Long-term Trend Of Monsoon Evolution And Driving Mechanism Research In Holocene Stalagmite Record From Western Fujian

The Holocene is a relatively warm period since the last glaciation,and there is a series of global monsoon-millennial weak monsoon events,but the weak monsoon events indicated by different isotopic carrier are slightly different.The holocene optimum is the warmest and humid period of Holocene,and is the most controversial in Holocene.At present,the research on Holocene paleoclimatic records in China is mainly concentrated in the southwest of China,the Yangtze River Basin,the northern region,but paleoclimatic records for the southeastern Holocene absolute dating,high-resolution is almost blank.Therefore,the study of Holocene paleoclimatic records in the southeast of China will help us to understand the Holocene climate change more deeply.In this paper,the stalagmite of Xianyun Cave was selected in western Fujian Province,China,as the object of study.By using U/Th dating and oxygen isotope measurements(33 230Th ages and 671 oxygen isotope data),we reconstructed east Asia summer monsoon intensity sequence of Holocene,discussed in depth the holocene optimum and the differences with other regions in China,and analyzed the driving factors of Holocene climate change.The main conclusions are as follows:(1)The Holocene record of the East Asian summer monsoon at Xianyun Cave was reconstructed by high-resolution,absolute dating of stalagmite oxygen isotopes.The record shows the Holocene change pattern of Xianyun Cave,which includes three stages:the summer monsoon weakening period(9.6-6.7ka BP),the ?18O average is-7.03‰,and the maximum value(-6.29‰)is reached at 7.1ka.The weak monsoon at this period is 8.6ka BP,but the "8.2ka BP event" is not captured.During the strong summer monsoon period(6.7-2.9ka B.P.),?18O averaged was-7.98 ‰,which was lower than the whole Holocene averaged(-7.24%o),showing strong East Asian summer monsoon stage and was Holocene optimum stage.The several weak summer monsoon events weakened the development of the East Asian summer monsoon,with 4.3?4.7ka B.P.and 5.6ka B.P.weak monsoon events,respectively,on the basis of the East Asian summer monsoon enhancement.During the summer monsoon weaked period(2.9?1.Oka B.P.),the ?18O average value is-7.28 ‰;?18O increases slowly,which indicates the weakening process of the East Asian summer monsoon.The weak summer monsoon is 1.6ka B.P.event.(2)The ?18O values of the stalagmite show a peak at 6.7 ka BP and 3.0 ka BP,which is the most suitable stage of Holocene.The ?18O values decrease rapidly at 3.0ka BP,and the ?18O values remain at a high level after 3.0ka BP,indicating that the intensity of East Asian summer monsoon ended.And the sediments of Fuzhou basin in this region showed that the East Asian monsoon weakened and the climate changed to dry and cold period since 4ka BP.The comparison of paleoclimatic records of the Indian monsoon region,the southeastern China and the Yangtze River Basin and north in China showed that the paleoclimatic record effected by Indian monsoon on the orbital scale expressed the same trend as the as the Yangtze River Basin.The changes are very close to the solar radiation in the northern hemisphere,The monsoon summer monsoon more enhanced and precipitation increased in early Holocene,and The monsoon summer monsoon weakened and precipitation decreased in mid Holocene,the late monocene summer monsoon was weaker and precipitation was less.The southeastern and northern regions of China's paleoclimate records change more close,showed that The summer monsoon and precipitation weakened and decreased in early Holocene,the middle monocene enhanced and increased,the late Holocene is weaker summer moonsoon and less precipitation.But the two regions of the paleoclimate records have different obvious with the northern hemisphere 25° solar radiation peak.(3)On the orbital scale,the stalagmite XY ? 13 expressed intensity of the East Asian summer monsoon was reduced due to the boundary condition of ice volume in the early Holocene.However,the East Asian summer monsoon peak laged the northern hemisphere solar radiation peak were about 7ka B.P.During the Holocene(10-7ka BP),the Indian summer monsoon increased by the Hadley circulation strengthened,the East Asian summer monsoon weakened and the precipitation decreased in the central and northern China.Since mid Holocene,Hadley circulation weakened and the Indian summer monsoon decreased,the East Asian summer monsoon strengthened and the precipitation increased in the central and northern China.The anomalous precipitation in the eastern part of China is positive-negative-positive,and the negative anomalies appear in the Yangtze River and southwestern China,and the positive anomalies appear in northern and southeast China.During the El Nino event,the western Pacific subtropical high shifted southward,leading to more precipitation in the Yangtze and Huaihe valleys,and less precipitation in the south China and north China.During the Holocene,the temperature of the western Pacific warm pool increased continuously,and the convective activity of the warm pool was enhanced.The climate was the wettest at about 5 to 4ka BP.However,the temperature contrast between the east and west Low latitude tropical Pacific changed suddenly minimum about 4ka BP,and convective activity in warm pool sea area is weakened,and the summer precipitation in the north and south of China is weakened.By comparing the sunspot number with the total solar irradiance,the East Asian summer monsoon record correlates with the solar activity,showing the solar cycle of 55a.Based on the analysis of water vapor traces,it is suggested that the Yangtze river basin is likely to be influenced by the Indian monsoon,while the East Asian summer monsoon is more sensitive to the north and south China.