Evolution Of The Asian Summer Monsoon During MIS 4 Period Inferred From A New Spliced High-resolution Stalagmite Record

The Asian summer monsoon?ASM?is an integral part of the global climatic system and plays a central role in rapid climate changes involving both the Northern Hemisphere?NH?and the Southern Hemisphere?SH?.In recent decades,the evolution of the ASM on centennial-millennial time scales since the last glacial period?LGP?has been studied using various paleoclimatic records,including stalagmite.However,high-resolution stalagmite records mainly cover Marine Isotope Stages?MIS?1-3,while little detailed information is available on the evolution of the ASM during MIS 4.With decreasing summer insolation in the North Hemisphere,increasing ice volume and sea level lowering,the MIS 4 interval was an extended cold period considered to be comparable to the last glacial maximum?LGM?.Therefore,a detailed study of the ASM changes during MIS 4 will improve our understanding of the progress and mechanisms of ASM evolution during the LGP and its role within the Eart h climate system.In addition,the age of the Heinrich event 6,which was first identified in North Atlantic sediments,remains controversial.Because the timing is beyond the limit of the 14 C dating method.Furthermore,even the most accurate ice-core age model – the annual layer-counted GICC05 timescale – is associated with uncertainties exceeding 2.6 ka at ages >60 ka.This lack of accurate and precise chronologies can be compensated by speleothems taking advantage of recent improvements in high-precision 230 Th dating.In this study,based on the precise U-Th dating and a new spliced high-resolution stalagmite Oxygen isotope record from Yangkou Cave and Xinya Cave,C hongqing,southwest China,we reconstructed the centennial-millennial-scale changes in Asian Summer monsoon?ASM?intensity during MIS 4 period.By the comparisons between the JFYK7 and other paleoclimatic records,we discussed the change patterns and the potential forcing mechanisms of ASM.Taking advantage of the 230 Th dating with uncertainties of less than 0.5%,we precisely constrained the timing of Greenland Interstadials and Heinrich events.It can provide a reference for the chronology calibration of the other paleoclimatic records.Although the variability of the ASM at millennial timescale recorded by stalagmites JFYK7 and XY2 is not completely consistent with the change in summer insolation at 30°N between 79.3 and 58.0 ka BP,we suggest that the variability of ASM is regulated by the summer insolation on orbital timescale.A series of millennial-scale climatic fluctuations are superimposed on the first-order trend,which may correlate to temperature changes recorded in the Greenland ice cores.The ?¹⁸O sequence of JFYK7 records at least five strong ASM events,indicated by negative ?¹⁸O excursions,which correspond to GIS 17-21.In addition,three positive excursions occurred at 59.4-61.5 ka BP,70.9-73.1 ka BP,73.9-75.4 ka BP,reflecting weak ASM events and their counterparts can also be found in the NGRIP records.These consistencies illustrate that the variability of ASM was mainly dominated by the climate change of the north high latitudes.Comparisons between stalagmites records from C hina and the sediment core record from Arabian Sea show that despite of these some differences,the variability of the ISM is consistent with the EASM on millennial timescale during MIS 4.This consistency probably should be attributed to that both the ISM and EASM are regulated by the climate change over the north high latitudes.Comparing the ?¹⁸O records of stalagmite JFYK7 and stalagmite BT2 from southeast Brazil reveals a prominent anti-phase relationship at millennial timescales between the ASM and the South American Summer Monsoon?SASM?,reflecting the migration of the ITC Z and the asymmetry of the Hadley circulation in the two hemispheres.We constrained the timing of the GIS 17-21 using the precise U-Th dating.By the comparison between NGRIP and JFYK7 record,we find that the timing of GIS 17 recorded in the JFYK7 is identical within error to stalagmites XY2,MSL and in agreement with that in NGRIP.But there are some small discrepancies in the timing of GIS 18-21 events.The coherence of the timing of GIS 17 among various archives underscores the accuracy of the GICC05 timescale back to 60 ka BP.However,conspicuous discrepancies exist for GIS 18-21,suggesting that the accuracy of the Greenland ice-core chronology should be slightly revised for the section prior to 60 ka.Both stalagmite JFYK7 and XY2 clearly record the evolution of H6.And the start and end time are constrained to 61.5±0.2 ka BP and 59.4±0.2 ka BP respectively.Based on the comparison between various paleoclimatic records we suggest that the Asian summer monsoon regions,including South Asia,North and South China,experienced a dry condition during H6 event.The transition leading from GIS 18 to H6 is clearly different between NGRIP and records from the mid to low NH latitudes.In NGRIP,GIS 18 is characterized as an abrupt warming and terminated by an equally rapid cooling.In contrast,the Chinese stalagmite records suggest a gradual,millennial-scale decrease in the intensity of the ASM subsequent to GIS 18.Comparing with the ice core record from Antarctica,we speculate that the gradual decline of the ASM at the end of GIS 18 should be attributed to the gradual warming in the southern hemisphere which may result in the southward migration of the mean position of ITCZ and the reduction of the transequatorial flow.