Stable oxygen isotope differences between the areas to the north and south of Qinling Mountains in China reveal different moisture sources

delta O-18 variability in daily precipitation at two stations (Lanzhou and Zhangye) north of the Qinling Mountains at the eastern margin of the Tibetan Plateau, China, and monthly precipitation delta O-18 at four stations to the south of the Qinling Mountains were examined. The data show that the delta O-18 composition of precipitation south of the Qinling Mountains is influenced strongly by the prevailing westerlies during winter and by the summer Asian monsoon during its most active periods. To the north, the westerlies prevail in winter, and delta O-18 trends coincide strongly with temperature. In summer, the Qinling Mountains block or weaken the Asian monsoon, which crosses them only during the most active periods. In these periods, the stations to the north experience relatively depleted delta O-18 values (<-7%). During weak monsoon periods, however, most of the delta O-18 values in summer precipitation at Lanzhou and Zhangye are relatively enriched (>-5%, and even as high as +9.2%). A positive northward weighted average delta O-18 gradient of summer precipitation with increasing distance from the coast suggests the influence of the prevailing westerlies and of continental recycling of moisture. Both the frequency of these heavily depleted delta O-18 values and their duration decrease from south to north, reaching minimums at Zhangye, which may have been in the monsoonal tail region during the sampling period. The data reveal that the Qinling Mountains act as an important climatic divide along the eastern margin of the Tibetan Plateau. The effects of the Asian summer monsoon and of different moisture sources should be considered for paleoclimatic studies.