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Paleo Climate Evolution Since the Late Early Pleistocene: an Evidence from a Loess Record, Garz(?), West Sichuan Plateau

Dongyan Liu  
【摘要】: The formation and evolution of Asian Monsoon is an important subject in the study of global changes.Loess is a research material characteristic of China.So far, while great achievements have been made in research on Asian monsoon utilizing loess in north China which locates in the south-east monsoon region as main research material, research on loess in south-west monsoon region is relatively weak. The westerly plays an important role in the transportation of loess and has an significant influence on climatic recording. Compared with Asian monsoon, research on the changes of the westerly is to be furthered. Former quantitative studies on paleoclimate using Chinese loess were mostly concentrated on the climate periodicity, while that on the abrupt changes needs to strengthen.This thesis studies the changes of paleoclimate since the late early Pleistocene,using the loess-paleosol sequences in Garze A section on West Sichuan Plateau as research materials.Previous research shows that the widely distributed loess on West Sichuan Plateau came into being since the late early Pleistocene; Southwest monsoon is the main transporter of water vapor,which can be studied using the magnetic susceptibility as its proxy;the westerly and the near-surface wind are the major driving forces for the transportation of loess in this region,which can be studied using the grain size as its proxy. This thesis studies the mathematical partitioned way of the bulk sample grain size frequency as well as its implementation; introduces quantitative periodical analysis and abrupt change analysis of paleoclimate proxies for their magnetic susceptibility and the grain size and discusses the mechanism of paleoclimatic evolutionary in this region. Both of stage feature of the magnetic susceptibility sequence in tectonic time scale and the feature of 100kyr filter curve suggest that there exists mid-Pleistocene Transition (MPT)in the evolutionary process of southwest monsoon. This transition was consistent in time with the dramatic uplift on the southeast rim of Qinghai-Tibet Plateau,which implies the contribution of the uplift of Qinghai-Tibet Plateau to the climatic transition in the mid-Pleistocene. The orbital period and precession beat periods detected from power spectrum analysis of magnetic susceptibility time sequence suggest that the evolution of southwest monsoon is influenced by astronomical factors, Indian monsoon system has nonlinear responds to solar insolation and southwest monsoon has tropical features. Experimental mode decomposing(EMD) results of magnetic susceptibility time sequence shows that there exists the 100kyr cycle whose waveform feature is consistent with the theoretical value of the fluctuation period of eccentricity (100kyr), suggesting that the main driving force of the Indian monsoon system is not global ice volume but solar insolation at low latitudes; there also exists quasi-periodic 200kyr cycle whose formation needs further study; a 400kyr cycle is detected and it probably corresponds to the long period of eccentricity whose theoretical value is 400kyr.In all,the southwest monsoon is probably driven by the orbital forcing but not the global ice volume. This thesis introduces the concrete way of data fitting of sample particle size frequency by superposing three lognormal distributions into a mixed distribution and use it to partition the bulk sample into three grained components mathematically.The partitioned component has the significance of single genesis. EMD is used for analyzing time-frequency characteristic of the fine grained component,which may indicate upper westerly. The results indicate that, the evolution of upper westerly is influenced by astronomical factors as well.The quasi-periodic 100kyr cycle has changed in sawtooth shape since 0.66MaBP,which suggests that evolution of upper westerly has close relationship with the change of global ice volum (quasi-periodic 100kyr cycle); The quasi-periodic 200kyr cycle and 400kyr cycle are also detected;and the 600-700kyr period is detected for the first time,which probably corresponds to the quasi-700kyr eccentricity long period. On a large time scale, the evolution of upper westerly responds remarkably to the uplift height of Qinghai-Tibet Plateau probably. The Qinghai-Tibet Plateau reached the freezing height above 3000 to 3500m, at about 0.66MaBP. Results of the abrupt change by moving-t technic(MTT) of the time sequence of the percentage of40μm bulk grain size component suggests that the abrupt change points of near-surface wind appeared at about 1.12,1.06,1.0,0.95,0.90,0.85,0.80, 0.73,0.64,0.59,0.54 (0.49),0.46,0.39,0.29,0.25,0.20,0.14(0.11),0.08,0.05,0.03 MaBP, at intervals of 100kyr and 50kyr. This abrupt change points sequence correspond well with ages from other geological records of tectonic uplift on the southeastern Qinghai-Tibet Plateau. The quasi-periodic cycle of 100kyr should be attributed to the tectonic-climatic circulation which results from the period of the earth's eccentricity. The time for abrupt change of paleoclimate is well correlated to the formation time of the Yangtze River and the Yellow River terraces as well as the tectonic activity of Qinghai-Tibet Plateau. The forming age of terraces was controlled by the climatic instadibility.River terraces with quasi-periodic cycle of 100kyr and 50kyr is the coupling product of the climatic change and tectonic uplift, so it should belong to the category of tectonic-climatic circulation.


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