The Environmental Processes Of The Faiyiun, Egypt And Its Relationship With The Nile During The Holocenes

African monsoon is an important component of the global monsoon system. The evolution of African monsoon, especially the African summer monsoon variations during the Holocene, profoundly affected the environment and civilization advancement of the Africa. Although substantial achievements have been obtained on the studies of African monsoon evolution during the Holocene, the studies on the linkages of African monsoon with the environmental changes of North African were still weak due to the lack of continuous and high resolution sedimentary record.Faiyum basin, a closed basin of tectonic origin, is located in north Egypt, lower reaches of Nile River. Presently, the basin is fed by the Nile River by artificial waterways from its southeast. The unique geographical location and relatively closed sedimentary environment provide ideal opportunities with the understanding of Holocene climate and environmental changes in regions of north Sahara desert and the historical hydrological linkages between the basin and the Nile River.Sediment magnetism, integrated with organic carbon, grain-size, soluble ions and ostracods analyses were conducted on a sediment core (FA-1) retrieved from Faiyum basin, north Egypt. The time sequences of sediment were established on the AMS 14C-dated chronology. The result showed that grain-size and secondary alternation had minor effects on the characteristics of the sediment 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 antiferromagnetic minerals, together with the grain-size distribution similar to the surrounding desert deposits, indicating an aeolian origin from local sources. 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 River. 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.The change of soluble ions of the sediment showed that high concentration of NH4+, NO3- appeared at the early-mid Holocene, reflecting high productivity of the lake and a relative freshwater setting. The concentration of soluble ions dramatically increased since ca.5.5 ka BP, combined with intensive gypsum layers, indicating a drier environment in the lake basin. During ca.5.0～2.1 ka BP, it witnessed higher concentrations of soluble ions especially K+, SO42-, Ca2+, Mg2+ and Na+ in the sediment, revealing the enhanced salinization in the basin possibly due to increasing aridity. Ions except CO32- and HCO3- showed low levels since the last ca.2.1 ka BP suggesting desalination processes likely due to fresh water input from the Nile River by man-made channel.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 Aeolian origin when no hydrological connection occurred between the basin and the Nile prior to the Holocene. During the early-to-middle Holocene (ca.10-5.4ka BP), the sediment source in the lake basin was mainly of fluvial origin when the Nile drained into the basin, largely driven by intensified monsoon activities in the Africa. High lake level and low salinity prevails during this period. With weakening hydrological linkage with the Nile in the late Holocene from declining monsoon rainfall, the basin was fed by relatively more magnetic materials from the Blue Nile. Lake level began to decline and the salinization enhanced. The rapid increase in sediment magnetism in the last ca.2.0 ka BP had recorded the accelerated basin soil erosion by increasing human activities such as reclamation and farming rather than natural forcing.