| The objective of paleoclimate study is making up for the deficiency of modern observation of environment and climate change,so as to obtain the reason and mechanism of global climate evolution which is beneficial for predicting the future climate change.However the study of older interstadial climate is not sufficient although the Holocene and the last interstadial climate study have made great progress.Otherwise,it is necessary to strengthen the study of older interstadial climate change.Based on 10 precise Th230 dates and 1320 stable oxygen isotope ratios(?18O),we present a high resolution record of Asian summer monsoon from 445.1 to 1 72.0 kyr BP,interrupted by two-80 kyr hiatus.Thus,the main deposited periods of this stalagmite are from 445.1 to 422.5 kyr BP,332.4to 283.7 kyr BP and 198.4 to 172.0 kyr BP respectively,and the periods of 198.4 to 172.0 kyr BP and 332.4to 283.7 kyr BP are roughly corresponding to the penultimate interglacial and antepenult interglacial.The time series of the ?18O is agree well with the summer insolation at 65°N,supporting the idea that tropical/intertropical monsoons respond dominantly and directly to changes in North Hemisphere summer insolation on orbital timescales.The cycles are punctuated by 8 millennial scale strong summer monsoon events and 9 events during penultimate interglacial and antepenult interglacial respectively.Meanwhile,there are many weak monsoon events corresponding to the ice rafted debris(IRD)of North Atlantic during those two periods.Giving similarities to the North Atlantic IRD,the proximate driving force producing millennial changes in the stalagmite record is postulated to be atmospheric circulation changes associated with changes in the Northern Atlantic region.Among which,an abrupt increasing of ?18O value denoting the significant weak ASM,the amplitude beyond 2‰ and duration about 1.5 kyr,happened in the middle of MIS9.0.This weak ASM may be caused by the coefficient of lower insolation of North Hemisphere and IRD event of Northern Atlantic.Moreover,the weak ASM happened in the transition between interstadial and stadial such as MIS9.2/9.1?MIS9.1/9.0?MIS9.0/8.5?MIS7.2/7.1 and MIS7.1/6.5 which agreed well with the unstable insolation.However none weak ASM events were happened during the transition between MIS9.3 and MIS9.2,suggesting that the climate boundary conditions,such as greenhouse gas,may play important role in forcing climate change.Although the magnitude of insolation is significantly different between the marine isotope stage MIS 9.3 and MIS9.1,the strength of the ASM are very similar,indicating the "dole effect"may influence the evolution of ASM in the late Pleistocene.The correlation coefficient was prominently increasing when detrend the depth sequence of?18O and ?13C,implying well connection between them.Time series of carbon isotope from this and another Holocene stalagmite were reanalyzed via the ensemble empirical mode decomposition(EEMD),so as to clarify the difference and connection between interglacials and the relationship between ?18O and ?13C on different time scales.The decomposed results of penultimate interglacial,intrinsic mode function,indicated three time scale changes of ?18O,e.g.centennial scale(al-a3),millennial scale(a4 and a5)and orbital scale(a6)and two time scale changes of ?13C,e.g.centennial scale(bl-b3)and millennial scale(b4-b6).The well correlation between a and b on the same periodicity means strong coupling between the elevation of ASM and the production of vegetation during this period.The decomposed results of antepenult interglacial,indicated three scale changes of ?18O,e.g.centennial scale(c1 and c2),millennial scale(c3-c5)and orbital scale(c6)and three time scales of ?13C,e.g.centennial scale(d1 and d2),millennial scale(d3-d5)and(d6 and d7).The prominent difference between the two periods is the lower correlation coefficient on the same periodicity.We suggest that the significantly increased concentration of CO2 during the antepenult interglacial may be the main reason.The millennial events may be regular in late Pleistocene,because the 1.6 kyr cycle were identified during the two interglacials which is likely to the 1.5 kyr cycle of DO events of the last glacial.Time series of oxygen-carbon isotopes from Dongge cave were reanalyzed via the same method(EEMD).The decomposed results of ?18O indicated three main time scale changes,inter-decadal scale(el-e3),centennial scale(e4-e6)and millennial scale(e7 and e8).The decomposed results of ?13C indicated three time scale changes as well,which is including inter-decadal scale(f1-f3),centennial scale(f4-f6)and millennial scale(f7-f9).At centennial to decadal scales,both isotopic records are closely coupled,among which the periodicities of 254 yr and 116 yr agree well with the atmospheric ?14C(proxy for solar activity),denoting both soil CO2 and the hydrologic circulation of low latitudes were controlled by solar activity.This relationship is further confirmed by the analysis of cross wavelet spectrum between the stalagmite ?13C values and the detrended atmospheric ?14C.The contribution of the imfc6 component is the highest,which presents a quasi-periodicity variability of 526 yr,and is similar to the 550 yr periodicity of the North Atlantic Deep Water formation,denoting a teleconnection between the carbon isotope variation and the environment change of the Northern high latitude region.The trend of ?13C variations is significantly different from ?18O record,suggesting the production of soil CO2 is not influenced directly by the change of solar irradiation.The comparison among the decomposed results at the same timescales of the last three interglacials,we can draw conclusions that the Asian summer monsoon was controlled by the procession on the orbital,and it is significantly different between the two older interglacials and the Holocene,which means that the increased greenhouse gas concentration by human activity was significantly influence the climate change of the mid-late Holocene. |
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