Spatio-temporal pattern of detrital clay-mineral supply to a lake system on the north-eastern Tibetan Plateau, and its relationship to late Quaternary paleoenvironmental changes
Introduction
The north-eastern Tibetan Plateau represents a key region for the recognition of climate processes as it represents one of earth's most vulnerable regions in respect to global warming and environmental changes. Its extreme setting at high altitude at the intersection between the climate regimes of the summer and winter monsoon and the westerly wind system gave rise to the establishment of a complex and sensitive geo-ecosystem since the late Pleistocene. In this context sedimentary archives give evidence of the pronounced effects of natural climate variability on environmental changes. Especially lake sediments provide valuable archives for the reconstruction of climatic and environmental conditions. However, to gain deeper insights into the lake system and its complex depositional history within the context of a geologically heterogeneous catchment area, it is important to determine the source areas of the different sediment deposits.
Our current understanding of the environmental history of the Tibetan Plateau and its relationship to climate change is largely derived from investigations of terrestrial records (Sun et al., 2007) and lake sediments (Mischke et al., 2010a, Mischke et al., 2010c). Recent investigations have integrated catchment area studies with lake research (Dietze et al., 2010, Dietze et al., 2012, Lehmkuhl and Haselein, 2000, Wünnemann et al., 2008, Stauch et al., 2014). Depending on the geomorphology and geology of the catchment area, the climatic signals preserved in sediments can be altered both during and after deposition. Physical processes such as weathering and relocation, and biological processes such as pedogenesis and vegetation changes, can therefore leave a multidimensional fingerprint on the sediments (Baumann et al., 2014, Chamley, 1989).
Clay-mineral assemblages have previously been used as proxies for regional paleoenvironmental reconstructions, and also for provenance analyses in marine (Diekmann and Kuhn, 1999), fluvial and lacustrine environments (Fagel et al., 2003, Popp et al., 2007, Yuretich et al., 1998). In tropical environments with efficient chemical weathering, clay minerals are widely used as paleoclimate proxies, but in cold temperate areas such as the Tibetan Plateau, with arid to semi-arid climatic conditions similar to those in polar regions, clay minerals are useful as source indicators (Chamley, 1989, Gao et al., 2002).
The objective of our research was to improve the understanding of the provenance of late Glacial and Holocene sediments in the complex depositional history of Lake Donggi Cona, in the north-eastern part of the Tibetan Plateau, using clay-mineralogy to investigate both terrestrial sediments and the modern and fossil lake sediments within a pilot study. To infer the source areas of the lake sediments, characteristic facies units (cover sediments and soils) from the catchment of the lake were studied concerning their clay-mineral compositions and through statistical analyses using the Fuzzy C-Means algorithm.
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Lake Donggi Cona and its regional setting
Lake Donggi Cona (35°18′N, 98°32′E) is located in the north-eastern part of the Tibetan Plateau within the Kunlun Mountain Range, at an altitude of 4090 m above sea level (a.s.l.) (Fig. 1). The lake and its catchment area fall in the Qinghai Province of the People's Republic of China. The mountainous terrain of the A'nyêmaqên and Burhan Budai ranges surrounds Lake Donggi Cona. The basin of the lake is a pull-apart basin within the active Kunlun Fault zone (van der Woerd et al., 2002). The 60 × 30
Surface sediment samples
To obtain information on clay-mineral assemblages 94 samples of lake sediment surface samples (DCS) were collected across the lake during the summers of 2003, 2006 and 2009, using a Hydro-Bios-Ekman grab. To trace the origins of the clay minerals 64 terrestrial surface reference samples (DC) were collected from characteristic facies units in the Donggi Cona catchment area (Fig. 2).
Sediment cores
For clay-mineralogy analyses altogether 197 samples were taken at 10 cm intervals from all sediment cores. The lake
Clay-mineral assemblages and FCM cluster analyses
The siliciclastic clay fraction of the lake and catchment sediments investigated includes both clay minerals and non-clay minerals. The non-clay minerals are mainly quartz, plagioclase, and potassium feldspar, which were not considered any further in these investigations. The clay-mineral spectrum is mainly dominated by illite (42–87%) and chlorite (up to 45%), with smaller quantities of smectite (1–13%) and kaolinite (1–24%; Fig. 3). Although clay-mineral variability in the investigated
Discussion
Any interpretation of the clay-mineral composition of lacustrine sediments needs to take into account a variety of environmental factors and sedimentary processes, the significance of which may vary from one lake system to another, as discussed below. To understand the clay mineral fingerprints of the lake sediments first modern processes have to be studied.
Assuming clays are delivered to the lake as detrital components, the geology and substrates of the catchment area are likely to be the
Conclusions
The main conclusions from our pilot study on the clay-mineral assemblages of the modern and late Quaternary sediments of Lake Donggi Cona, and the terrestrial sediments of its catchment area, can be summarized as follows:
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The clay-mineral assemblages in the lake sediments can be used to infer the sediment sources within the catchment area. The clay-mineral fingerprints are related to the detrital sediment supplies from the rocks, soils, and cover sediments of different geological provinces.
Acknowledgments
Fieldwork was carried out in conjunction with German and Chinese partners, within the German Research Foundation (Deutsche Forschungsgemeinschaft — DFG) priority project entitled ″Tibetan Plateau: Formation–Climate–Ecosystems“ (TiP). We are grateful to our colleagues for their help during the fieldwork and for the fruitful discussions. We would also like to thank two anonymous reviewers for their very valuable comments. The DFG and the Alfred Wegener Institute for Polar and Marine Research (AWI)
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