中中新世秦安黄土-古土壤序列磁性增强机制及其环境意义  

Magneticenhancement mechanism of loess-soil sequence at Qinan during Middle Miocene and its paleoclimate significance

作　　者：周鸣亮 张春霞 郭正堂[2,3] ZHOU MingLiang;ZHANG ChunXia;GUO ZhengTang(College of Earth Science and Engineering,Shandong University of Science and Technology,Qingdao Shandong 266590,China;Key Laboratory of Cenozoic Geology and Environment,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China;InnovationAcademy for Earth Science,Chinese Academy of Sciences,Beijing 100029,China)

机构地区：[1]山东科技大学地球科学与工程学院,山东青岛266590 [2]中国科学院地质与地球物理研究所,中国科学院新生代地质与环境重点实验室,北京100029 [3]中国科学院地球科学研究院,北京100029

出　　处：《地球物理学报》2023年第8期3316-3334,共19页Chinese Journal of Geophysics

基　　金：国家自然基金项目(42072209和41888101);中国科学院(B类)战略性先导科技专项项目(XDB26000000)资助。

摘　　要：中中新世黄土高原风尘堆积序列磁化率显著增高,在区域内已成为地层对比标志,然而其磁性增强机制依然不清楚.本文对秦安地区QA-I剖面约14~17.1 Ma期间的黄土-古土壤样品进行了系统的环境磁学、岩石磁学(热磁性质、磁滞性质、非磁滞剩磁与饱和等温剩磁比值χARM/SIRM等)和色度指标分析.结果表明,QA-I剖面黄土-古土壤序列样品中磁性矿物为准单畴(PSD)磁铁矿、超顺磁/单畴(SP/SD)磁赤铁矿和赤铁矿,古土壤中细粒磁赤铁矿含量高于上下相邻黄土层.CBD(Citrate-Bicarbonate-Dithionite)处理方法显示,中中新世风尘序列中碎屑成因的PSD磁铁矿含量变化不大,约14.5~16.0 Ma期间成壤成因SP/SD磁赤铁矿含量的增多,是导致该时期风尘堆积序列磁化率增强的主要原因;这与该时期较低的土壤发育程度并不一致,表明CBD处理方法虽然可有效获取成壤成因SP/SD磁赤铁矿信息,却无法区分其为当地还是源区成壤成因.低温磁学研究表明,QA-I剖面约14.5~16.0 Ma期间SP/SD磁赤铁矿可能为碎屑磁铁矿颗粒的风化外壳,其含量高低变化与风尘样品中值粒径粗细变化一致,揭示出此类磁性颗粒含量受控于风力强弱,可能为源区风化成因.尽管中中新世古土壤样品的磁化率与上下黄土层相比可反映其成壤强度,但约14.5~16.0 Ma期间SP/SD磁赤铁矿含量变化可能同时受到当地成壤强度以及源区风化的影响,导致磁化率与当地成壤强度之间关系较为复杂.因此,中新世风尘堆积序列长尺度的磁化率趋势并不反映东亚夏季风强度的变化.Magnetic susceptibility of loess-soil sequence increased significantly during Mid-Miocene,which has been used as a regional marker for stratigraphic correlation on the Loess Plateau.However,the mechanism of magnetic enhancement is still unclear.In order to reveal the magnetic enhancement mechanism of Miocene's Red clay during the Mid-Miocene and its paleoclimatic significance,347 samples from 14 to 17.1 Ma are collected from QA-I loess-soil sequence in the Qin'an Area.In this paper,environmental parameters and rock magnetic measurements(thermomagnetic properties,hysteresis properties,χARM/SIRM,etc.)and color parameters of all samples are systematically analyzed.The results show that magnetic minerals of the loess-soil sequence are pseudo-single domain(PSD)magnetite,superparamagnetic/single domain(SP/SD)maghemite and hematite.More SP/SD maghemite presents in soil than in upper and lower loess layers.Magnetic characters of loess-soil samples before and after CBD(Citrate-Bicarbonate-Dithionite)treatment show that concentrations of detrital PSD magnetite of the loess-soil sequence are almost stable.The increase of pedogenic fine particle(SP/SD)maghemite concentrations is the main reason for the magnetic susceptibility enhancement of the loess-soil sequence during about 14.5~16.0 Ma.However,the redness of the loess-soil sequence correlated with a pedogenetic degree did not increase significantly during this period.This inconsistency indicates that these SP/SD maghemite minerals removed by CBD treatment may not be formed by local pedogenetic weathering.Low-temperature magnetic properties of samples before and after CBD treatment during about 14.5~16.0 Ma show that SP/SD maghemite may be the weathering crust of detrital magnetite particles,and their variations are consistent with the median grainsize of samples during about 14.5~16.0 Ma.These phenomena reveal that these magnetic particles with SP/SD maghemite possibly are formed in the source area,and their contents in loess-paleosol are mainly controlled by the streng

关 键 词：中中新世 环境磁学 CBD处理 色度 成壤强度 

分 类 号：P318[天文地球—固体地球物理学]