A Study On The Evolution Characteristics Of The Asian Monsoon On The Millennium Scale Of The Glacial Cycle Before And After The Middle Bron Event

The coupling mechanism of insolation and the boundary conditions of the climate system before and after the Middle-Brunhes event has undergone significant changes,which has a profound impact on the Earth's climate system.However,there are still many doubts about the specific process of this influence.The high resolution record across the Middle-Brunhes event is expected to further advance the research in this aspect.Based on the 15 ²³⁰Th ages,1224 stable isotopes and 1586 trace element datas of two stalagmites(No.LS20,LS74)from Luoshui Cave,Hubei Provence,we established the high-resolution records of Asian monsoon from 510.92 ka to 458.07 ka and from 405.52 ka to 377.77 ka as well as the records of karst ecological environment from 510.92 ka to 420.7 ka and 405.52 ka to 377.77 ka.Combined with the reconstructed oxygen and carbon isotope sequences of LS10,LS26,and LS60 to synthesize the stable isotope sequences which spanning the reciprocal fifth to fourth glacial cycle,it's a key period across the Middle-Brunhes event.And then discuss the characteristics of millennial-scale evolution of Asian monsoon before and after the Middle-Brunhes event and how they response to the change of boundary conditions of climate system.The oxygen isotope records of stalagmites from Luoshui Cave show that there are multiple millennial-scale positive shifts from the reciprocal fifth to fourth glacial cycles,which roughly correspond to the cold events in North Atlantic area and the warm peaks of the Antarctic temperatur e recorded by ice core.Further analysis found that the frequency of millennial-scale weak monsoon events is significantly different in the glacial cycles before and after the Middle-Brunhes event,that is,the frequency of weak monsoon events in the reciprocal fifth glacial cycle is lower than that in the reciprocal fourth glacial cycle.There are obvious differences in oscillation characteristics of insolation on the precession cycle before and after the Middle-Brunhes event:insolation during the reciprocal fourth glacial cycle shows the characteristics of high frequency and low amplitude,while low frequency and high amplitude during the reciprocal fifth glacial cycle.In addition,the boundary conditions of the climate system have changed significantly after the Middle-Brunhes event,especially the instability of the ice sheet has increased,which directly influence the frequency of cold events at the northern high latitude and in turn affects the evolution of the low-latitude monsoon system.They may be the important reasons for the difference in the frequency of weak monsoon events before and after the Middle-Brunhes event.The carbon isotope records of the stalagmites from Luoshui cave are basically consistent with the synthetic Greenland temperature on the orbital scale,and show a similar period to the ice volume cycle,indicating that the?¹³C of stalagmite in the Asian monsoon region reflects the biomass and the soil CO₂ yield overlying the cave.The evolution characteristics of?¹³C are controlled by change of global ice volume on the orbital scale.The?¹³C record exhibited significant positive shift during the weak monsoon events,indicating that the weaken of the Asian monsoon intensity caused the overlying biomass and the soil CO₂ yield decreased.Like the stalagmite?¹⁸O record,the decrease of?¹³C value during the weak monsoon events reflects that it was also affected by the temperature changes of northern high latitude.The trace element ratios of stalagmites(Mg/Ca,Sr/Ca,Ba/Ca)from Luoshui Cave have a complicated relationship on the orbital-millennial scale.The Sr/Ca and Ba/Ca ratios are similar to the changes of?¹³C records on the orbital scale,indicating that the Sr/Ca,Ba/Ca and?¹³C of stalagmites may have similar control mechanisms,they all reflected the evolution of the regional karst ecological environment.However,the Mg/Ca has no obvious trend on the orbital scale and there is no correlation with other indicators.The Sr/Ca,Ba/Ca,and Mg/Ca were low in varying degrees during the weak monsoon events,indicating that the evolution of the regional ecological environment reflected by the trace elements of stalagmites from Luoshui Cave was sensitive to change of the monsoon intensity.