Late Holocene Paleoclimatic And Paleoenviron-Mental Evolutions Recorded By Stalagmite From The Songya Cave,Zhejiang Province

The evolution of global climate and environment is closely related to the development of human society.Natural disasters caused by extreme climate anomalies not only cause the loss of human social property,but also threaten the safety of human life.Therefore,paleoclimate change has become one of the important research fields in the scientific community.The monsoon system has a decisive influence on the climate evolution process of the northern and southern hemispheres.The Asian monsoon is an important carrier connecting the Eurasian continent and the Pacific Ocean.The study of the late Holocene,especially the internal law and driving mechanism of the evolution of the Asian monsoon since 2000,has obvious theoretical and practical significance.Cave stalagmites,with their independent absolute age scale and high resolution,stand out among many climate carriers and are favored by paleoclimatologists.Reconstructing paleoclimate with stalagmites has become a hot topic in the field of global climate and environmental change.At present,the research on the stalagmite records of the evolution of the Holocene Asian monsoon mainly focuses on the discussion of the overall change trend of the monsoon on the Millennium scale.However,there are few discussions on the driving mechanism of the late Holocene interdecadal-centennial monsoon,and the internal structure,occurrence frequency and fluctuation amplitude of the short-time-scale weak monsoon events are still controversial.Using stalagmites to study the multi-scale evolution mechanism of the late Holocene monsoon and to explore the causes of abrupt climate change events can lay a theoretical foundation for the diagnosis of future climate evolution trends.In this study,eight high-precision chronological data and 670 sets of oxygen and carbon isotope data were obtained from a stalagmite SY5 in Songya Cave,Beishan area,northeastern Jinqu Basin,Zhejiang Province.The evolution sequence of summer monsoon in Jinqu Basin since the Late Holocene has been reconstructed with a resolution of 6.3 years.Oxygen isotopes of Songya Cave stalagmites were tested by equilibrium fractionation,and showed good reproducibility with Dongge Cave stalagmites DA and D4,Lianhua Cave LHD,Sanbao Cave SB43 and Heshang Cave HS4,respectively.It shows that theδ¹⁸O reflects the external climate signals well and basically meets the conditions of climate substitution index.Based on the analysis of oxygen and carbon isotope curves of SY5 stalagmites in Songya Cave,the climatic significance of the different time scales of stalagmites referred to asδ¹⁸O andδ¹³C was further studied,and compared with other geological records of Late Holocene,combined with power spectrum analysis,the possible driving mechanism of Asian monsoon and the internal structure of weak monsoon events in interdecadal-centennial time scales were discussed.Songya Cave SY5 stalagmiteδ¹⁸O fluctuates between-8.464‰and-6.531‰,the maximum amplitude of the sequence is 1.933‰,with an average of-7.435‰,the variation of stalagmite’s value ofδ¹⁸O is obvious,indicating that the monsoon first weakens and then strengthens.The Songya Cave stalagmite’s value curve ofδ¹⁸O shows five significant weak monsoon events,namely 4110aBP-3802 a BP、2559aBP-2247 a BP、1975 aBP-1688a BP、1104aBP-942 a BP and 697aBP-399 a BP,it shows that stalagmiteδ¹⁸O is sensitive to climate change.The overall variation trend of the stalagmite’sδ¹⁸O curve on the Millennium scale is consistent with that of the northern hemisphere’s 65°N summer solar radiation,which indicates that the millennium-scale variation of Asian monsoon is regulated by solar radiation.The thousand-year time scale of SY5 Stalagmite in Songya Cave indicates the intensity change of East Asian summer monsoon,and the interdecadal-centennial time scale indicates the precipitation change caused by the intensity of summer monsoon.Songya Cave stalagmiteδ¹³C is closely related to soil CO₂ yield,indicating the change of surface biomass,and is a good index of surface temperature and humidity.Oxygen and carbon isotopes of the stalagmites in Songya Cave indicate that the climate was warm and dry during the MWP,there were two distinct drought periods,897-1006 AD and 1105-1201 AD.The climate of the LIA was cold and dry compared with MWP.The drought events recorded by Songya Cave Stalagmite in the historical period are consistent with the literature records,and the decline of Liangzhu and Sanxingdui civilizations is also recorded,suggesting that climate and environmental changes have a significant impact on the development of human society.Significant solar cycle was detected in the stable oxygen isotope and carbon isotope power spectrum analysis of stalagmites in Songya Cave,the de Vries period of 213a,Gleissberg period of 84a and double Haier period of 20a are respectively observed,the solar activity cycles of 633a,500a,335a and 139a are slightly lower than those of the others,it shows that the evolution of Asian monsoon is strongly influenced by solar activity on the interdecadal-centennial time scale.Oxygen and carbon isotopes of the stalagmites in Songya Cave show the 50-year and 68-year cycle components that characterize the PDO 50-70a cycle,the ENSO-2000 cycle and the 500-year cycle of the North Atlantic thermohaline circulation,which further illustrates that the East Asian summer monsoon has complex influence factors and closely related to PDO,ENSO and NAO.