits occur frequently in elevated mountain areas, similar to the glacial landforms. In addition, glacial deposits occur at lower elevations some distance beyond the mapped glacial geomorphology. This indicates that icefields which extended beyond the area of visible glacial geomorphology were apparently incapable of significant glacial erosion, possibly because these where short-lived events. In the lowest-lying plateau areas, there is a striking absence of glacial deposits. The observed distribution of glacial deposits indicates that glaciations were controlled by altitude, that the maximum glacial extent exceeds that inferred from glacial geomorphological alone, and that it offers no support for the presence of paleo-ice sheet configurations. 1. Definition of the ELA pattern of the study area. 2. ELA reconstructions for a part of the study area with well-defined ice marginal positions. 3. Reconstruction of ice marginal positions for the entire study area employing inversion of the ELA reconstruction (point 2) in combination with the ELA pattern (point 1). 4. Extrapolation of the ice marginal outline between the ice marginal locations given by point 3. The timing of former glaciations in Bayan Har Shan is still to be revealed. To constrain the glacial chronology we are performing cosmogenic isotope exposure dating as well as optically stimulated luminescence dating of glacial deposits. We have collected a large number of samples from an extensive area, and we intend to adopt varying cosmogenic analysis techniques, such as dating of boulder surfaces, sediment depth profiles and sets of surface cobbles. In addition, cosmogenic exposure dating of river sediment samples may provide a measure of erosion rates which can confirm or disprove the idea of a relict plateau surface being consumed by fluvial incision (P II). • The northeastern Tibetan Plateau is composed of a low-relief relict plateau surface with elevated mountain areas. The relict surface is being consumed by fluvial incision of the Chang Jiang and Huang He tributary rivers. The size and depth of glacial valleys testify of glacial erosion surpassing fluvial erosion in the elevated mountain areas of the plateau surface when integrated over glacial time spans. At the plateau margin this relationship is reversed, with fluvial erosion eradicating whatever glacial landforms may have existed. The outline of the relict plateau surface broadly resembles the outline of proposed Huang He ice sheet, indicating that the triangular regional ice sheet may have come about as a misinterpreted relict plateau surface with scattered glacial traces. Acknowledgements First I want to thank my supervisors, Arjen Stroeven, Clas Hättestrand and Helena Alexanderson, who have guided me in a positive, helpful, inspiring and friendly atmosphere. My principle supervisor Arjen has been a fantastic support with very valuable comments on • The glacial geology of the Bayan Har Shan indicates that the northeastern Tibetan Plateau paleoglaciology was restricted to elevated mountain areas. In its most expansive outline it probably 7 various topics, enthusiasm over glacial issues and a dinner-at-University company. Similarly, Clas and Helena have been excellent co-supervisors always ready to help with different issues. Clas was also to a great extent involved in getting me into paleoglaciological research, as he guided me in such a good way during my undergraduate-degree project on Swedish marginal moraines. All of my supervisors also participated in the Bayan Har fieldwork of 2006, and have given valuable comments for this summary. I am looking forward to continuing working on this project together with you! courses, conferences and research visits, group meetings and cosmogenic exposure dating (in progress) has been provided by PRIME Lab, Purdue University, by courtesy of Prof. Marc Caffee, VR/SIDA – Swedish Research Links grant 348-2004-5684 to Arjen Stroeven and the Royal Swedish Academy of Sciences (Margit Althins stipendiefond and Hierta-Retzius stipendiefond), the Swedish Society for Anthropology and Geography (Andréefonden and Vegafonden), Carl Mannerfelts fond, A.E.W. Smitts stipendiestiftelse, Ahlmanns fond, Peking University (SU/PKU Academic Exchange Agreement), C.F. Liljevalch J:ors resestipendium, and Gerard De Geers stiftelse to Jakob Heyman. This project has involved a large number of helpful people who have contributed to this thesis in many different ways. Zhou Liping, at Peking University, has been helping out in a splendid way with the logistics of my two fieldworks 2006 and 2007. Zhou Liping was also an excellent host for my visit at Peking University after the fieldwork of 2007, providing an open and friendly atmosphere and giving me possibilities to conduct research. Liu Gengnian introduced me to the Tibetan Plateau in a very friendly and favourable way, during some memorable days in August 2006 from Geermu (Golmud) and across the plateau to Lhasa. Li Yingkui, Jon Harbor and Marc Caffee have been helping out with several issues, although for sure being overwhelmed by other commitments, and they have offered very nice company during several meetings and in the field. Jon and Marc have also been extremely helpful and hospitable during my stay at PRIME Lab, Purdue University, for processing TCN samples. Johan Kleman has added sharp analysis to the project in an inspiring way, and he has also helped out with practical details. During my two trips to China, two persons have helped out in an extraordinary way, translating, explaining, discussing and helping me with anything I could ever imagine. Liu Feng during 2007 and Dong Jianyi during 2006 have helped me in a way far exceeding what could be expected, and for this I am deeply thankful. In addition to Feng and Jianyi, I have had very nice company during the fieldworks with Martin Machiedo and Ma Luyi in 2006 and Daniel Veres in 2007, making the sometimes harsh fieldworks to very joyful and nice experiences. Göran Alm has been a fantastic resource when it comes to DEM and satellite imagery work. Åke Nagrelius at Stockholm University and Li Yun at Peking University have been very helpful in connection with my exchange visit to Peking University. References An ZS, Kutzbach JE, Prell WL, Porter SC. 2001: Evolution of Asian monsoons and phased uplift of the Himalayan Tibetan plateau since late Miocene times. Nature, 411, 62-66. Benn DI, Owen LA. 1998: The role of the Indian summer monsoon and the mid-latitude westerlies in Himalayan glaciation: review and speculative discussion. Journal of the Geological Society, London, 155, 353-363. Benn DI, Owen LA. 2002: Himalayan glacial sedimentary environments: a framework for reconstructing and dating the former extent of glaciers in high mountains. Quaternary International, 97-98, 325. Bierman P. 2007: Cosmogenic glacial dating, 20 years and counting. Geology, 35, 575-576. Boulton GS, Clark CD. 1990: A highly mobile Laurentide Ice Sheet revealed by satellite images of glacial lineations. Nature, 346, 813-817. 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Quaternary Research, 32, 121-123. Li BY, Li JJ, Cui ZJ, Zheng BX, Zhang QS, Wang FB, Zhou SZ, Shi ZH, Jiao KQ, Kang JC. 1991: Quaternary glacial distribution map of Qinghai-Xizang (Tibet) plateau. Science Press, Beijing. (Map scale: 1:3.000.000) Zheng BX, Rutter N. 1998: On the problem of Quaternary glaciations, and the extent and patterns of Pleistocene ice cover in the Qinghai-Xizang (Tibet) plateau. Quaternary International, 45/46, 109-122. Molnar P, England P. 1990: Late Cenozoic uplift of mountain ranges and global climate change: Chicken or egg?. Nature, 346, 29-34. Zhou SZ, Li JJ. 1998: The sequence of Quaternary glaciation in the Bayan Har mountains. Quaternary International, 45/46, 135-142. Napieralski J, Harbor J, Li YK. 2007: Glacial geomorphology and geographic information systems. Earth-Science Reviews, 85, 1-22. 9