Chemical and isotopic (O, H, C) composition of surface waters in the catchment of Lake Donggi Cona (NW China) and implications for paleoenvironmental reconstructions

摘要

The oxygen, hydrogen, and carbon isotope ratios and major and trace element concentrations of surface waters are reported for the catchment of Lake Donggi Cona, Qinghai Province, China. The chemistry of the surface waters in the southeastern catchment is reflecting the dissolution of carbonate rocks, whereas thermal waters add sodium and chlorine to the waters in the northern catchment. The Dongqu River, draining the southeast catchment, contributes 87 to 94% to the water budget of the lake. Thermal waters and waters from the northern catchment add 6 to 13%. The combination of hydrogen and oxygen isotopes provides evidence that only small amounts of recycled lake water contribute to the precipitation in the catchment. Most of the moisture may be transported by the East Asian summer monsoon or an eastern branch of the South Asian monsoon into the Donggi Cona region. From oxygen isotope ratios it is calculated that about 45% of the lake water input is evaporated and 55% leaves the lake through a channeled outflow. The carbon isotope ratios identifies microbial respiration of organic matter, probably in soils and sediments of the catchment, as the major source of dissolved inorganic carbon in most surface waters and the lake. An isotope effect, from biological processes, in the lake is minor, and can be neglected in order to explain isotope ratios of inorganic carbon in lake water. The results imply that oxygen isotope ratios in the lake sediment archives mainly mirror the relation between precipitation and evaporation (P/E), which may be controlled by changes in the global water cycle like monsoon intensity or by tectonically driven local changes affecting the outflow flux. A comparison between lake archives based on ostracod shells and the modern system indicates similar delta O-18 values for the lake water since 4.3 ka but different delta C-13(DIC) values. This discrepancy between carbon isotope ratios in ostracod shells andmodern lakewater samplesmay be due to thermal waters with high delta C-13(DIC) emerging near the coring site. It is suggested that carbon isotopes of ostracods from a sediment drill core reflect the activity of a local thermal spring at the lake bottom, whereas oxygen isotopes provide evidence for an open lake systemsince about 11 ka and that the climate and environment of theDonggi Cona region changed very little over the last 4300 years. (C) 2016 Elsevier B.V. All rights reserved.