末次间冰期以来临夏地区气候变化的黏土矿物学及地球化学记录

CLAY MINERALOGICAL AND GEOCHEMICAL RECORDS OF PALEOCLIMATE IN LINXIA SINCE THE LAST INTERGLACIAL

临夏盆地塔湾剖面黄土-古土壤沉积物的黏土矿物学及常量元素特征分析表明,末次间冰期以来临夏地区气候变化经历了从相对温暖湿润(Ⅰ阶段:92.4～78.8 ka)—寒冷干燥(Ⅱ阶段:78.8～59.8ka)—相对温暖湿润(Ⅲ阶段:59.8～47.7 ka)—寒冷干燥(Ⅳ阶段:47.7 ka以来)等4个阶段的演化。黄土-古土壤中黏土矿物以伊利石、绿泥石、伊/蒙混层及坡缕石等为主,并且坡缕石在沉积物中广泛发育,表明末次间冰期以来临夏地区总体上以相对干燥为主。w(TiO2)/w(Al2O3)比值表明沉积物剖面中黄土-古土壤层可能具有相同的物源,黄土-古土壤交替的发育特征应该为不同时期风化程度差别所致,与当时的大气降水量及温度密切相关。在相对温暖湿润的时期,黏土矿物相对含量、化学蚀变指数CIA值及w(Al2O3)/w(Fe2O3)、w(SiO2)/w(Al2O3)、w(SiO2)/w(Fe2O3)和w(MgO)/w(Al2O3)等氧化物比率变化波动较大;在寒冷干燥的时期,黏土矿物相对含量、CIA值及各氧化物比率较为稳定。黏土矿物学及常量元素变化特征共同揭示92.4～78.8 ka期间和47.7 ka以来出现明显的降温事件,说明临夏地区末次间冰期以来气候环境的不稳定性,可能与高原冬、夏季风的此消彼长密切相关。

Clay mineralogical and chemical elements analyses of the loess-paloesol deposit of Tawan Profile in Linxia Basin indicated that the climate of Linxia experienced a four-phase alteration, i.e. relatively warm/humid （92.4 - 78.8 ka） -cold/arid （78.8- 59.8 ka） -relatively warm/humid （59.8 - 47.7 ka） -cold/arid （47.7 ka to present） since the last interglacial period. The loess-paleosol deposit of the profile was dominated with illite, chlorite, mixed layer illite-smectite and minor palygorskite. Palygorskite was ubiquitous in the sediment, suggesting that on the whole the region was relatively arid throughout the period, w（TiO2）/w（ Al2O3 ） ratio of the loess-paleosol deposits indicated of the same origin. The loess-paleosol alternation was attributed to different weathering intensities, which were closely related to rainfall and temperature conditions in the period. Relative proportions of clay minerals, CIA values, w （ Al2O3 ）/w （ Fe2O3 ）, w（ SiO2 ）/w（Al2O3 ）, w（ SiO2 ）/w（ Fe2O3 ） and w（ MgO）/w（ Al2O3 ） varied quite sharply during the warm and humid period, but rather stable during the cold and arid period. The variation of the clay mineralogical features and major elements indicated significant cooling events occurred at the period of 92.4 - 78.8 ka and 47.7 ka - present, suggesting unstability of the climate environment in Linxia since the last interglacial period, which was probably related to the winter and summer monsoons in the plateau.