Climate And Environmental Changes In Southwest China During The Heinrich Event 4 Recorded By A Stalagmite In Yangkou Cave,Chongqing

Heinrich Events(HE)is one of the important characteristics of climate change during the last glacial.Due to the uncertainty of ocean sediments and ice core chronology,it has not been possible to accurately calibrate the exact time of each HE occurrence so far.The phase relationship and influence mechanism between iceberg discharge,low-latitude climate change,and Greenland temperature change during HE are still controversial.The response process of the low latitude atmospheric circulation system to HE may be different from that of Younger Dryas(YD)and Dansgaard/Oeschger(DO)events.Discussions on these issues need to be supported by higher resolution paleoclimate records from lower latitudes.Although the high-precision U-Th dating provides chronological advantages for cave stalagmites,only a few stalagmite studies have reconstructed the abrupt process and internal structure of the Asian summer monsoon(ASM)intensity during HE.Subject to the influence of dating errors and time resolution,even within the Asian monsoon region,the start and end times and internal structure of HE is different.In this study,the Stalagmite YK1 taken from Yangkou Cave in southwestern China were discussed by analyzing the stalagmites?¹⁸O,?¹³C and Mg,Ba,Sr,Ca,combined with high-precision U-Th and stalagmite fluorescence laminae age model,high resolution,and multiple indicators reconstructed the change process of ASM intensity and regional hydrological conditions in Southwest China during 37.14-42.00 ka BP.The main conclusions are as follows:(1)The intensity of ASM recorded by?¹⁸O of Yangkou cave stalagmite decreased/increased rapidly at the beginning/end of HE4.The mutation time of ASM intensity at the beginning and end of HE4 was 39.44 ka BP and 37.85 ka BP,and it reached the weakest during 39.34-38.27 ka BP.During the HE4,the responses of stalagmite?¹⁸O records in different regions of the monsoon region of China to the abrupt climate events in the North Atlantic are consistent on the millennium scale.The main performance is that the ASM intensity during the HE4 decreases significantly,and has the characteristics of abrupt change at the beginning and end of HE4,as well as internal instability,which indicates that the ASM is greatly weakened/enhanced in the form of abrupt change during the HE4,and the ASM intensity decreases significantly in the hundred years on the annual scale,it has the instability of oscillatory change.The response of stalagmite?¹⁸O to HE4 is different in the time and process of abrupt climate event.The mutation time of?¹⁸O in stalagmites from Yangkou cave and Wulu cave was later than that in northern China at the beginning and end of HE4.In the aspect of the mutation process,compared with the stalagmite?¹⁸O records in the low latitudes of Southwest China,the stalagmite?¹⁸O records from the north and central China have a shorter mutation duration,and the process of ASM weakening and strengthening is more rapid.The regional differences of stalagmite?¹⁸O records during HE4 in the South and North monsoon regions of China are mainly due to the differences of ASM intensity response to the North Atlantic abrupt climate events in the north and south regions of China.Northern China is affected by the westerly,and the atmospheric process can quickly spread the abrupt climate change signal,while the climate change in southern China is mainly controlled by the tropical ocean hydrology and low latitude atmospheric circulation process.The North Atlantic climate change affects the hydrological change process of the tropical Indian Ocean through the reorganization of Hadley circulation,including the North-South movement of ITCZ,and its climate change is relatively stable the atmospheric process is slower.(2)Compared with the Greenland ice core?¹⁸O records,the change in different stages of paleoclimate records such as stalagmites and marine cores in low latitude areas including Yangkou cave stalagmites?¹⁸O are more obvious during HE4.During the HE4,the?¹⁸O of Greenland ice core showed a break at the beginning and end of HE4,but there was no significant oscillation within HE4.However,this stable state has hardly been observed in other global climate records at different latitudes,including YK1 stalagmite?¹⁸O.The paleoclimate records at low latitudes can identify three different stages,which indicates that the ASM and the atmospheric and hydrological cycles at low latitudes may have undergone large-scale reorganization when the temperature in Greenland remains stable,which is similar to that in Greenland the temperature change is decoupled.The decoupling between high latitude and low latitude climate records means that the climate change in other parts of the world during HE4 period can not be completely attributed to the temperature change in the high latitude region of the Northern Hemisphere(Greenland),and there are significant differences in the response process and mode between high latitude and low latitude to HE4.When HE4 occurred,stalagmite?¹⁸O records in northern China were consistent with the rapid changes of ice core?¹⁸O and dust concentration in Greenland.However,at the beginning of HE4,the low latitude ocean and hydrological cycle processes,including the tropical western Pacific,are not rapidly affected by the abrupt climate change in the North Atlantic region.The atmospheric process may transmit the abrupt climate change signal generated by the Atlantic meridional overturning circulation(AMOC)faster and more effectively than the oceanic process.The abrupt climate change signal in the North Atlantic region mainly propagates to the global scale through atmospheric and oceanic processes.The tropical hydrological cycle process and monsoon intensity change in the low latitude region related to ITCZ also play an important role in the process of millennium scale abrupt climate change.(3)The?¹³C and element ratios(Mg/Ca,Ba/Ca,Sr/Ca)of the stalagmites from Yangkou cave recorded the surface hydrological environment and the process of prior calcite precipitation(PCP).The results show that?¹⁸O,?¹³C and X/Ca PC1 have strong covariance in the period of 40.25-42.00 ka BP,which indicates that the change of ASM intensity can affect the change of?¹³C and X/Ca of stalagmites by affecting regional precipitation and hydrological conditions,ecological environment and biological activity.However,similar coupling characteristics were not observed in stalagmite?¹⁸O and X/Ca PC1 at 37.14-40.25 ka BP.It indicates that the change of the regional hydrological environment is decoupled from ASM intensity at this time,which may be related to a relatively mild surface hydrological environment and weak PCP process in Yangkou cave area during HE4.