Speleothem Evidence for Temporal-Spatial Variation in the East Asian Summer Monsoon Since the Medieval Warm Period
NAVAL POSTGRADUATE SCHOOL MONTEREY CA DEPT OF OCEANOGRAPHY
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Published annual-to-decadal-resolution stalagmite dexpn 18O 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 wetdry 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, dexpn 18O anomalies of each record were computed by subtracting the base trend. Mean dexpn 18O 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 wetdry conditions has been identified. Positive values of the mean dexpn 18O 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 drynesswetness 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 dexpn 18O record could represent the monsoonal climate in this vast region. Climatic patterns in the monsoonal region can be either a combination of warmwet and colddry or a combination of coldwet and warmdry 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.