Speleothem Evidence for Temporal-Spatial Variation in the East Asian Summer Monsoon Since the Medieval Warm Period

Published annual-to-decadal-resolution stalagmite d(expn 18)O records since AD 900 from six caves (Dongge, Furong, Heshang, Buddha, Shihua and Wanxiang) in China were analyzed to detect temporal and spatial variability of the East Asian Summer Monsoon strength, which strongly affects wet/dry conditions in eastern China. The empirical mode decomposition method was used to obtain trends of the six cave records. After the base trend was determined, d(expn 18)O anomalies of each record were computed by subtracting the base trend. Mean d(expn 18)O anomaly values of the detrended time series for each cave record were calculated for four periods: (i) Medieval Warm Period (MWP; AD 900 1250): (ii) Little Ice Age phase 1 (LIA-1; AD 1250 1550); (iii) Little Ice Age phase 2 (LIA-2; AD 1550 1850); and (iv) modern period (MD; AD 1850 2000). From these anomalies, the temporal and spatial variability of wet/dry conditions has been identified. Positive values of the mean d(expn 18)O anomalies indicating drier conditions appeared in the lower Yangtze River Drainage Area and Southeast Coast Area during MD-1, LIA-1 and MWP, whereas negative values existed in north, south and Yangtze areas of eastern China during LIA-2. The results agree with the dryness/wetness index reconstructed by Chinese historic records in general. These results illustrate that wet and dry conditions in different regions of eastern China could be opposite under monsoon influence, so that no single speleothem d(expn 18)O record could represent the monsoonal climate in this vast region. Climatic patterns in the monsoonal region can be either a combination of warm/wet and cold/dry or a combination of cold/wet and warm/dry on annual-tocentennial scales. A 128-year periodic cycle exists in all six cave records, whereas 64-year (possibly a harmonic of 128-year periodicity) and 42-year periodicities appear in Shihua, Heshang and Dongge records.
Published in the Journal of Quaternary Science, v27 n9 p901-910, 2012. Prepared in cooperation with the Department of Geosciences, National Taiwan University, Taipei, Taiwan, and the School of Environmental Science and Engineering, Guangzhou University, China.