The Evolution Of Summer Monsoon In Southwest China During 97-59 Ka B.P.as Revealed By Stalagmite ?¹⁸O Record

Stalagmite,an important proxy in paleoclimatic research,is increasingly valued by the scientific community since the far-reaching Hulu Cave stalagmites known to the public.The Chinese stalagmite standing in the field of international paleoclimatic research gradually established,due to Chinese scientists endless efforts of the past decade,and the high accurate chronology of stalagmite can become one of the potential references for chronological refinement for other records.The most prominent scale of variability is the centennial-millennial scale variability during the last glacial period.The Dansgaard-Oeschger?D/O?oscillations and Heinrich?H?events are the most typical characteristic of climate change on the millennial scale.Within D/O cycles and consist of abrupt warmings either preceding individual Greenland Interstadials?GIS??named Precursor-type events?or occurring toward the end of some of the Greenland stadials?GS??named Rebound-type events?representing centennial-scale climate change.However,due to the lack of high resolution and accurate dating of geological records,the timing and pattern of these events have not yet been fully resolved and the climate-driven mechanisms need further exploration.Stalagmite is characterized with long growth time,complete storage information,accurate dating and independent absolute chronology,so its position and function becomes increasingly evident.We reconstruct the centennial-millennial scale changes in Asian Summer Monsoon?ASM?intensity from 97 to 59 ka B.P.?ka B.P.,before 1950 AD?,based on 780 oxygen isotope data and 36²³⁰Th dates from a continuous growing stalagmite collected in Yangkou Cave,Chongqing,southwestern China.The Yangkou?¹⁸O record shows five strong ASM periods named Chinese Interstadials?CIS?18-22,corresponding to GIS 18-22.However,the GIS18 event shown in the Yangkou?¹⁸O record was an extremely enhanced summer monsoon in Marine Isotope Stage?MIS?4,but the duration and intensity of the GIS19 event might not as strong as previous.Besides,the GIS20,GIS21and GIS 22 are also different from Greenland ice core records.The high degree of similarity of the?¹⁸O records between Yangkou Cave and Greenland supports the notion that climate change of the Northern Hemisphere?NH?high latitudes has exerted significant influences on the ASM variations,but there may be different patterns in response to specific climate event.The structure of the GIS21 event appears to be three sections,and the GIS22 event manifests bimodal pattern in the Yangkou?¹⁸O record,indicating that there will still be oscillations within the millennium-scale event.It also shows that the Precursor-type events and Rebound-type events recurred within the cold GS and warm GIS successions.These suggest that the instability of the last glacial period may be more intense than previous comprehension.The identification of the centennial-millennial scale events in ASM region suggests a large-scale climatic footprint of the NH high latitudes highlighting the interactions among the constituent elements of the earth's climate system,especially the coupling of the ocean,atmosphere,and cryosphere systems.The H6 event is further confirmed by the Yangkou?¹⁸O record,indicating that it does exist in ASM region.Based on the comparison between various published paleoclimatic records and the Yangkou?¹⁸O record,we suggest that in ASM regions,including South Asia,North and South China,experienced a dry condition during H6 event.The Yangkou?¹⁸O record manifested a weak ASM interval from 73 to 70.9 ka B.P.,which was punctuated by a short-lived abrupt warming event at 72.3 ka B.P..We considered that the two weak ASM intervals might be in response to the Toba Super-eruption and the 72 ka Event respectively.The Toba eruption may have a slight impact on the 72 ka Event but no means substantive.The 72 ka Event may not as obvious as the high latitudes at NH mid-low latitudes.We constrained the timing of the centennial-millennial scale events using the precise stalagmite U-Th dating.The NGRIP?¹⁸O record?using the GICC05modelext timescale?is compared with the Yangkou stalagmite record for the interval 97 to 59 ka B.P.,we find that the onset and end of the GIS22 and GIS18 in the Yangkou record are older than those of the NGRIP record.On the contrary,the timing of the GIS19,GIS20 and GIS21 are younger in Yangkou stalagmite record.Based on the Yangkou stalagmite U-Th data combined with other published stalagmite records in ASM region,the mid-point at transitions of the MIS 5a/MIS 4 was tentatively constrained at 76.54±0.3 ka B.P.,and the end of the Marine Isotope Stage 4 occurred at 59.4±0.2 ka B.P..The Yangkou?¹⁸O record follows the 65°N summer insolation,suggesting that the change of the ASM in Chongqing basically responded to the variation of the summer insolation in orbit-scale.Furthermore,there may also be other factors influencing the ASM.