Spatial distribution and controlling factors of stable isotopes in river waters on the Tibetan Plateau

Abstract:

Debates persist about the interpretations of stable-isotope based proxies for the surface uplift of the north-central Tibetan Plateau. These disputes arise from the uncertain relationship between elevation and the δ¹⁸O values of meteoric waters, based on modern patterns of isotopes in precipitation and surface waters. We present a comprehensive river water data set (1,315 samples) covering most parts of Tibet to characterize the spatial variability and controlling factors of their isotopic compositions. Compared with the annual mean oxygen isotope values of precipitations, we conclude that river waters are a good substitute for isotopic studies of precipitations on the high flat Tibetan Plateau on a mean annual scale, but experience predictable negative shifts in the marginal piedmont lowlands. In the marginal mountainous regions of the plateau, especially the southern and eastern margins, the d¹⁸O and d-excess values of river waters decrease and increase respectively with increasing mean catchment elevation, which can be modeled as a Rayleigh distillation process with decreasing condensation temperature and under relative humidity lower than 100%. However, in the interior of the plateau (north of the Trans-Himalaya range), northward increasing trends in d¹⁸O and dD values are pronounced and present robust linear relations; d-excess values are lower than the marginal mountainous regions and exhibit distinct contrasts between the eastern (8‰–12‰) and western parts of the plateau (<8‰). We suggest that these isotopic features of river waters in the interior of the plateau result from combined effects of: 1) mixing of different moisture sources transported by the Indian summer monsoon and Westerly winds; 2) contribution of moisture from recycled local surface water; and 3) sub-cloud evaporation under low relative humidity, especially in regions with small annual precipitation amount.