A new approach to stable isotope-based paleoaltimetry: implications for paleoaltimetry and paleohypsometry of the High Himalaya since the Late Miocene

The change in oxygen isotopic composition of precipitation is modeled using equilibrium fractionation during Rayleigh distillation linked to the therm odynamics of atmospheric ascent and water vapor condensation. The primary c ontrollers of the vertical variation in isotopic composition with elevation are the low elevation temperature and relative humidity as these control t he vertical distribution of condensation. An empirical fit of precipitation versus model condensation based on Alpine stations is derived. This fit is represented in the model as the weighted mean composition of condensation within a 1000 m thick air parcel 1500 +/- 500 m above the ground surface an d is used for all other regions. Comparison of model versus observed modern precipitation reveals a close fit. particularly of more highly elevated si tes. Comparison of modern waters in the Himalayas and southern Tibet with m odel predictions, particularly as revealed by comparison of observed and pr edicted hypsometry provides additional support to the validity of the model . Finally. application of this model to estimates of paleo-waters in the Hi malayas and southern Tibet reveals that this region had already achieved it s present hypsometry by the Late Miocene, about 10 Ma ago. (C) 2001 Publish ed by Elsevier Science B.V.