Stable isotopic characteristics of hydrogen and oxygen in different water bodies in the Qinghai-Tibet Plateau
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Abstract:
Over the years, great progress was made in the study of δ(D) and δ(18O) in the Qinghai-Tibet Plateau. However, a comprehensive systematic study of the entire Qinghai-Tibet Plateau was lacking, and much of the research focused on the spatial and temporal patterns and evolutionary mechanisms of precipitation stable isotopes. There was a lack of systematic research at the basin scale for runoff source analysis, and further exploration was needed on the relationships between different runoff components.The available data was used to analyze the δ(D) and δ(18O) values of different types of water samples on the Qinghai-Tibet Plateau to investigate their characteristics. Hydrogen and oxygen stable isotopic data were obtained mainly from the following aspects: (1) Water samples were collected in the Naqu River basin from August 2016 to June 2023, and the stable isotope values of hydrogen and oxygen were measured; (2) Global Network of Isotope in Precipitation (GNIP; https://www.iaea.org/services/networks/gnip); (3) National Qinghai-Tibet Plateau / Third Pole Environment Data Center (TPDC; https://data.tpdc.ac.cn); (4) Stable isotope values of hydrogen and oxygen in the Qinghai-Tibet Plateau studied by predecessors. Water samples collected in the Naqu River basin were analyzed and tested using the L2130-i Water Isotope Analyzer (Picarro, USA), which indicated the stable isotope ratio with respect to the Vienna Standard Mean Ocean Water (V-SMOW). The Local Meteoric Water Line (LMWL) was δ(D)=8.25δ(18O)+16.85. The δ(18O) value increased from February to May and decreased from May to October. A high deuterium surplus value was observed in winter, while it was low in summer. An "inverse altitude effect" was demonstrated by the stable isotopes in the northern part of the Qinghai-Tibet Plateau, while other areas exhibited an obvious "altitude effect". When comparing with the local atmospheric precipitation line and the global atmospheric precipitation line, it was found that both the slope and intercept of the river water line of the Minjiang River and the Daqu-Xianshui River were larger, while the slope of the river water line of the remaining basins was smaller. The Lake Water Line was represented by the equation δ(D)=6.12δ(18O）?23.29, and the Groundwater Line was represented by the equation δ(D)=9.45δ(18O)+28.25. In the isotopic composition of different water bodies on the Qinghai-Tibet Plateau,δ(D) varied from ?212.21‰ to 60.10‰ with a range of 272.31‰ and an average of ?85.80‰,δ(18O) varied from ?36.86‰ to 6.9‰ with a range of 43.76‰ and an average of ?11.71‰. Therefore,δ (18O) was more stable than δ(D) on the Qinghai-Tibet Plateau. The D-excess varied from ?78.55‰ to 41.84‰, with a range of 120.39‰ and an average of 0.95‰. Among all water bodies, the average value of D-excess followed the order: river water > groundwater > lake water > precipitation. The δ(18O) value of Naqu River basin and Daqu-Xianshui River basin in the Qinghai-Tibet Plateau increased first and then decreased along the river channel, the δ(18O) value of Shule River basin and Lhasa River basin decreased along the river channel, and the δ(18O) value of Minjiang River basin increased along the river channel. The Naqu River basin was primarily influenced by lake regulation, while the Shule River basin and the Minjiang River basin located near the Longmen Mountains were primarily affected by evaporation. On the other hand, the Daqu-Xianshuihe River basin and the Lhasa River basin were primarily influenced by atmospheric precipitation recharge. The characteristics of stable isotopes of hydrogen and oxygen varied among different types of water samples. The river water and groundwater were minimally influenced by evaporation, exhibited a close hydraulic relationship, and had a mutual recharge relationship. Precipitation was the main source of water supply in the Lhasa River basin. The average contribution rates of precipitation and groundwater to the water supply in the Shule River basin were similar. The Minjiang River basin was dominated by groundwater recharge. In the upper reaches of the Cuona Lake in the Naqu River basin, precipitation was the main source of water supply, while in the lower reaches, it was mainly from lake water recharge. As the distance increased, the contribution rate of lake water decreased.
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