【摘要】 黄土高原第四纪黄土伊利石结晶度是东亚夏季风的有效指标,但以前对该指标的研究主要集中在末次间冰期以来的黄土沉积。本文采用X射线衍射技术对西峰第四纪黄土(0~2.6Ma B.P.)和三趾马红土上部(2.6~3.6Ma B.P.)代表性样品的粘土矿物(<2μm)进行了测试,结合伊利石化学指数I(002)/I(001)值对伊利石结晶度K(u|¨)bler指数(KI值)指示的环境信息进行了分析。结果显示,第四纪黄土伊利石结晶度主要继承了原始风尘特征,而第四纪古土壤伊利石结晶度指数KI值均显著高于相邻黄土,与伊利石化学指数I(002)/I(001)值和成壤特征有较好的一致性,表明KI指数可以有效地指示整个第四纪黄土沉积的风化成壤强度,对研究东亚夏季风长期演化具有重要的潜力;上新世三趾马红土KI值较高的特征表明上新世期间我国北方夏季风环流整体上较第四纪时期强盛。进一步分析KI值与I(002)/I(001)值的关系后发现二者具有较好的线性正相关,表明伊利石结晶度的变化是由于风尘堆积在风化成壤过程中,伴随着Fe-Mg离子的不断淋失,伊利石晶体结构发生改变而引起的。更多还原

【Abstract】 Illite crystallinity of loess deposits is a useful proxy for the East Asian summer monsoon in the Chinese Loess Plateau.However,a few studies have only mainly centered on the record since the last interglacial periods.Examination of the paleoenvironmental information of illite crystallinity for the eolian dust deposits over a longer time span would significantly improve our understanding of the East Asian monsoon climate.In this paper,illite crystallinity and illite chemical index were investigated on the <2μm of samples from Quaternary loess-palesol and Hipparion Red Clay sequences.The Xifeng section（35°53’N,107°58’E）,the typical eolian section in the eastern Loess Plateau and is about 228.4m in thickness is selected for this study.The section can be divided into two portions;the upper Quaternary loess-soil sequence（172.5m in thickness） and the lower Late Miocene-Pliocene red clay（55.9m in thickness）.Magnetostratigraphic measurements have dated the boundary between the Quaternary loess-soil sequence and the red clay at around 2.6Ma B.P.,and the bottom age of the red clay sequence was dated at around 7.6Ma B.P.Previous studies show that the red clay sequence consists of three parts.The upper part（2.6 3.6Ma B.P.） is an eolian formation,similar to Quaternary loess-soil sequence;the middle part（6.2 3.6Ma B.P.） was significantly affected by groundwater oscillations;and the lower part（6.2 ⁷.6Ma B.P.） is water-reworked deposits related to alluvial and slope process.18 and 8 representative samples were selected from the Quaternary loess-soil sequence and the upper part of red clay（2.6³.6Ma B.P.）,respectively.In this study,all samples for clay minerals analysis were disaggregated in deionized water and treated with10%H₂O₂ and lmol/L HAC to remove organic materials and carbonate,respectively.The particles less than 2|xm were then subject to pipetting based on Stokes’ law.Oriented specimens were prepared on glass slides by the pipette method at room temperature.Clay minerals were measured by X-ray diffraction（XRD） on oriented mounts of claysized particles（<2μm）.Illite crystallinity（Kiibler index,KI） was obtained from the full width half maximum of the 1.Onm peak（Δ°2θ）.Lower values indicate higher crystallinity,characteristic of weak hydrolysis under dry and cold climate conditions in continental environments.Illite chemistry index I（002）/I（001） was determined by the ratio of the Illite（002） and Illite（001） peak areas.Ratios lower than 0.5 are found in Fe-Mg-rich illites,which are characteristics for physical erosion;while ratios higher than 0.5 are found in Al-rich illites,which are formed by strong hydrolysis.Results indicate that in the samples from Quaternary loess layers the KI values range from 0.310° ⁰.412°Δ2θ（mean of 0.356° Δ2θ）,mainly inherited from the characteristics of original dust.In contrast,the KI values（0.339°⁰.638°Δ2θ,with a mean of 0.514°Δ2θ） for samples from the Quaternary palesol units are significantly higher than the Quaternary loess layers（0.310° ⁰.412°Δ2θ,with a mean of 0.356°Δ2θ）.The changes in KI values in the Quaternary palesols are highly relevant to I（002）/I（001） values,which shows that KI can effectively recorded the strength of soil weathering in the whole Quaternary loess deposits and is a potential proxy for the changes in the intensity of East Asian Summer monsoon.The KI values（0.426° 0.794° Δ2θ,with a mean of0.627°Δ2θ） in Pliocene Hipparion Red Clay is rather higher than in the Quaternary loess deposits（0.310° ⁰.638°Δ2θ,with a mean of 0.435°Δ2θ）,indicating that summer monsoon in Pliocene is stronger than Quaternary in Northern China.Furthermore,the clear linear positive correlation between the KI and the I（002）/I（001）indicates that the changes of illite crystallinity may caused by the by Fe²⁺ and Mg²⁺ constantly leaching in soil weathering,which altered the illite crystal structure.The results also provide new evidence for the weathering sequence of illite in well-drained soils.更多还原