Lake water isotope variation linked with the in-lake water cycle of the alpine Bangong Co, arid western Tibetan Plateau

Water isotopes play an important role in the study of the alpine lake water budget and the hydrological cycle in the arid, far western Tibetan Plateau. These isotope records, derived from well-preserved sediments, are believed to reflect climatic and environmental changes. Using two years of delta O-18 and delta D data from precipitation, river water, underground water, and lake water at the long alpine lake Bangong (LBG), together with local meteorological observations in the arid western Tibetan Plateau, this study reveals that the delta O-18 in the lake is over 10 parts per thousand more enriched than that in the local precipitation due to evaporation of the lake water. Spatial changes in both the lake water delta O-18 and d-excess (= delta D - 8 * delta O-18) are apparent, ranging from similar to-4.9 parts per thousand to +0.9 parts per thousand for delta O-18, and -13.22%o to -30.85%0 for d-excess, respectively, from east to west of the lakes. Simulation with the Craig-Gordon model shows that the isotopes in alpine inland lake water are controlled to a great extent by local relative humidity. Using a modified partly mixed isotope fractionation model, we rebuilt the spatial change of the evaporation/inflow (E/I) ratios from east to west of the lake. A quantitative estimation shows that the E/I ratio of the lake water increases from 0.73 similar to 0.83 in the eastern part and 0.90 similar to 0.93 in the western part of LBG. We also found that by assuming a full development of kinetic fractionation of the environmental vapor isotopes, our simulation result matched the observed spatial change for both delta O-18 and d-excess, confirming the strong inland evaporation enrichment in the northern part of the plateau. This research may increase our understanding of inland water movement in the alpine Tibetan lakes, and also will improve our understanding of the lake sediment isotope record.