柴达木盆地SG-1孔1．0Ma以来碳酸盐同位素记录的亚洲内陆干旱化及成因

ARIDIFICATION OF THE ASIAN INLAND SINCE 1.OMa:EVIDENCES FROM CARBONATE ISOTOPE RECORDS OF DEEP CORE FROM THE QAIDAM BASIN

亚洲内陆干旱区是连接赤道和中高纬地区的过渡地带,也是西风气候和季风气候的相互作用区,其形成演化与青藏高原的隆起和全球变化等因素密切相关。因此,揭示其干旱化过程和趋势具有重大的理论和现实意义。文章对来自柴达木盆地察汗斯拉图干盐湖的深钻SG-1孔沉积物进行了初步的碳酸盐碳氧同位素测试与分析,结果清晰地指示了柴达木盆地自1.0Ma以来的持续干旱化及约0.6Ma以来的加速干旱化过程,我们认为1.0Ma以来的持续干旱化可能是全球气候变化和构造活动共同作用的结果,而约0.6Ma以来的加速干旱化可能是昆黄运动所导致的高原北部强烈隆升和区域环流系统变化或强化所造成的。

As the largest temperate arid region,the vast Asian arid inland has attracted wide interests due to its great impacts on global climatic change.However,its evolution history and its relation with the westerlies,Tibet uplifts and global change are still unknown.Although many related studies have been carried out,most of the studies are mainly based on the eolian records from the surrounding areas,and only few records are directly from this arid region. The Qaidam basin,located in the eastern part of this vast arid region,develops continuous and well-exposed Cenozoic sediments,providing a unique opportunity to study aridity of the Asian inland.We obtained a 938.5m long lake sediment core（SG-1）（38°24＇35.3＂N,92°30＇32.6＂E;2700m a.s.l.）with an average recovery rate of 95% from the saline playa in the depocenter of the Chahansilatu sub-basin between the Eboliang anticline and the Jianshan anticline in the western Qaidam basin.A combined detailed paleomagnetic dating, OSL and U series dating of the core determined its age at about 2.77Ma to 0.10Ma. The core is mainly comprised of evaporate-bearing fine mudstone-siltstone sequence.In the lower 390.0~938.5m part of the core,the sediments consist mainly of alternation of dark-gray or bluish-gray generally laminated mudstone and siltstone and gray calcareous mudstone and marl/limestone.In the upper 0~390m part of the core,the lithology is characterized by cycles of thick gray-white salt and grayish black massive-laminated mudstone-siltstone mostly containing gypsum crystals or thin gypsum layers.This study mainly carries out a more detailed analysis of carbon and oxygen isotopes for the upper 0~350m part of the core with 0.1~1.0Ma.Samples with high carbonate contents （ 〉30wt% ） were systematically collected at an average interval of ca.3m and 135 samples were selected.We collected 12 rock samples（including limestone and marble）from exposed strata since Precambrian in the surrounding Kunlun,Qilian and Altun major river basins to determine the carbon and oxygen isotopes in lakes debris and further to learn whether the lake debris can effect the isotopes of the lacustrine carbonate.All samples were first oven-dried at 40℃ and ground into fine powder.After standard routine of treatment,the carbon isotope and oxygen isotope results were analyzed using a MAT -252 Mass Spectrometer,with a precision of better than 0.1‰ . The analyses of stable isotopes of carbonates from the SG-1 core suggest that the carbonates in this core are authigenic and the oxygen isotope mainly reflects the regional aridity.The carbonate isotope records indicate that there exists an evident long-term persistent aridification of Asian inland since ca.1.0Ma,followed by an accelerated drying since ca.0.6Ma.The Qaidam paleo-lake changed from brackish lake to salt lake and finally playa accordingly. From 1.0Ma to 0.6Ma,the carbon and oxygen isotope values change from -1.4‰ to 3.7‰ and -2.9‰ to 3.9‰,respectively,indicating a minor drying trend.We speculate that both the northern hemisphere glaciation and the uplift of the Tibetan Plateau in the mid-Pleistocene（Kunhuang Movement）occurred at about 1.0Ma might have collectively prompted the drying of the Asian inland during this time. Since 0.6Ma,both the carbon and oxygen isotope values show sharp synchronous increasing trends（changed from -0.6‰ to 7.3‰ and -1.7‰ to 11.9‰,respectively）,which are discordant with the marine oxygen isotope records.We suggest that the rapid uplift of the Tibetan Plateau（Kunhuang Movement C）might have contributed to the aridification of the Asian inland through changing or enhancing the westerly jet and blocking the moisture input from the westerlies and monsoons.