Holocene climate change inferred from stratigraphy and OSL chronology of aeolian sediments in the Qaidam Basin,northeastern Qinghai–Tibetan Plateau

Paleoclimatic reconstruction based on aeolian sediments in the eastern Qaidam Basin (QB) has been hindered by the limited chronological data. Here we present 61 Optically Stimulated Luminescence (OSL) ages. On the basis of these OSL ages and the lithologic stratigraphy, we propose the ‘effective moisture index (EMI)’ for aeolian sediments to reconstruct the effective moisture change. Based on the EMI from twelve sections, the effective moisture change, moisture sources and relevant mechanisms for paleoclimatic change in the eastern QB are discussed. The results indicate that (1) aeolian deposition started at least before 12.4 ± 0.7 ka during the deglaciation, the paleosols developed at the early and mid-Holocene, and aeolian sand and loess accumulated at mid- and late Holocene; (2) effective moisture history was: hyper-arid at 12.8–11.6 ka, humid and variable at 11.6–8.3 ka, moderately humid and stable at 8.3–3.5 ka, and increasingly arid at 3.5–0 ka; (3) the effective moisture change was mainly controlled by the Asian summer monsoon (ASM), which mainly followed the change of Northern Hemispheric summer insolation, and the westerlies strengthened and increased the aridity in the QB when the ASM shrank.     

Timing of glacier fluctuations and trigger mechanisms in eastern Qinghai–Tibetan Plateau during the late Quaternary

It is highly debated whether glacial advances on the Qinghai–Tibetan Plateau (QTP) occurred as a response to temperature cooling, or whether they were forced by an increase in moisture brought by the intensive Indian summer monsoon. We here report a case study investigating this issue. Multiple moraine series in the Yingpu Valley, Queer Shan ranges of the Hengduan Mountains, and eastern QTP, provide an excellent archive for examining the timing and trigger mechanism of glacier fluctuations. Twenty-seven optically stimulated luminescence (OSL) samples of glacial sediments were collected from this valley. The quartz OSL ages show that the moraine series of Y-1, I, M and O were formed during the Late Holocene, Late Glacial, the global Last Glacial Maximum (LGM) and Marine Oxygen Isotope Stage (MIS) 3 (likely mid-MIS-3). The youngest Y-2 moraines probably formed during the Little Ice Age (LIA). The oldest H moraines formed before MIS-3. We found that glacial advances during the late Quaternary at the Yingpu Valley responded to cold stages or cold events rather than episodes of enhanced summer monsoon and moisture. As a result, glaciers in the monsoonal Hengduan Mountains were mainly triggered by changes in temperature. Millennial time scale temperature oscillations might have caused the multiple glacial advances.     

Deformation at the Easternmost Altyn Tagh Fault: Constraints on the Growth of the Northern Qinghai–Tibetan Plateau

How the Altyn Tagh fault(ATF) extends eastwards is one of the key questions in the study of the growth of the Qinghai–Tibetan Plateau. Detailed fieldwork at the easternmost part of the ATF shows that the ATF extends eastward and bypasses the Kuantan Mountain; it does not stop at the Kuantan Mountain, but connects with the northern Heishan fault in the east. The ATF does not enter the Alxa Block but extends eastward along the southern Alxa Block to the Jintanan Mountain. The Heishan fault is not ...     

Different response of vegetation to permafrost change in semi-arid and semi-humid regions in Qinghai–Tibetan Plateau

The effects of the depth of the active layer of permafrost on aboveground vegetation in semi-arid and semi-humid regions of the Qinghai–Tibetan Plateau were studied. The depth of active permafrost was measured and aboveground vegetation recorded. Differences in correspondence between permafrost depth and aboveground vegetation in semi-arid and semi-humid regions were analyzed. Vegetation cover and biomass were well correlated with permafrost depth in both semi-arid and semi-humid regions, but the correlation coefficient in the semi-arid region was larger than in the semi-humid region. With the increase in permafrost depth, vegetation cover and biomass decreased in both regions. Species richness and diversity decreased with increasing depth of permafrost in the semi-arid region. In the semi-humid region, these at first increased and then decreased as permafrost depth increased. It seems likely that vegetation on the Qinghai–Tibetan Plateau will degenerate to different degrees due to permafrost depth increasing as a result of climatic warming. The influence would be especially remarkable in the semi-arid region.     

Thermal hazards zonation and permafrost change over the Qinghai–Tibet Plateau

The Qinghai–Tibet Plateau is the largest permafrost region at low latitude in the world. Climate warming may lead to permafrost temperature rise, ground ice thawing and permafrost degradation, thus inducing thermal hazards. In this paper, the ARCGIS method is used to calculate the changes of ground ice content and active layer thickness under different climate scenarios on the Qinghai–Tibet Plateau, in the coming decades, thus providing the basis for hazards zonation. The method proposed by Nelson in 2002 was used for hazards zonation after revision, which was based on the changes of active layer thickness and ground ice content. The study shows that permafrost exhibits different degrees of degradation in the different climate scenarios. The thawing of ground ice and the change from low-temperature to high-temperature permafrost were the main permafrost degradation modes. This process, accompanied with thinning permafrost, increases the active layer thickness and the northward movement of the permafrost southern boundary. By 2099, the permafrost area decreases by 46.2, 16.01 and 8.5% under scenarios A2, A1B and B1, respectively. The greatest danger zones are located mainly to the south of the West Kunlun Mountains, the middle of the Qingnan Valley, the southern piedmont of the Gangdise and Nyainqentanglha Mountains and some regions in the southern piedmont of the Himalayas. The Qinghai–Tibet Plateau permafrost region is in the low-risk category. Climate warming exacerbates the development of thermal hazards. In 2099, the permafrost region is mainly in the middle-risk category, and only a small portion is in the low-risk category.     

The Holocene climatic optimum and pollen records of sapropel 1 in the eastern Mediterranean, 9000–6000 BP

The most recent sapropel in the deep eastern Mediterranean Sea has been deposited between 9 and 6 ka bp. Climate conditions, as revealed by the pollen records of this sapropel in marine cores, were most favorable for temperate deciduous trees, which is in agreement with the inferences from records of peripheral land pollen sites. The abundance of deciduous oak pollen is much higher than that of Artemisia (sage-brush), indicating that annual precipitation in the mid-elevation borderlands was at least 550 mm without summer drought, but more probably in the range 800–1300 mm. The pollen of Pistacia, which formed a savanna at low elevations, is also at its highest abundance and signals the absence of frost in winter, while being capable of withstanding summer drought. The early Holocene therefore appears as the post-glacial climatic optimum with the highest moisture and mildest winters. In southwest Asia, this is also the time of the Neolithic population explosion with incipient domestication of cereals, possibly following natural selection of the ‘tough rachis’ mutation in wheat and barley by the extreme aridity of the preceding Younger Dryas.     

Aeolian desertification and its causes in the Zoige Plateau of China’s Qinghai–Tibetan Plateau

Studies and efforts to control aeolian desertification in China have focused on the arid and semiarid lands in the north. However, the aeolian desertification that is occurring on the high-altitude Qinghai–Tibetan Plateau, which has a cold and humid climate, has received attention only in recent years. In this paper, we report the results of monitoring this aeolian desertification between 1975 and 2005 and of our analysis of its causes on the Zoige Plateau, which is located in the northeastern part of China’s Qinghai–Tibetan Plateau. Aeolian desertified lands expanded at a compound annual rate of 4.07% between 1975 and 2005. They expanded most rapidly between 1975 and 1990, at an annual rate of 7.73%. Factors responsible for this expansion include increasing temperature, decreasing precipitation, over-grazing, drainage of water systems, and land reclamation for agriculture. Increasing temperature, over-grazing, and the drainage of water systems were the key factors. The climatic variations between 1975 and 2005 were not sufficient by themselves to lead to aeolian desertification. Human disturbances such as over-grazing and drainage of water systems must thus have been primarily responsible for the observed changes, and human behavior must be adjusted to control the expansion of aeolian desertification and rehabilitate the desertified lands.     

Holocene vegetation and climate histories in the eastern Tibetan Plateau: controls by insolation-driven temperature or monsoon-derived precipitation changes?

The climates on the eastern Tibetan Plateau are strongly influenced by direct insolation heating as well as monsoon-derived precipitation change. However, the moisture and temperature influences on regional vegetation and climate have not been well documented in paleoclimate studies. Here we present a well-dated and high-resolution loss-on-ignition, peat property and fossil pollen record over the last 10,000 years from a sedge-dominated fen peatland in the central Zoige Basin on the eastern Tibetan Plateau and discuss its ecological and climatic interpretations. Lithology results indicate that organic matter content is high at 60–80% between 10 and 3 ka (1 ka = 1000 cal yr BP) and shows large-magnitude fluctuations in the last 3000 years. Ash-free bulk density, as a proxy of peat decomposition and peatland surface moisture conditions, oscillates around a mean value of 0.1 g/cm³, with low values at 6.5–4.7 ka, reflecting a wet interval, and an increasing trend from 4.7 to 2 ka, suggesting a drying trend. The time-averaged mean carbon accumulation rates are 30.6 gC/m²/yr for the last 10,000 years, higher than that from many northern peatlands. Tree pollen (mainly from Picea), mostly reflecting temperature change in this alpine meadow-forest ecotonal region, has variable values (from 3 to 34%) during the early Holocene, reaches the peak value during the mid-Holocene at 6.5 ka, and then decreases until 2 ka. The combined peat property and pollen data indicate that a warm and wet climate prevailed in the mid-Holocene (6.5–4.7 ka), representing a monsoon maximum or “optimum climate” for the region. The timing is consistent with recent paleo-monsoon records from southern China and with the idea that the interplays of summer insolation and other extratropical large-scale boundary conditions, including sea-surface temperature and sea-level change, control regional climate. The cooling and drying trend since the mid-Holocene likely reflects the decrease in insolation heating and weakening of summer monsoons. Regional synthesis of five pollen records along a south–north transect indicates that this climate pattern can be recognized all across the eastern Tibetan Plateau. The peatland and vegetation changes in the late Holocene suggest complex and dramatic responses of these lowland and upland ecosystems to changes in temperature and moisture conditions and human activities.     

Active layer thickness variations on the Qinghai–Tibet Plateau under the scenarios of climate change

Climate change has greatly influenced the permafrost regions on the Qinghai–Tibet Plateau (QTP). Most general circulation models (GCMs) project that global warming will continue and the amplitude will amplify during the twenty-first century. Climate change has caused extensive degradation of permafrost, including thickening of the active layer, rising of ground temperature, melting of ground ice, expansion of taliks, and disappearance of sporadic permafrost. The changes in the active layer thickness (ALT) greatly impact the energy balance of the land surface, hydrological cycle, ecosystems and engineering infrastructures in the cold regions. ALT is affected by climatic, geographic and geological factors. A model based on Kudryavtsev’s formulas is used to study the potential changes of ALT in the permafrost regions on the QTP. Maps of ALT for the year 2049 and 2099 on the QTP are projected under GCM scenarios. Results indicate that ALT will increase with the rising air temperature. ALT may increase by 0.1–0.7 m for the year 2049 and 0.3–1.2 m for the year 2099. The average increment of ALT is 0.8 m with the largest increment of 1.2 m under the A1F1 scenario and 0.4 m with the largest increment of 0.6 m under the B1 scenario during the twenty-first century. ALT changes significantly in sporadic permafrost regions, while in the continuous permafrost regions of the inland plateau ALT change is relatively smaller. The largest increment of ALT occurs in the northeastern and southwestern plateaus under both scenarios because of higher ground temperatures and lower soil moisture content in these regions.     

The effect of permafrost changes on embankment stability along the Qinghai–Xizang Railway

After construction of the Qinghai–Xizang Railway, the resultant heat exchange between soil and atmosphere caused changes in the thermal and mechanical stability of permafrost beneath the railway embankment. Monitoring from 2005 to 2010 indicated 12 sections of embankment that experienced more than 5 cm of settlement, with three showing deformations of more than 10 cm and signs of continuing settlement. Embankment stability is closely related to permafrost changes beneath the embankment. Large-scale deformations have contributed to permafrost thaw and artificial permafrost table deepening, and this deformation has not stabilized over the short term. In contrast, small-scale deformations have contributed to a warming of the permafrost that has gradually stabilized as soil temperature decreases. Only three sections of the Qinghai–Xizang Railway have exhibited settlement deformation that exceeds 10 cm, through a deepening of the artificial permafrost table and a gradual increase in permafrost thawing result in embankment settlement deformation. However, with climate warming trends and the long-term operation of the railway, the long-term thermal and mechanical stability of the embankment needs to be carefully monitored to ensure the safe operation of the Qinghai–Xizang Railway.     

Late Pleistocene lake and glaciation evolution on the northeastern Qinghai–Tibetan Plateau: a review

Late Pleistocene paleoclimatic history on northeastern Qinghai–Tibetan Plateau (QTP) has been reconstructed mainly from lake sediments; however, data regarding dry–wet climate changes reported in this region are still not clear and controversial. Based on shoreline features and highstand lacustrine sediments around lakes on the QTP, high lake level histories in this paper were summarized and compared with paleoclimatic records from lake sediments, ice core and glaciation evolution surrounding mountains on the NE QTP during late Pleistocene. The results indicate that periods of high lake level occurred at MIS 5, MIS 3 and early-middle Holocene and most likely corresponding to warm and wet climate periods, while periods of low lake level existed in intervening intervals, corresponding to cold and dry climate periods, which most likely coincide with glacial advances surrounding high mountains. With an exception, no wide glacial advance in study area was found during MIS 3, possibly suggesting that effective moisture is lower than that in the other region of NE QTP in this period.     

Interactions between freeze–thaw actions,wind erosion desertification,and permafrost in the Qinghai–Tibet Plateau

The unique natural environment of the Qinghai–Tibet Plateau has led to the development of widespread permafrost and desertification. However, the relationship between desertification and permafrost is rarely explored. Here we study the interaction between desertification and permafrost using a combination of simulations, experiments, and field observations in the Qinghai–Tibet Plateau. Results show the cohesion values of the test samples that experienced 1, 3, and 6 freeze–thaw cycle times decreased by 65.9, 46.0, and 35.5 %, respectively, and the compressive strength of the test samples decreased by 69.6, 39.6, and 34.7 %, respectively, compared to the test samples that did not experience freeze–thaw cycles. The wind erosion rate of the test block eroded by sand-bearing wind was far larger than that by clean wind under the same conditions; the maximum value was 50 times higher than that by clean wind. The wind erosion rate increased with an increasing number of freeze–thaw cycles, water content, and freeze–thaw temperature difference. The ground temperature below the sand layer was decreased, compared to the natural ground surface that without sand layer covering, the drop amplitude of yearly average temperature was roughly maintained at 0.2 °C below the thick sand layer (1.2 m), and the maximum drop of yearly average temperature was 0.7 °C below the thin sand layer (0.1 m). Therefore, with the presence of water, the destruction of surface soil structure caused by repeated and fierce freeze–thaw actions is the main cause of wind erosion desertification in the permafrost region of Qinghai–Tibet Plateau, and sand-bearing wind is the main dynamic force. The development of eolian sand deposits after the desertification emerges. As a result, the properties of the underlying surface are altered. Due to the high reflectivity and poor heat conductivity of the sand layer, the heat exchange of the land–atmosphere system is impeded, causing a drop in the ground temperature of the underlying permafrost that subsequently preserves the permafrost.     

Eco-environmental degradation in the northeastern margin of the Qinghai–Tibetan Plateau and comprehensive ecological protection planning

The regional hydrology and ecosystems of the northeastern margin of the Qinghai–Tibetan Plateau have changed over the past 40 years driven by intense human activity and regional climate changes. Annual mean air temperature has increased in the region. Streamflow from the northeastern margin of the Qinghai–Tibetan Plateau has decreased significantly. Overall, a number of Alpine step meadows and Alpine frigid meadows have seriously degraded. Degeneration of vegetation and grassland led to desertification and frequently induced dust storms. With the continuous increase in cultivated land area, grassland area in the region has dropped significantly since the 1960s. At present, degraded grassland occupies about 83% of total usable grassland area. As the number of livestock increased, range condition deteriorated and the carrying capacity was reduced. The forest area in the northeastern margin of the Qinghai–Tibetan Plateau has decreased by 20%, and the local ecosystem has become very fragile. Given the relatively stable weather conditions, the northeastern margin of the Qinghai–Tibetan Plateau can be characterized by its three major ecosystems: grassland ecosystem, forest ecosystem and wetland ecosystem, which are crucial in maintaining the ecological stability. Changes in these ecosystems could influence sustainable development in the region. To avoid further deterioration of the environment and ecosystems, it is important to establish and implement ecosystem protection planning. Some effective measures are essential in this respect, including technical and political considerations.     

Vegetation and peat accumulation steer Holocene tidal–fluvial basin filling and overbank sedimentation along the Old Rhine River,The Netherlands

In the transformation from tidal systems to freshwater coastal landscapes, plants act as eco-engineering species that reduce hydrodynamics and trap sediment, but nature and timing of the mechanisms of land creation along estuaries remains unclear. This article focuses on the Old Rhine estuary (The Netherlands) to show the importance of vegetation in coastal landscape evolution, predominantly regarding tidal basin filling and overbank morphology. This estuary hosted the main outflow channel of the river Rhine between ca 6500 to 2000 cal bp , and was constrained by peat during most of its existence. This study reconstructs its geological evolution, by correlating newly integrated geological data and new field records to varying conditions. Numerical modelling was performed to test the inferred mechanisms. It was found that floodbasin vegetation and resulting organic accumulation strongly accelerated back-barrier infill, by minimizing tidal influence. After tidal and wave transport had already sufficiently filled the back-barrier basin, reed rapidly expanded from its edges under brackish conditions, as shown by diatom analysis and datings. Reed growth provided a positive infilling feedback by reducing tidal flow and tidal prism, accelerating basin infilling. New radiocarbon dates show that large-scale crevassing along the Old Rhine River – driven by tidal backwater effect – only started as nutrient-rich river water transformed the floodbasin into an Alder carr in a next phase of estuary evolution. Such less dense vegetation promotes crevassing as sediments are more easily transported into the floodbasin. As river discharge increased and estuary mouth infilling progressed, crevasse activity diminished around 3800 to 3000 cal bp , likely due to a reduced tidal backwater effect. The insights from this data-rich Holocene study showcase the dominant role that vegetation may have in the long-term evolution of coastal wetlands. It provides clues for effective use of vegetation in vulnerable wetland landscapes to steer sedimentation patterns to strategically adapt to rising water levels.     

Distribution,stock, and influencing factors of soil organic carbon in an alpine meadow in the hinterland of the Qinghai–Tibetan Plateau

Understanding the spatial distribution, stocks, and influencing factors of soil organic carbon (SOC) is important for understanding the current situation of SOC in alpine meadow ecosystems on the Qinghai–Tibetan Plateau (QTP). We sampled 23 soil profiles to a depth of 50 cm in a 33.5 hm\(^{2}\) plot in a typical meadow on the central QTP. The distribution, stock and influencing factors of SOC was then analyzed. The mean density of soil carbon content (SOCD) was 2.28 kg m\(^{-2}\) with a range of 5.99 kg  m\(^{-2}\). SOCD in the 0–10 cm layer was 3.94 kg m\(^{-2}\) and decreased quadratically with depth. The total stock of SOC to a depth of 50 cm was ca. 2950 t, the 0–10 and 0–30 cm layers accounting for 38 and 80%, respectively. SOCD varied moderately spatially and was distributed more homogeneously in the 0–10 and 40–50 cm layers but was more variable in the middle three layers. SOCD was significantly correlated positively with soil-water content, total porosity, and silt content and negatively with soil pH, bulk density, stone content and sand content. This study provides an important contribution to understanding the role of alpine meadows in the global carbon cycle. It also provides field data for model simulation and the management of alpine meadow ecosystems.     

Simulation analysis of the impacts of underground mining on permafrost in an opencast coal mine in the northern Qinghai–Tibet Plateau

Based on two-dimensional heat-conduction equations with a phase-change component, this study investigates the impact of underground mining on the permafrost environment in an opencast coal mining pit. The dynamics of the maximum thawed and freezing depths at different depths around a borehole wall are determined. The spatial distributions of these dynamics are also determined at different locations of the wall profile. The results show that (1) the maximum freezing depth tends to increase over 100 years; (2) the maximum thawed depth increases along a borehole wall over 100 years. In particular, the maximum thawed depth increases faster near the junctions of permafrost and seasonally frozen soil; (3) due to the small cross section of mining laneways, coal mining does not cause rapid increases in permafrost temperature around borehole walls. Once disturbance to permafrost around a borehole wall decreases, the once-insignificant effect of temperature will become more obvious. Underground mining does have some impacts on permafrost surrounding borehole walls, but it does not cause large areas of deformation due to thermal disturbance.     

Garnet amphibolites from the Ganzi–Litang fault zone,eastern Tibetan Plateau: mineralogy,geochemistry, and implications for evolution of the eastern Palaeo-Tethys Realm

The Ganzi–Litang fault zone, an outstanding tectonic element in the eastern Tibetan Plateau has been intensively debated as an in-situ suture zone marking relict of a subducted Palaeo-Tethyan oceanic crust or a failed intracontinental rift. This paper reports the garnet amphibolites discovered along the Ganzi–Litang fault zone, eastern Tibetan Plateau. These garnet amphibolites are characterized by the garnet–hornblende–rutile–sphene–plagioclase–quartz assemblage. Conventional geothermobarometry figures out the metamorphic temperature and pressure conditions at 582–626°C and 1.61–1.82 GPa, respectively. Geochemical analysis (no Nb–Ta deletions and left-inclined to flat patterns of rare-earth elements) indicates that the garnet amphibolites could represent metamorphic product of the mid-ocean-ridge (MORB)-type mafic rocks that were contaminated by a mantle plume. The protolith of the garnet amphibolites was dated at 236 Ma using in-situ U–Pb zircon method, and the retrograde metamorphism was dated at 218 Ma using in-situ U–Pb sphene method. A comprehensive analysis combined with the development of the Palaeo-Tethys Ocean and the Yidun arc through geologic time indicates a Triassic to Early Jurassic age (236–195 Ma) for the metamorphism of the garnet amphibolites. The low geothermal gradient of 9.8ºC/km and the N-MORB nature of the garnet amphibolites suggest a subduction-zone environment for the high-pressure metamorphism. Therefore, the Ganzi–Litang fault zone is a Palaeo-Tethyan suture separating the Yidun arc and the Songpan–Ganzi terrane, representing the relics of a branch of the Palaeo-Tethys Ocean that was contaminated by a mantle plume.