The Natural Variability Of The Asian Monsoon At Interannual Resolution And The Diagnosis Of Extreme Events Recorded By Stalagmites In Shennongjia 5ka BP

The Asian Summer Monsoon(ASM)is an integral component of the coupled system of oceans,atmospheres and land surfaces.The previous studies suggest that the insolation deceasing and ITCZ southward displacement over the Northern Hemisphere were important in the Asian monsoon deceasing during the mid-late Holocene.On hundred and thousand scales,the variation in the East Asian monsoon intensity agrees with changes in the North Atlantic climate and solar activity.However,annual and inter-decadal climate changes and the underlying driving mechanism remain unclear.Investigating collectively climate changes on the typical three scales helps provide further insights into the coupling between the Earth climate system,such as oceanic/atmospheric interactions,and solar forcing.Also,the thorough investigation helps advance our understanding of whether human activities result in the global warming over the recent several hundred years on the basis of high-resolution Asian monsoon reconstructions.Here we provide stable isotope records measured on a stalagmite(No.LFG1)in Longfugong Cave,Shennongjia Forestry District,Hubei province.These records,constrained by precise 230Th ages,cover a time span of 4.67-0.32 ka with averaging resolution of 0.4a.High uranium concentrations and high-growth rates of the stalagmite contribute to the better ages with small age errors,and the unparalleled resolution,respectively.Thus our records can characterize in great detail decadal-centennial climate anomalies over the Asian continent.The stalagmite δ18O record ranges from-7.5‰ to-9.7‰,with an amplitude of 2‰ across the whole record.A good replication is observed between the LFG1 and the other Asian stalagmite records.Further supportive evidence for the isotope equilibrium condition is a weak correlation between the LFG1 δ18O and δ13O records,passing the Hendy test.Over the mid-late Holocene,amplitudes of the LFG1 δ18O variation are greater than other stalagmite records,suggesting the sensitivity of the δ18O variation to climate change outside the cave and finer reconstructions of the Asian monsoon circulation.Holocene climate change is characterized by gradually cooling and drying conditions in the Asian monsoon realm,following the gradual decrease in the Northern Hemisphere summer insolation since 9000 years.However,the LFG1 high-resolution,well-dated δ18O record seems to exhibit an abnormal increase in Asian summer monsoon strength since 2ka BP,against the long-term monsoon weakening trend.This abnormality appears to be a robust feature,due to its identification in a wealth of monsoon records.The plausible reversed trend in Asian summer monsoon strength,preceding insolation minima,seems to have also occurred during previous interglacials.If this is the case,the monsoon abnormality during the late Holocene is difficult to be explained using the traditional boreal-insolation-driven mechanism.We suggest that this phenomenon might be linked to austral summer insolation changes,Tibet Plateau warming and/or greenhouse gas increases.The monsoon strengthening period can be divided into two stages:Dark Ages Cold Period(DACP,1.9-0.9 ka BP)and Medieval Warm Period(MWP,0.9-0.6 ka BP).The two periods can be identified by some other monsoon records,but with different starting and ending timings.The asynchrony of the reversal in different regions suggests a time difference of the Asian monsoon precipitation.Nevertheless,two monsoon peaks are identified within WMP for the first time in our high-resolution LFG1 δ18O record,and these variations show a strong resemblance to historical document records.The so-called ’2.8 ka event’ in the stalagmite coincided with the 2800 cal.BP’solar’ event(since identified as a Grand Solar Minimum),is well-attested from much of Europe and from some other parts of the global.Until now,no high resolution and well dating data were available to help describe its structure and to test the climate-sun link.The 2.8 ka event in our δ18O record,between 2.93 and 2.63 ka BP,exhibits an asymmetrical structure.The ASM decline at the onset lasts about 230 yr,and the subsequent monsoon amelioration at the end is accomplished within 60 yr.Such an asymmetrical structure was also found during the 8.2 ka BP event and Little Ice Age in other records.An impact of tropical ocean warmings may explain the asymmetrical ASM pattern.Two additional dry periods are recorded in the LFG1 record around 4.2ka BP and 3.5ka BP.On the centennial scale,a strikingly inverse correlation can be observed between the ASM and Indonesian-Australian summer monsoon(IASM)intensity,including the three prominent ASM failure events related to weak solar activity.Our results support the interpretation that ocean-atmosphere interactions amplified the solar signal and caused abrupt cooling events in widespread areas during the Holocene.It is noted that the influence of the North Atlantic cannot be overlooked.With the global warming,extreme events show a more frequent tendency than before.The situation threatens seriously people’s lives and property.The modern meteorological records are so short that the frequency and mechanism of these events are not clear.Furthermore,timescale uncertainties and low resolution in proxy-climate series reconstruction also restrict the investigation of extreme events.Here a series of extreme event are found in the LFG1 record using the technique of the multi-scale scanning T test.We find that the frequency of occurrences of these events varies with the climate background.We also find that the cycle of 20-30 yr is punctuated by the 2.8ka BP weak monsoon event,and that the structure of DACP is reshaped by multiple short-scale changes.These extreme events are accompanied by historical floods and droughts.In particular,some climate abrupt changes are correlative with some replacements of Chinese dynasties.Wavelet spectrum analysis suggest that solar activity,volcanic activity and/or internal factors of the Earth climate system possibly modulate the ASM precipitation on the interannual scale.This study,for the first time,provides a continuous sub-annual-resolution stalagmite isotope record recovered from Longfugong Cave,Hubei,which covers a period of 4350a spanning the transition of the middle to late Holocene.We discussed the causes of the abnormal increase in Asian summer monsoon intensity since 2ka BP,against the Holocene long-term weakening insolation trend.The LFG1 record first in great detail depict the structure of the 2.8ka event.This event corroborates the influence of solar activity and the North Atlantic climate on global climate change.Finally,a series of finer-scale climate variations provide insights into understanding the driving mechanism of extreme events.