TY - JOUR A1 - Zhang Chengjun, A1 - Fan Rong, A1 - Li Jun, A1 - Mischke, Steffen A1 - Dembele, Blaise A1 - Hu Xiaolan, T1 - Carbon and oxygen isotopic compositions - how lacustrine environmental factors respond in northwestern and northeastern China JF - Acta geologica Sinica : english edition N2 - Surface lake sediments, 28 from Hoh Xil, 24 from northeastern China, 99 from Lake Bosten, 31 from Ulungur and 26 from Heihai were collected to determine C-13 and O-18 values. Considering the impact factors, conductivity, alkalinity, pH, TOC, C/N and carbonate-content in the sediments, Cl, P, S, and metal element ratios of Mg/Ca, Sr/Ca, Fe/Mn of bulk sediments as environmental variables enable evaluation of their influences on C-13 and O-18 using principal component analysis (PCA) method. The closure and residence time of lakes can influence the correlation between C-13 and O-18. Lake water will change from fresh to brackish with increasing reduction and eutrophication effects. Mg/Ca in the bulk sediment indicates the characteristic of residence time, Sr/Ca and Fe/Mn infer the salinity of lakes. Carbonate formation processes and types can influence the C-13-O-18 correlation. O-18 will be heavier from Mg-calcite and aragonite formed in a high-salinity water body than calcite formed in freshwater conditions. When carbonate content is less than 30%, there is no relationship with either C-13 or O-18, and also none between C-13 and O-18. More than 30%, carbonate content, however, co-varies highly to C-13 and O-18, and there is also a high correlation between C-13 and O-18. Vegetation conditions and primary productivity of lakes can influence the characteristics of C-13 and O-18, and their co-variance. Total organic matter content (TOC) in the sediments is higher with more terrestrial and submerged plants infilling. In northeastern and northwestern China, when organic matter in the lake sediments comes from endogenous floating organisms and algae, the C-13 value is high. C-13 is in the range of -4%o to 0 parts per thousand when organic matter comes mainly from floating organisms (C/N<6); in the range of -4 parts per thousand to 8 parts per thousand when organic matter comes from diatoms (C/N=6 to 8); and -8 parts per thousand to -4 parts per thousand when organic matter comes from aquatic and terrestrial plants (C/N>8). KW - Limnology KW - isotopic analysis KW - carbonates KW - organic matter KW - PCA KW - Tibet KW - Xinjiang KW - Northeastern China Y1 - 2013 U6 - https://doi.org/10.1111/1755-6724.12133 SN - 1000-9515 SN - 1755-6724 VL - 87 IS - 5 SP - 1344 EP - 1354 PB - Wiley-Blackwell CY - Hoboken ER - TY - JOUR A1 - Tian, Fang A1 - Herzschuh, Ulrike A1 - Dallmeyer, Anne A1 - Xu, Qinghai A1 - Mischke, Steffen A1 - Biskaborn, Boris T1 - Environmental variability in the monsoon-westerlies transition zone during the last 1200 years - lake sediment analyses from central Mongolia and supra-regional synthesis JF - Quaternary science reviews : the international multidisciplinary research and review journal N2 - A high resolution multi proxy (pollen, grain size, total organic carbon) record from a small mountain lake (Lake Khuisiin; 46.6 degrees N, 101.8 degrees E; 2270 m a.s.l.) in the south eastern Khangai Mountains of central Mongolia has been used to explore changes in vegetation and climate over the last 1200 years. The pollen data indicates that the vegetation changed from dry steppe dominated by Poaceae and Artemisia (ca AD 760-950), to Larix forest steppe (ca AD 950-1170), Larix Betula forest steppe (ca AD 1170-1380), meadow dominated by Cyperaceae and Poaceae (ca AD 1380-1830), and Larix Betula forest steppe (after similar to AD 1830). The cold-wet period between AD 1380 and 1830 may relate to the Little Ice Age. Environmental changes were generally subtle and climate change seems to have been the major driver of variations in vegetation until at least the early part of the 20th century, suggesting that either the level of human activity was generally low, or the relationship between human activity and vegetation did not alter substantially between AD 760 and 1830. A review of centennial scale moisture records from China and Mongolia revealed that most areas experienced major changes at ca AD 1500 and AD 1900. However, the moisture availability since AD 1500 varied between sites, with no clear regional pattern or relationship to present day conditions. Both the reconstructions and the moisture levels simulation on a millennium scale performed in the MPI Earth System Model indicate that the monsoon-westerlies transition area shows a greater climate variability than those areas influenced by the westerlies, or by the summer monsoon only. KW - Pollen KW - Grain size KW - TOC KW - Asian monsoon KW - Westerlies KW - Late Holocene KW - Vegetation change KW - Mongolia Y1 - 2013 U6 - https://doi.org/10.1016/j.quascirev.2013.05.005 SN - 0277-3791 VL - 73 IS - 2 SP - 31 EP - 47 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Opitz, Stephan A1 - Ramisch, Arne A1 - Mischke, Steffen A1 - Diekmann, Bernhard T1 - Holocene lake stages and thermokarst dynamics in a discontinuous permafrost affected region, north-eastern Tibetan Plateau JF - Journal of Asian earth sciences N2 - Sediments of a thermokarst system on the north-eastern Tibetan Plateau were studied to infer changes in the lacustrine depositional environment related to climatic changes since the early Holocene. The thermokarst pond with a length of 360 m is situated in a 14.5 x 6 km tectonically unaffected intermontane basin, which is underlain by discontinuous permafrost. A lake sediment core and bankside lacustrine onshore deposits were analysed. Additionally, fossil lake sediments were investigated, which document a former lake-level high stand. The sediments are mainly composed of marls with variable amounts of silt carbonate micrite, and organic matter. On the basis of sedimentological (grain size data), geochemical (XRF), mineralogical (XRD) and micropaleontological data (ostracods and chironomide assemblages) a reconstruction of a paleolake environment was achieved. Lacustrine sediments with endogenic carbonate precipitation suggest a lacustrine environment since at least 19.0 cal ka BP. However, because of relocation and reworking processes in the lake, the sediments did not provide distinct information about the ultimate formation of the lake. The high amount of endogenic carbonate suggests prolonged still-water conditions at about 9.3 cal ka BP. Ostracod shells and chironomid head capsules in fossil lake sediments indicate at least one former lake-level high stand, which were developed between the early and middle Holocene. From the late Holocene the area was possibly characterized by a lake-level decline, documented by a hiatus between lacustrine sediments and a reworked loess or loess-like horizon. After the lake-level decline and the following warming period, the area was affected by thermally-induced subsidence and a re-flooding of the basin because of thawing permafrost. KW - Palaeoenvironmental reconstruction KW - Palaeolimnology KW - Lake level KW - XRD Y1 - 2013 U6 - https://doi.org/10.1016/j.jseaes.2013.08.006 SN - 1367-9120 SN - 1878-5786 VL - 76 IS - 17 SP - 85 EP - 94 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Zhang, Wanyi A1 - Mischke, Steffen A1 - Zhang, Chengjun A1 - Gao, Dou A1 - Fan, Rong T1 - Ostracod distribution and habitat relationships in the Kunlun Mountains, northern Tibetan Plateau JF - Quaternary international : the journal of the International Union for Quaternary Research N2 - Surface sediment samples were collected from the lakes Heihai, Kusai, Haiding Nuur and Yan Hu, and from streams and ponds in the Kunlun Mountains at the northern margin of the Tibetan Plateau to investigate the sub-fossil ostracod (micro-crustacean) fauna of the region. Among 65 collected samples, 46 ostracod shell-rich samples were used to study the relationship between the ostracod distribution and specific conductivity (SC) of the water, which ranged from 0.6 to 53.0 mS cm(-1). A total of eleven ostracod species was identified from this region, with about half of the species restricted to the Tibetan Plateau and its adjacent mountain areas, and the other half representing Holarctic taxa. Tonnacypris cf. estonica and Tonnacypris tonnensis are reported from the Tibetan Plateau for the first time. Leucocythere sp. is the dominant species and Ilyocypris cf. bradyi is also relatively abundant. The other seven species were recorded with limited abundances apparently due to lower SC tolerances. Leucocythere sp. was recorded over the full SC range from 0.6 to 53 mS cm(-1). Eucypris mareotica is a typical brackish and saline water species, which was found at sample sites with high SC (2.8-53.0 mS cm(-1)). In contrast, Leucocythere dorsotuberosa, Candona candida and Eucypris afghanistanensis prefer freshwater to slightly oli-gohaline waters with SC < 1.8 mS cm(-1). The SC optimum and tolerance range for each species were determined and compared to earlier reported data from other regions of Central Asia. The results indicate that species assemblage data from fossil ostracod shells have a large potential to provide information on past SC levels and more general climate-determined moisture conditions. (C) 2013 Elsevier Ltd and INQUA. All rights reserved. Y1 - 2013 U6 - https://doi.org/10.1016/j.quaint.2013.06.020 SN - 1040-6182 VL - 313 SP - 38 EP - 46 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Taft, Linda A1 - Wiechert, Uwe A1 - Zhang, Hucai A1 - Lei, Guoliang A1 - Mischke, Steffen A1 - Plessen, Birgit A1 - Weynell, Marc A1 - Winkler, Andreas A1 - Riedel, Frank T1 - Oxygen and carbon isotope patterns archived in shells of the aquatic gastropod Radix - hydrologic and climatic signals across the Tibetan Plateau in sub-monthly resolution JF - Quaternary international : the journal of the International Union for Quaternary Research N2 - The Tibetan Plateau (TP), including its surrounding mountain ranges, represents the largest store of ice outside the polar regions. It hosts numerous lakes as well as the head waters of major Asian rivers, on which billions of people depend, and it is particularly sensitive to climate change. The moisture transport to the TP is controlled by the Indian and Pacific monsoon and the Westerlies. Understanding the evolution of the interaction of these circulation systems requires studies on climate archives in different spatial and temporal contexts. The objective of this study is to learn more about the interannual variability of precipitation patterns across the TP and how different hydrologic systems react to different climatic factors. Aragonite shells of the aquatic gastropod Radix, which is widely distributed in the region, may represent suitable archives for inferring hydrologic and climatic signals in particularly high resolution. Therefore, sclerochronological studies of delta O-18 and delta C-13 ratios in Radix shells from seven lakes were conducted, each representing a different hydrologic and climatic setting, on a transect from the Pamirs across the TP. The shell patterns exhibit an increasing influence of precipitation and a decreasing influence of evaporation on the isotope compositions from west to east. delta O-18 values of shells from lakes on the eastern and central TP (Donggi Cona, Yamdrok Yumco, Tarab Co) mirror monsoon signals, indicated by more negative values and higher variabilities compared to the more western lakes (Karakul, Bangong/Nyak, Manasarovar). In Yadang Co, located on the central southern TP, the monsoon rains did not reach the lake in the sampling year, although it is located in a region which is usually affected by monsoon circulation. The delta O-18 values are used to differentiate the annual hydrological cycle into ice cover period, melt water period, precipitation period and evaporation period. delta C-13 compositions in the shells particularly depend on specific habitats, which vary in biological productivity and in carbon sources. delta O-18 and delta C-13 patterns show a positive covariance in shells originating from large closed basins. The results show that Radix shells mirror general climatic differences between the seven lake regions. These differences reflect both regional and local climate signals in sub-seasonal resolution, without noticeable dependence on the particular lake system. Y1 - 2013 U6 - https://doi.org/10.1016/j.quaint.2012.10.031 SN - 1040-6182 VL - 290 IS - 1 SP - 282 EP - 298 PB - Elsevier CY - Oxford ER - TY - INPR A1 - Mischke, Steffen A1 - Wünnemann, Bernd A1 - Appel, Erwin T1 - Proxies for quaternary monsoon reconstruction on the tibetan plateau T2 - Quaternary international : the journal of the International Union for Quaternary Research Y1 - 2013 U6 - https://doi.org/10.1016/j.quaint.2013.10.001 SN - 1040-6182 VL - 313 SP - 1 EP - 2 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Mischke, Steffen A1 - Weynell, Marc A1 - Zhang, Chengjun A1 - Wiechert, Uwe T1 - Spatial variability of C-14 reservoir effects in Tibetan Plateau lakes JF - Quaternary international : the journal of the International Union for Quaternary Research N2 - Radiocarbon dating of lake sediments is often hampered by the presence of a lake reservoir effect (LRE, also 'dead carbon' or 'old carbon' effect) especially in dry and cold regions with a sparse plant cover in the catchment. The Tibetan Plateau became a hotspot of the palaeoenvironmental and climate research community in recent years and the assessment of present-day LREs is a crucial prerequisite for the establishment of reliable radiocarbon age depth relationships for lake sediment cores. This paper examines the spatial variability of LREs within individual lakes, through a discussion of new data for Lake Donggi Cona and a compilation of previously published data for five additional lakes where LRE data are available for different sites. Lake reservoir effects for Lake Donggi Cona on the northeastern Tibetan Plateau were determined for shells of aquatic snails collected alive close to the lake's shore. The largest determined LRE of 20,000 C-14 years is significantly larger than previously reported LREs from the central part of the lake, and larger than any previously published LRE for the Tibetan Plateau. Relatively low LREs in the central regions of lakes, higher LREs towards the margins, and high LREs in tributaries and spring waters are apparently a common pattern of Tibetan Plateau lakes. The differences in LREs within individual lakes or catchment areas are attributed to the more prolonged exchange of the lake water's dissolved inorganic carbon with the atmospheric CO2 in central lake regions on the one hand and the increasing influence of C-14 free or poor stream and groundwater due to the dissolution of carbonaceous basement rocks towards its margins. Generally higher LREs were recorded in the three tectonically active lake regions of the six examined catchments, and it is speculated that rising crustal CO2 further contributes to the LREs in these catchments. In addition to these observations and inferences, elevated C-14 levels of the atmosphere as a result of nuclear bomb testing are often ignored if LREs for modern materials are reported by convention relative to the atmospheric C-14 activity of the year 1950. LRE data reported in this way represent unrealistic minimum estimates. (C) 2013 Elsevier Ltd and INQUA. Y1 - 2013 U6 - https://doi.org/10.1016/j.quaint.2013.01.030 SN - 1040-6182 VL - 313 IS - 45 SP - 147 EP - 155 PB - Elsevier CY - Oxford ER -