A Study On Palaeoenvironmental Changes Recorded By Organic Geochemistry Proxies Of Loess In The Western Loess Plateau Since The Last-glacial

As the last glaciation is the most recent Ice Age,it is very important to understand the last glacial climat change to predict future climat change.Due to the special characteristics of the climatic and environmental change,the research on the Marine Isotope Stage 3(MIS3) attracts numerous researchers' attention.In China,the distinct climate change in MIS3 is demonstrated by records from Guliya ice core in Qinghai-Tibetan Plateau and a number of lake sediments in west China.Even loess deposition plays an important role in reconstructing the past climate change of the whole globe,the research on palaeoclimatic and palaeoenvironmental change during MIS3 in the western Loess Plateau is still relatively undeveloped.The totalδ¹³Corgs results of loess from east and west Loess Plateau are shown very different during MIS3,since there is still a hot debate about the controling factors of the totalδ¹³Corgs variation in loess deposition through the last glacial period.In this study,we integrated the totalδ¹³Corgs and other multiple proxy records from loess deposits to reconstruct the last glacial climatic and environmental change during MIS3 in the western Loess Plateau.The spatial and temporal variation of the totalδ¹³Corgs of loess is established,together with a exploration of the corresponding controlling factors,under a close comparison of multiple profile records since the last glaciation. Thus we get a good understanding of the evolutionary history of vegetation types in the western Loess Plateau since the last glaciation.1,The spatial-temporal variation of the totalδ¹³Corgs in Loess Plateau since the last glaciation displays the following features:(1) the totalδ¹³Corgs is consistent in east and west Loess Plateau on the glacial-interglacial scale;(2) In glacial period the totalδ¹³Corgs become lighter,but heavier in interglacial period;(3) During the last glaciation the totalδ¹³Corgs changes in Loess Plateau are different from east to west. The totalδ¹³Corgs changes of the last glaciation in the eastern Loess Plateau have the same characters as on the glacial-interglacial scale,which is relatively heavier in MIS3 but lighter in MIS2 and MIS4.However,the results from the three sections in the western Loess Plateau show that the totalδ¹³Corgs in loess is relatively lighter in MIS3 but heavier in MIS2 and MIS4.2,The dominant controlling factors of the totalδ¹³Corgs changes in loess since the last glaciation are explored as follows:(1) From glaciation to interglaciation the changes of totalδ¹³Corgs is consistent between west and east Loess plateau due to relative abundance changes of C₃ and C₄ plants.(2) In the last glaciation,the controlling factors of the totalδ¹³Corgs from the eastern Loess Plateau show the same pattern with the glacial-interglacial scale,which are relative abundance changes of C₃ and C₄ plants.But the controlling factors of totalδ¹³Corgs changes in the western Loess Plateau are something else.By comparing the totalδ¹³Corgs from the three sections in the western Loess Plateau with the records of temperature,precipitation and the concerntration of atmospheric CO₂ during the last glaciation,we found that, as the temperature of last glaciation was too low for C₄ plants to grow,the vegetation was represented by pure C₃ plants.Then the totalδ¹³Corgs change is caused by the pure C₃ vegetation in response to precipitation variation.Therefore,the totalδ¹³Corgs change in C₃ plants gradually becomes lighter under the increasing precipitation.So we conclude that during the last galciation the controling factors of the totalδ¹³Corgs are different between the western and eastern Loess Plateau.Moreover,the temperature may influence the change of the totalδ¹³Corgs,although it is not the main controlling factor.3,There are three sub-stages in MIS3 in the western Loess Plateau according to the records of the totalδ¹³Corgs,grain size,and magnetic susceptibility from Caoxian, Dadiwan and Yuanbao sections.But the precipitation decreases in the later period compared to the earlier period during MIS3,Which is coincident with SPECMAP,but different from Guliya ice core records.High temperature and high precipitation in late MIS3 is reflected in the records of lake sediments from Tibetan Plateau and West China,and records of Guliya ice core.respectively,but not showed distinctively in loess sections in the western Loess Plateau.4,The research of modern plants and organic compounds lipid in surface soil, namely,the distribution model of n-alkanes in the western Loess Plateau showed:10 wood samples are the heavy-molecular-weight(HMW) homologues of the n-alkanes distribution with the main peak of C₂₇ or C₂₉,while in 3 kinds of herb n-alkanes,the HMW homologues distributions of C₃₁ or C₃₃ are the main peaks,which show a completely different distribution feature with woody plants.For the modern study of surface soil,surface soil n-alkanes distribution is directly related to the vegetation cover.Study shows that there is a direct relationship between HMW homologues distributions of the surface of the vegetation.The wood vegetation except for the highly mixed forest by herbaceous shrub has a distribution of n-alkanes C₂₇ and C₃₁ as the main peak.The highly mixed forest by herbaceous shrub has a distribution of n-alkanes C₂₇ or C₂₉ as the main peak,It is indicated that mixed herbs in the forest leads to the difference in this distribution,because of its high volume of n-alkanes. Grassland surface soil shows the same characteristics.When there are fallen leaves around grassland,the input of deciduous plants would be greater than the herbal input. The n-alkanes from grass show different main peak.As to the surface soil of large grass land,the n-alkanes distribution reflects the characteristic of herbaceous plants with the main peak of C₃₁.This study for conifer shows the characteristics of wood plants with the main peak of C₂₉ or C₂₇.5,The results of lipid organic compounds n-alkanes from three sections in the western Loess Plateau during MIS3 shows the heavy-molecular-weight(HMW) homologues of the n-alkanes distribution pattern:except for two samples from Caoxian section,C₂₉ is the main peak,while the other samples showing the performance of the C₃₁ as the main peak,and the C₂₇ content of the lowest(C₂₇＜C₂₉＜C₃₁),which presents the same distribution as a pure herbal vegetation types of modern surface soil samples but the different distribution from the wood plants and modern wood surface soil distribution of n-alkanes.It is preliminarily proposed that the vegetation of western part of Loess Plateau in MIS3 is dominated in grass but not forest.In combination with the analysis of totalδ¹³Corgs,all results suggest that C₃ grass is dominant in the vegetation of the western Loess Plateau during MIS3.6.Two Holocene sections,a swamp-alluvial loess section and a typical loess-paleosol section in Qin'an area,are used to reconstruct the history of past regional vegetation changes.It is found that the heavy-molecular-weight(HMW) homologues of the n-alkanes in all samples exhibit a pronounced odd-over-even predominance,maximizing at C₃₁ and the abundance of the nC₂₇-alkanes is the lowest, nC₂₇,nC₂₉,and nC₃₁ in turns(that is C₂₇＜C₂₉＜C₃₁),which is a typical grass n-alkanes model.The results are the same as those of modern surface soil samples under grass cover but different from forest cover and modern pine leaves,which yield preferentially nC₂₉-alkanes peak.Therefore,it is preliminary inferred that the area is dominated by grass cover but not forest cover during the whole Holocene.