湖泊沉积物磁学特征指示的埃及 Faiyum 盆地全新世以来的环境演变∗

摘要

Sediment magnetism, integrated with organic carbon, grain⁃size and ostracods analyses were conducted on a sediment core ( FA⁃1) retrieved from Faiyum basin, north Egypt. The time sequences of sediment magnetism were established on the AMS14 C⁃dated chronology. The result showed that grain⁃size and secondary alternation had minor effects on the characteristics of the sedi⁃ment magnetism, and changes in sediment magnetism basically reflected the relative proportion of different sources contributing to the basin. Prior to the Holocene, weak sediment magnetism featured by incomplete antiferro magnetic minerals, together with the grain⁃size distribution similar to the surrounding desert deposits, indicating an aeolianorigin. The sediment magnetism kept relatively stable with high organic content during the early⁃to⁃middle Holocene at ca. 10-5.4 ka BP, suggesting the stable material inputs to the basin from the Nile. The last ca. 5.4 ka BP especially since the last ca. 4.2 ka BP, evident variations in sediment magnetism indicated the increasing contribution of material from the Blue Nile when rainfall declined in the Nile catchment. Of note, the change of sediment magnetism since the last ca. 2.0 ka BP largely exhibited the intensified human involves.Briefly, the environment settings of Faiyum basin since the Holocene were controlled by the hydrological linkage between the basin and the Nile. The sediment was primarily of Aeolianorigin when no hydrological connection occurred between the basin and the Nile prior to the Holocene.During the early⁃to⁃middle Holocene (ca.10-5.4 ka BP), the sediment source in the lake basin was mainly of fluvial ori⁃gin when the Nile drained into the basin, largely driven by intensified monsoon activities in the Africa.With weakening hydrological linkage with the Nilein the late Holocene from declining monsoon rainfall, the basin was fed by relatively more magnetic materials from the Blue Nile.The rapid increase in sediment magnetism in the last ca. 2.0 ka BP had recorded the accelerated basin soil ero⁃sion by increasing human activities such as reclamation and farming rather than natural forcing.%利用埃及北部Faiyum盆地获得的高取芯率沉积物岩芯，进行沉积物多种磁性参数的测量，结合有机碳、介形虫、粒度等分析，在AMS14 C加速器测年的基础上，建立全新世以来湖泊沉积物磁性特征变化的时间序列．结果表明，粒度效应以及沉积后的各种次生作用对沉积物的磁性特征没有明显的影响，磁性变化主要反映了沉积物不同来源组成的相对变化．全新世前沉积物磁性较弱，主要含不完全反铁磁性矿物，与周边沙漠的物质相似，结合其粒度特征，沉积物来源应以近源物质为主．全新世早中期（约105．4 ka BP）沉积物磁性变化相对稳定，有机质含量也较高，指示了来自尼罗河较为稳定的物质供应；而大约5．4 ka BP尤其最近约4．2 ka BP以来，磁性的明显变化反映了流域降水减少情况下，来自青尼河物质贡献的相对增加；最近约2．0 ka BP以来沉积物的磁性变化，则更多地与盆地人类活动的强化有关．总体而言， Faiyum盆地全新世以来的环境演变主要受控于全新世以来尼罗河与盆地的水力学联系．即：全新世前盆地未与尼罗河连通时，沉积物主要来源于周边沙漠的风成物质；而受全新世早－中期来流域季风降水增加的影响，泛滥的尼罗河为盆地提供了相对稳定的物质供应，湖泊也处于高湖面；全新世晚期以来，随着流域干旱化的加剧，尼罗河与盆地的连通性开始减弱，来自高磁性的青尼罗河物质贡献开始相对增加．最近约2．0 ka BP以来，虽然仍有人工运河连接尼罗河与盆地，但沉积物磁性的显著变化更多地反映了盆地人类活动的不断强化．