A study of longitudinal and altitudinal variations in surface water stable isotopes in West Pamir, Tajikistan

In an effort to establish a clear relation between stable isotopes and altitude in the Pamir region, as well as to improve the understanding of stable isotope spatial variations found along the routes taken by the westerlies, surface river water samples were collected along a roughly west–east profile in Tajikistan. Here we present the spatial changes in modern surface water δ18O and deuterium excess (d-excess), and their links with snow melt, glacier melt and seasonal precipitation patterns in Tajikistan. The results show a close relation between river water δ18O and δD above the GMWL (Global Meteoric Water Line), implying that surface evaporation exerts a weaker influence. Spatially, river water δ18O is gradually depleted from west to east due mainly to the effect of altitude, yielding a river water δ18O vertical lapse rate of 0.09‰/100 m. River water d-excess shows a decreasing trend from west to east in Tajikistan. This spatial d-excess pattern in surface water is explained by the different moistures influenced by the Mediterranean and other in-land seas with higher d-excess in west, and no-Mediterranean moisture in east. Another possible reason is the differences in precipitation seasonality between the west and east of Tajikistan.