East Asian Summer Monsoon Variations in the Last 1,700 Years Inferred from δ18o of Ostracode Shells

Abstract:

Oxygen isotope ratios of ostracode shells (δ¹⁸O_ostracode) from the central part of Lake Nakaumi, southwest Japan were examined to clarify intensity of East Asian summer monsoon (EASM) in the last 1700 years. Adult whole shell of Bicornucythere bisanensis, which grow into adulthood from April to August in the present Nakaumi, was used for δ¹⁸O measurements. Long-term trend in δ¹⁸O_ostracode correspond with salinity changes in Lake Nakaumi caused by regional geographic events based on analysis of fossil ostracode assemblages. Five short-term increases in δ¹⁸O_ostracode were recognized in 500, 900, 1350, 1650 and 1850 AD, which correspond with weak periods of the EASM intensity based on δ¹⁸O of stalagmite in Wanxiang cave. Intensity of EASM is representative of precipitation around the Japan. For recent ostracodes, δ¹⁸O_ostracode positively correlated with bottom water salinity, which related negatively to precipitation and positively to δ¹⁸O of lake water in the recent Nakaumi. Thus, it might be interpreted that the short-term fluctuations in δ¹⁸O_ostracode were caused by intensity change of EASM, since δ¹⁸O_ostracode will increase due to low precipitation during weak periods of EASM. Further, the compiled data demonstrate that at least five EASM weakening periods, in 500, 900, 1350, 1650 and 1850 AD, occurred between 30° and 40° N in East Asia in the last 1700 years. These timings accord with data from sites, which suggests that oscillations of the EASM have been synchronous across the region.