The Physical And Chemical Characteristics Of The Late Quaternary Loess In Two Climatic Zones In China And Its Environmental Significance

As one of the important indicators for the Paleoclimate and Paleoenvironment,the improved research on loess sequence could be an effective approach to reconstruct the history of global palaeoclimate changes.So it has very important scientific values for the study of loess in typical climate region S hengshan island locates in East China sea,which climate has been mainly controlled by East Asian monsoon.Tianshan mountain locates in middle Asia arid areas,which climate has been mainly controlled by west wind zone system.In this work the loess sections on Shengshan island and the north slope of Tianshan mountain were studied.The physical and chemical characteristics of the loess profiles,the comparison of these features and the regional responses to the global climate change in the last Pleistocene were investigated.All these results will improve the studies of palaeoclimate changes in quaternary period.Based on the time scale built by Optical Stimulated Luminescence dating,the component of particle size,magnetic properties,geochemical properties of elements and carbon isotope content in two loess sections were analyzed.Furthermore,combined with regional vegetation coverage sensitivity simulation based on the Intermediate complexity climate model(UVic_ECSM),paleoclimatic changes induced by land surface change in the last glacial epoch in two study areas were discussed.The main results are listed as follows:(1)Results of the optical stimulated luminescence dating shown,the loesses in Shengshan and northern Tianshan were deposited in the last glacial epoch(2)Results of grain size analysis of these two sections shown:the main component of these two losses sections were silty sand.The content of clay and sand were comparatively less.Transporting modes of particles in these two sections were mainly suspension movement by wind.In S hengshan island section of western China,components of grain size above 32?m can reflect the strength of winter monsoon.In northern Tianshan section of western China,components of grain size above 25?m can reflect the strength of winter monsoon.(3)The results of environmental magnetism analysis indicate that the magnetization and saturation isothermal remanent magnetization(SIRM)of Shengshan island and northern of the Tianshan were relatively high,and these two parameters significant positive correlation(R=0.95,p<0.01).Compositions of magnetic mineral analyzed shown that the main mineral in Shengshan Island of eastern China was ferrimagnetic minerals.Meanwhile,the main mineral in northern of Tianshan section was ferrimagnetic minerals and a certain amount of incomplete antiferromagnetic minerals.Particle characteristics of magnetic mineral analysis shown,the main magnetic mineral particle in Shengshou Island section of eastern China was single domain magnetic mineral particles,and the superparamagnetism mineral particles was relatively high.The main magnetic mineral particle in northern Tianshan section was multi-domain magnetic mineral particles,and the magnetic mineral particles were comparatively less.These results indicate that the magnetization of Shengshan island sections was significant affected by the fine component produced during pedogenesis.In northern Tianshan section,the coarse particle fraction was increased with the high magnetization.It indicated that the main reason for enhance of magnetization in arid loess area was controlled by original source.(4)The macro and micro geochemical element characteristics indicate that the loess section of the eastern shengshan island experiences a strong weathering effect and the chemical alteration index(CIA)is 85.1.The happen of obvious eluviation and migration of Ca,Mn,Na,P and other elements reflect the cold and warm fluctuations during the formation of the section.The analysis of the chemical elements of the loess section in the north west of Tianshan mountain which experiences the primary weathering effect in the process of forming shows that the chemical alteration index(CIA)of the section is 54.66 and Na,Ca and Mn elements leaching and migrating significantly.During the migration section forming process,relatively weak hydrothermal condition,whinch shows that the difference of post-depositional environment is the important factor for the differences of element distributions.The distribution curve of rare earth elements shows that the loess sections have similar sediment sources in Shengshan island and north piedmont of Tianshan.(5)The results of organic carbon isotope(?13C)studies show that the characteristics of the organic carbon isotope changes in two sections were obviously different ?13C in Shengshan Island section ranged between-21.63‰ and-27.56‰,with an average value of-24.86‰.Quantitative calculation results show that the regional depositional environment had been dominated by C3 type vegetation development since the lately last glacial period.The range of ?13C in north Tianshan section was-15.6‰ to-27‰ and its average value was-22.03‰.The quantitative calculation results show that since the late last glacial period,both the C3 and C4 type plants had developed in the depositional environment in the northern Tianshan.In Shengshan Island section,lighter value of organic carbon isotopes indicated the climate was rich in precipitation,and heavier value of organic carbon isotopes indicated that precipitation was less.In northern Tianshan loess profile,lighter value of organic carbon isotopes indicated dry and cold condition and heavier value of organic carbon isotopes indicated humidity and warm condition.The results above are consistent with the characteristics of paleoclimate change revealed by element geochemical indicators.(6)Through the comparative analysis of index characteristics such as geochemical elements and organic carbon isotopes among Shengshan Island loess section,Huining section and Xiayuanbao section,it seems that the climate change reflected by Shengshan Island loess section during the last glacial period is relatively consistent with the overall climate change characteristic of the oxygen isotope of deep-sea sediments at MIS3 stage.The general variation characteristics,warm and wet-cold and dry-warm and wet,show that the formation and evolution of Shengshan Island loess section which located in the East China monsoon region and typical loess of Western China have consistency at the scale of glacial period.In addition,at roughly the same time scale from the late last glacial period to early Holocene,the sedimentary environment and climate change of Shengshan Island loess and north Tianshan loess section experienced presented difference of asynchrony.It is relatively consistent with the existing researches which show that the climate change of Northwest area under the control of westerly system and Southeast area under the control of East Asian monsoon system appears antiphase change regulation in the Quaternary geological history.(7)The results of the intermediate complexity climate model UVic_ECSM show that the vegetation covers in the eastern Yangtze River Delta and its epitaxial area changes,by simulating the sensitivity of the effects of climate coupling on vegetation cover in eastern China.The change shows the reduction of regional surface vegetation coverage,the enhancement of regional shortwave radiation and the drop in temperature.The cooling of Siberia may result in the strengthening of winter monsoon,the increases of dust in Yangze reviver,meanwhile,the temperature deduced,which indicates that the Yangtze Delta region belongs to the sensitive region to the regional environment change and monsoon change.These simulation results are consistent with the reconstruction results from carbon isotope and the median particle size change in two profile above.This shows the efficiency of the intermediate complexity climate model to simulate the response of the climate to the land cover change.