| The reoccurrence of millennial-scale oscillations is one of the remarkable characters during the last glacial climate,which hav e been focused by scientists over the past half a century.However,these globally-distributed climatic events seem to display various responses in spatially-seperated regions,such as in the form ofvariation mode and structure characteristic.With the development of more high-resolved and absolute-dated geologic records wherever from the ocean or the continent,the "icon" of Greenland record on the variability features of millennial-scale events and associated periodicity topics has been faced with challenges.On the other hand,the increasing but open controversy of the phase(lead and lag)between northern and southern hemispheres probably results from different responses to a common climate signature,and/or from age model offsets for dividual records.Now we are recognizing that hydroclimate in the low-latitudes is playing a significant role in modulating global vapor and heat redistribution.Especially for the need of high-quality tropic/subtropic monsoon record,if so,it is conducive to reveal a more detailed process of the propagation of millennial-scale signatures around the world;and by extension,it provides a new insight into the behavior response to two hemispheric climates and relevant dynamics on abrupt climate changes in the frame of so called "seesaw" mode.Here we present a composite and thus a continuous summer monsoon record from Luoshui Cave,central China,with three pieces of stalagmite constrained by 122 U/Th dates and 5761 isotopic subsamples,to reconstruct millennial-orbital scales climate evolution during the last glacial period(95.8?10.8 ka BP,1950 AD).Relative to the recently published data from Hulu and Yongxing Caves(Wang et al.,2001,2008;Cheng et al.,2016a;Chen et al.,2016),Luoshui Cave record has the highest resolution with an average of 20 a,some portions of which can be as high as 2-5 a.The repeatability tests among stalagmite records within Luoshui Cave and other caves indicate that ?18O data from Luoshui Cave are suitable for the reconstruction of the variability of Asian summer monsoon intensity,and in turn,it provides another valuable geologic record for studying the timing,duration,structure of millennial-scale climate events and the relationship with the orbital-scale backgrounds.Comparing with cave and ice core records,we found that the dates of Dansgaard-Oeschger(DO)events onset for Luoshui Cave agree well with those of Sofular Cave,north Turkey(Fleitmann et al.,2009);and seven of which are in concert with each other within 100 a.In addition,the distinct offset relative to this study is Bittoo Cave record,north India(Kathayat et al.,2016),where the divergence between them ranges from 500?2200 a but had not ocurred on the such as Hulu,Yongxing and Sofular records,suggesting the need of correcting for Bittoo Cave time series.Comparison among Luoshui,Yongxing and Hulu records,they are virtually compatible within 300 and 500 a,respectively,further supporting of that thetimescales of all three speleothem records are reliable.However,the timings of DO22 and 18 events for Hulu Cave probably need correcting,and the former seems olderand the latter is younger by over 1 ka,respectively.Moreover,age disparities between cave and ice core records demonstrate that the chronology for NGRIP on theAICC2012 timescale(North Greenland Ice Core Project members,2004;Veres et al.,2013)seem to bo consistently too young while they donot show increase systematically with depth.We thus support the previous results(Fleitmann et al.,2009)and by extension,AICC2012 timescale,to a certain degree,is of fidelity due to its harmonious matches with Luoshui and recently Yongxing stalagmite chronologies within 400 a.The duration of trasitions into Chinese interstadials(CIS)ranges from severalhundred to thousand years imprinted on Luoshui Cave.These transition phases are overally characterized by slow onsets and gradual ends,similar to those of Yongxing record.More importantly,a linear relationship between the suborbital AM variability and Antarctic time series implies the influence of latent heat released from the South Ocean on north-tropical monsoon,and indicates that the amount of moisture transport from which is proportional to changes in the Atlantic meridional overturning circulation(AMOC)strength.Then through the tight linkages between the Asian monsoon and CH4,we find that early change in Antarctic temperature relative to Greenland abrupt warming were identified in the speleothem record by 0.1?0.2 ka.Analogous to CIS events,the onset variations for six Heirich events(Hs)are also display a slow change mode and character,mirrored to remoted temperature changesin the Antarctic region.Combining U/Th dates with mathmetical analysis,we pin dwon the timings of H1?H6 events are 16.69±0.04,24.48±0.04,30.41±0.03,39.61±0.04,48.31±0.02 and 61.91 ka,respectively.In terms of internal structures of these climateevents,"V"-like(H1,H2 and H4)and "U"-like(H3,H5 and H6)styles are included,implying that the climate response of summer monsoon in the low-latitudes is of dual attributes with south and north.The speleothem record demonstrates an extra-strong forcing from high-north ern latitudes during the late part of the "Mystery interval"(MI).This forcing,along with the southern forcing dugring and after then MI,led to oceanic and atmospheric circulations coupled as the pacemaker of the last ice-age termination.Spectral analysis for the Luoshui record shows that besides predominant period of precession,statistically significant periodicities,such as 2-3 ka,4 ka,6-8 ka,at the 90?95%confidence levels,are also recognized,similar to the results from the Atlantic sediment records and newly analysis for Grenland ice cores(Bond et al.,1992;Bond et al.,1993;Bond and Lotti,1995;Long and Stoy,2013).Among,4 ka and 8 ka are confirmed as primary periods except the previously epidemic 1470 a,consistent with those of Yongxing record.Of interst is that the precession signature is still obvious although the removal of the insolation component from Luoshui record,indicating the significant influence and role of insolation in modulating millennial-scale climate oscillations.Notwithstanding Luoshui ?18O excursions are in parallel with other cave indicators,however,during the interval of 72?64 ka,Luoshui ?18O-based summer monsoon strength emerged a persistently increasing trend.And superposed on this tendcy are high-frequency millennial-centennial scales oscillations.These features result in a very strongcontrast with previous geological records and climate background during marrine isotope stage 4(MIS4),whilst it well matches with that of Yongxing Cave located on the vicinity of Luoshui Cave,and with Santiago Cave record of South America summer monsoon(Mosblech et al.,2012),but in an opposite phase.Whether the "anomalies" result from specific cave systems or reflect virtually monssonal hydroclimate variability and evolution in the low-latitudes need to be further examed by more high-resolution proxies.On the other hand,multi-indices of Luoshui including ?13C,trace element 238U and ?234Uintial manifest that they are not only affected by climatic background and trend,but also are of dependence on the local environment and therefore,it is cautious in the interpretation of its climatic significance.However,they could produce an amplification effect in indicationg major abrupt climate changes or catastrophic environmental events.Especially for ?234 Uinitial index,its values,as a whole,get more positivewhenever weak summer monsoon occurs.The relationship demonstrates that ?234Uinitial can be used as a potentially helpful indicator on surface weathering intension,chemical environment in the soil and karstic plumbing system.Future work should focus on the related mechanism study,and finally make a breakthrough to provide new and reliable monsoonal proxies. |
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