| The Holocene is a period of rapid development of human civilization.The emergence and rapid rise of agricultural civilization triggered a series of environmental problems,such as environmental deterioration and human-land conflict.The dramatic changes in climate in a short period of time often lead to a series of major natural disasters,among which the most extensive and profound are the impacts of geological disasters(collapses,landslides,and mudslides).The prehistorical catastrophes in the Lajia Ruins in the Guanting Basin are currently one of the hot topics in the field of global environmental change research.However,many scholars have different views on the cause of the Lajia disaster.In essence,it is not clear about the properties and sources of sediments,and stratigraphic sequence and Holocene climate change of the Lajia Ruins in the Guanting basin of the upper Yellow River.Our field investigations were carried through along the upper reaches of the Yellow River in 2010-2018.Detailed pedological,sedimentological and stratigraphic observations were carried out over the Lajia Ruins within the Guanting Basin.A comparative study of the Lajia Ruins profile(LJYZ)and the other related locations in the Guanting Basin and Jishixia Gorge.Fresh sediments without being disturbed by past human activities were collected systematically,the sedimentary features(grain-size,magnetic susceptibility,loss on ignition,total organic carbon),geochemical features and age of sediments were analyzed in the laboratory.We focus on the features and source of the two layers of conglomerated red clay(RC1,RC2),and the sand of the archaeological agent in XI district of Lajia Ruins;the surface process of mudflow disasters;the analysis of the Holocene climate changes in the Guanting basin in the upper Yellow River and climate deterioration period in the middle Holocene;the analysis of the the connection between the destruction of Qijia culture at the Lajia Ruins and human activities further revealed the fundamental cause of the destruction of the Lajia Ruins in the Guanting Basin.Following is the main conclusions:(1)Identifying the properties and sources of different sediments:Comparative study on the two layers of conglomerated red clay in the Lajia Ruins withn the Guanitng Basin,the deposits at the Ganggou Gully head site at the foot of the Great Red Hills behind the Lajia Ruins within the Guanting Basin and Yellow River flood slackwater deposits(SWD)at the Jishixia Gorge has been made in this paper.Comparative study on the sand of the archaeological agent in XI district of Lajia Ruins with the deposits at the Lvjiagou Gully,sediments derived from the floodplain of the Yellow Rriver and the ancient dammed lake fine sediments in the Jishixia Gorge has been made in this paper.we can conclude that the two layers of conglomerated red clay and the sand of the archaeological agent in XI district of Lajia Ruins were formed by the flash floods and debris flows in the gullies behind the Lajia Ruins,where the hills consisted of the unconsolidated red clay and gravel from the Tertiary period.(2)The two layers of conglomerated red clay(RC1,RC2)were dated to around 3850 a BP and aroud 3600 a BP,respectively,using the stratigraphic contrast,OSL and the 14C method.Therefore,such a conclusion can be deduced:Around 3850 a BP,a major palaeo-earthquake first devastated the settlement,and then a rainstorm-caused rain-mudflow buried the settlement.In the following 200 years,several rainstorm-caused rain-mudflow occurred intermittently until about 3600 a BP,immense mudflows coming along the tributary gullies comes back again,which ended the second large-scale rain-mudflow.(3)Holocene climate change in the Guanting basin of the upper Yellow River:At late Pleistocene(11,500 a BP);the climate was dry and cold,fluctuates greatly,aeolian dust accumulated,the biological soiling effect was weak,presenting a semi-desert grassland landscape.At early Holocene(11,500-8500 a BP),the climate was relatively dry and unstable.During the middle Holocene(8500-3100a BP),the climate was wetter and warmer,the vegetation was lush,and then was active bio-pedogenic processes.However,the climate during this period was not stable,but there was a period of deteriorating climate.During the late Holocene(3100 a BP),the accumulation rate of dust was accelere,the climate was relatively cold and dry,weak bio-pedogenic processes,with the enhancement of human activities,which became another relatively arid stage during Holocene.(4)Implications of the early human impact and the feedbacks:Through the analysis of the prehistoric climate change and excessive human activities in the Guanting Basin,it is revealed that the fundamental cause of the destruction of the Qijia Culture in the Lajia Ruins was unreasonable human activities.The analysis of the interconnections between global climate change and the disappearance of human civilization from 4200?4000 a BP reveals that there is a negative feedback effect between excessive human activities and climate change.Enhanced human activities and over-exploitation of natural resources increased the vulnerability of the communities to detrimental environmental change and catastrophe.These results provides important guiding significance for us to rationally develop and utilize land resources in the future,handle people-land relations well,avoid natural disasters,and realize the sustainable development of human-earth relations. |
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