Spectral Analysis On The Climatic Periods Indicated By Clay Minerals

In order to analyse the climate change and mechanism on the clay minerals periods of the core of Kuhai Lake,Qingzang-Tibet Plateau during 15 cal ka BP since the late glacial,multi-signals of smectite/(illite+chlorite),kaolinite/smectite,chlorite/illite,illite crystallinity,illite chemical index,and total contents of clay mineral were studied with TMT and wavelet analysises,which focuse on analyzing the evolution of significant oscillation signals conveyed and concealed in clay curves.It is difficult to divide and distingguishish the clay changing phases such as total content of clay minerals,chemical index of illite,crystallinity by the general visual method in high resolution.Through the two spectral analysises,accurate cycles could be detected and the inflection point of the cycle curves have been found.These signal reconstruction of significant periods had improved the accuracy of the cycle divisions and avoided the ininflences of different person's thinkings when dividing climatic phases.The two researches methods show that there are obvious and signifivcant major climate change cycles:443a,487a,579a,579-634a,7514a,etc.in the area.By signal reconstruction of significant periods and the energy spectrum of TMT and wavelet,three climatic phases have been divided in the core of Kuhai:the first stage is 14.9 cal ka BP-6.1 cal ka BP,which is warm-humid;the second one is 6.1 cal ka BP-5.4 cal ka BP,which shows that the stage is a transition form warm-humid climate environment to cold-dry climate environment;the third one is 5.4-0 cal ka BP,the climate characteristics are cold and wet.The two researches methods also illustrate that the periods indicated by clay minerals are consistent with those of the other indicators in the core.This study demonstrates that the Qinghai-Tibet Plateau is affected by the combined effects of westerly and monsoon.Among a series of carbon isotope cycles such as 952a,557a,475a,312a,211a and 77a of the southwest monsoon,the two cycles of 557a and 475a are similar to the two significant periods of 579a and 487a of the clay mineral sequences of Kuhai Lake,which indicate there is a certain correlation between monsoon activity and climate change in the area.Comparing the cyclical changes of the Holocene climate fluctuation reflected by the other indexes of the northwest lakes with the core,their cycles are similar very well,meaning that this area was influenced by westly.Besides,there are a series of cycles 435a,544a,610a,692a and 1219a in the ?18O cycles of the Atlantic Ocean,and the significant cycles of 610a and 435a are similar to 630a and 1219a.Their contribution rates of variance were 18.14%and 7.2%,respectively.It was found that the contribution rate of the major cycle variance in millennium was less during the climate evolution,indicating that the influence of the millennium cycle was not much,which indicated that there is a certain climatic correlation between the plateau and the North Atlantic.Finally,the 241a and 561a periods in the sunspot number sequence are in good agreement with the 443a and 579a climate cycles of the clay minerals of the core,indicating that the climate change in the Qinghai-Tibet Plateau was affected by sunspot activity.