Water isotope variations in the snow pack and summer precipitation at July 1 Glacier, Qilian Mountains in northwest China

This paper presents the stable isotope data of the snow pack and summer precipitation collected at the July 1 Glacier, Qilian Mountains in northwest China and analyses their relationships with meteorological factors. On an event scale, there is no temperature effect on the delta O-18 values in the summer precipitation, whereas the amount effect is shown to be clear. By tracing the moisture transport history and comparing the precipitation with its isotopic composition, it is shown that this amount effect not only reflects the change in moisture trajectory, which is related to the monsoon activities, but is also associated with the cooling degree of vapor in the cloud, the evaporation of falling raindrops and the isotopic exchange between the falling drops and the atmospheric vapor. As very little precipitation occurs in winter, the snow pack profile mainly represents the precipitation in the other three seasons. There are low precipitation delta O-18 ratios in summer and high ratios in spring and autumn. The Meteoric Water Line (MLW) for the summer precipitation is delta D = 7.6 delta O-18 + 13.3, which is similar to that at Delingha, located in the south rim of the Qilian Mountains. The MWL for the snow pack is delta D = 10.4 delta O-18 + 41.4, showing a large slope and intercept. The deuterium excess (d) of the snow pack is positively correlated with delta O-18, indicating that both d and delta O-18 decrease from spring to summer and increase from early autumn to early spring. This then results in the high slope and intercept of the MWL. Seasonal fluctuations of d in the snow pack indicate the change of moisture source and trajectory. During spring and autumn, the moisture originates from continental recycling or rapid evaporation over relatively warm water bodies like Black, Caspian and Aral Seas when the dry westerly air masses pass over them, hence very high d values in precipitation are formed. During summer, the monsoon is responsible for the low d values. This indicates that the monsoon can reach the western part of the Qilian Mountains.