The Millennial-scale Abrupt Climate Events During The Last Glacial Recorded By Stalagmites From Xianyun Cave In Western Fujian

A series of millennial-scale abrupt climate events are the main climatic features of the last glacial.Since their discovery,they have been a hot topic for paleoclimatologists worldwide.Especially during the Marine Isotope Stages（MIS）3,millennial-scale abrupt climate events are characterized by high frequency and large amplitude.With the development of dating techniques and research methods,high resolution and high precision records are constantly emerging in different regions.However,the transformation pattern and internal characteristics of these events recorded in different regions are different may due to different climate mechanisms,which challenges the traditional theory of northern high latitude driving mechanisms.Low latitude hydrological processes play an important role in linking the climate between the northern and southern hemisphere and regulating global heat and moisture.In particularly,high precision and high-resolution records from low-latitude region are very important for researching the structure,transition pattern and climate mechanism of millennial-scale abrupt climate events.We reconstructed the evolution history of East Asian summer monsoon（EASM）during MIS3-2（58.2～23 kyr BP）of the last glacial,based on 123 U/Th dates and 5841sets of carbon and oxygen isotope data from 7 stalagmites from Xianyun Cave,Fujian province,southeast China,covering Chinese interstadial（CIS）3-15 and Heinrich Stadial（HS）2-5.The average resolution was 10 years and dating errors are less than 200 years（80%less than 150 years）.Our record is the highest resolution record in the monsoonal China at present and is also the first continuous record of long-timescale in southeast China.Due to the differences in dating methods and accuracy between different records,it is difficult to make comparison between different records,which also limits our discussion on the driving mechanism of climate abrupt events.Combined with previous studies,we choose the break-point rather than the mid-point as the timing of onset and termination.The results show that the onset of CIS and HS of Xianyun Cave is consistent with that of Yongxing Cave and new Hulu Cave within error.Moreover,compare the CIS events of Xianyun Cave,Yongxing Cave,Hulu Cave and GIS（Greenland interstadial）of NGRIP（North Greenland Ice Core Project）of GICC05 timescale in the research period of this paper,the latter seems generally younger than the stalagmite time scale.Considering the accuracy and resolution in our record are better than previous records,we anchored the onset and termination of HS2 to HS5 recorded in Xianyun Cave stalagmite are as follows:24.45±0.06～23.42±0.05 kyr BP,30.51±0.09～29.09±0.05 kyr BP,40.04±0.07～37.76 kyr BP,48.72±0.15～46.26±0.11 kyr BP.Due to the cumulative counting error of GICC05 time scale during 58.2-23 kyr BP is close to 2,000 years,this time scale needs to be shifted in order to make comparison between ice core and stalagmite records.Based on the accuracy of the onset time of typical millennial-scale climate abrupt events and the synchronization of centennial scale climate events in different records,the identify and accuracy of the onset time of the events are considered,we determined six tie-points that can be used to shift ice core record,which are the break-points of onset of HS2-5 events,the onset of GIS15.2 and the peak point of GIS15.1.We can draw the following conclusions:The GICC05 time scale of the Greenland ice core is in good agreement with the Xianyun Cave age model within the error.The absolute age error between the Greenland ice core and Xianyun Cave is less than 500 years during the study period（58.2-23 kyr BP）,but there are differences in specific events.Such as,the GICC05 time scale of NGRIP is about 460 years old at the HS2 event,relatively reliable in HS3～HS5（30～50 kyr BP）and about 270 years younger in GIS 15.The time scale of the Greenland ice core can be further optimized by the Xianyun stalagmite records based on their high precision age control at both the onset and termination of each event.The transition pattern and internal variation characteristics of millennial-scale climate events are important for understanding the driven mechanism.A typical feature of CIS in Xianyun record is the transition pattern of most CIS events is more gradual than that in NGRIP and northern China.e.g.,the durations of the CIS 14,12 and 8 are 1490 yr,670 yr and 720 yr,respectively,while that in NGRIP and northern China are 20 yr,60 yr and 40yr（27 yr,160 yr and 350 yr）.The gradual pattern in Xianyun Cave is similar to that in the stalagmite record of low-latitude Borneo caves and in the Sea Surface Temperature（SST）record.During the onset of typical CIS events,our results show that the southeastern China,which is close to the water vapor source,show a gradual pattern due to the influence of the low latitude region coupled ocean-atmosphere process.However,the northern China are greatly influenced by westerlies.Only when the Atlantic Meridional overturning circulation（AMOC）recovers,the monsoon in north China can enhance rapidly and shows a abrupt pattern.This makes the onset of typical CIS events in China show two patterns:the rapid change pattern in the north and the gradual pattern in the southeast.On the other hand,similar to CIS events,the durations of onset and termination of HS 2,4 and 5 in Xianyun Cave also show gradual transition pattern（the duration of the transition is longer than 200years）,while that in NGRIP shows an abrupt pattern（less than 100 yr）and the climatic state within events is unstable.In a word,the HS 2,4 and 5 in Xianyun Cave present three-phased feature with gradual onset and termination,that is agrees with the shift of Intertropical Convergence Zone（ITCZ）.The results suggest that the structure of the HS events in southeastern China affected by atmospheric/oceanic circulations in both high and low latitudes influenced by both the variation of AMOC and the low-latitude coupled ocean-atmosphere processes.The study of the long-term evolution trend and structure of paleoclimate records is helpful to better understand the climate mechanism in long time scale.Firstly,in the long timescale,the amplitude of Xianyun record（only 2.8‰）is smaller than that of other stalagmite records in the monsoonal China.This may be related to the small seasonal variations of precipitationδ¹⁸O in Xianyun Cave,which is located in the front zone of EASM,strongly influenced by low latitude ocean hydrological cycle.Moreover,comparing our records with other paleoclimate records,it is found that,although climate changes of many geological records show a cooling/weaken pattern in the long-term trend,the small amplatitude of Xianyun stalagmites records is closer to the change of ITCZ and SST in the Western Pacific.This indicates that not only the climate in the northern high latitudes,but also the changes of EASM reflected by Xianyun records are also affected by the contraction and movement of ITCZ and the SST of low latitude tropical ocean,which provides evidence for the previous discussion that the hydrological climate in low latitude plays an important role in the millennium-scale climate events.Secondly,the structures of the typical mid-CIS event recorded in Xianyun record present a relatively gentle“flat”structure,is different from the“jagged”cooling structure of Greenland ice cores.Moreover,a linear relationship between the EASM variability and Antarctic tempreature implies the influence of the strengthening of the cross-equatorial flow from the southern hemisphere on north-tropical monsoon.In conclusion,the monsoon changes recorded by stalagmites closer to the lower latitudes may be affected by combinated climate factors,and the quantitative influence mechanism among these factors needs to be further studied by more climate models.In summary,we present a long-timescale and continuous evolution history of EASM in southeast China near the low latitude water vapor source region,with a 10-year resolution and high dating accuracy.That is important for understanding the relationship between EASM and high and low latitude climate.Our record indicates that the low latitude monsoon change will be affected by combinated climate factors.The transition patterns of millennial scale event between Xianyun Cave and northern China and north high latitude are different,show unique low-latitude climate signals.That confirmed that the low-latitude coupled ocean-atmosphere processes plays an important role in the transition of millennium-scale climate events.