Precipitation δ18O on the Himalaya-Tibet orogeny and its relationship to surface elevation

The elevation history of the Himalaya-Tibet orogen is central to understanding the evolution and dynamics of both the India-Asia collision and the Asian monsoons. The surface elevation history of the region is largely deduced from stable isotope (delta O-18, delta D) paleoaltimetry. This method is based on the observed relationship between the isotopic composition of meteoric waters (delta O-18(p), delta D-p) and surface elevation, and the assumption that precipitation undergoes Rayleigh distillation under forced ascent. Here we evaluate how elevation-induced climate change influences the delta O-18(p)-elevation relationship and whether Rayleigh distillation is the dominant process affecting delta O-18(p). We use an isotope-enabled climate model, ECHAM-wiso, to show that the Rayleigh distillation process is only dominant in the monsoonal regions of the Himalayas when the mountains are high. When the orogen is lowered, local surface recycling and convective processes become important, as forced ascent is weakened due to weaker Asian monsoons. As a result, the delta O-18(p) lapse rate in the Himalayas increases from around -3 to above -0.1% km(-1), and has little relationship with elevation. On the Tibetan Plateau, the meridional gradient of delta O-18 decreases from similar to 1 to similar to 0.3%o (o-1) with reduced elevation, primarily due to enhanced sub-cloud reevaporation under lower relative humidity. Overall, we report that using delta O-18(p) or delta D-p to deduce surface elevation change in the Himalayan-Tibetan region has severe limitations and demonstrate that the processes that control annualmean precipitation-weighted delta O-18(p) vary by region and with surface elevation. In summary, we determine that the application of delta O-18 paleoaltimetry is only appropriate for 7 of the 50 sites from which delta O-18 records have been used to infer past elevations.