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

• We analyzed element concentrations and H, O, C isotopes in a lake system
• Lake Donggi Cona is located on the Kunlun fault in the northeast of the Tibetan Plateau (TP)
• Thermal waters provide significant amounts of Ca, Na, Cl and HCO3 to surface waters in the northern catchment and the lake
• The East Asian monsoon is the dominant source of moisture
• The climate and environment changed little during the last 4300 a on the northeastern TP

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 δ18O values for the lake water since 4.3 ka but different δ13CDIC values. This discrepancy between carbon isotope ratios in ostracod shells and modern lake water samples may be due to thermal waters with high δ13CDIC 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 system since about 11 ka and that the climate and environment of the Donggi Cona region changed very little over the last 4300 years.