末次盛冰期和全新世大暖期呼伦贝尔沙地的环境变化

呼伦贝尔沙地保存的风沙层-古土壤沉积序列,是认识过去环境变化的重要地质记录。沙地沉积具有速率快、沉积与侵蚀并存的特点,因此,通过大量独立的光释光年代控制对不同的剖面进行拼接,可以重建沙地在千年尺度上干湿变化过程以及特征时期的边界。本文获得了呼伦贝尔沙地8个沉积剖面的47个光释光年代和近200个样品的环境代用指标数据,结合已发表文献资料,揭示了16ka以来呼伦贝尔沙地千年尺度的干湿变化,并估算了末次盛冰期(LGM)和全新世大暖期(HO)沙地流沙的边界。初步认为,LGM期间,呼伦贝尔沙地相对于现代沙带边界,最远向北移动了约60km,向东移动了约50km,并且南北沙带之间的区域全部沙化; LGM期间沙漠化面积达到22337km2,相对现代沙漠化面积扩大了约2.7倍。而在HO期间,整个沙地被植被所固定,发育砂质古土壤层,只在局部区域存在短暂的风沙活动。

ENVIRONMENTAL CHANGES OF HULUN BUIR DUNEFIELD IN NORTHEASTERN CHINA DURING THE LAST GLACIAL MAXIMUM AND HOLOCENE OPTIMUM

Hulun Buir dunefield is located at the northwest border of East Asian monsoon area in Northeastern China,where the environment is sensitive response to advance and retreat of the monsoon precipitation belt during Late Quaternary.Modern active dune area is situated in the northern and southern part of the Hailar Basin; the other areas are grassland cover.The dominant aeolian landform types are the stabilized and semi-stabilized sand dunes,blowouts,sand sheets and shrub-coppice dunes.The prairie in Hulun Buir region is one of the most important stockbreeding bases in China,but the vegetation degeneration and desertification has become the main eco-environmental problem.Recent researches have showed that the modern aeolian landform in Hulun Buir dunefield formed under joint action of natural and human impacts.Therefore,in order to better understand the natural background of desertification and to guide the production activities of local people,to reconstruct the aeolian environment change during the characteristic paleoclimate period of Last Glacial Maximum(LGM)and Holocene Optimum(HO)has important scientific and social values.The deposition sequences of aeolian sand beds and sandy plaeosols in Hunlun Buir dunefield are important geological archive of past environmental changes.Aeolian sand beds often lack the organic matter indicating active sand transport with limited vegetation cover.The paleosols represent wetter conditions with much less aeolian activity.With the development of optically stimulated luminescence dating in recent years,it has provided the possibility of dating the aeolian sand beds directly,which are difficult to be dated by radiocarbon dating because of their low organic carbon content.We had explored the dunefield and took the samples from eight typical sand-paleosol sections.In this study,47 OSL ages were obtained and nearly 200 bulk samples were measured for grain-size and magnetic susceptibility as the environmental proxies.The equivalent doses(De)were determined by the single aliquot regeneration(SAR)protocol using the purified quartz grains(90~150μm).Combined with previously published data,it reveals wet-dry variations in Hulun Buir dunefield since 16ka and the desertification area during the LGM and HO.Our preliminary conclusions are that the Hulun Buir dunefield was almost completely desertificated during the LGM,and the boundary of desertificated area extended about 60km northward and 50km eastward compared to the modern boundaries,respectively; the total desertificated area reached at about 22337km2 and it is 3.7 times than the modern desertificated area.The mobile sand dunes were nearly completely stabilized during the HO,there was only local short-lived aeolian activity existed in that time.However,these preliminary conclusions need be tested by more OSL ages from the buried sand beds and palaeosols in this sand field where the desertification is still developing.