Stable isotopes of modern water across the Himalaya and eastern Tibetan Plateau: Implications for estimates of paleoelevation and paleoclimate

Paleoaltimetry based on stable isotopes (delta O-18 and delta H-2) of paleowater from the central and northern Tibetan Plateau is challenged by the lack of a clear relationship between isotopic composition and elevation north of the Himalaya. In order to determine the environmental factor(s) responsible for temporal changes in isotopic composition revealed in the geologic record, an understanding of the modern controls on isotope evolution in the continental interior is necessary. Here, we present new delta O-18 and deuterium excess (d excess) data from modern surface water along a roughly south-north transect on the eastern part of the Himalaya and Tibetan Plateau. Results corroborate an inverse relationship between delta O-18 and elevation in the Himalaya. Northward across the plateau, there is a positive trend in meteoric water delta O-18 that is linear (similar to 1.5% per degree latitude) and robust (R-2 = 0.94). A positive trend northward is also observed in d excess of surface water from large rivers. We show that Rayleigh distillation modified by surface water recycling can account for the observed spatial distribution of both delta O-18 and d excess across the plateau. HYSPLIT modeling of air parcel back trajectories suggests that air mass mixing varies from east to west across the plateau. However, isotopic trends along the plateau's eastern margin are consistent with roughly parallel transects to the west, suggesting that a local process like moisture recycling exerts control over the isotopic evolution across the entire plateau, regardless of origin of air masses. Assuming the northern Tibetan Plateau was equally far from an oceanic source during late Eocene-Miocene time, paleoelevations of the Hoh Xil Basin are recalculated to account for recycling, increasing elevation estimates by 1100-2700 m.