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Late glacial and Holocene development of Lake Donggi Cona on the NE Tibetan Plateau.

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Disciplines
  • Earth Science
  • Ecology
  • Geography

Abstract

Lake Donggi Cona is located in the north-eastern part of the Tibetan Plateau at the boundary between monsoon and westerly climate influence on an elevation of 4090 m. Donggi Cona is an oligotrophic freshwater lake with a recent open-basin system. The 30 km long, 8 km wide, and up to 98 m deep lake basin is a tectonic pull-apart structure as part of Kunlun fault. Sub-bottom profiling of the lake basin revealed the presence of graben structures, conjugate faults, subaquatic terraces, and ancient fan systems, draped by 4 to 5 m thick postglacial lacustrine sediments. The aim of our investigations is the reconstruction of palaeo-environmental changes of the lake catchment.After analysis of the seismic pre-survey, five sediment cores have been retrieved at 35-40 m and at 2 m water depth. The cores are partly laminated and composed of calcareous muds with changing amounts of carbonate micrite, organic matter and detrital silt and clay. Furthermore, the cores contains remarkable amounts of ostracods and plant remains. Dating results from the top of three sediment cores obtain a hardwater effect of about 2000 years. Nineteen AMS dates from the entire cores suggest a sedimentary history throughout the Holocene. Despite possible reservoir errors the sandy bottom sediments were deposited during or after the last glacial maximum (LGM). With a multi-proxy approach using grain size, XRF, XRD, CNS, TOC and multivariate statistical endmember-modelling (EMMA) we hereby represent an interpretation concerning changes in hydrological conditions and sedimentation processes during the last about 20 kyears: Prior to 18 kyears, terrestrial loess and sand suggest a very low lake level. From 18 to 13 kyrs. BP the lake level rise during a period of climate-driven glacier melt. For the same time period the detrital supply is at its maximum and seems to control the high sedimentation rate.Grain-size coarsening and hiatuses in four sediment cores, document a repeated lake-level fall after 13 kyrs. BP. From 12.3 to 4.3 kyrs, the lake level rose to above its present level as evidenced by the different terrace levels in the littoral of Lake Donggi Cona and by the increase of clay content within all cores. For the same time interval we assume increasing salinity because of the accumulation of aragonite. The lake changes in that time from a full ventilated to a stratified system in response to warm and moist conditions during the Holocene climate optimum. At 4.3 kyrs. BP the system shifted from an aragonite- to a calcite- dominated system. This sudden change can be attributed to a threshold response during prolonged lake-level rise and overspill at the western end of the lake, an tectonic event and/or climate induced processes.

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