Evolution Of The East Asian Summer Monsoon In The Last Two Millennia From High-resolution Stalagmite Records In Western Fujian

The variation characteristics of the East Asian summer monsoon（EASM）have an important impact on the economic development and ecological environment of the East Asian monsoon region.Fujian Province is located in the pioneer area landfall of the East Asian summer monsoon and in the south foothill area of the East Asian winter monsoon.The pre-flood season in South China is an important precipitation process in Fujian,closely related to the EASM development.Investigating the EASM evolutionary features in the Fujian region over the past 2000 years can deepen the understanding of the evolution of the EASM system.In this article,high-resolution stalagmite oxygen isotope records for the past 2000 years and tree-ring cellulose oxygen isotope records for the past200 years were obtained using stalagmites and Cryptomeria fortunei near Xianyun Cave in Fujian province.By comparing them with the existing climate records in southern China,the evolution of the East Asian summer monsoon in the southeast monsoon region over the past 2000 years was analyzed on the interdecadal-century scale,and its possible driving mechanism was discussed.Through the research of this paper,the main conclusions are as follows:（1）Based on the 926 cellulose samples extracted from Cryptomeria fortunei tree ring cores around Xianyun Cave in western Fujian,δ¹⁸O chronology of tree-ring cellulose（LSM）from 1816 to 2016 with annual resolution was established.Theδ¹⁸O chronology LSM was significantly negatively correlated with the relative humidity and precipitation amount in April and August-September recorded by meteorological stations.Spatial correlation analysis showed that LSM could be used to represent the precipitation amount in South China and effectively extended the paleoclimate records of EASM in western Fujian.When the EASM is strong（weak）,tree ringδ¹⁸O is negative（positive）.In comparison with other tree-ring cellulose oxygen isotope records in the East Asian monsoon region,it is found that the variation of LSM is closely related to the East Asian summer monsoon,and the low frequency signal is well preserved.On the interannual scale,LSM are mainly influenced by Ni?o 3.4（region:170°W–120°W,5°S–5°N）SST（sea surface temperature）,and on the interdecadal scale,LSM are regulated by the AMO（Atlantic Multidecadal Oscillation）.（2）Based on 771δ¹⁸O data and 19 high-precision ²³⁰Th ages of the stalagmites XYD-A and XYD-B in Xianyun Cave,the stalagmiteδ¹⁸O sequence with an average resolution of～0.7 years in the southeast monsoon area during the past 300 years was established.Theδ¹⁸O of stalagmite was negatively correlated with precipitation amount from March to June and September,and the correlation coefficient with Yong’an meteorological station was-0.40（n=62,p<0.01）,indicating that the stalagmiteδ¹⁸O is mainly controlled by the rainfall effect.The greater the precipitation from March to June and September,the more negative the stalagmiteδ¹⁸O value,and vice versa.The oxygen isotope of the stalagmite was positively correlated with LSM.The cross-wavelet analysis also indicated that stalagmiteδ¹⁸O and LSM vary in the same phase on the interdecadal timescale,which further supported that the oxygen isotope of the stalagmite in Xianyun Cave was mainly influenced by the EASM.（3）Based on 998δ¹⁸O data and 22 high-precision ²³⁰Th ages of stalagmites XYD-01and XYD-02 in Xianyun Cave,theδ¹⁸O sequences of stalagmites with an average resolution of 3～4 years in the past 2000 years was established.The stalagmiteδ¹⁸O presents obvious decadal to centennial scale variation characteristics.In AD350-500,AD600—750,AD1150—1250,AD1550—1750 and AD1850—2020,precipitation amount during the flood season decreased and the East Asian summer monsoon weakened.In AD1—150,AD800—1060,AD1350—1460 and AD1740—1840,precipitation amount during the flood season increased and the East Asian summer monsoon strengthened.Theδ¹⁸O records of stalagmites in the East Asian monsoon region show a significant difference between AD 400—950 and AD 1350—1500.Compared with the series of dry and wet indices in different regions（Eastern and Central China,North China,Jianghuai and Jiangnan）integrated with historical records,on the multidecadal scale,the north-south dry and wet changes are more consistent in cold period,while the north-south dry and wet changes are more complex in warm period.（4）The EASM（flood season precipitation）in southeast China is influenced by several mechanisms on different time scales.On the interdecadal and multidecadal scales,the changes of the EASM are controlled by AMO and ENSO（El Ni?o-Southern Oscillation）.And PDO（Pacific Decadal Oscillation）plays a tuning role.On the interdecadal-century scale,theδ¹⁸O records of stalagmites in Xianyun Cave in the last2000 years are related to the TSI（total solar irradiance）,indicating that solar activity is the dominant factor of precipitation variation in this area on the interdecadal-centennial scale.