δ18O and δD of streamwaters across the Himalaya and Tibetan Plateau: Implications for moisture sources and paleoelevation reconstructions

This study presents new δ18O and δD data from 191 streams across the Himalaya and Tibetan Plateau to better constrain the spatial variability of stable isotopes in modern precipitation over this region. Moisture penetrating into the southeastern Tibetan Plateau is predominantly derived from monsoonal airmasses originating from the Bay of Bengal and transported into the eastern Himalayan syntaxis along the Brahmaputra River. Progressive rainout during orographic lifting and cooling results in clear relationships between δ18O and δD and catchment hypsometric elevation on the plateau margin. However, monsoonal-derived moisture is progressively mixed with central Asian airmasses in more western and northern parts of the Tibetan Plateau. As a result, predicted isotope–elevation relationships that are based on empirical lapse rates or thermodynamic models of the isotopic evolution of an airmass produce large (1–3 km) misfits between measured and predicted catchment elevations for much of the Tibetan Plateau, including some areas directly north of the central Himalayan crest. This suggests that changes in the δ18O or δD of paleoprecipitation on the central and southwestern Tibetan Plateau may reflect surface uplift along moisture transport pathways or changes in the penetration of monsoonally-derived moisture rather than regional surface uplift histories.