Holocene Environment Evolution In The Alashan Desert And Its Adjacent Area Inferred From Calcareous Root Tubes

Desert is a key area of global change research because of its high sensitivity to climate change and important feedback function.The reconstruction of desert environment will reveal the difference of regional environment response to global change and provide scientific and technological support for us to better adapt to future climate change.However,environmental reconstruction is difficult in the deserts.The paleoenvironment reconstruction materials in the deserts are relatively scarce due to the strong wind erosion.Especially in the desert hinterlands,the paleoenvironment reconstruction work is very weak.Excavating new materials for environmental reconstruction in the deserts and reconstructing the history of environmental change in desert hinterlands will further promote the study of desert environmental change process and its response mechanism to global change.The Alashan Desert,located in the mid-latitude regions of China,is sensitive to climate change and is an ideal region for studying climate change and environmental evolution at different time scales,because it is located at the junction of Asian summer monsoon(ASM)and Westerlies.Calcareous root tubes(CRTs)are secondary carbonate crusts formed by the roots of terrestrial plants and widely occur in the hinterlands of Alashan Desert.The formation of CRTs is influenced not only by soil water content and evaporation,but also by the plant roots and microorganisms.Thus,the regional paleoclimate and paleovegetation information could be recorded in CRTs.In addition,compared with other paleoenvironmental reconstruction materials in this area,CRTs have the advantages of relatively high reliability of dating results,wide distribution in the desert hinterlands,and clear environmental indication.However,at present,only the chronology,formation mechanism and mineralogy of CRTs in Alashan Desert have been studied,the paleoenvironmental information has not been systematically extracted for palaeoenvironmental research in this area.Therefore,it remains to be further studied whether the paleoenvironmental signals can be extracted from CRTs in the hinterlands of Alashan Desert and the paleoenvironmental change history can be reconstructed accordingly,so as to further reveal the characteristics and possible driving mechanism of the environmental change during the characteristic period in the hinterlands of Alashan Desert.In this study,chemical elements,lipid molecular proxies and carbon and oxygen isotopes were analyzed and phytolith assemblages were extracted from CRTs to reconstruct the paleoclimate and paleoenvironment in this area during the Holocene.Accordingly,the environmental change pattern and its possible driving mechanism were discussed in the Asian monsoon margin area during the Holocene.The achievements of this thesis are as follows:(1)The chemical composition and content of CRTs during the Holocene in the Alashan Desert were mainly controlled by the content of elements in soil parent materials and regional climate change.Ca,Mg and Sr,which were obviously migrated and enriched,were affected by climate factors.According to Rayleigh fractionation model,the element ratios Mg/Ca and Sr/Ca could be used to reconstruct the effective moisture variation in this area.Moreover,based on the different distribution coefficient of Mg and Sr in calcite,the Mg/Sr ratio indicated the temperature change.That is,the decrease of Mg/Ca and Sr/Ca ratios in CRTs from middle Holocene to late Holocene indicated the decrease of effective moisture in this period.Similarly,the significantly lower Mg/Sr ratio in the middle Holocene than in the late Holocene indicated that the temperature decreased during this period.(2)N-alkanes in CRTs were derived from the mixture of terrestrial higher plant roots and rhizosphere microorganisms.N-alkanes from higher plants could effectively distinguish woody vegetation(shrub)from herbaceous vegetation.In this study,the main peak carbon of long-chain n-alkanes(C>25)was mainly C₂₇,indicating that shrub vegetation was dominant in the Alashan Desert during the Holocene,and the increase of(C₂₇+C₂₉)/(C₃₁+C₃₃)ratio indicated the gradual increase of shrub vegetation during 8.0-2.0 cal kyr BP.Due to the lack of spatial coverage and controversial representativeness of palynological reconstruction of paleovegetation in this area,the data of n-alkanes in CRTs could further improve the credibility of paleovegetation reconstruction in this area.In addition,the change of short chain to long chain n-alkanes ratio(L/H)mainly reflected the intensity of microbial activity in CRTs.Therefore,the decrease in the L/H ratio of n-alkanes during 8.0-2.0 cal kyr BP indicated a gradual decrease in effective moisture during this period.(3)In this study area,almost no primary carbonate was incorporated into the CRTs,and the?¹³C and?¹⁸O values of the inner belt of the CRTs were not affected by recrystallization.Therefore,the carbonate inside the CRTs could be used for paleoenvironmental reconstruction in this area.The carbon source of the CRTs was a mixture of CO₂ produced by root respiration of C₃ plants with atmospheric CO₂.Furthermore,?¹³C values indicated that C₃ vegetation was dominant in this area during the middle and late Holocene.The oxygen isotope of CRTs reflected the oxygen isotope composition of summer precipitation.Due to be mainly influenced by the"amount effect",the oxygen isotope of CRTs could be used to reconstruct the precipitation change in this area.The?¹⁸O value of CRTs was negative in the middle Holocene,indicating high precipitation.The positive?¹⁸O values in the late Holocene indicated a decrease in precipitation.(4)Phytolith within CRTs from this study area mainly came from local surface vegetation during the formation of CRTs,which could be used to quantitatively reconstruct the Holocene millennial-scale precipitation changes in this area.WA-PLS model was used to quantitatively reconstruct the millennial-scale annual precipitation of Tengger Desert as follows:in the 10.0-7.0 cal kyr BP stage,the annual average precipitation was 138±16-149±18 mm;In the 7.0-5.0 cal kyr BP stage,the average annual precipitation was 179±26-192±26 mm;In the 5.0-1.0 cal kyr BP stage,the average annual precipitation was 129±20-161±13 mm.The above quantitative precipitation reconstruction results reflect the Holocene climate change pattern in the monsoon margin of northern China.That is,the early Holocene precipitation was less,the middle Holocene precipitation gradually increased and reached the maximum(about 30%more than the modern precipitation),and the late Holocene precipitation subsequently decreased.In the early and middle Holocene,This precipitation change pattern in the Asian monsoon margin area described an“out-of-phase”relationship with that observed in the typical Asian monsoon area.(5)The Holocene millennial-scale precipitation in the Alashan Desert was mainly affected by the position change of the north boundary of the ASM.During the early Holocene,the northern boundary of ASM was southward due to the large area of high latitude ice sheet in the northern hemisphere,which resulted in less precipitation in the monsoon margin area.In the middle Holocene,with the melting of ice sheet and the rise of sea levels,the northern boundary of ASM gradually moved northward,resulting in maximum precipitation in the monsoon margin area.According to the spatial variation of Holocene precipitation in the region,the northern boundary of ASM moved at least 250 km northwest during the middle Holocene,reaching the southeastern Badain Jaran Desert.In the late Holocene,with the decrease of summer solar radiation in the northern hemisphere,the intensity of monsoon weakened,the northern boundary of ASM moved southward,and precipitation decreased.This research provided a new reconstruction materials and the research direction of palaeoclimate and palaeoenvironment in desert area.Moreover,it provided a new research perspective and method for multi-source and quantitative reconstruction of palaeoclimate in desert hinterlands,revealed regional differences in response to global change in arid desert areas,and deepened the understanding of climate driving mechanism at high and low latitudes.