全新世以来毛乌素沙地东南缘成壤环境演变研究——以榆林镇北台为例

Study on the evolution of pedogenesis environment on the southeast margin of the Mu Us Desert since Holocene

古土壤是重建古环境重要的信息源。毛乌素沙地东南缘的古土壤在发育程度上尚未有明确解释。陕西省榆林市的镇北台地区位于毛乌素沙地东南边缘,对气候响应敏感。本文选取厚度为524 cm的镇北台(ZBT)剖面,按照古土壤2 cm、风成砂5 cm的间隔共采集古土壤样品156个,风成砂样品58个。基于6个AMS14C和3个OSL年代样品,推测剖面古土壤底部年龄为10 cal.ka B.P.,顶部年龄为3.2 cal.ka B.P.;并结合土壤微形态特征,对镇北台剖面沉积物中常量元素特征与成壤强度进行分析探讨。结果表明,镇北台(ZBT)剖面常量元素组成以SiO2、Al2O3和Na2O为主。CIA指数指示剖面处于脱Ca、Na为主的初级风化阶段,反映了毛乌素沙地东南缘全新世以来可能以相对干旱的气候环境为主,风化成壤作用较弱,但其中也不乏幅度较小的冷暖气候旋回。土壤微形态特征也表明ZBT剖面风化程度较弱,主要矿物由石英、长石、云母和岩屑矿物构成,可见胶结物和泥岩岩屑等土壤形成物。其成壤强度变化与化学元素特征表现一致:10.0~8.1 cal.ka B.P.左右,成壤较弱,气候向暖湿转变;6.9 cal.ka B.P.左右,成壤作用稍有加强;5.5~3.7 cal.ka B.P.左右,成壤作用最强,气候相对暖湿,化学、生物风化加强,但此阶段具有明显的不稳定性.

alaeosol is an important source of information for the reconstruction of paleoenvironment because of its rich environmental information.There is no clear explanation for the degree of development of palaeosol in the southeastern margin of the Mu Us Desert.This paper selected a typical aeolian-palaeosol sequence(ZBT profile:38°20'N,109°44'E)located at junction of the southeastern margin of the Mu Us Desert and the Loess Plateau.ZBT profile belongs to Zhenbeitai area of Yulin City,Shaanxi Province.ZBT profile was 524 cm in depth,and 156 samples were taken at an interval of 2 cm in palaeosol section,while 58 samples were taken at an interval of 5 cm in aeolian section.Based on the 6 dates of AMS 14C and 3 dates of OSL,it is speculated that the age of bottom of the paleosol is 10 cal.ka B.P.,and the age of the top is 3.2 cal.ka B.P.The characteristics of major elements in the sediments and the intensity of the soil were analyzed.The results showed that the major elements of ZBT profile were SiO2,Al2O3 and Na2O,indicated that the section is always in the primary weathering stage with leaching Ca and Na from CIA index,which indicated that may be a relatively arid climate environment since the Holocene in the region,and the weathering effect was weak,but there were a small cold and warm climate cycling.The soil micromorphology also showed that the weathering of the ZBT profile was relatively weak.The main minerals were quartz,feldspar,mica and lithic minerals of palaeosol.Pedogenesis product such as cements and mudstones can be seen.The changes in the intensity of pedogenesis were consistent with the characteristics of chemical elements:From 10.0 cal.ka B.P.to 8.1 cal.ka B.P.,the pedogenesis were weak,and the climate shifted to warm and humid;At about 6.9 cal.ka B.P.,the pedogenesis were slightly strengthened;From 5.5 cal.ka B.P.to 3.7 cal.ka B.P.,pedogenesis were the strongest,the climate was relatively warm and humid,chemical and biological weathering strengthened,but this stage was obvious instability.