TY - THES A1 - Theuring, Philipp Christian T1 - Suspended sediments in the Kharaa River, sources and impacts T1 - Feinsedimente im Kharaa Fluss, Quellen und Auswirkungen N2 - Anthropogenically amplified erosion leads to increased fine-grained sediment input into the fluvial system in the 15.000 km2 Kharaa River catchment in northern Mongolia and constitutes a major stressing factor for the aquatic ecosystem. This study uniquely combines the application of intensive monitoring, source fingerprinting and catchment modelling techniques to allow for the comparison of the credibility and accuracy of each single method. High-resolution discharge data were used in combination with daily suspended solid measurements to calculate the suspended sediment budget and compare it with estimations of the sediment budget model SedNet. The comparison of both techniques showed that the development of an overall sediment budget with SedNet was possible, yielding results in the same order of magnitude (20.3 kt a- 1 and 16.2 kt a- 1). Radionuclide sediment tracing, using Be-7, Cs-137 and Pb-210 was applied to differentiate sediment sources for particles < 10μm from hillslope and riverbank erosion and showed that riverbank erosion generates 74.5% of the suspended sediment load, whereas surface erosion contributes 21.7% and gully erosion only 3.8%. The contribution of the single subcatchments of the Kharaa to the suspended sediment load was assessed based on their variation in geochemical composition (e.g. in Ti, Sn, Mo, Mn, As, Sr, B, U, Ca and Sb). These variations were used for sediment source discrimination with geochemical composite fingerprints based on Genetic Algorithm driven Discriminant Function Analysis, the Kruskal–Wallis H-test and Principal Component Analysis. The contributions of the individual sub-catchment varied from 6.4% to 36.2%, generally showing higher contributions from the sub-catchments in the middle, rather than the upstream portions of the study area. The results indicate that river bank erosion generated by existing grazing practices of livestock is the main cause for elevated fine sediment input. Actions towards the protection of the headwaters and the stabilization of the river banks within the middle reaches were identified as the highest priority. Deforestation and by lodging and forest fires should be prevented to avoid increased hillslope erosion in the mountainous areas. Mining activities are of minor importance for the overall catchment sediment load but can constitute locally important point sources for particular heavy metals in the fluvial system. N2 - Durch Landnutzung erhöhte Erosion führt zu verstärktem Eintrag von Feinsedimenten im 15.000 km2 großen Einzugsgebiet des Kharaa in der nördlichen Mongolei. Diese Einträge stellen einen starken Stressor für das aquatische Ökosystem dar. Im Rahmen dieser Studie werden auf neuartige Weise verschiedene Ansätze zur Untersuchung der Feinsedimentdynamik im Einzugsgebiet kombiniert. Der Vergleich der Ergebnisse aus zeitlich hochaufgelösten Abfluss- und Schwebstoffanalysen, Sedimentbudgetmodellierung und „Sediment Source Fingerprinting“ Techniken erlaubt eine Bewertung der Genauigkeit und Anwendbarkeit jeder einzelnen Methodik im Untersuchungsgebiet. Das anhand von Abflussund Schwebstoffmessdaten berechnete Sedimentbudget am Gebietsauslass wurde mit dem mittels des Sedimentbudgetmodels SedNet berechneten Sedimentbudget verglichen. Die Ergebnisse von des aus Abfluss- und Schwebstoffmessdaten berechneten Sedimentbudgets (20,3 kt a- 1) und der mittels SedNet berechneten Werte (16,2 kt a- 1) zeigen eine befriedigende Übereinstimmung und bekräftigen die Anwendbarkeit des Models. Zur Unterscheidung der Herkunft der Feinsedimente hinsichtlich des dominanten Erosionsprozesses wurde die Sedimentfraktion < 10μm hinsichtlich der Konzentrationen der Radioisotope Be-7, Cs-137 und Pb-210 analysiert. Die Ergebnisse zeigen, dass Gerinneerosion für 74.5% der Suspensionsfracht verantwortlich ist und Einträge durch Flächenerosion 21.7% und durch Gully-erosion nur 3.8% beitragen. Zur Untersuchung der Anteile der Teileinzugsgebiete an der Gesamtsuspensionsfracht am Gebietsauslass wurden die Sedimente der Teilgebiete hinsichtlich der Variation ihrer geochemischen Zusammensetzung untersucht (z.B. Ti, Sn, Mo, Mn, As, Sr, B, U, Ca und Sb). Anhand der Zusammensetzung wurde jeweils ein spezifischer “Fingerabdruck” für Sedimente aus spezifischen Teileinzugsgebieten definiert. Mittels Diskriminanzanalyse, Kruskal–Wallis H-test und Hauptkomponentenanalyse konnten die Beiträge der Teileinzugsgebiete an der Gesamtfracht mit einem Mischungsmodel berechnet werden. Generell trugen die oberläufigen Teileinzugsgebiete am geringsten, und die mittelläufigen am stärksten zur Sediment Fracht bei. Die Ergebnisse der Studie zeigen, dass durch Überweidung begünstigte Ufererosion für den Hauptteil der Suspensionsfracht im Kharaa Fluss verantwortlich ist. Zur Verringerung des Schwebstoffeintrags in den Fluss werden Maßnahmen zur Uferbefestigung, insbesondere der Schutz und die Wiederherstellung der Ufervegetation als primäre Maßnahmen empfohlen. KW - sediment source fingerprinting KW - Mongolia KW - suspended sediments KW - sediment transport modelling KW - Mongolei KW - Sedimentquellenidentifizierung KW - Feinsedimente KW - Sedimentfracht Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-410550 ER - TY - JOUR A1 - Zhang, Chengjun A1 - Zhang, Wanyi A1 - Feng, Zhaodong A1 - Mischke, Steffen A1 - Gao, Xiang A1 - Gao, Dou A1 - Sun, Feifei T1 - Holocene hydrological and climatic change on the northern Mongolian Plateau based on multi-proxy records from Lake Gun Nuur JF - Palaeogeography, palaeoclimatology, palaeoecology : an international journal for the geo-sciences N2 - A multi-proxy study including analyses of delta C-13(org) for the lake sediment core GN-02 and grain size, TOC. CaCO3 content, delta C-13(carb) and delta O-18(carb) of bulk carbonate, and the mineralogy of the parallel core GN-04 from Gun Nuur was performed to reconstruct the Holocene hydrology and climate on the northern Mongolian Plateau. The chronology was established using 40 C-14 dates of bulk organic matter in addition to nine previously published radiocarbon dates for core GN-02, and further five C-14 dates for the new core GN-04. A lake reservoir effect of 1060 C-14 years was determined as the intercept of the high-resolution GN-02 age-depth model at the modern sediment surface. The size of the reservoir effect is supported by the age of the core-top sample (1200 +/- 40 C-14 years) and the determined difference between a wood-derived radiocarbon age from the GN-02 core base and the age-model inferred age for bulk organic matter at the same stratigraphic level (1000 C-14 years). Low lake level and prevailing aeolian sediment deposition at Gun Nuur under dry conditions were recorded during the earliest Holocene (> 10,800-10,300 cal a BP). Gun Nuur expanded under significantly wetter conditions between 10,300 and 7000 cal a BP. Unstable climate conditions existed in the mid Holocene (7000-2500 cal a BP) and three periods of low lake-levels and significantly drier conditions were recorded between 7000-5700, 4100-3600 and 3000-2500 cal a BP. Intermediate lake levels were inferred for the intervening periods. Around 2500 cal a BP, the climate change and wetter conditions were established again. As a consequence, the lake level of Gun Nuur rose again due to higher effective moisture and the relatively wet present conditions were achieved ca. 1600 cal a BP. Our results suggest that the initial Holocene climate change on the northern Mongolian Plateau was not accompanied by a rapid increase in precipitation as on the Tibetan Plateau. The establishment of wetter conditions in northern Mongolia lagged behind the early Holocene moisture increase on the Tibetan Plateau by ca. 1000 years. Subsiding dry air in the north of the Tibetan Plateau resulted from the strengthened summer monsoon on the Tibetan Plateau during the period of maximum summer insolation and probably inhibited a significant precipitation increase in Mongolia. The significant moisture increase in the Gun Nuur region at ca. 10.3 cal ka BP is probably not related to the northward shift of the present summer monsoon boundary or the moisture delivery from the northern Atlantic through the westerlies. Instead, water from melting snow, ice and frozen ground and the generation of precipitation from the local recycling of moisture are discussed as possible moisture source for the early onset of wetter conditions on the Mongolian Plateau. KW - Multi-proxy record KW - Sediment geochemistry KW - Mineralogy KW - Paleohydrology KW - Holocene KW - Mongolia Y1 - 2012 U6 - https://doi.org/10.1016/j.palaeo.2012.01.032 SN - 0031-0182 VL - 323 IS - 6 SP - 75 EP - 86 PB - Elsevier CY - Amsterdam 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 -