Lake system development on the northern Tibetan Plateau during the last ~12 ka

Bernhard.Diekmann [ at ]


Changing environmental conditions associated with a predicted change in global temperatures is believed to affect water availability on the Tibetan Plateau, which provides freshwater to approximately 20% of the global population. The reconstruction of past environmental conditions may provide past climatic analogues to modern warming scenarios and help to elucidate the environmental response to climate forcing. One of the most abundant archives of Late Quaternary environmental conditions on the Tibetan Plateau are lacustrine sediments associated with lake systems. However, lake systems are known to reveal complexity in their response to climate forcing resulting in individualized sedimentary patterns for different lake systems. This individuality delimits the potential to infer supra-regional patterns of environmental change. This thesis presents a reconstruction of environmental conditions during the last ~12 ka around Lake Heihai, situated on the northern fringe of the Tibetan Plateau. Special focus is given on analyzing the climatic sensitivity of inorganic sediment fluxes, i.e. fluxes of allogenic minerals originating from the terrestrial surroundings of the lake as well as fluxes of endogenic minerals precipitated in the water column of the lake. A cluster analysis on the mineralogical composition of terrestrial reference samples revealed four major source areas, distributed evenly on two bordering mountain ranges: the Kunlun Mountains in the south and the Burhan Buda Mountains in the north of the lake. Especially sediment source areas on the Kunlun range differ in their main mode of activation, which is predominantly precipitation generated and glacial runoff. An end-member analysis of grain size distributions of the siliciclastic detrital fraction revealed that sediment supply from the Burhan Buda range is a consequence of eolian dynamics. The findings suggests a blockage effect of the Kunlun range on southerly air masses, leading to an extended rain shadow north of the Kunlun range. Endogenic mineral formation in Lake Heihai is dominated by carbonate precipitation. The geochemical analysis of carbonate mineral phases apparent within lacustrine sediments revealed, that the main control on the predominant endogenic carbonate mineral phase is the ratio between solute magnesium and calcium (Mg/Ca) in lake waters. The solute lake water Mg/Ca is controlled by solute composition of inflowing waters as well as lake internal Mg enrichment relative to Ca by the removal of Ca due to carbonate precipitation. The solute composition of inflowing waters, in turn, is mainly influenced by weathering intensities of limestone outcrops in the drainage basin of the lake. Changes in the inorganic mineral assemblage of a sediment core from the deepest part of the lake revealed major variations in sediment fluxes during the past ~12 ka induced by varying environmental conditions. After a prolonged dry and cold phase during the Late Glacial, allogenic sediment supply to Lake Heihai is characterized by a sudden intensification of precipitation generated sediment supply around ~10.7 cal. ka BP and a gradual decrease from ~7.9 cal. ka BP to present. This Holocene trend is in phase with known variations in atmospheric circulation systems over monsoonal Asia. The gradual long-term trend to more arid conditions is accompanied by several changes in the predominant endogenic carbonate phase, one of which evidences the onset of permafrost formation at ~3.9 cal. ka BP and hence a complete disappearance of the lake body. The disappearance is synchronous to a phase of weakened monsoonal precipitation as recorded in several continental and terrestrial archives throughout the Asian continent. For the central Kunlun Fault system, this climate event is the driest and coldest of the past 12 ka. However, while allogenic sediment supply is directly affected by climatic variations in the study area, endogenic mineral precipitation is additionally controlled by lake-internal feedbacks not directly influenced by climatic variations, like e.g. geomorphologically controlled changes in the hydrological condition. The observed local paleoenvironmental evolution in the region of Lake Heihai was compared to two well-studied lake systems: Lake Donggi Cona situated on the north eastern Tibetan Plateau and Lake Nam Co on the southern Tibetan Plateau. The supra-regional comparison was based on the allogenic sediment input because of the climatic sensitivity of the allogenic sediment supply system. Variations in allogenic sediment supply were characterized by factor analysis of elemental concentration in core sediments measured by X-Ray fluorescence scans. To account for a potential individual response of lake systems to climate forcing, a phase space reconstruction was carried out on the previously calculated elemental factors. Phase space reconstruction revealed that despite the individuality in the environmental setting of each lake, a similar sedimentary system consisting of three (yet unidentified) essential variables controls the allogenic sediment supply in each drainage basin. The Holocene dynamics in the allogenic sediment flux suggests the presence of a universal attracting state for lakes situated on the Tibetan Plateau. The attracting state is mainly characterized by short-term variation synchronous to solar output variations. Forcing mechanisms not originating from solar variations destabilize the systems dynamics and lead to unpredictable responses in sedimentary patterns of the lake systems. The last major destabilization phase was evident in the Early Holocene under increased monsoonal rainfall intensities caused by orbital variations. The results have great implications for temporal and spatial variations in atmospheric circulation patterns over monsoonal Asia and will help to facilitate the prediction of future monsoonal precipitation patterns under varying climatic conditions.

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Ramisch, A. (2015): Lake system development on the northern Tibetan Plateau during the last ~12 ka , PhD thesis, University Potsdam.

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