Calibration of Chinese speleothem δ18O to the tropical summer monsoon precipitation

Climate simulations and atmospheric precipitation monitoring show that oxygen isotopes of stalagmites in eastern China reflect the precipitation variations in the upstream tropic regions-along the moisture transport pathway, but the quantitative relationship between Chinese speleothem δ ¹⁸O and tropical precipitation is still not estalbished. Based on a 21-year-monitoring record of the precipitation δ ¹⁸O and temperature at monthly resolution from East China, the quantitative relationship between tropical precipitation and stalagmite oxygen isotope at seasonal to decadal time scales was established, then being used to reconstruct the tropical summer precipitation since 640 ka BP. The study suggests that the oxygen isotope compositions of Chinese stalagmites are negatively correlated with summer monsoon precipitation (MJJAS) between 0°-25°N and 80°-110°E, which could be described as a formula: P(mm/month) =-16.768*δ ¹⁸O+75.34 (r=0.7077,n=105), i.e. a decrease of δ ¹⁸O by 1 ‰ responding to an increase of precipitation by 16.77mm/month. Furthermore, in the last 100 years, the above relationship has been supported on interannual-interdecadal scale by δ ¹⁸O compositions of an annual laminated stalagmite from Heshang cave located in the middle reaches of Yangtze River. Therefore, the oxygen isotope composition of stalagmites could indicate the variation of tropical monsoon precipitation from seasonal to interdecadal time scale. The monthly average precipitation amount during the summer (MJJAS) ,reconstructed from Chinese speleothem δ ¹⁸O, is 222 mm/month in last 640ka, which is less than that in the past 100 years (251 mm/month). The precipitation generally follows the variation of precession period with an average amplitude of ~32% between glacial and interglacial stages. This reconstruction of the tropical precipitation provides a new evidence for climate modelling to predict the tropical precipitation in the future under global warming.