Frank Lehmkuhl

Mass balance and equilibrium-line altitudes of glaciers in high-mountain environments

Abstract The mass-balance characteristics of glaciers in high-mountain environments complicate the relationship between glacier equilibrium-line altitudes (ELAs) and climatic variables such as precipitation and air temperature. Therefore, methods of ELA reconstruction employed in low-relief environments are commonly not applicable in high mountains, or require some modification. We review the concept of the ELA, with reference to the mass balance of a range of glacier types found in high-mountain regions. We examine the applicability of several commonly used methods of ELA reconstruction for different glacier types, and propose some general principles to guide the choice of appropriate methods.

Quaternary paleoenvironmental change on the Tibetan Plateau and adjacent areas (Western China and Western Mongolia)

Abstract This paper summarises the nature of climatic change during the Last Glacial–Interglacial cycle on the Tibetan Plateau and adjacent areas. The results are derived from inland glacier fluctuations, lake level changes and dust (loess) records of Central Asia including Tibet. These are based on our field investigations from 1988 to 1998 and literature surveys. The changing extents of different climatic controlled geomorphic landscapes and vegetation zones help to provide estimates of the magnitude of climatic change. Features, such as ice wedge casts, loess, and palaeosols, ELA-reconstructions and palaeobotanical data are used to help reconstruct palaeoprecipitation and palaeotemperatures. Weathering characteristics, the overlying strata and some luminescence dates indicate that there are two main glacial ice advances during the last glacial cycle. These correspond to marine Oxygen Isotope Stages (OIS) 2 and 4. In some areas, as the eastern or northern margin of the Tibetan Plateau, remnants of older glaciations are preserved. Higher lake levels in the deserts of Central Asia and on the Tibetan Plateau are dated to >40 to 25 ka (OIS 3) and to Late Glacial / Early to Mid Holocene periods. Our work supports the view that in many areas of Central Asia cold phases during the last glacial correspond with the maximum extent of glaciers and the periglacial activity. The last glaciation produced large alluvial fans alternating with periods of high lake stands on the Tibetan Plateau. However, at the northern margin of the Plateau in the Qaidam Basin and in some particular desert areas in Western China, high lake levels occurred also during the Pleistocene and are related with alluvial fans.

Extent and spatial distribution of Pleistocene glaciations in eastern Tibet

As a contribution to the controversial discussion regarding the nature and extent of the Pleistocene glaciation on the Tibetan Plateau, additional observations and results were obtained during several Chinese-German joint expeditions. In the investigation area of the eastern and central Tibetan Plateau, it will be shown that the maximum extent of the last glaciation was limited to the glaciation of isolated mountain groups or smaller plateau glaciations. These glaciations are characterized by glacial erosional features such as U-shaped valleys in the inner mountain massifs and cirques in the higher summit areas. Accumulation forms, such as terminal and lateral moraines (less common than basal moraines) are marked by granitic and basaltic erratics. Using two examples from the eastern margin of the Tibetan Plateau (Nianbaoyeze Shan and northern Chola Shan), the extent of Pleistocene glaciations can be mapped by the distribution of terminal and lateral moraines and erratic granite boulders on the slopes of the surrounding schists. Two to three main Pleistocene glaciations can be separated using weathering criteria, paleosols, and the overlying stratum on the moraines and associated terrace sequences. Overlying stratum consist of aeolian sandy silt or solifluction debris. The younger moraines are found to be characterized by large erratics, were therefore named “Big-Boulder-Moraine”, and can be found over the investigated area. The ice free areas are characterized by Pleistocene terraces along the main rivers, loess deposits with sand and ice wedges at higher elevations (e.g. the basin of Zoige), and paleosols in the basins below 3500 m a.s.l. (e.g. Aba basin or Garze basin). The hypothesis of an extensive plateau glaciation could not be verified for this region. A rising snowline (equilibrium-line altitudes = ELAs) is revealed in two cross-sections during the Last Glacial Maximum (LGM) at the eastern and northern margin of the Tibetan Plateau. Snowlines migrated from about 4000 m in the east (Minshan), to 4300 m in the Nianbaoyeze, and to about 4700 m in the west (Bayan Har Shan). A rise was also recorded from about 3800 m in the north (central Qilian Shan), up to a range of 4300–4950 m (different exposures) in the Kunlun Shan, to approximately 5200 m in the Tanggula Shan in the center of the Tibetan Plateau. The general increase of modern and LGM snowlines (or ELAs) towards the plateau corresponds with a decrease of precipitation from east to west and from north to south respectively. Precipitation was more pronounced at the eastern margin during glacial times than compared to present.

Geomorphological investigations and luminescence dating in the southern part of the Khangay and the Valley of the Gobi Lakes (Central Mongolia)

Geomorphological investigations in the catchment area of the Baydragiyn river along the southern slope of the Khangay and in the Valley of the Gobi Lakes in western Mongolia provide evidence for Late Quaternary glaciations and lake-level changes. Thermoluminescence (TL) and infrared optically stimulated luminescence (IRSL) dating of aeolian and colluvial sediments from the central Khangay place the sediments that overlie fluvial and glaciofluvial terraces in the Holocene. An age of 21 ka is determined for a sand deposit overlying the terrace that is related to the last glacial ice-margin. Lacustrine sediments from higher beach lines in the Valley of the Gobi Lakes provide evidence for a slightly more humid period around 1.5 ka, and a larger extent of the lakes in the Early Holocene at about 8.5 ka, as also reported from other parts of Central Asia. Remnants of lacustrine sediments buried by alluvial gravel, and indicating a huge palaeolake in the basin of the Orog Nuur, however, date to the early stage of the last glaciation around 70 ka. Copyright

Late pleistocene, Late-glacial and Holocene glacier advances on the Tibetan Plateau

Abstract Many end-moraine sequences in various mountain areas of the Tibetan Plateau have already been investigated in detail, but a homogeneous chronostratigraphy is still lacking. The ice-marginal limits are commonly given local names and often the classification does not seem to be secure. This is one reason for controversial discussions on the ice extent at the LGM. Because most organic material in Tibet developed at the beginning of the Holocene, only a few older radiocarbon dates are available. Up to now, Late-glacial terminal moraines have been found in the western Kunlun Shan and in the Tian Shan. Their radiocarbon ages (ca. 15 ka BP) correspond to a glacier advance south of the Qaidam basin, which can be correlated with Late-glacial lacustrine sediments. In Eastern Tibet different ages of peat development in different tongue-like basins have been obtained. Even the bases of peat bogs in basins at Nianbaoyeze mountains enclosed by (apparent) LGM end moraines gave Holocene ages. Three main Holocene intervals have been radiocarbon dated from palaeosols and basal peat layers in the area. Except for a younger glacier re-advance at about 3000 BP, and those of the Little Ice Age, the age of many last glaciation glacier advances on the Tibetan Plateau are constrained only on the basis of organic matter lying on top of the moraines and therefore might be of Late-glacial or Pleistocene age.

An outline of physical geography including Pleistocene glacial landforms of Eastern Tibet (provinces Sichuan and Qinghai)

The relief of the expedition area in NW-Sichuan and SE-Qinghai can simply be separated into two main areas: the catchment area of the Yangtze with deeply incised rivers in narrow V-shaped valleys up to about 3,000 m and the plateau area with old peneplains in the Huang He catchment with meandering rivers up to more than 4,000 m. Precipitation and temperature decrease towards the NW. All zonal boundaries, eg the vegetation boundaries or the snowline, are ascending in elevation towards the interior of the plateau. The natural vegetation, especially the forests, are modified by anthropo-zoogenic influences. Within the zone of alpine meadows and the uppermost region of natural forest as well as in the Red Basin evidence of pasture and agriculture goes back by at least 2,000 years. During the last 20 years within the transition zone of the deeply incised valleys towards the plateau one third of the forests have been cut down.The main investigation area of Nianbaoyeze is built up by a granite-dome being about 500 to 800 m higher than the surrounding peneplains. The extent of Pleistocene glaciations can be mapped by the distribution of terminal and lateral moraines and erratic granite boulders on the slopes of the surrounding schists. The altitude of terminal moraines indicates a snowline of the last glaciation at about 4,300 m, which is about 800 m below the present snowline. Three main Pleistocene glaciations can be separated due to palaeosols, the overlying stratum being either aeolian sandy silt or solifluction debris. The ice free areas shows several Pleistocene terraces and in addition loess deposits with palaeosols in the deeper basins. In summary there is a rising of the snowline of the last glaciation at the E margin of the Tibetan Plateau from about 4,000 m in the E (Minshan) to 4,300 m at Nianbaoyeze and to about 4,700 m in the W (Bayan Har Shan). The hypothesis of an extensive plateau glaciation could not be verified for this region.

Tracing the influence of Mediterranean climate on Southeastern Europe during the past 350,000 years

Loess-palaeosol sequences are valuable archives of past environmental changes. Although regional palaeoclimatic trends and conditions in Southeastern Europe have been inferred from loess sequences, large scale forcing mechanisms responsible for their formation have yet to be determined. Southeastern Europe is a climatically sensitive region, existing under the strong influence of both Mediterranean and continental climates. Establishment of the spatial and temporal evolution and interaction of these climatic areas is essential to understand the mechanisms of loess formation. Here we present high-resolution grain-size, environmental magnetic, spectrophotometric and geochemical data from the Stalać section in the Central Balkans (Serbia) for the past ~350,000 years. The goal of this study is to determine the influence of the Mediterranean climate during this period. Data show that the Central Balkans were under different atmospheric circulation regimes, especially during Marine Isotope Stages 9 and 7, while continental climate prevailed further north. We observe a general weakening of the Mediterranean climate influence with time. Our data suggest that Marine Isotope Stage 5 was the first interglacial in the Central Balkans that had continental climate characteristics. This prominent shift in climatic conditions resulted in unexpectedly warm and humid conditions during the last glacial.

Aeolian sedimentation in arid and semi-arid environments of Western Mongolia

Research on aeolian sediments in Mongolia shows two main cycles of aeolian sedimentation: first the accumulation of major sand fields neighbouring the eastern bank of rivers and lakes, and second the distribution of loess-like sediments on the mountain slopes. The first is resulting from strong westerly winds, being more strength especially during the glacial periods. The latter is resulting in the erosion and accumulation of silt in this region in more humid periods in Interstadial stages and at the end of glacial periods. Both cycles are described on the case study area of the Uvs Nuur Basin in Western Mongolia.

The extent of Late Pleistocene glaciations in the Altai and Khangai Mountains

Summary The current state of research concerning the extent of Late Pleistocene glaciations in Mongolia and the Russian Altai is summarised. Pleistocene moraines resulting from valley glaciers, plateau glaciers, cirque glaciers and ice streams can be found. The most extensive Late Pleistocene glaciation occurred in the western part of the Altai. However, the extent of Late Pleistocene ice in the Russian Altai is still under debate. In the eastern Altai and especially in the northern part of the Mongolian Altai, Pleistocene glaciers were restricted to several isolated mountain systems. To show the variations in the ice extent, the modern and Late Pleistocene equilibrium-line altitudes (ELAs) and the glaciated area are presented in two cross-sections through the Altai and Western Mongolia. The ELAs are relatively low in the more humid outermost ranges of the arid and semi-arid regions of Central Asia and rise towards the central part of Mongolia. The limited extent of present and Pleistocene glaciers in the eastern part of the Russian Altai and in the Mongolian Altai is the result of decreasing precipitation from west to east. This results in a rise of both the present-day and Pleistocene ELAs towards the east. However, it was more pronounced during the Pleistocene than today. There is an essential lack of absolute dating of glacial sediments in this particular region. Nevertheless, on the basis of present knowledge, most Late Pleistocene glacier advances in Mongolia and in the Russian Altai took place during Marine Isotope Stages (MIS) 2 and 4.

Twentieth-century summer temperature variability in the southern Altai Mountains: A carbon and oxygen isotope study of tree-rings

The Altai mountains, southern Siberia, represent an area of significant scientific interest and exceptional palaeoecological potential. To assess the influence of climate on the stable isotopic composition of tree-ring cellulose and the potential of this record as a palaeoclimate proxy, replicated stable oxygen and carbon isotope time-series were developed for the twentieth century from four Siberian Pine (Pinus sibirica Du Tour) trees growing near the settlement of Aktash, Russian Altai mountains, southern Siberia. Bootstrapped calibrations for the local instrumental period (AD 1954—2000) reveal strong correlations between summer (July—August) growing season temperatures and tree-ring oxygen (r 2=0.55) and carbon (r 2=0.30) isotopes. Covariance observed between both carbon and oxygen isotope data suggest a common (stomatal) control. The resulting empirical model was used to reconstruct regional summer temperatures for the twentieth century. No divergence is observed between the non-detrended tree-ring isotope series and instrumental data nor is there any twentieth-century summer warming trend. The strong isotopic signal preserved in the tree-ring series supports the wider application of this approach to explore climatic variability and environmental trends during past millennia through analysis of new and existing long tree-ring chronologies from this region.