Climate Change In The Early Holocence Revealed By Stalagmite From Lianhua Cave In Hunan Province

With the frequent occurrence of global warming and extreme climate events, it is necessary to make a research on the change of global climate. Based on the short time of modern climatic instrument data, studying the changes of the ancient climate can obtain the significant data for the study of the modern climate, the prediction of the future climate change and climate simulation. Holocene is a period of the outbreak of the human civilization, and the early Holocene is an important period in the transition from the Paleolithic Culture to the Neolithic culture. The strong climatic fluctuations in the early Holocene have an important impact on the production and life of the prehistoric people. In the early Holocene, there were differences in the time of the occurrence of cold/dry events（or not reflect）, besides, the mechanism of cold/dry events is not clear. The establishment of high resolution records of the Asian monsoon region in the early Holocene is helpful to reveal the characteristics of climate change and the mechanism of its formation and has a great significance for the study of the links among the Asian monsoon, ocean and polar regions. The author takes the Lianhua Cave in Longshan county of Hunan province as the research area, which is controlled by the South Asian monsoon and the East Asian monsoon, and records LHD5 stalagmite from Lianhua Cave measured 28 U/Th age and 535 oxygen isotopic data to reconstruct the characteristics of the Asian monsoon’s evolution in Holocene. In the early Holocene,the resolution lasts 8 years. Conclusions are as follows:（1） The δ¹⁸O values of LHD5 range from-7.82‰ ^-4.67‰ and the average value is-6.61%, whose fluctuation amplitude is 3.15‰. The data Indicates that the monsoon intensity enhances first, and then decreases when it reaches the strongest level in 9-7 ka BP. The characteristic of the δ¹⁸O values of LHD5 varies obviously. In 12.0-11.75 ka BP（YD event）, the average value of δ¹⁸O is-5.18‰; in 11.75-11.68 ka BP the δ¹⁸O values decreases from the-4.84‰to-6.69‰, whose fluctuation amounts to1.85‰. It can be regarded as the monsoon rapidly increasing period（the Younger Dryas eventfrom to Holocene period）; in 11.68-9.0 ka BP, the fluctuation of the δ¹⁸O values is small;in 9.0-7.0 ka BP, the δ¹⁸O values becomes the most light, with an average value of-7.26‰, and is the strongest monsoon period; in 7.0-2.8 ka BP, δ¹⁸O value continues to become heavier, and the variation range of δ¹⁸O is from-7.26 to-5.78‰, whose fluctuation is 1.48‰. It can be regarded as the monsoon weakening period; in 2.8-0.05 ka BP, the fluctuation of δ¹⁸O is small and the δ¹⁸O becomes heavier, with an average value of-5.79‰, as the weak monsoon. At the same time, there are a number of weak monsoon events throughout the whole Holocene. The changing trend of LHD5 δ¹⁸O value is consistent with the solar radiation in the northern hemisphere in general.However, the degree of agreement with 65°N solar radiation is stronger than that of30°N, which mainly reflects in the small age difference, between the LHD5 δ¹⁸O peak value and 65 °N solar radiation peak value and the difference between the two peaks is about one thousand years. What’s more, after 7 ka BP the change of δ¹⁸O from LHD5 is almost identical to that of solar radiation at 65°N.（2） In11.75-11.68 ka BP, the δ¹⁸O values become rapidly lighter from-4.84‰ to-6.69‰, whose fluctuation is 1.85‰, as the monsoon rapidly increasing period（the Younger Dryas event from to Holocene period）. According to the LHD5 stalagmite records, the end of Younger Dryas is at 11748±30 a BP, the start of the Holocene is at11684±39 a BP, and the conversion time is about 64 years, which is consistent with the records of the gicc05 ice core from Greenland in the error range. The starting time of the Holocene of LHD5 is 100 years earlier than most geological records, and the conversion time from Younger Dryas to Holocene is not consistent with the all kinds of geological records. This shows that the difference of the starting time and conversion time of the Holocene is influenced by the age error of measuring, climate proxies or local environment.（3） The record of LHD5 stalagmite in the early Holocene of the Asian monsoon is gradually increasing, compared with studies of other geological records, which has shown that phase of South Asia monsoon and East Asia monsoon in early Holocene is consistent, mainly because the Solar radiation enhancement in the northern hemisphere drives the ITCZ to shift northward greatly resulting in monsoon strengthening. However,the peak monsoon intensity recorded in LHD5 lags the peak of Northern Hemisphere insolation by 2.0-3.0 ka. LHD 5 and NGRIP ice core record is very consistent with the evolution of trends. And a possible explanation is that the glacial boundary force plays amore dominant role in modulating the monsoon than insolation during the early Holocene.（4） The weak monsoon events can be found six times in LHD 5 stalagmite records in early Holocene, and the central age of the six weak summer monsoon events is at11461±34 a BP（1.08‰）, 10354±36 a BP（0.94‰）, 9957±25 a BP（0.66‰）, 9062±36 a BP（0.90‰）, 8744±23 a BP（0.55‰） and 8144±24 a BP（1.02‰）, the fluctuation δ¹⁸O value of 1461±34 a BP and 8144±24 a BP is greater than 1‰, but the fluctuation of theδ¹⁸O value of other weak monsoons is less than 1‰. These weak monsoon events last for about 100-200 years, and all of them have a rapid entry and a fluctuating exit. The weak monsoon events at 11461±34 a BP、10354±36 a BP、and 8144±24 a BP have regional scale climate significance; while the weak monsoon events at 9957±25 a BP、9062±36 a BP、and 8744±23 a BP are recorded only by few geological records. The record of LHD5 stalagmite 9.2-9.3 ka BP weak monsoon event is absent, probably due to an age error or the influence of local climate. In addition, according to the analysis of the 8.2 ka BP event, the weaker summer monsoon events before 10 ka BP are not only affected by the summer insolation but also by the ice-rafted debris（IRD） events in the North Atlantic. And the following weak summer monsoon events are mostly influenced by solar activity and the southward migration of the mean position of the Inter-Tropical Convergence Zone（ITCZ）.