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The East Asian summer monsoon (EASM) precipitation from May to August over the Helan Mountains, located in the northwest margin of the EASM, has been investigated through tree-ring delta O-18 for the last 120 years. The dry and wet intervals derived from the delta O-18 series can be verified by the tree-ring width-based May-July precipitation reconstruction. Most of the time, the delta O-18 series and ring width-derived precipitation reconstruction vary synchronously at low frequency. High values of delta O-18 correspond to high precipitation periods, and vice versa. Since the 1970s, the East Asian summer monsoon precipitation has been increasing, which was confirmed by other records and documents. Our results show that tree-ring delta O-18 can serve as an indicator of the strength of the East Asian summer monsoon variation in the past. We also found that tree-ring delta O-18 varies inversely to delta C-13 in the study area. (C) 2008 Elsevier B.V. All rights reserved.