Holocene Climate Change Of The Nile River Basin And The Response Of Early Agriculture In Its Delta

The Nile Delta is of geographical and morphodynamical characteristics,developed as wave-dominated pattern and in an arid climate setting.The delta is also hosting more than 50%of Egyptian population,to be one of the most densely-populated areas in the world.The environment setting of the delta is closely related to the hydroclimate change of the Nile River catchment,which has been continuously fed by a huge amount of water and sediment in the lower Nile and delta coast.This provides rich natural resrouces that had nourished the early human occupation and agricutlral civilization.Numerous archaeological sites that were found in the deltaplian that witnessed the long-lasting history of human activities since the Early-Middle Holocene,making the Nile Delta an ideal place for studying the relationship between climate change and early human activities.In this paper,core FA-1 in Faiyum Basin of the lower Nile region and core SH-1in Sais Hagar site located in the central-western Nile Delta were obtained for their continuous sedimentary and archaeological records,ideal for establishing the relationship between the Holocene climate fluctuations and early agricultural adaptation.A comprehensive analysis of AMS ¹⁴C dating,lithology,grain size,quartz morphology,palynology and other relevant proxies were applied to reconstruct the hydroclimatic history of the Nile catchment,and also established the connection between the environmental changes and human activities.In addition,the spatial and temporal distribution pattern of the ancient sites on the Nile delta was revealed,in order to discuss mechanism of the ancient human activities in response to the environmental changes.Main conclusions were as drawn below:1.Quartz roundness index was used to distinguish different sediment sources in core FA-1.The stained-rounded quartz grains in the sediment are mostly from the surrounding Sahara Desert by short-distance windblown process,while the transparent-angular quartz grains are mostly from the Nile Basin.Accordingly,it is believed that the content of transparent-angular quartz is related to the humid climate of the Nile basin,while the content of stained-rounded quartz represents the arid climate of the basin.Meanwhile,the Nile headwater pollen consist of Podocarpaceae and Polypodiaceae can indicate the humid climate in the basin on the basis of pollen analysis of core SH-1.The variation of quartz roundness and the Nile headwater pollen,revealed that the wet period in Africa started from ca.9700 cal yr BP and lasted to ca.5800 cal yr BP and then following a dry period during ca.5800–5500 cal yr BP.T extreme drought event formed in ca.4200 cal yr BP,leading to the decline of the Old Kingdom civilization?ca.4650–4110 cal yr BP?and onset of the long-time drought.The paleohydrological changes in the Nile basin reflected by the quartz roundness and Nile headwater pollen are closely related to the fluctuation of the African summer monsoon,which controlled by the northwrad-southward migration of the Intertropical Convergence Zone?ITCZ?affected by solar radiation changes since the Holocene.2.Wetland Indicators represented by Cyperaceae,Typha,Nymphaea and Myriophyllum was used to indicate the freshwater condition.While the Xerophytic Indicators mainly consists of Asteraceae,Artemisia,Chenopodiaceae/Amaranthaceae and Poaceae?<35?m?was used to indicate dry environment.The peak value of Wetland Indicators?43.3%?with the lowest percentage of Xerophytic Indicators?26.1%?during cal.9000–7000 cal yr BP indicates the Nile Delta was dominated by floodplain with a relative high water level.During ca.7000–5800 cal yr BP,the proportion of the Wetland Indicators decreased to 32.7%with a slight increase of Xerophytic Indicators?29.0%?indicates the transition from floodplain to wetland environment in the Nile Delta.After ca.5800 cal yr BP,the wetlands in the Nile Delta gradually shrank as recorded by the sharp decrease of Wetland Indicators?27.3%?,together with a high propotion of Xerophytic Indicators?40%?.As a result,the wetland was relaced by dry meadows.A peak value of Xerophytic Indicators?53%?indicates a hyper arid condition during ca.4200–4000 cal yr BP,also evidenced by the lowest value of Wetland Indicators?17.6%?.3.The first appearance of Poaceae??35?m?with a sharp increase of micro-charcoals??100?m?indicate the start of early agriculture in the Nile Delta at ca.6700cal yr BP,which is perhaps the earlist early agriculture in the Nile Delta.In addition,our pollen-spore spectrum revealed that Poaceae?<35?m?was present throughout the Holocene at this site,but not Poaceae??35?m?which appeared suddenly ca.6700 cal yr BP.This lack of correlation in the occurrence of the two species suggests that the cereal in the cultural layer was brought into the delta from somewhere outside of Egypt.Although many studies revealed that the early farming was introduced into the Nile Delta at ca.6000 cal yr BP.This study may change the start time of early agriculture in the Nile delta in terms of 700 years earlier than previously reported.4.In the early stage of agricultural activities?ca.6700–5500 cal yr BP?,Poaceae??35?m?decreased accompanied with the increase of Xerophytic Indicators.This indicates that early agriculture declined under dry climate.It can be seen that human activities were mainly to adapt to the environment in this period.While in the following period?ca.5500–2250 cal yr BP?,Poaceae??35?m?was in an increasing tendency when the climate was drier.This indicates that the human activities changed from passive adaptation to natural conditions to remaking nature under the long term dry condition.During the Predynastic-Early Dynastic period?ca.6000–4650 cal yr BP?,the growth of ancient human settlements and the expansion of site area may be associated with this transition.Due to the intensification of the drought,the ancient human settlements reduced and gradually concentrated in the eastern Nile Delta,especially during the extreme drought event occurred during ca.4200–4000 cal yr BP.Notably,the increase of Azolla after ca.4000 cal yr BP,suggesting expanded water bodies and agriculture-based needs in the delta area.This implies the improvement of capacity in terms of irrigation and related water management in the process of remaking nature.