Oxygen and deuterium isotope characteristics of Teesta river catchment from Sikkim Himalaya, India: Implications of different moisture sources

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The present investigations of δ¹⁸O and δD values of Teesta river system (glacier fed; Eastern Indian Himalaya) along a steep altitudinal gradient has helped to better understand the spatial variability of the isotopic composition in water and moisture sources. We have used the stable isotope composition of the main Teesta River, its major tributaries, glacial melt streams (nallah) and few high altitude lakes. As the region is dominated by Indian Summer Monsoon precipitation brought by the Bay of Bengal Branch, the δ¹⁸O values ranges from ~–11 to –15‰ and δD from ~–41 to –128‰ in the main Teesta River. We propose that as the monsoonal moisture penetrates along the river valleys towards higher Himalaya, a progressive depletion in ¹⁸O and D isotopes take place during orographic lifting, resultant cooling and progressive rainout. It is observed that d-excess values range between –1 to 18 and suggests mixed moisture source having contributions from moisture originating in Bay of Bengal and mid-latitude westerlies.