The isotopic composition of meteoric water along altitudinal transects in the Tian Shan of Central Asia

Abstract

Abstract The Tian Shan in Central Asia are a unique mountain range in that they are in the world s most continental location. Seasonal precipitation in the northern Tian Shan is segregated into distinct elevation bands where high elevations receive precipitation primarily during summer and low elevations to the north receive precipitation primarily during the late winter and spring. In this study, we sampled stream water along multiple altitudinal transects to determine the effect unique seasonal patterns of precipitation have on the isotopic composition of surface water. Our results suggest that the northern Tian Shan exhibits an isotopic lapse rate for waters sampled in late spring, but not for those sampled in late summer, when stream water budgets are dominated by high elevation precipitation and snow melt. Deuterium excess results suggest that subcloud evaporation significantly affects the isotopic composition of precipitation at low elevations in spring and that sublimation of snow has a minor impact on δ18O values of summer melt water. Because high and low elevation δ18O values are similar, conventional paleoaltimetry based on Rayleigh distillation of an air mass is not applicable to the Kyrgyz Tian Shan. Stream water proxies from the rock record are likely to reflect changes in the seasonal distribution of precipitation which occur on the same spatial scale as altitudinal changes. These results highlight the need to understand modern controls on local stable isotopes of meteoric water in reconstructions of past climate or topography using geologic proxy materials.